[리눅스 명령어]CentOS : man nmap

[리눅스 명령어]CentOS : man nmap

1. 용도 및 목적

네트워크 탐색 및 보안 도구, 포트 스캔(minimal 설치 시 포함 안됨 yum 설치 또는 단일 패키지이니 다운 받아 설치)

2. 자주 쓰는 옵션

-ping 확인

#nmap -sP 192.168.35.11

-TCP 포트 범위 확인

#nmap -sT -p 1-65535 192.168.35.11

-UDP 포트 범위 확인

#nmap -sU -p 1-65535 192.168.35.11

-일반적인 icmp 가 아닌 ack 패킷을 보낸 후 rst 로 응답 받기

#nmap -sT -PT -p 22 192.168.35.11

-일반적인 icmp 가 아닌 syn 패킷을 보내 스캔

#nmap -sT -PS -p 22 192.168.35.11

3. 활용 방법

-설치

#yum install nmap

4. 메뉴얼

#man nmap

NMAP(1) Nmap Reference Guide NMAP(1)

NAME

nmap - Network exploration tool and security / port scanner

SYNOPSIS

nmap [Scan Type...] [Options] {target specification}

DESCRIPTION

Nmap ("Network Mapper") is an open source tool for network exploration

and security auditing. It was designed to rapidly scan large networks,

although it works fine against single hosts. Nmap uses raw IP packets

in novel ways to determine what hosts are available on the network,

what services (application name and version) those hosts are offering,

what operating systems (and OS versions) they are running, what type of

packet filters/firewalls are in use, and dozens of other

characteristics. While Nmap is commonly used for security audits, many

systems and network administrators find it useful for routine tasks

such as network inventory, managing service upgrade schedules, and

monitoring host or service uptime.

The output from Nmap is a list of scanned targets, with supplemental

information on each depending on the options used. Key among that

information is the "interesting ports table".. That table lists the

port number and protocol, service name, and state. The state is either

open, filtered, closed, or unfiltered. Open. means that an

application on the target machine is listening for connections/packets

on that port. Filtered. means that a firewall, filter, or other

network obstacle is blocking the port so that Nmap cannot tell whether

it is open or closed. Closed. ports have no application listening on

them, though they could open up at any time. Ports are classified as

unfiltered. when they are responsive to Nmap´s probes, but Nmap cannot

determine whether they are open or closed. Nmap reports the state

combinations open|filtered. and closed|filtered. when it cannot

determine which of the two states describe a port. The port table may

also include software version details when version detection has been

requested. When an IP protocol scan is requested (-sO), Nmap provides

information on supported IP protocols rather than listening ports.

In addition to the interesting ports table, Nmap can provide further

information on targets, including reverse DNS names, operating system

guesses, device types, and MAC addresses.

A typical Nmap scan is shown in Example 1. The only Nmap arguments used

in this example are -A, to enable OS and version detection, script

scanning, and traceroute; -T4 for faster execution; and then the two

target hostnames.

Example 1. A representative Nmap scan

# nmap -A -T4 scanme.nmap.org

Nmap scan report for scanme.nmap.org (64.13.134.52)

Host is up (0.045s latency).

Not shown: 993 filtered ports

PORT STATE SERVICE VERSION

22/tcp open ssh OpenSSH 4.3 (protocol 2.0)

| ssh-hostkey: 1024 60:ac:4d:51:b1:cd:85:09:12:16:92:76:1d:5d:27:6e (DSA)

|_2048 2c:22:75:60:4b:c3:3b:18:a2:97:2c:96:7e:28:dc:dd (RSA)

25/tcp closed smtp

53/tcp open domain

70/tcp closed gopher

80/tcp open http Apache httpd 2.2.3 ((CentOS))

|_html-title: Go ahead and ScanMe!

| http-methods: Potentially risky methods: TRACE

|_See http://nmap.org/nsedoc/scripts/http-methods.html

113/tcp closed auth

31337/tcp closed Elite

Device type: general purpose

Running: Linux 2.6.X

OS details: Linux 2.6.13 - 2.6.31, Linux 2.6.18

Network Distance: 13 hops

TRACEROUTE (using port 80/tcp)

HOP RTT ADDRESS

[Cut first 10 hops for brevity]

11 80.33 ms layer42.car2.sanjose2.level3.net (4.59.4.78)

12 137.52 ms xe6-2.core1.svk.layer42.net (69.36.239.221)

13 44.15 ms scanme.nmap.org (64.13.134.52)

Nmap done: 1 IP address (1 host up) scanned in 22.19 seconds

The newest version of Nmap can be obtained from http://nmap.org. The

newest version of this man page is available at

http://nmap.org/book/man.html. It is also included as a chapter of

Nmap Network Scanning: The Official Nmap Project Guide to Network

Discovery and Security Scanning (see http://nmap.org/book/).

OPTIONS SUMMARY

This options summary is printed when Nmap is run with no arguments, and

the latest version is always available at

http://nmap.org/data/nmap.usage.txt. It helps people remember the most

common options, but is no substitute for the in-depth documentation in

the rest of this manual. Some obscure options aren´t even included

here.

Nmap 5.51 ( http://nmap.org )

Usage: nmap [Scan Type(s)] [Options] {target specification}

TARGET SPECIFICATION:

Can pass hostnames, IP addresses, networks, etc.

Ex: scanme.nmap.org, 192.168.0.1; 10.0.0-255.1-254

-iL : Input from list of hosts/networks

-iR : Choose random targets

--exclude : Exclude hosts/networks

--excludefile : Exclude list from file

HOST DISCOVERY:

-sL: List Scan - simply list targets to scan

-sn: Ping Scan - disable port scan

-Pn: Treat all hosts as online -- skip host discovery

-PS/PA/PU/PY[portlist]: TCP SYN/ACK, UDP or SCTP discovery to given ports

-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes

-PO[protocol list]: IP Protocol Ping

-n/-R: Never do DNS resolution/Always resolve [default: sometimes]

--dns-servers : Specify custom DNS servers

--system-dns: Use OS´s DNS resolver

--traceroute: Trace hop path to each host

SCAN TECHNIQUES:

-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans

-sU: UDP Scan

-sN/sF/sX: TCP Null, FIN, and Xmas scans

--scanflags : Customize TCP scan flags

-sI : Idle scan

-sY/sZ: SCTP INIT/COOKIE-ECHO scans

-sO: IP protocol scan

-b : FTP bounce scan

PORT SPECIFICATION AND SCAN ORDER:

-p : Only scan specified ports

Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080,S:9

-F: Fast mode - Scan fewer ports than the default scan

-r: Scan ports consecutively - don´t randomize

--top-ports : Scan most common ports

--port-ratio : Scan ports more common than

SERVICE/VERSION DETECTION:

-sV: Probe open ports to determine service/version info

--version-intensity : Set from 0 (light) to 9 (try all probes)

--version-light: Limit to most likely probes (intensity 2)

--version-all: Try every single probe (intensity 9)

--version-trace: Show detailed version scan activity (for debugging)

SCRIPT SCAN:

-sC: equivalent to --script=default

--script=: is a comma separated list of

directories, script-files or script-categories

--script-args=: provide arguments to scripts

--script-trace: Show all data sent and received

--script-updatedb: Update the script database.

OS DETECTION:

-O: Enable OS detection

--osscan-limit: Limit OS detection to promising targets

--osscan-guess: Guess OS more aggressively

TIMING AND PERFORMANCE:

Options which take are in seconds, or append ´ms´ (milliseconds),

´s´ (seconds), ´m´ (minutes), or ´h´ (hours) to the value (e.g. 30m).

-T<0-5>: Set timing template (higher is faster)

--min-hostgroup/max-hostgroup : Parallel host scan group sizes

--min-parallelism/max-parallelism : Probe parallelization

--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout : Specifies

probe round trip time.

--max-retries : Caps number of port scan probe retransmissions.

--host-timeout : Give up on target after this long

--scan-delay/--max-scan-delay : Adjust delay between probes

--min-rate : Send packets no slower than per second

--max-rate : Send packets no faster than per second

FIREWALL/IDS EVASION AND SPOOFING:

-f; --mtu : fragment packets (optionally w/given MTU)

-D : Cloak a scan with decoys

-S : Spoof source address

-e : Use specified interface

-g/--source-port : Use given port number

--data-length : Append random data to sent packets

--ip-options : Send packets with specified ip options

--ttl : Set IP time-to-live field

--spoof-mac : Spoof your MAC address

--badsum: Send packets with a bogus TCP/UDP/SCTP checksum

OUTPUT:

-oN/-oX/-oS/-oG : Output scan in normal, XML, s|

and Grepable format, respectively, to the given filename.

-oA : Output in the three major formats at once

-v: Increase verbosity level (use -vv or more for greater effect)

-d: Increase debugging level (use -dd or more for greater effect)

--reason: Display the reason a port is in a particular state

--open: Only show open (or possibly open) ports

--packet-trace: Show all packets sent and received

--iflist: Print host interfaces and routes (for debugging)

--log-errors: Log errors/warnings to the normal-format output file

--append-output: Append to rather than clobber specified output files

--resume : Resume an aborted scan

--stylesheet : XSL stylesheet to transform XML output to HTML

--webxml: Reference stylesheet from Nmap.Org for more portable XML

--no-stylesheet: Prevent associating of XSL stylesheet w/XML output

MISC:

-6: Enable IPv6 scanning

-A: Enable OS detection, version detection, script scanning, and traceroute

--datadir : Specify custom Nmap data file location

--send-eth/--send-ip: Send using raw ethernet frames or IP packets

--privileged: Assume that the user is fully privileged

--unprivileged: Assume the user lacks raw socket privileges

-V: Print version number

-h: Print this help summary page.

EXAMPLES:

nmap -v -A scanme.nmap.org

nmap -v -sn 192.168.0.0/16 10.0.0.0/8

nmap -v -iR 10000 -Pn -p 80

SEE THE MAN PAGE (http://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES

TARGET SPECIFICATION

Everything on the Nmap command-line that isn´t an option (or option

argument) is treated as a target host specification. The simplest case

is to specify a target IP address or hostname for scanning.

Sometimes you wish to scan a whole network of adjacent hosts. For this,

Nmap supports CIDR-style. addressing. You can append /numbits to an

IPv4 address or hostname and Nmap will scan every IP address for which

the first numbits are the same as for the reference IP or hostname

given. For example, 192.168.10.0/24 would scan the 256 hosts between

192.168.10.0 (binary: 11000000 10101000 00001010 00000000) and

192.168.10.255 (binary: 11000000 10101000 00001010 11111111),

inclusive. 192.168.10.40/24 would scan exactly the same targets. Given

that the host scanme.nmap.org. is at the IP address 64.13.134.52, the

specification scanme.nmap.org/16 would scan the 65,536 IP addresses

between 64.13.0.0 and 64.13.255.255. The smallest allowed value is /0,

which targets the whole Internet. The largest value is /32, which scans

just the named host or IP address because all address bits are fixed.

CIDR notation is short but not always flexible enough. For example, you

might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or

.255 because they may be used as subnet network and broadcast

addresses. Nmap supports this through octet range addressing. Rather

than specify a normal IP address, you can specify a comma-separated

list of numbers or ranges for each octet. For example,

192.168.0-255.1-254 will skip all addresses in the range that end in .0

or .255, and 192.168.3-5,7.1 will scan the four addresses 192.168.3.1,

192.168.4.1, 192.168.5.1, and 192.168.7.1. Either side of a range may

be omitted; the default values are 0 on the left and 255 on the right.

Using - by itself is the same as 0-255, but remember to use 0- in the

first octet so the target specification doesn´t look like a

command-line option. Ranges need not be limited to the final octets:

the specifier 0-255.0-255.13.37 will perform an Internet-wide scan for

all IP addresses ending in 13.37. This sort of broad sampling can be

useful for Internet surveys and research.

IPv6 addresses can only be specified by their fully qualified IPv6

address or hostname. CIDR and octet ranges aren´t supported for IPv6

because they are rarely useful.

Nmap accepts multiple host specifications on the command line, and they

don´t need to be the same type. The command nmap scanme.nmap.org

192.168.0.0/8 10.0.0,1,3-7.- does what you would expect.

While targets are usually specified on the command lines, the following

options are also available to control target selection:

-iL inputfilename (Input from list) .

Reads target specifications from inputfilename. Passing a huge list

of hosts is often awkward on the command line, yet it is a common

desire. For example, your DHCP server might export a list of 10,000

current leases that you wish to scan. Or maybe you want to scan all

IP addresses except for those to locate hosts using unauthorized

static IP addresses. Simply generate the list of hosts to scan and

pass that filename to Nmap as an argument to the -iL option.

Entries can be in any of the formats accepted by Nmap on the

command line (IP address, hostname, CIDR, IPv6, or octet ranges).

Each entry must be separated by one or more spaces, tabs, or

newlines. You can specify a hyphen (-) as the filename if you want

Nmap to read hosts from standard input rather than an actual file.

The input file may contain comments that start with # and extend to

the end of the line.

-iR num hosts (Choose random targets) .

For Internet-wide surveys and other research, you may want to

choose targets at random. The num hosts argument tells Nmap how

many IPs to generate. Undesirable IPs such as those in certain

private, multicast, or unallocated address ranges are automatically

skipped. The argument 0 can be specified for a never-ending scan.

Keep in mind that some network administrators bristle at

unauthorized scans of their networks and may complain. Use this

option at your own risk! If you find yourself really bored one

rainy afternoon, try the command nmap -Pn -sS -p 80 -iR 0 --open.

to locate random web servers for browsing.

--exclude host1[,host2[,...]] (Exclude hosts/networks) .

Specifies a comma-separated list of targets to be excluded from the

scan even if they are part of the overall network range you

specify. The list you pass in uses normal Nmap syntax, so it can

include hostnames, CIDR netblocks, octet ranges, etc. This can be

useful when the network you wish to scan includes untouchable

mission-critical servers, systems that are known to react adversely

to port scans, or subnets administered by other people.

--excludefile exclude_file (Exclude list from file) .

This offers the same functionality as the --exclude option, except

that the excluded targets are provided in a newline-, space-, or

tab-delimited exclude_file rather than on the command line.

The exclude file may contain comments that start with # and extend

to the end of the line.

HOST DISCOVERY

One of the very first steps in any network reconnaissance mission is to

reduce a (sometimes huge) set of IP ranges into a list of active or

interesting hosts. Scanning every port of every single IP address is

slow and usually unnecessary. Of course what makes a host interesting

depends greatly on the scan purposes. Network administrators may only

be interested in hosts running a certain service, while security

auditors may care about every single device with an IP address. An

administrator may be comfortable using just an ICMP ping to locate

hosts on his internal network, while an external penetration tester may

use a diverse set of dozens of probes in an attempt to evade firewall

restrictions.

Because host discovery needs are so diverse, Nmap offers a wide variety

of options for customizing the techniques used. Host discovery is

sometimes called ping scan, but it goes well beyond the simple ICMP

echo request packets associated with the ubiquitous ping tool. Users

can skip the ping step entirely with a list scan (-sL) or by disabling

ping (-Pn), or engage the network with arbitrary combinations of

multi-port TCP SYN/ACK, UDP, SCTP INIT and ICMP probes. The goal of

these probes is to solicit responses which demonstrate that an IP

address is actually active (is being used by a host or network device).

On many networks, only a small percentage of IP addresses are active at

any given time. This is particularly common with private address space

such as 10.0.0.0/8. That network has 16 million IPs, but I have seen it

used by companies with less than a thousand machines. Host discovery

can find those machines in a sparsely allocated sea of IP addresses.

If no host discovery options are given, Nmap sends an ICMP echo

request, a TCP SYN packet to port 443, a TCP ACK packet to port 80, and

an ICMP timestamp request. These defaults are equivalent to the -PE

-PS443 -PA80 -PP options. An exception to this is that an ARP scan is

used for any targets which are on a local ethernet network. For

unprivileged Unix shell users, the default probes are a SYN packet to

ports 80 and 443 using the connect system call.. This host discovery

is often sufficient when scanning local networks, but a more

comprehensive set of discovery probes is recommended for security

auditing.

The -P* options (which select ping types) can be combined. You can

increase your odds of penetrating strict firewalls by sending many

probe types using different TCP ports/flags and ICMP codes. Also note

that ARP discovery (-PR). is done by default against targets on a

local ethernet network even if you specify other -P* options, because

it is almost always faster and more effective.

By default, Nmap does host discovery and then performs a port scan

against each host it determines is online. This is true even if you

specify non-default host discovery types such as UDP probes (-PU). Read

about the -sn option to learn how to perform only host discovery, or

use -Pn to skip host discovery and port scan all target hosts. The

following options control host discovery:

-sL (List Scan) .

The list scan is a degenerate form of host discovery that simply

lists each host of the network(s) specified, without sending any

packets to the target hosts. By default, Nmap still does

reverse-DNS resolution on the hosts to learn their names. It is

often surprising how much useful information simple hostnames give

out. For example, fw.chi is the name of one company´s Chicago

firewall. Nmap also reports the total number of IP addresses at

the end. The list scan is a good sanity check to ensure that you

have proper IP addresses for your targets. If the hosts sport

domain names you do not recognize, it is worth investigating

further to prevent scanning the wrong company´s network.

Since the idea is to simply print a list of target hosts, options

for higher level functionality such as port scanning, OS detection,

or ping scanning cannot be combined with this. If you wish to

disable ping scanning while still performing such higher level

functionality, read up on the -Pn (skip ping) option.

-sn (No port scan) .

This option tells Nmap not to do a port scan after host discovery,

and only print out the available hosts that responded to the scan.

This is often known as a "ping scan", but you can also request that

traceroute and NSE host scripts be run. This is by default one step

more intrusive than the list scan, and can often be used for the

same purposes. It allows light reconnaissance of a target network

without attracting much attention. Knowing how many hosts are up is

more valuable to attackers than the list provided by list scan of

every single IP and host name.

Systems administrators often find this option valuable as well. It

can easily be used to count available machines on a network or

monitor server availability. This is often called a ping sweep, and

is more reliable than pinging the broadcast address because many

hosts do not reply to broadcast queries.

The default host discovery done with -sn consists of an ICMP echo

request, TCP SYN to port 443, TCP ACK to port 80, and an ICMP

timestamp request by default. When executed by an unprivileged

user, only SYN packets are sent (using a connect call) to ports 80

and 443 on the target. When a privileged user tries to scan targets

on a local ethernet network, ARP requests are used unless --send-ip

was specified. The -sn option can be combined with any of the

discovery probe types (the -P* options, excluding -Pn) for greater

flexibility. If any of those probe type and port number options are

used, the default probes are overridden. When strict firewalls are

in place between the source host running Nmap and the target

network, using those advanced techniques is recommended. Otherwise

hosts could be missed when the firewall drops probes or their

responses.

In previous releases of Nmap, -sn was known as -sP..

-Pn (No ping) .

This option skips the Nmap discovery stage altogether. Normally,

Nmap uses this stage to determine active machines for heavier

scanning. By default, Nmap only performs heavy probing such as port

scans, version detection, or OS detection against hosts that are

found to be up. Disabling host discovery with -Pn causes Nmap to

attempt the requested scanning functions against every target IP

address specified. So if a class B target address space (/16) is

specified on the command line, all 65,536 IP addresses are scanned.

Proper host discovery is skipped as with the list scan, but instead

of stopping and printing the target list, Nmap continues to perform

requested functions as if each target IP is active. To skip ping

scan and port scan, while still allowing NSE to run, use the two

options -Pn -sn together.

For machines on a local ethernet network, ARP scanning will still

be performed (unless --send-ip is specified) because Nmap needs MAC

addresses to further scan target hosts. In previous versions of

Nmap, -Pn was -P0. and -PN..

-PS port list (TCP SYN Ping) .

This option sends an empty TCP packet with the SYN flag set. The

default destination port is 80 (configurable at compile time by

changing DEFAULT_TCP_PROBE_PORT_SPEC. in nmap.h).. Alternate

ports can be specified as a parameter. The syntax is the same as

for the -p except that port type specifiers like T: are not

allowed. Examples are -PS22 and -PS22-25,80,113,1050,35000. Note

that there can be no space between -PS and the port list. If

multiple probes are specified they will be sent in parallel.

The SYN flag suggests to the remote system that you are attempting

to establish a connection. Normally the destination port will be

closed, and a RST (reset) packet sent back. If the port happens to

be open, the target will take the second step of a TCP

three-way-handshake. by responding with a SYN/ACK TCP packet. The

machine running Nmap then tears down the nascent connection by

responding with a RST rather than sending an ACK packet which would

complete the three-way-handshake and establish a full connection.

The RST packet is sent by the kernel of the machine running Nmap in

response to the unexpected SYN/ACK, not by Nmap itself.

Nmap does not care whether the port is open or closed. Either the

RST or SYN/ACK response discussed previously tell Nmap that the

host is available and responsive.

On Unix boxes, only the privileged user root. is generally able to

send and receive raw TCP packets.. For unprivileged users, a

workaround is automatically employed. whereby the connect system

call is initiated against each target port. This has the effect of

sending a SYN packet to the target host, in an attempt to establish

a connection. If connect returns with a quick success or an

ECONNREFUSED failure, the underlying TCP stack must have received a

SYN/ACK or RST and the host is marked available. If the connection

attempt is left hanging until a timeout is reached, the host is

marked as down. This workaround is also used for IPv6 connections,

as raw IPv6 packet building support is not yet available in Nmap..

-PA port list (TCP ACK Ping) .

The TCP ACK ping is quite similar to the just-discussed SYN ping.

The difference, as you could likely guess, is that the TCP ACK flag

is set instead of the SYN flag. Such an ACK packet purports to be

acknowledging data over an established TCP connection, but no such

connection exists. So remote hosts should always respond with a RST

packet, disclosing their existence in the process.

The -PA option uses the same default port as the SYN probe (80) and

can also take a list of destination ports in the same format. If an

unprivileged user tries this, or an IPv6 target is specified, the

connect workaround discussed previously is used. This workaround is

imperfect because connect is actually sending a SYN packet rather

than an ACK.

The reason for offering both SYN and ACK ping probes is to maximize

the chances of bypassing firewalls. Many administrators configure

routers and other simple firewalls to block incoming SYN packets

except for those destined for public services like the company web

site or mail server. This prevents other incoming connections to

the organization, while allowing users to make unobstructed

outgoing connections to the Internet. This non-stateful approach

takes up few resources on the firewall/router and is widely

supported by hardware and software filters. The Linux

Netfilter/iptables. firewall software offers the --syn convenience

option to implement this stateless approach. When stateless

firewall rules such as this are in place, SYN ping probes (-PS) are

likely to be blocked when sent to closed target ports. In such

cases, the ACK probe shines as it cuts right through these rules.

Another common type of firewall uses stateful rules that drop

unexpected packets. This feature was initially found mostly on

high-end firewalls, though it has become much more common over the

years. The Linux Netfilter/iptables system supports this through

the --state option, which categorizes packets based on connection

state. A SYN probe is more likely to work against such a system, as

unexpected ACK packets are generally recognized as bogus and

dropped. A solution to this quandary is to send both SYN and ACK

probes by specifying -PS and -PA.

-PU port list (UDP Ping) .

Another host discovery option is the UDP ping, which sends a UDP

packet to the given ports. For most ports, the packet will be

empty, though for a few a protocol-specific payload will be sent

that is more likely to get a response.. The payload database is

described at http://nmap.org/book/nmap-payloads.html.

The --data-length. option sends a fixed-length random payload for

all ports.

The port list takes the same format as with the previously

discussed -PS and -PA options. If no ports are specified, the

default is 40125.. This default can be configured at compile-time

by changing DEFAULT_UDP_PROBE_PORT_SPEC. in nmap.h.. A highly

uncommon port is used by default because sending to open ports is

often undesirable for this particular scan type.

Upon hitting a closed port on the target machine, the UDP probe

should elicit an ICMP port unreachable packet in return. This

signifies to Nmap that the machine is up and available. Many other

types of ICMP errors, such as host/network unreachables or TTL

exceeded are indicative of a down or unreachable host. A lack of

response is also interpreted this way. If an open port is reached,

most services simply ignore the empty packet and fail to return any

response. This is why the default probe port is 40125, which is

highly unlikely to be in use. A few services, such as the Character

Generator (chargen) protocol, will respond to an empty UDP packet,

and thus disclose to Nmap that the machine is available.

The primary advantage of this scan type is that it bypasses

firewalls and filters that only screen TCP. For example, I once

owned a Linksys BEFW11S4 wireless broadband router. The external

interface of this device filtered all TCP ports by default, but UDP

probes would still elicit port unreachable messages and thus give

away the device.

-PY port list (SCTP INIT Ping) .

This option sends an SCTP packet containing a minimal INIT chunk.

The default destination port is 80 (configurable at compile time by

changing DEFAULT_SCTP_PROBE_PORT_SPEC. in nmap.h). Alternate ports

can be specified as a parameter. The syntax is the same as for the

-p except that port type specifiers like S: are not allowed.

Examples are -PY22 and -PY22,80,179,5060. Note that there can be no

space between -PY and the port list. If multiple probes are

specified they will be sent in parallel.

The INIT chunk suggests to the remote system that you are

attempting to establish an association. Normally the destination

port will be closed, and an ABORT chunk will be sent back. If the

port happens to be open, the target will take the second step of an

SCTP four-way-handshake. by responding with an INIT-ACK chunk. If

the machine running Nmap has a functional SCTP stack, then it tears

down the nascent association by responding with an ABORT chunk

rather than sending a COOKIE-ECHO chunk which would be the next

step in the four-way-handshake. The ABORT packet is sent by the

kernel of the machine running Nmap in response to the unexpected

INIT-ACK, not by Nmap itself.

Nmap does not care whether the port is open or closed. Either the

ABORT or INIT-ACK response discussed previously tell Nmap that the

host is available and responsive.

On Unix boxes, only the privileged user root. is generally able to

send and receive raw SCTP packets.. Using SCTP INIT Pings is

currently not possible for unprivileged users.. The same

limitation applies to IPv6, which is currently not supported for

SCTP INIT Ping..

-PE; -PP; -PM (ICMP Ping Types) .

In addition to the unusual TCP, UDP and SCTP host discovery types

discussed previously, Nmap can send the standard packets sent by

the ubiquitous ping program. Nmap sends an ICMP type 8 (echo

request) packet to the target IP addresses, expecting a type 0

(echo reply) in return from available hosts.. Unfortunately for

network explorers, many hosts and firewalls now block these

packets, rather than responding as required by RFC 1122[2].. For

this reason, ICMP-only scans are rarely reliable enough against

unknown targets over the Internet. But for system administrators

monitoring an internal network, they can be a practical and

efficient approach. Use the -PE option to enable this echo request

behavior.

While echo request is the standard ICMP ping query, Nmap does not

stop there. The ICMP standards (RFC 792[3]. and RFC 950[4]. "a

host SHOULD NOT implement these messages". Timestamp and address

mask queries can be sent with the -PP and -PM options,

respectively. A timestamp reply (ICMP code 14) or address mask

reply (code 18) discloses that the host is available. These two

queries can be valuable when administrators specifically block echo

request packets while forgetting that other ICMP queries can be

used for the same purpose.

-PO protocol list (IP Protocol Ping) .

One of the newer host discovery options is the IP protocol ping,

which sends IP packets with the specified protocol number set in

their IP header. The protocol list takes the same format as do port

lists in the previously discussed TCP, UDP and SCTP host discovery

options. If no protocols are specified, the default is to send

multiple IP packets for ICMP (protocol 1), IGMP (protocol 2), and

IP-in-IP (protocol 4). The default protocols can be configured at

compile-time by changing DEFAULT_PROTO_PROBE_PORT_SPEC. in nmap.h.

Note that for the ICMP, IGMP, TCP (protocol 6), UDP (protocol 17)

and SCTP (protocol 132), the packets are sent with the proper

protocol headers. while other protocols are sent with no

additional data beyond the IP header (unless the --data-length.

option is specified).

This host discovery method looks for either responses using the

same protocol as a probe, or ICMP protocol unreachable messages

which signify that the given protocol isn´t supported on the

destination host. Either type of response signifies that the target

host is alive.

-PR (ARP Ping) .

One of the most common Nmap usage scenarios is to scan an ethernet

LAN. On most LANs, especially those using private address ranges

specified by RFC 1918[5], the vast majority of IP addresses are

unused at any given time. When Nmap tries to send a raw IP packet

such as an ICMP echo request, the operating system must determine

the destination hardware (ARP) address corresponding to the target

IP so that it can properly address the ethernet frame. This is

often slow and problematic, since operating systems weren´t written

with the expectation that they would need to do millions of ARP

requests against unavailable hosts in a short time period.

ARP scan puts Nmap and its optimized algorithms in charge of ARP

requests. And if it gets a response back, Nmap doesn´t even need to

worry about the IP-based ping packets since it already knows the

host is up. This makes ARP scan much faster and more reliable than

IP-based scans. So it is done by default when scanning ethernet

hosts that Nmap detects are on a local ethernet network. Even if

different ping types (such as -PE or -PS) are specified, Nmap uses

ARP instead for any of the targets which are on the same LAN. If

you absolutely don´t want to do an ARP scan, specify --send-ip.

--traceroute (Trace path to host) .

Traceroutes are performed post-scan using information from the scan

results to determine the port and protocol most likely to reach the

target. It works with all scan types except connect scans (-sT) and

idle scans (-sI). All traces use Nmap´s dynamic timing model and

are performed in parallel.

Traceroute works by sending packets with a low TTL (time-to-live)

in an attempt to elicit ICMP Time Exceeded messages from

intermediate hops between the scanner and the target host. Standard

traceroute implementations start with a TTL of 1 and increment the

TTL until the destination host is reached. Nmap´s traceroute starts

with a high TTL and then decrements the TTL until it reaches zero.

Doing it backwards lets Nmap employ clever caching algorithms to

speed up traces over multiple hosts. On average Nmap sends 5–10

fewer packets per host, depending on network conditions. If a

single subnet is being scanned (i.e. 192.168.0.0/24) Nmap may only

have to send two packets to most hosts.

-n (No DNS resolution) .

Tells Nmap to never do reverse DNS resolution on the active IP

addresses it finds. Since DNS can be slow even with Nmap´s built-in

parallel stub resolver, this option can slash scanning times.

-R (DNS resolution for all targets) .

Tells Nmap to always do reverse DNS resolution on the target IP

addresses. Normally reverse DNS is only performed against

responsive (online) hosts.

--system-dns (Use system DNS resolver) .

By default, Nmap resolves IP addresses by sending queries directly

to the name servers configured on your host and then listening for

responses. Many requests (often dozens) are performed in parallel

to improve performance. Specify this option to use your system

resolver instead (one IP at a time via the getnameinfo call). This

is slower and rarely useful unless you find a bug in the Nmap

parallel resolver (please let us know if you do). The system

resolver is always used for IPv6 scans.

--dns-servers server1[,server2[,...]] (Servers to use for reverse DNS

queries) .

By default, Nmap determines your DNS servers (for rDNS resolution)

from your resolv.conf file (Unix) or the Registry (Win32).

Alternatively, you may use this option to specify alternate

servers. This option is not honored if you are using --system-dns

or an IPv6 scan. Using multiple DNS servers is often faster,

especially if you choose authoritative servers for your target IP

space. This option can also improve stealth, as your requests can

be bounced off just about any recursive DNS server on the Internet.

This option also comes in handy when scanning private networks.

Sometimes only a few name servers provide proper rDNS information,

and you may not even know where they are. You can scan the network

for port 53 (perhaps with version detection), then try Nmap list

scans (-sL) specifying each name server one at a time with

--dns-servers until you find one which works.

PORT SCANNING BASICS

While Nmap has grown in functionality over the years, it began as an

efficient port scanner, and that remains its core function. The simple

command nmap target scans 1,000 TCP ports on the host target. While

many port scanners have traditionally lumped all ports into the open or

closed states, Nmap is much more granular. It divides ports into six

states: open, closed, filtered, unfiltered, open|filtered, or

closed|filtered.

These states are not intrinsic properties of the port itself, but

describe how Nmap sees them. For example, an Nmap scan from the same

network as the target may show port 135/tcp as open, while a scan at

the same time with the same options from across the Internet might show

that port as filtered.

The six port states recognized by Nmap

An application is actively accepting TCP connections, UDP datagrams

or SCTP associations on this port. Finding these is often the

primary goal of port scanning. Security-minded people know that

each open port is an avenue for attack. Attackers and pen-testers

want to exploit the open ports, while administrators try to close

or protect them with firewalls without thwarting legitimate users.

Open ports are also interesting for non-security scans because they

show services available for use on the network.

A closed port is accessible (it receives and responds to Nmap probe

packets), but there is no application listening on it. They can be

helpful in showing that a host is up on an IP address (host

discovery, or ping scanning), and as part of OS detection. Because

closed ports are reachable, it may be worth scanning later in case

some open up. Administrators may want to consider blocking such

ports with a firewall. Then they would appear in the filtered

state, discussed next.

Nmap cannot determine whether the port is open because packet

filtering prevents its probes from reaching the port. The filtering

could be from a dedicated firewall device, router rules, or

host-based firewall software. These ports frustrate attackers

because they provide so little information. Sometimes they respond

with ICMP error messages such as type 3 code 13 (destination

unreachable: communication administratively prohibited), but

filters that simply drop probes without responding are far more

common. This forces Nmap to retry several times just in case the

probe was dropped due to network congestion rather than filtering.

This slows down the scan dramatically.

The unfiltered state means that a port is accessible, but Nmap is

unable to determine whether it is open or closed. Only the ACK

scan, which is used to map firewall rulesets, classifies ports into

this state. Scanning unfiltered ports with other scan types such as

Window scan, SYN scan, or FIN scan, may help resolve whether the

port is open.

Nmap places ports in this state when it is unable to determine

whether a port is open or filtered. This occurs for scan types in

which open ports give no response. The lack of response could also

mean that a packet filter dropped the probe or any response it

elicited. So Nmap does not know for sure whether the port is open

or being filtered. The UDP, IP protocol, FIN, NULL, and Xmas scans

classify ports this way.

This state is used when Nmap is unable to determine whether a port

is closed or filtered. It is only used for the IP ID idle scan.

PORT SCANNING TECHNIQUES

As a novice performing automotive repair, I can struggle for hours

trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to

the task at hand. When I fail miserably and tow my jalopy to a real

mechanic, he invariably fishes around in a huge tool chest until

pulling out the perfect gizmo which makes the job seem effortless. The

art of port scanning is similar. Experts understand the dozens of scan

techniques and choose the appropriate one (or combination) for a given

task. Inexperienced users and script kiddies,. on the other hand, try

to solve every problem with the default SYN scan. Since Nmap is free,

the only barrier to port scanning mastery is knowledge. That certainly

beats the automotive world, where it may take great skill to determine

that you need a strut spring compressor, then you still have to pay

thousands of dollars for it.

Most of the scan types are only available to privileged users.. This

is because they send and receive raw packets,. which requires root

access on Unix systems. Using an administrator account on Windows is

recommended, though Nmap sometimes works for unprivileged users on that

platform when WinPcap has already been loaded into the OS. Requiring

root privileges was a serious limitation when Nmap was released in

1997, as many users only had access to shared shell accounts. Now, the

world is different. Computers are cheaper, far more people have

always-on direct Internet access, and desktop Unix systems (including

Linux and Mac OS X) are prevalent. A Windows version of Nmap is now

available, allowing it to run on even more desktops. For all these

reasons, users have less need to run Nmap from limited shared shell

accounts. This is fortunate, as the privileged options make Nmap far

more powerful and flexible.

While Nmap attempts to produce accurate results, keep in mind that all

of its insights are based on packets returned by the target machines

(or firewalls in front of them). Such hosts may be untrustworthy and

send responses intended to confuse or mislead Nmap. Much more common

are non-RFC-compliant hosts that do not respond as they should to Nmap

probes. FIN, NULL, and Xmas scans are particularly susceptible to this

problem. Such issues are specific to certain scan types and so are

discussed in the individual scan type entries.

This section documents the dozen or so port scan techniques supported

by Nmap. Only one method may be used at a time, except that UDP scan

(-sU) and any one of the SCTP scan types (-sY, -sZ) may be combined

with any one of the TCP scan types. As a memory aid, port scan type

options are of the form -sC, where C is a prominent character in the

scan name, usually the first. The one exception to this is the

deprecated FTP bounce scan (-b). By default, Nmap performs a SYN Scan,

though it substitutes a connect scan if the user does not have proper

privileges to send raw packets (requires root access on Unix) or if

IPv6 targets were specified. Of the scans listed in this section,

unprivileged users can only execute connect and FTP bounce scans.

-sS (TCP SYN scan) .

SYN scan is the default and most popular scan option for good

reasons. It can be performed quickly, scanning thousands of ports

per second on a fast network not hampered by restrictive firewalls.

It is also relatively unobtrusive and stealthy since it never

completes TCP connections. SYN scan works against any compliant TCP

stack rather than depending on idiosyncrasies of specific platforms

as Nmap´s FIN/NULL/Xmas, Maimon and idle scans do. It also allows

clear, reliable differentiation between the open, closed, and

filtered states.

This technique is often referred to as half-open scanning, because

you don´t open a full TCP connection. You send a SYN packet, as if

you are going to open a real connection and then wait for a

response. A SYN/ACK indicates the port is listening (open), while a

RST (reset) is indicative of a non-listener. If no response is

received after several retransmissions, the port is marked as

filtered. The port is also marked filtered if an ICMP unreachable

error (type 3, code 1, 2, 3, 9, 10, or 13) is received. The port is

also considered open if a SYN packet (without the ACK flag) is

received in response. This can be due to an extremely rare TCP

feature known as a simultaneous open or split handshake connection

(see http://nmap.org/misc/split-handshake.pdf).

-sT (TCP connect scan) .

TCP connect scan is the default TCP scan type when SYN scan is not

an option. This is the case when a user does not have raw packet

privileges or is scanning IPv6 networks. Instead of writing raw

packets as most other scan types do, Nmap asks the underlying

operating system to establish a connection with the target machine

and port by issuing the connect system call. This is the same

high-level system call that web browsers, P2P clients, and most

other network-enabled applications use to establish a connection.

It is part of a programming interface known as the Berkeley Sockets

API. Rather than read raw packet responses off the wire, Nmap uses

this API to obtain status information on each connection attempt.

When SYN scan is available, it is usually a better choice. Nmap has

less control over the high level connect call than with raw

packets, making it less efficient. The system call completes

connections to open target ports rather than performing the

half-open reset that SYN scan does. Not only does this take longer

and require more packets to obtain the same information, but target

machines are more likely to log the connection. A decent IDS will

catch either, but most machines have no such alarm system. Many

services on your average Unix system will add a note to syslog, and

sometimes a cryptic error message, when Nmap connects and then

closes the connection without sending data. Truly pathetic services

crash when this happens, though that is uncommon. An administrator

who sees a bunch of connection attempts in her logs from a single

system should know that she has been connect scanned.

-sU (UDP scans) .

While most popular services on the Internet run over the TCP

protocol, UDP[6] services are widely deployed. DNS, SNMP, and DHCP

(registered ports 53, 161/162, and 67/68) are three of the most

common. Because UDP scanning is generally slower and more difficult

than TCP, some security auditors ignore these ports. This is a

mistake, as exploitable UDP services are quite common and attackers

certainly don´t ignore the whole protocol. Fortunately, Nmap can

help inventory UDP ports.

UDP scan is activated with the -sU option. It can be combined with

a TCP scan type such as SYN scan (-sS) to check both protocols

during the same run.

UDP scan works by sending a UDP packet to every targeted port. For

some common ports such as 53 and 161, a protocol-specific payload

is sent, but for most ports the packet is empty.. The

--data-length option can be used to send a fixed-length random

payload to every port. If an ICMP port unreachable error (type 3,

code 3) is returned, the port is closed. Other ICMP unreachable

errors (type 3, codes 1, 2, 9, 10, or 13) mark the port as

filtered. Occasionally, a service will respond with a UDP packet,

proving that it is open. If no response is received after

retransmissions, the port is classified as open|filtered. This

means that the port could be open, or perhaps packet filters are

blocking the communication. Version detection (-sV) can be used to

help differentiate the truly open ports from the filtered ones.

A big challenge with UDP scanning is doing it quickly. Open and

filtered ports rarely send any response, leaving Nmap to time out

and then conduct retransmissions just in case the probe or response

were lost. Closed ports are often an even bigger problem. They

usually send back an ICMP port unreachable error. But unlike the

RST packets sent by closed TCP ports in response to a SYN or

connect scan, many hosts rate limit. ICMP port unreachable

messages by default. Linux and Solaris are particularly strict

about this. For example, the Linux 2.4.20 kernel limits destination

unreachable messages to one per second (in net/ipv4/icmp.c).

Nmap detects rate limiting and slows down accordingly to avoid

flooding the network with useless packets that the target machine

will drop. Unfortunately, a Linux-style limit of one packet per

second makes a 65,536-port scan take more than 18 hours. Ideas for

speeding your UDP scans up include scanning more hosts in parallel,

doing a quick scan of just the popular ports first, scanning from

behind the firewall, and using --host-timeout to skip slow hosts.

-sY (SCTP INIT scan) .

SCTP[7] is a relatively new alternative to the TCP and UDP

protocols, combining most characteristics of TCP and UDP, and also

adding new features like multi-homing and multi-streaming. It is

mostly being used for SS7/SIGTRAN related services but has the

potential to be used for other applications as well. SCTP INIT scan

is the SCTP equivalent of a TCP SYN scan. It can be performed

quickly, scanning thousands of ports per second on a fast network

not hampered by restrictive firewalls. Like SYN scan, INIT scan is

relatively unobtrusive and stealthy, since it never completes SCTP

associations. It also allows clear, reliable differentiation

between the open, closed, and filtered states.

This technique is often referred to as half-open scanning, because

you don´t open a full SCTP association. You send an INIT chunk, as

if you are going to open a real association and then wait for a

response. An INIT-ACK chunk indicates the port is listening (open),

while an ABORT chunk is indicative of a non-listener. If no

response is received after several retransmissions, the port is

marked as filtered. The port is also marked filtered if an ICMP

unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is received.

-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans) .

These three scan types (even more are possible with the --scanflags

option described in the next section) exploit a subtle loophole in

the TCP RFC[8] to differentiate between open and closed ports. Page

65 of RFC 793 says that "if the [destination] port state is CLOSED

.... an incoming segment not containing a RST causes a RST to be

sent in response." Then the next page discusses packets sent to

open ports without the SYN, RST, or ACK bits set, stating that:

"you are unlikely to get here, but if you do, drop the segment, and

return."

When scanning systems compliant with this RFC text, any packet not

containing SYN, RST, or ACK bits will result in a returned RST if

the port is closed and no response at all if the port is open. As

long as none of those three bits are included, any combination of

the other three (FIN, PSH, and URG) are OK. Nmap exploits this with

three scan types:

Null scan (-sN)

Does not set any bits (TCP flag header is 0)

FIN scan (-sF)

Sets just the TCP FIN bit.

Xmas scan (-sX)

Sets the FIN, PSH, and URG flags, lighting the packet up like a

Christmas tree.

These three scan types are exactly the same in behavior except for

the TCP flags set in probe packets. If a RST packet is received,

the port is considered closed, while no response means it is

open|filtered. The port is marked filtered if an ICMP unreachable

error (type 3, code 1, 2, 3, 9, 10, or 13) is received.

The key advantage to these scan types is that they can sneak

through certain non-stateful firewalls and packet filtering

routers. Another advantage is that these scan types are a little

more stealthy than even a SYN scan. Don´t count on this though—most

modern IDS products can be configured to detect them. The big

downside is that not all systems follow RFC 793 to the letter. A

number of systems send RST responses to the probes regardless of

whether the port is open or not. This causes all of the ports to be

labeled closed. Major operating systems that do this are Microsoft

Windows, many Cisco devices, BSDI, and IBM OS/400. This scan does

work against most Unix-based systems though. Another downside of

these scans is that they can´t distinguish open ports from certain

filtered ones, leaving you with the response open|filtered.

-sA (TCP ACK scan) .

This scan is different than the others discussed so far in that it

never determines open (or even open|filtered) ports. It is used to

map out firewall rulesets, determining whether they are stateful or

not and which ports are filtered.

The ACK scan probe packet has only the ACK flag set (unless you use

--scanflags). When scanning unfiltered systems, open and closed

ports will both return a RST packet. Nmap then labels them as

unfiltered, meaning that they are reachable by the ACK packet, but

whether they are open or closed is undetermined. Ports that don´t

respond, or send certain ICMP error messages back (type 3, code 1,

2, 3, 9, 10, or 13), are labeled filtered.

-sW (TCP Window scan) .

Window scan is exactly the same as ACK scan except that it exploits

an implementation detail of certain systems to differentiate open

ports from closed ones, rather than always printing unfiltered when

a RST is returned. It does this by examining the TCP Window field

of the RST packets returned. On some systems, open ports use a

positive window size (even for RST packets) while closed ones have

a zero window. So instead of always listing a port as unfiltered

when it receives a RST back, Window scan lists the port as open or

closed if the TCP Window value in that reset is positive or zero,

respectively.

This scan relies on an implementation detail of a minority of

systems out on the Internet, so you can´t always trust it. Systems

that don´t support it will usually return all ports closed. Of

course, it is possible that the machine really has no open ports.

If most scanned ports are closed but a few common port numbers

(such as 22, 25, 53) are filtered, the system is most likely

susceptible. Occasionally, systems will even show the exact

opposite behavior. If your scan shows 1,000 open ports and three

closed or filtered ports, then those three may very well be the

truly open ones.

-sM (TCP Maimon scan) .

The Maimon scan is named after its discoverer, Uriel Maimon.. He

described the technique in Phrack Magazine issue #49 (November

1996).. Nmap, which included this technique, was released two

issues later. This technique is exactly the same as NULL, FIN, and

Xmas scans, except that the probe is FIN/ACK. According to RFC

793[8] (TCP), a RST packet should be generated in response to such

a probe whether the port is open or closed. However, Uriel noticed

that many BSD-derived systems simply drop the packet if the port is

open.

--scanflags (Custom TCP scan) .

Truly advanced Nmap users need not limit themselves to the canned

scan types offered. The --scanflags option allows you to design

your own scan by specifying arbitrary TCP flags.. Let your

creative juices flow, while evading intrusion detection systems.

whose vendors simply paged through the Nmap man page adding

specific rules!

The --scanflags argument can be a numerical flag value such as 9

(PSH and FIN), but using symbolic names is easier. Just mash

together any combination of URG, ACK, PSH, RST, SYN, and FIN. For

example, --scanflags URGACKPSHRSTSYNFIN sets everything, though

it´s not very useful for scanning. The order these are specified in

is irrelevant.

In addition to specifying the desired flags, you can specify a TCP

scan type (such as -sA or -sF). That base type tells Nmap how to

interpret responses. For example, a SYN scan considers no-response

to indicate a filtered port, while a FIN scan treats the same as

open|filtered. Nmap will behave the same way it does for the base

scan type, except that it will use the TCP flags you specify

instead. If you don´t specify a base type, SYN scan is used.

-sZ (SCTP COOKIE ECHO scan) .

SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes

advantage of the fact that SCTP implementations should silently

drop packets containing COOKIE ECHO chunks on open ports, but send

an ABORT if the port is closed. The advantage of this scan type is

that it is not as obvious a port scan than an INIT scan. Also,

there may be non-stateful firewall rulesets blocking INIT chunks,

but not COOKIE ECHO chunks. Don´t be fooled into thinking that this

will make a port scan invisible; a good IDS will be able to detect

SCTP COOKIE ECHO scans too. The downside is that SCTP COOKIE ECHO

scans cannot differentiate between open and filtered ports, leaving

you with the state open|filtered in both cases.

-sI zombie host[:probeport] (idle scan) .

This advanced scan method allows for a truly blind TCP port scan of

the target (meaning no packets are sent to the target from your

real IP address). Instead, a unique side-channel attack exploits

predictable IP fragmentation ID sequence generation on the zombie

host to glean information about the open ports on the target. IDS

systems will display the scan as coming from the zombie machine you

specify (which must be up and meet certain criteria). This

fascinating scan type is too complex to fully describe in this

reference guide, so I wrote and posted an informal paper with full

details at http://nmap.org/book/idlescan.html.

Besides being extraordinarily stealthy (due to its blind nature),

this scan type permits mapping out IP-based trust relationships

between machines. The port listing shows open ports from the

perspective of the zombie host. So you can try scanning a target

using various zombies that you think might be trusted. (via

router/packet filter rules).

You can add a colon followed by a port number to the zombie host if

you wish to probe a particular port on the zombie for IP ID

changes. Otherwise Nmap will use the port it uses by default for

TCP pings (80).

-sO (IP protocol scan) .

IP protocol scan allows you to determine which IP protocols (TCP,

ICMP, IGMP, etc.) are supported by target machines. This isn´t

technically a port scan, since it cycles through IP protocol

numbers rather than TCP or UDP port numbers. Yet it still uses the

-p option to select scanned protocol numbers, reports its results

within the normal port table format, and even uses the same

underlying scan engine as the true port scanning methods. So it is

close enough to a port scan that it belongs here.

Besides being useful in its own right, protocol scan demonstrates

the power of open-source software. While the fundamental idea is

pretty simple, I had not thought to add it nor received any

requests for such functionality. Then in the summer of 2000,

Gerhard Rieger. conceived the idea, wrote an excellent patch

implementing it, and sent it to the nmap-hackers mailing list.. I

incorporated that patch into the Nmap tree and released a new

version the next day. Few pieces of commercial software have users

enthusiastic enough to design and contribute their own

improvements!

Protocol scan works in a similar fashion to UDP scan. Instead of

iterating through the port number field of a UDP packet, it sends

IP packet headers and iterates through the eight-bit IP protocol

field. The headers are usually empty, containing no data and not

even the proper header for the claimed protocol. The exceptions are

TCP, UDP, ICMP, SCTP, and IGMP. A proper protocol header for those

is included since some systems won´t send them otherwise and

because Nmap already has functions to create them. Instead of

watching for ICMP port unreachable messages, protocol scan is on

the lookout for ICMP protocol unreachable messages. If Nmap

receives any response in any protocol from the target host, Nmap

marks that protocol as open. An ICMP protocol unreachable error

(type 3, code 2) causes the protocol to be marked as closed Other

ICMP unreachable errors (type 3, code 1, 3, 9, 10, or 13) cause the

protocol to be marked filtered (though they prove that ICMP is open

at the same time). If no response is received after

retransmissions, the protocol is marked open|filtered

-b FTP relay host (FTP bounce scan) .

An interesting feature of the FTP protocol (RFC 959[9]) is support

for so-called proxy FTP connections. This allows a user to connect

to one FTP server, then ask that files be sent to a third-party

server. Such a feature is ripe for abuse on many levels, so most

servers have ceased supporting it. One of the abuses this feature

allows is causing the FTP server to port scan other hosts. Simply

ask the FTP server to send a file to each interesting port of a

target host in turn. The error message will describe whether the

port is open or not. This is a good way to bypass firewalls because

organizational FTP servers are often placed where they have more

access to other internal hosts than any old Internet host would.

Nmap supports FTP bounce scan with the -b option. It takes an

argument of the form username:password@server:port. Server is the

name or IP address of a vulnerable FTP server. As with a normal

URL, you may omit username:password, in which case anonymous login

credentials (user: anonymous password:-wwwuser@) are used. The port

number (and preceding colon) may be omitted as well, in which case

the default FTP port (21) on server is used.

This vulnerability was widespread in 1997 when Nmap was released,

but has largely been fixed. Vulnerable servers are still around, so

it is worth trying when all else fails. If bypassing a firewall is

your goal, scan the target network for port 21 (or even for any FTP

services if you scan all ports with version detection) and use the

ftp-bounce. NSE script. Nmap will tell you whether the host is

vulnerable or not. If you are just trying to cover your tracks, you

don´t need to (and, in fact, shouldn´t) limit yourself to hosts on

the target network. Before you go scanning random Internet

addresses for vulnerable FTP servers, consider that sysadmins may

not appreciate you abusing their servers in this way.

PORT SPECIFICATION AND SCAN ORDER

In addition to all of the scan methods discussed previously, Nmap

offers options for specifying which ports are scanned and whether the

scan order is randomized or sequential. By default, Nmap scans the most

common 1,000 ports for each protocol.

-p port ranges (Only scan specified ports) .

This option specifies which ports you want to scan and overrides

the default. Individual port numbers are OK, as are ranges

separated by a hyphen (e.g. 1-1023). The beginning and/or end

values of a range may be omitted, causing Nmap to use 1 and 65535,

respectively. So you can specify -p- to scan ports from 1 through

65535. Scanning port zero. is allowed if you specify it

explicitly. For IP protocol scanning (-sO), this option specifies

the protocol numbers you wish to scan for (0–255).

When scanning both TCP and UDP ports, you can specify a particular

protocol by preceding the port numbers by T: or U:. The qualifier

lasts until you specify another qualifier. For example, the

argument -p U:53,111,137,T:21-25,80,139,8080 would scan UDP ports

53, 111,and 137, as well as the listed TCP ports. Note that to scan

both UDP and TCP, you have to specify -sU and at least one TCP scan

type (such as -sS, -sF, or -sT). If no protocol qualifier is given,

the port numbers are added to all protocol lists. Ports can also

be specified by name according to what the port is referred to in

the nmap-services. You can even use the wildcards * and ? with the

names. For example, to scan FTP and all ports whose names begin

with "http", use -p ftp,http*. Be careful about shell expansions

and quote the argument to -p if unsure.

Ranges of ports can be surrounded by square brackets to indicate

ports inside that range that appear in nmap-services. For example,

the following will scan all ports in nmap-services equal to or

below 1024: -p [-1024]. Be careful with shell expansions and quote

the argument to -p if unsure.

-F (Fast (limited port) scan) .

Specifies that you wish to scan fewer ports than the default.

Normally Nmap scans the most common 1,000 ports for each scanned

protocol. With -F, this is reduced to 100.

Nmap needs an nmap-services file with frequency information in

order to know which ports are the most common. If port frequency

information isn´t available, perhaps because of the use of a custom

nmap-services file, -F means to scan only ports that are named in

the services file (normally Nmap scans all named ports plus ports

1–1024).

-r (Don´t randomize ports) .

By default, Nmap randomizes the scanned port order (except that

certain commonly accessible ports are moved near the beginning for

efficiency reasons). This randomization is normally desirable, but

you can specify -r for sequential (sorted from lowest to highest)

port scanning instead.

--port-ratio ratio

Scans all ports in nmap-services file with a ratio greater than the

one given. ratio must be between 0.0 and 1.1.

--top-ports n

Scans the n highest-ratio ports found in nmap-services file. n

must be 1 or greater.

SERVICE AND VERSION DETECTION

Point Nmap at a remote machine and it might tell you that ports 25/tcp,

80/tcp, and 53/udp are open. Using its nmap-services. database of

about 2,200 well-known services,. Nmap would report that those ports

probably correspond to a mail server (SMTP), web server (HTTP), and

name server (DNS) respectively. This lookup is usually accurate—the

vast majority of daemons listening on TCP port 25 are, in fact, mail

servers. However, you should not bet your security on this! People can

and do run services on strange ports..

Even if Nmap is right, and the hypothetical server above is running

SMTP, HTTP, and DNS servers, that is not a lot of information. When

doing vulnerability assessments (or even simple network inventories) of

your companies or clients, you really want to know which mail and DNS

servers and versions are running. Having an accurate version number

helps dramatically in determining which exploits a server is vulnerable

to. Version detection helps you obtain this information.

After TCP and/or UDP ports are discovered using one of the other scan

methods, version detection interrogates those ports to determine more

about what is actually running. The nmap-service-probes. database

contains probes for querying various services and match expressions to

recognize and parse responses. Nmap tries to determine the service

protocol (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC

BIND, Apache httpd, Solaris telnetd), the version number, hostname,

device type (e.g. printer, router), the OS family (e.g. Windows, Linux)

and sometimes miscellaneous details like whether an X server is open to

connections, the SSH protocol version, or the KaZaA user name). Of

course, most services don´t provide all of this information. If Nmap

was compiled with OpenSSL support, it will connect to SSL servers to

deduce the service listening behind that encryption layer.. When RPC

services are discovered, the Nmap RPC grinder. (-sR). is

automatically used to determine the RPC program and version numbers.

Some UDP ports are left in the open|filtered state after a UDP port

scan is unable to determine whether the port is open or filtered.

Version detection will try to elicit a response from these ports (just

as it does with open ports), and change the state to open if it

succeeds. open|filtered TCP ports are treated the same way. Note that

the Nmap -A option enables version detection among other things. A

paper documenting the workings, usage, and customization of version

detection is available at http://nmap.org/book/vscan.html.

When Nmap receives responses from a service but cannot match them to

its database, it prints out a special fingerprint and a URL for you to

submit if to if you know for sure what is running on the port. Please

take a couple minutes to make the submission so that your find can

benefit everyone. Thanks to these submissions, Nmap has about 6,500

pattern matches for more than 650 protocols such as SMTP, FTP, HTTP,

etc..

Version detection is enabled and controlled with the following options:

-sV (Version detection) .

Enables version detection, as discussed above. Alternatively, you

can use -A, which enables version detection among other things.

--allports (Don´t exclude any ports from version detection) .

By default, Nmap version detection skips TCP port 9100 because some

printers simply print anything sent to that port, leading to dozens

of pages of HTTP GET requests, binary SSL session requests, etc.

This behavior can be changed by modifying or removing the Exclude

directive in nmap-service-probes, or you can specify --allports to

scan all ports regardless of any Exclude directive.

--version-intensity intensity (Set version scan intensity) .

When performing a version scan (-sV), Nmap sends a series of

probes, each of which is assigned a rarity value between one and

nine. The lower-numbered probes are effective against a wide

variety of common services, while the higher-numbered ones are

rarely useful. The intensity level specifies which probes should be

applied. The higher the number, the more likely it is the service

will be correctly identified. However, high intensity scans take

longer. The intensity must be between 0 and 9.. The default is 7..

When a probe is registered to the target port via the

nmap-service-probes ports directive, that probe is tried regardless

of intensity level. This ensures that the DNS probes will always be

attempted against any open port 53, the SSL probe will be done

against 443, etc.

--version-light (Enable light mode) .

This is a convenience alias for --version-intensity 2. This light

mode makes version scanning much faster, but it is slightly less

likely to identify services.

--version-all (Try every single probe) .

An alias for --version-intensity 9, ensuring that every single

probe is attempted against each port.

--version-trace (Trace version scan activity) .

This causes Nmap to print out extensive debugging info about what

version scanning is doing. It is a subset of what you get with

--packet-trace.

-sR (RPC scan) .

This method works in conjunction with the various port scan methods

of Nmap. It takes all the TCP/UDP ports found open and floods them

with SunRPC program NULL commands in an attempt to determine

whether they are RPC ports, and if so, what program and version

number they serve up. Thus you can effectively obtain the same info

as rpcinfo -p even if the target´s portmapper is behind a firewall

(or protected by TCP wrappers). Decoys do not currently work with

RPC scan.. This is automatically enabled as part of version scan

(-sV) if you request that. As version detection includes this and

is much more comprehensive, -sR is rarely needed.

OS DETECTION

One of Nmap´s best-known features is remote OS detection using TCP/IP

stack fingerprinting. Nmap sends a series of TCP and UDP packets to the

remote host and examines practically every bit in the responses. After

performing dozens of tests such as TCP ISN sampling, TCP options

support and ordering, IP ID sampling, and the initial window size

check, Nmap compares the results to its nmap-os-db. database of more

than 2,600 known OS fingerprints and prints out the OS details if there

is a match. Each fingerprint includes a freeform textual description of

the OS, and a classification which provides the vendor name (e.g. Sun),

underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type

(general purpose, router, switch, game console, etc).

If Nmap is unable to guess the OS of a machine, and conditions are good

(e.g. at least one open port and one closed port were found), Nmap will

provide a URL you can use to submit the fingerprint if you know (for

sure) the OS running on the machine. By doing this you contribute to

the pool of operating systems known to Nmap and thus it will be more

accurate for everyone.

OS detection enables some other tests which make use of information

that is gathered during the process anyway. One of these is TCP

Sequence Predictability Classification. This measures approximately how

hard it is to establish a forged TCP connection against the remote

host. It is useful for exploiting source-IP based trust relationships

(rlogin, firewall filters, etc) or for hiding the source of an attack.

This sort of spoofing is rarely performed any more, but many machines

are still vulnerable to it. The actual difficulty number is based on

statistical sampling and may fluctuate. It is generally better to use

the English classification such as "worthy challenge" or "trivial

joke". This is only reported in normal output in verbose (-v) mode.

When verbose mode is enabled along with -O, IP ID sequence generation

is also reported. Most machines are in the "incremental" class, which

means that they increment the ID field in the IP header for each packet

they send. This makes them vulnerable to several advanced information

gathering and spoofing attacks.

Another bit of extra information enabled by OS detection is a guess at

a target´s uptime. This uses the TCP timestamp option (RFC 1323[10]) to

guess when a machine was last rebooted. The guess can be inaccurate due

to the timestamp counter not being initialized to zero or the counter

overflowing and wrapping around, so it is printed only in verbose mode.

A paper documenting the workings, usage, and customization of OS

detection is available at http://nmap.org/book/osdetect.html.

OS detection is enabled and controlled with the following options:

-O (Enable OS detection) .

Enables OS detection, as discussed above. Alternatively, you can

use -A to enable OS detection along with other things.

--osscan-limit (Limit OS detection to promising targets) .

OS detection is far more effective if at least one open and one

closed TCP port are found. Set this option and Nmap will not even

try OS detection against hosts that do not meet this criteria. This

can save substantial time, particularly on -Pn scans against many

hosts. It only matters when OS detection is requested with -O or

-A.

--osscan-guess; --fuzzy (Guess OS detection results) .

When Nmap is unable to detect a perfect OS match, it sometimes

offers up near-matches as possibilities. The match has to be very

close for Nmap to do this by default. Either of these (equivalent)

options make Nmap guess more aggressively. Nmap will still tell you

when an imperfect match is printed and display its confidence level

(percentage) for each guess.

--max-os-tries (Set the maximum number of OS detection tries against a

target) .

When Nmap performs OS detection against a target and fails to find

a perfect match, it usually repeats the attempt. By default, Nmap

tries five times if conditions are favorable for OS fingerprint

submission, and twice when conditions aren´t so good. Specifying a

lower --max-os-tries value (such as 1) speeds Nmap up, though you

miss out on retries which could potentially identify the OS.

Alternatively, a high value may be set to allow even more retries

when conditions are favorable. This is rarely done, except to

generate better fingerprints for submission and integration into

the Nmap OS database.

NMAP SCRIPTING ENGINE (NSE)

The Nmap Scripting Engine (NSE) is one of Nmap´s most powerful and

flexible features. It allows users to write (and share) simple scripts

(using the Lua programming language[11],

Tasks we had in mind when creating the system include network

discovery, more sophisticated version detection, vulnerability

detection. NSE can even be used for vulnerability exploitation.

To reflect those different uses and to simplify the choice of which

scripts to run, each script contains a field associating it with one or

more categories. Currently defined categories are auth, default.

discovery, dos, exploit, external, fuzzer, intrusive, malware, safe,

version, and vuln. These are all described at

http://nmap.org/book/nse-usage.html#nse-categories.

Scripts are not run in a sandbox and thus could accidentally or

maliciously damage your system or invade your privacy. Never run

scripts from third parties unless you trust the authors or have

carefully audited the scripts yourself.

The Nmap Scripting Engine is described in detail at

http://nmap.org/book/nse.html

and is controlled by the following options:

-sC .

Performs a script scan using the default set of scripts. It is

equivalent to --script=default. Some of the scripts in this

category are considered intrusive and should not be run against a

target network without permission.

--script filename|category|directory|expression|all[,...] .

Runs a script scan using the comma-separated list of filenames,

script categories, and directories. Each element in the list may

also be a Boolean expression describing a more complex set of

scripts. Each element is interpreted first as an expression, then

as a category, and finally as a file or directory name. The special

argument all makes every script in Nmap´s script database eligible

to run. The all argument should be used with caution as NSE may

contain dangerous scripts including exploits, brute force

authentication crackers, and denial of service attacks.

File and directory names may be relative or absolute. Absolute

names are used directly. Relative paths are looked for in the

following places until found:

--datadir

$NMAPDIR.

~/.nmap (not searched on Windows).

NMAPDATADIR.

the current directory

A scripts subdirectory is also tried in each of these.

When a directory name is given, Nmap loads every file in the

directory whose name ends with .nse. All other files are ignored

and directories are not searched recursively. When a filename is

given, it does not have to have the .nse extension; it will be

added automatically if necessary. Nmap scripts are stored in a

scripts subdirectory of the Nmap data directory by default (see

http://nmap.org/book/data-files.html).

For efficiency, scripts are indexed in a database stored in

scripts/script.db,. which lists the category or categories in

which each script belongs. When referring to scripts from

script.db by name, you can use a shell-style '*' wildcard.

nmap --script "http-*"

Loads all scripts whose name starts with http-, such as

http-auth.nse and http-open-proxy.nse. The argument to --script

had to be in quotes to protect the wildcard from the shell.

More complicated script selection can be done using the and, or,

and not operators to build Boolean expressions. The operators have

the same precedence[12] as in Lua: not is the highest, followed by

and and then or. You can alter precedence by using parentheses.

Because expressions contain space characters it is necessary to

quote them.

nmap --script "not intrusive"

Loads every script except for those in the intrusive category.

nmap --script "default or safe"

This is functionally equivalent to nmap --script

"default,safe". It loads all scripts that are in the default

category or the safe category or both.

nmap --script "default and safe"

Loads those scripts that are in both the default and safe

categories.

nmap --script "(default or safe or intrusive) and not http-*"

Loads scripts in the default, safe, or intrusive categories,

except for those whose names start with http-.

--script-args n1=v1,n2={n3=v3},n4={v4,v5} .

Lets you provide arguments to NSE scripts. Arguments are a

comma-separated list of name=value pairs. Names and values may be

strings not containing whitespace or the characters '{', '}', '=',

or ','. To include one of these characters in a string, enclose the

string in single or double quotes. Within a quoted string, '\'

escapes a quote. A backslash is only used to escape quotation marks

in this special case; in all other cases a backslash is interpreted

literally. Values may also be tables enclosed in {}, just as in

Lua. A table may contain simple string values or more name-value

pairs, including nested tables. A complex example of script

arguments is The online NSE Documentation Portal at

http://nmap.org/nsedoc/ lists the arguments that each script

accepts.

--script-trace .

This option does what --packet-trace does, just one ISO layer

higher. If this option is specified all incoming and outgoing

communication performed by a script is printed. The displayed

information includes the communication protocol, the source, the

target and the transmitted data. If more than 5% of all transmitted

data is not printable, then the trace output is in a hex dump

format. Specifying --packet-trace enables script tracing too.

--script-updatedb .

This option updates the script database found in scripts/script.db

which is used by Nmap to determine the available default scripts

and categories. It is only necessary to update the database if you

have added or removed NSE scripts from the default scripts

directory or if you have changed the categories of any script. This

option is generally used by itself: nmap --script-updatedb.

TIMING AND PERFORMANCE

One of my highest Nmap development priorities has always been

performance. A default scan (nmap hostname) of a host on my local

network takes a fifth of a second. That is barely enough time to blink,

but adds up when you are scanning hundreds or thousands of hosts.

Moreover, certain scan options such as UDP scanning and version

detection can increase scan times substantially. So can certain

firewall configurations, particularly response rate limiting. While

Nmap utilizes parallelism and many advanced algorithms to accelerate

these scans, the user has ultimate control over how Nmap runs. Expert

users carefully craft Nmap commands to obtain only the information they

care about while meeting their time constraints.

Techniques for improving scan times include omitting non-critical

tests, and upgrading to the latest version of Nmap (performance

enhancements are made frequently). Optimizing timing parameters can

also make a substantial difference. Those options are listed below.

Some options accept a time parameter. This is specified in seconds by

default, though you can append 'ms', 's', 'm', or 'h' to the value to

specify milliseconds, seconds, minutes, or hours. So the --host-timeout

arguments 900000ms, 900, 900s, and 15m all do the same thing.

--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel

scan group sizes) .

Nmap has the ability to port scan or version scan multiple hosts in

parallel. Nmap does this by dividing the target IP space into

groups and then scanning one group at a time. In general, larger

groups are more efficient. The downside is that host results can´t

be provided until the whole group is finished. So if Nmap started

out with a group size of 50, the user would not receive any reports

(except for the updates offered in verbose mode) until the first 50

hosts are completed.

By default, Nmap takes a compromise approach to this conflict. It

starts out with a group size as low as five so the first results

come quickly and then increases the groupsize to as high as 1024.

The exact default numbers depend on the options given. For

efficiency reasons, Nmap uses larger group sizes for UDP or

few-port TCP scans.

When a maximum group size is specified with --max-hostgroup, Nmap

will never exceed that size. Specify a minimum size with

--min-hostgroup and Nmap will try to keep group sizes above that

level. Nmap may have to use smaller groups than you specify if

there are not enough target hosts left on a given interface to

fulfill the specified minimum. Both may be set to keep the group

size within a specific range, though this is rarely desired.

These options do not have an effect during the host discovery phase

of a scan. This includes plain ping scans (-sn). Host discovery

always works in large groups of hosts to improve speed and

accuracy.

The primary use of these options is to specify a large minimum

group size so that the full scan runs more quickly. A common choice

is 256 to scan a network in Class C sized chunks. For a scan with

many ports, exceeding that number is unlikely to help much. For

scans of just a few port numbers, host group sizes of 2048 or more

may be helpful.

--min-parallelism numprobes; --max-parallelism numprobes (Adjust probe

parallelization) .

These options control the total number of probes that may be

outstanding for a host group. They are used for port scanning and

host discovery. By default, Nmap calculates an ever-changing ideal

parallelism based on network performance. If packets are being

dropped, Nmap slows down and allows fewer outstanding probes. The

ideal probe number slowly rises as the network proves itself

worthy. These options place minimum or maximum bounds on that

variable. By default, the ideal parallelism can drop to one if the

network proves unreliable and rise to several hundred in perfect

conditions.

The most common usage is to set --min-parallelism to a number

higher than one to speed up scans of poorly performing hosts or

networks. This is a risky option to play with, as setting it too

high may affect accuracy. Setting this also reduces Nmap´s ability

to control parallelism dynamically based on network conditions. A

value of 10 might be reasonable, though I only adjust this value as

a last resort.

The --max-parallelism option is sometimes set to one to prevent

Nmap from sending more than one probe at a time to hosts. The

--scan-delay option, discussed later, is another way to do this.

--min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout

time (Adjust probe timeouts) .

Nmap maintains a running timeout value for determining how long it

will wait for a probe response before giving up or retransmitting

the probe. This is calculated based on the response times of

previous probes.

If the network latency shows itself to be significant and variable,

this timeout can grow to several seconds. It also starts at a

conservative (high) level and may stay that way for a while when

Nmap scans unresponsive hosts.

Specifying a lower --max-rtt-timeout and --initial-rtt-timeout than

the defaults can cut scan times significantly. This is particularly

true for pingless (-Pn) scans, and those against heavily filtered

networks. Don´t get too aggressive though. The scan can end up

taking longer if you specify such a low value that many probes are

timing out and retransmitting while the response is in transit.

If all the hosts are on a local network, 100 milliseconds

(--max-rtt-timeout 100ms) is a reasonable aggressive value. If

routing is involved, ping a host on the network first with the ICMP

ping utility, or with a custom packet crafter such as Nping. that

is more likely to get through a firewall. Look at the maximum round

trip time out of ten packets or so. You might want to double that

for the --initial-rtt-timeout and triple or quadruple it for the

--max-rtt-timeout. I generally do not set the maximum RTT below

100 ms, no matter what the ping times are. Nor do I exceed 1000 ms.

--min-rtt-timeout is a rarely used option that could be useful when

a network is so unreliable that even Nmap´s default is too

aggressive. Since Nmap only reduces the timeout down to the minimum

when the network seems to be reliable, this need is unusual and

should be reported as a bug to the nmap-dev mailing list..

--max-retries numtries (Specify the maximum number of port scan probe

retransmissions) .

When Nmap receives no response to a port scan probe, it could mean

the port is filtered. Or maybe the probe or response was simply

lost on the network. It is also possible that the target host has

rate limiting enabled that temporarily blocked the response. So

Nmap tries again by retransmitting the initial probe. If Nmap

detects poor network reliability, it may try many more times before

giving up on a port. While this benefits accuracy, it also lengthen

scan times. When performance is critical, scans may be sped up by

limiting the number of retransmissions allowed. You can even

specify --max-retries 0 to prevent any retransmissions, though that

is only recommended for situations such as informal surveys where

occasional missed ports and hosts are acceptable.

The default (with no -T template) is to allow ten retransmissions.

If a network seems reliable and the target hosts aren´t rate

limiting, Nmap usually only does one retransmission. So most target

scans aren´t even affected by dropping --max-retries to a low value

such as three. Such values can substantially speed scans of slow

(rate limited) hosts. You usually lose some information when Nmap

gives up on ports early, though that may be preferable to letting

the --host-timeout expire and losing all information about the

target.

--host-timeout time (Give up on slow target hosts) .

Some hosts simply take a long time to scan. This may be due to

poorly performing or unreliable networking hardware or software,

packet rate limiting, or a restrictive firewall. The slowest few

percent of the scanned hosts can eat up a majority of the scan

time. Sometimes it is best to cut your losses and skip those hosts

initially. Specify --host-timeout with the maximum amount of time

you are willing to wait. For example, specify 30m to ensure that

Nmap doesn´t waste more than half an hour on a single host. Note

that Nmap may be scanning other hosts at the same time during that

half an hour, so it isn´t a complete loss. A host that times out is

skipped. No port table, OS detection, or version detection results

are printed for that host.

--scan-delay time; --max-scan-delay time (Adjust delay between probes)

.

This option causes Nmap to wait at least the given amount of time

between each probe it sends to a given host. This is particularly

useful in the case of rate limiting.. Solaris machines (among many

others) will usually respond to UDP scan probe packets with only

one ICMP message per second. Any more than that sent by Nmap will

be wasteful. A --scan-delay of 1s will keep Nmap at that slow rate.

Nmap tries to detect rate limiting and adjust the scan delay

accordingly, but it doesn´t hurt to specify it explicitly if you

already know what rate works best.

When Nmap adjusts the scan delay upward to cope with rate limiting,

the scan slows down dramatically. The --max-scan-delay option

specifies the largest delay that Nmap will allow. A low

--max-scan-delay can speed up Nmap, but it is risky. Setting this

value too low can lead to wasteful packet retransmissions and

possible missed ports when the target implements strict rate

limiting.

Another use of --scan-delay is to evade threshold based intrusion

detection and prevention systems (IDS/IPS)..

--min-rate number; --max-rate number (Directly control the scanning

rate) .

Nmap´s dynamic timing does a good job of finding an appropriate

speed at which to scan. Sometimes, however, you may happen to know

an appropriate scanning rate for a network, or you may have to

guarantee that a scan will be finished by a certain time. Or

perhaps you must keep Nmap from scanning too quickly. The

--min-rate and --max-rate options are designed for these

situations.

When the --min-rate option is given Nmap will do its best to send

packets as fast as or faster than the given rate. The argument is a

positive real number representing a packet rate in packets per

second. For example, specifying --min-rate 300 means that Nmap will

try to keep the sending rate at or above 300 packets per second.

Specifying a minimum rate does not keep Nmap from going faster if

conditions warrant.

Likewise, --max-rate limits a scan´s sending rate to a given

maximum. Use --max-rate 100, for example, to limit sending to 100

packets per second on a fast network. Use --max-rate 0.1 for a slow

scan of one packet every ten seconds. Use --min-rate and --max-rate

together to keep the rate inside a certain range.

These two options are global, affecting an entire scan, not

individual hosts. They only affect port scans and host discovery

scans. Other features like OS detection implement their own timing.

There are two conditions when the actual scanning rate may fall

below the requested minimum. The first is if the minimum is faster

than the fastest rate at which Nmap can send, which is dependent on

hardware. In this case Nmap will simply send packets as fast as

possible, but be aware that such high rates are likely to cause a

loss of accuracy. The second case is when Nmap has nothing to send,

for example at the end of a scan when the last probes have been

sent and Nmap is waiting for them to time out or be responded to.

It´s normal to see the scanning rate drop at the end of a scan or

in between hostgroups. The sending rate may temporarily exceed the

maximum to make up for unpredictable delays, but on average the

rate will stay at or below the maximum.

Specifying a minimum rate should be done with care. Scanning faster

than a network can support may lead to a loss of accuracy. In some

cases, using a faster rate can make a scan take longer than it

would with a slower rate. This is because Nmap´s

adaptive retransmission algorithms will detect the network

congestion caused by an excessive scanning rate and increase the

number of retransmissions in order to improve accuracy. So even

though packets are sent at a higher rate, more packets are sent

overall. Cap the number of retransmissions with the --max-retries

option if you need to set an upper limit on total scan time.

--defeat-rst-ratelimit .

Many hosts have long used rate limiting. to reduce the number of

ICMP error messages (such as port-unreachable errors) they send.

Some systems now apply similar rate limits to the RST (reset)

packets they generate. This can slow Nmap down dramatically as it

adjusts its timing to reflect those rate limits. You can tell Nmap

to ignore those rate limits (for port scans such as SYN scan which

don´t treat non-responsive ports as open) by specifying

--defeat-rst-ratelimit.

Using this option can reduce accuracy, as some ports will appear

non-responsive because Nmap didn´t wait long enough for a

rate-limited RST response. With a SYN scan, the non-response

results in the port being labeled filtered rather than the closed

state we see when RST packets are received. This option is useful

when you only care about open ports, and distinguishing between

closed and filtered ports isn´t worth the extra time.

-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing

template) .

While the fine-grained timing controls discussed in the previous

section are powerful and effective, some people find them

confusing. Moreover, choosing the appropriate values can sometimes

take more time than the scan you are trying to optimize. So Nmap

offers a simpler approach, with six timing templates. You can

specify them with the -T option and their number (0–5) or their

name. The template names are paranoid (0), sneaky (1), polite (2),

normal (3), aggressive (4), and insane (5). The first two are for

IDS evasion. Polite mode slows down the scan to use less bandwidth

and target machine resources. Normal mode is the default and so -T3

does nothing. Aggressive mode speeds scans up by making the

assumption that you are on a reasonably fast and reliable network.

Finally insane mode. assumes that you are on an extraordinarily

fast network or are willing to sacrifice some accuracy for speed.

These templates allow the user to specify how aggressive they wish

to be, while leaving Nmap to pick the exact timing values. The

templates also make some minor speed adjustments for which

fine-grained control options do not currently exist. For example,

-T4. prohibits the dynamic scan delay from exceeding 10 ms for TCP

ports and -T5 caps that value at 5 ms. Templates can be used in

combination with fine-grained controls, and the fine-grained

controls will you specify will take precedence over the timing

template default for that parameter. I recommend using -T4 when

scanning reasonably modern and reliable networks. Keep that option

even when you add fine-grained controls so that you benefit from

those extra minor optimizations that it enables.

If you are on a decent broadband or ethernet connection, I would

recommend always using -T4. Some people love -T5 though it is too

aggressive for my taste. People sometimes specify -T2 because they

think it is less likely to crash hosts or because they consider

themselves to be polite in general. They often don´t realize just

how slow -T polite. really is. Their scan may take ten times

longer than a default scan. Machine crashes and bandwidth problems

are rare with the default timing options (-T3) and so I normally

recommend that for cautious scanners. Omitting version detection is

far more effective than playing with timing values at reducing

these problems.

While -T0. and -T1. may be useful for avoiding IDS alerts, they

will take an extraordinarily long time to scan thousands of

machines or ports. For such a long scan, you may prefer to set the

exact timing values you need rather than rely on the canned -T0 and

-T1 values.

The main effects of T0 are serializing the scan so only one port is

scanned at a time, and waiting five minutes between sending each

probe. T1 and T2 are similar but they only wait 15 seconds and 0.4

seconds, respectively, between probes. T3 is Nmap´s default

behavior, which includes parallelization.. -T4 does the equivalent

of --max-rtt-timeout 1250ms --initial-rtt-timeout 500ms

--max-retries 6 and sets the maximum TCP scan delay to 10

milliseconds. T5 does the equivalent of --max-rtt-timeout 300ms

--min-rtt-timeout 50ms --initial-rtt-timeout 250ms --max-retries 2

--host-timeout 15m as well as setting the maximum TCP scan delay to

5 ms.

FIREWALL/IDS EVASION AND SPOOFING

Many Internet pioneers envisioned a global open network with a

universal IP address space allowing virtual connections between any two

nodes. This allows hosts to act as true peers, serving and retrieving

information from each other. People could access all of their home

systems from work, changing the climate control settings or unlocking

the doors for early guests. This vision of universal connectivity has

been stifled by address space shortages and security concerns. In the

early 1990s, organizations began deploying firewalls for the express

purpose of reducing connectivity. Huge networks were cordoned off from

the unfiltered Internet by application proxies, network address

translation, and packet filters. The unrestricted flow of information

gave way to tight regulation of approved communication channels and the

content that passes over them.

Network obstructions such as firewalls can make mapping a network

exceedingly difficult. It will not get any easier, as stifling casual

reconnaissance is often a key goal of implementing the devices.

Nevertheless, Nmap offers many features to help understand these

complex networks, and to verify that filters are working as intended.

It even supports mechanisms for bypassing poorly implemented defenses.

One of the best methods of understanding your ne

from http://systemeng.tistory.com/167 by ccl(A) rewrite - 2020-03-11 11:54:17