Nmap

Nmap Overview

How to check your network With Nmap.
What is network Scanning?
Network scanning is an important part of network That any system administrator must be done it.
network scanning is the process of detect active hosts on the network and Find out some
information about the hosts, such as operating system, active ports, services, and applications.
Who Uses Network Scanning?
System administrators, network engineers, auditors,… all use network scanners for following
reasons :
Security auditing
Compliance testing
Asset management
Network and system inventory

What you will learn?
•

At the end of this article, You can examine a website or IP address for Security Hole.

•

To become an ethical hacker ,You should be familiar a software scanning like Nmap.
Generally, a hacker spends 90 percent of the time for gathering information on a target and
10 percent of the time For launching the attack.

Figure 1-Phases of hacking

What you should know?
•

You should be familiar with the discussions basic network, Like Network+. In this article We
can not teach these topics.

•

It is true that Scanning not crime, but in The company or office for Run Software Like
Nmap, you must Get permission of network administrator.

network scanning contain of Port scanner and Vulnerability scanner.
Port scanner is a software that designed to probe a server or host for open ports. this is often used
by administrators to verify security policies of their networks and can be used by an attacker to
identify running services on a host with the view to compromise it. a port scan sends client requests
to a server port addresses on a host for finding an active port. The design and operation of the
Internet is based on TCP/IP. A port can be have some behavior like below :
1. Open or Accepted: The host sent a reply indicating that a service is listening on the port.
2. Closed or Denied or Not Listening: The host sent a reply indicating that connections will be
denied to the port.
3. Filtered, Dropped or Blocked: There was no reply from the host.
Ports in Nmap have other modes that more of them being.
Port scanning has several types such as : TCP scanning ,SYN scanning ,UDP scanning ,ACK scanning
,Window scanning ,FIN scanning, X-mas , Protocol scan, Proxy scan, Idle scan, CatSCAN , ICMP scan.
In below we explain number of this scan :
•

TCP scanning

The simplest port scanners use the operating system's network functions and is generally the next
option to go to when SYN is not a feasible option.
•

SYN scanning

SYN scan is another form of TCP scanning. Rather than use the operating system's network
functions, the port scanner generates raw IP packets itself, and monitors for responses. This scan
type is also known as "half-open scanning", because it never actually opens a full TCP connection.
•

UDP scanning

UDP is a connectionless protocol so there is no equivalent to a TCP SYN packet. if a UDP packet is
sent to a port that is not open, the system will respond with an ICMP port unreachable message. if a
port is blocked by a firewall, this method will falsely report that the port is open. If the port
unreachable message is blocked, all ports will appear open.
•

ACK scanning

This kind of scan does not exactly determine whether the port is open or closed, but whether the
port is filtered or unfiltered. This kind of scan can be good when attempting to probe for the
existence of a firewall and its rule sets.

•

FIN scanning

Usually firewalls blocking packets in the form of SYN packets. FIN packets are able to pass by
firewalls with no modification to its purpose. Closed ports reply to a FIN packet with the appropriate
RST packet, whereas open ports ignore the packet on hand.
Nmap Support large number of this scanning.
A vulnerability scanner is a computer program designed to assess computers, computer systems,
networks or applications for weaknesses. It is important that the network administrator is familiar
with these methods.
There are many software For Scanning network ,some of this software are free and some are not
free, at http://sectools.org/vuln-scanners.html you can find list of this software.
The significant point about nmap(Network Mapper) is Free and Open Source. Nmap is a security
scanner originally written by Gordon Lyon (also known by his pseudonym Fyodor Vaskovich) for
discover hosts and services on a computer network. Nmap runs on Linux, Microsoft Windows,
Solaris, HP-UX and BSD variants (including Mac OS X), and also on AmigaOS and SGI IRIX.
Nmap Includes the following features :
•
•
•
•
•

Host Discovery
Port Scanning
Version Detection
OS Detection
Scriptable interaction with the target

Nmap was originally released in September of 1997 via an article Fyodor posted in Phrack,
www.phrack.org/issues.html?issue=51&id=11# Article. Nmap began drawing more and more
interest. Nmap used In a film called The Matrix Reloaded , One of the movie characters executes
a perfect example of an Nmap port scan.

Figure 2-Nmap in The Matrix Reloaded Movie
Nmap Works in two modes, In command line mode and GUI mode.Graphic version of Nmap known
as Zenmap . official GUI for Nmap versions 2.2 to 4.22 known as NmapFE, originally written by Zach

Smith. For Nmap 4.50, NmapFE was replaced with Zenmap, a new graphical user interface based on
,
UMIT, developed by Adriano Monteiro Marques
Marques.

Figure 3-Anatomy of Zenmap
Work with Zenmap is easy and have a good Environment for work.

Is Nmap Good or Bad?
A Powerful Tools Like Nmap can be a double
double-edged sword. Some people known this tool as a tool for
hacking And computer security experts known Nmap as a tools for Scanning a Computer for
Vulnerabilities. Eventually, The bad guys are already using Nmap for reconnaissance. Nmap scan
Can provide useful information about the open doors and OS running and Etc.
The good guys use Nmap to make their network safer. Network Experts use Nmap to identify IP
addresses and specific open port that may Be used by hackers, spyware infestation and other
malicious tools. Nevertheless Nmap is a Tools that can be used for Useful things and Bad things. If
you employ Nmap In a company or a local network Make sure you know about it and permission of
network,
the network owners.
There are many features about nmap that We can not say all in this article. We just tell some of the
important features.

Scan a Single Target

For Scan a single target, your target can be specified as an IP address or host name.
Usage syntax: nmap [target]
$ nmap 192.168.10.1
Starting Nmap 5.00 ( http://nmap.org ) at 2009-08-07 19:38 CDT
Interesting ports on 192.168.10.1:
Not shown: 997 filtered ports
PORT
STATE SERVICE
20/tcp closed ftp-data
21/tcp closed ftp
80/tcp open
http
Nmap done: 1 IP address (1 host up) scanned in 7.21 seconds

In above example, PORT show port number/protocol and STATE show state of port and SERVICE
show type of service for the port.
You might think that a port can only be two modes: open and closed. Nmap can detect other
occurrences effecting state. Nmap detects the following six port states:
Open : Open ports have an active application accepting TCP connections
or UDP packets.
Closed : Closed ports are accessible, but they do not have a listening
application.
Filtered : Responses are blocked by a packet filter, therefore Nmap
cannot determine if the port is open.
Unfiltered : Unfiltered ports are accessible, but Nmap is unable to
determine if they are open or closed. (ACK scan only)
Open|filtered : Nmap is unable to determine if the port is open or
filtered for scan types where open ports do not respond. (UDP, IP
Proto, FIN, Null, Xmas scans)
Closed|filtered : Nmap is unable to determine if a port is closed or
filtered. (IP ID idle scan only)

You can scan Multiple Targets with flowing syntax :
Usage syntax: nmap [target1 target2 etc]
$ nmap 192.168.10.1 192.168.10.100 192.168.10.101

Scan a Range of IP Addresses
A range of IP addresses can be used for target specification as in the example below.
Usage syntax: nmap [Range of IP addresses]
$ nmap 192.168.10.1-100

Scan an Entire Subnet
Nmap can be used to scan an entire subnet using CIDR.
Usage syntax: nmap [Network/CIDR]

$ nmap 192.168.10.1/24

You can create a text file that contain of your victim and give this file to Nmap for Scan, see below example :
Usage syntax: nmap -iL [list.txt]
$ nmap -iL list.txt

Exclude Targets from a Scan
For Exclude a target from scan, you can use this syntax :
Usage syntax: nmap [targets] --exclude [target(s)]
$ nmap 192.168.10.0/24 --exclude 192.168.10.100

Scan an IPv6 Target
Addition of IPv4,Nmap can be scan IPv6.the -6 parameter is used to perform IPv6 scan.
Usage syntax: nmap -6 [target]
# nmap -6 fe80::29aa:9db9:4164:d80e

A summary of some features about Discovery Options for Quick read , Exist in the below table :
Feature

Option

Don’t Ping

-PN

Perform a Ping Only Scan

-sP

TCP SYN Ping

-PS

TCP ACK Ping

-PA

UDP Ping

-PU

SCTP INIT Ping

-PY

ICMP Echo Ping

-PE

ICMP Timestamp Ping

-PP

ICMP Address Mask Ping

-PM

IP Protocol Ping

-PO

ARP Ping

-PR

Traceroute

--traceroute

Table 1 - Discovery Options
In this paper we refrain from explaining details and For example, only show general form for using.

Don’t Ping
Usage syntax: nmap -PN [target]
$ nmap -PN 10.10.5.11

Other features are used similarly. Let, examine Advanced Scanning Options.
Nmap supports a number of user selectable scan types. By default, Nmap will perform a basic TCP
scan on each target system. In some situations, it may be necessary to perform more complex TCP
(or even UDP) scans to find uncommon services or to evade a firewall. In below table we show some
option that you need to perform advanced scanning.

Feature

Option

TCP SYN Scan

-sS

TCP Connect Scan

-sT

UDP Scan

-sU

TCP NULL Scan

-sN

TCP FIN Scan

-sF

Xmas Scan

-sX

TCP ACK Scan

-sA

Custom TCP Scan

--scanflags

IP Protocol Scan

-sO

Send Raw Ethernet Packets
Send IP Packets

--send-eth
--send-ip

Table 2- Advanced Scanning Options

For example ,We only show general form of scanning and some of options that require special
settings, we explain they.
advanced scanning used like other scanning, in below example we show you how use options to
scan target.
Note:
You must login with root/administrator privileges (or use the sudo command) to execute many of
the scans .

TCP SYN Scan

The TCP SYN scan uses common methods of port-identification and allow Nmap to gather
information about open.

Figure 4- TCP SYN Scan

To performs a TCP SYN scan you must use The -sS option.
Usage syntax: nmap -sS [target]
# nmap -sS 10.10.1.48

Other options are used in the above form, Only some of the options require special settings.
Advantages of the TCP SYN Scan is, The TCP SYN scan never actually creates a TCP session and not
logged by the destination and Disadvantages of the TCP SYN Scan is privileged access to the system
for use it.

Custom TCP Scan
The --scanflags option is used perform a custom TCP scan.
Usage syntax: nmap --scanflags [flag(s)] [target]
# nmap --scanflags SYNURG 10.10.1.127

The --scanflags option allows users to define a custom scan using one or more TCP header flags.

Flag

Usage

SYN

Synchronize

ACK

Acknowledgment

PSH

Push

URG

Urgent

RST

Reset

FIN

Finished
Table 3- TCP header flags

Port Scanning Options
There are a total of 131,070 TCP/IP ports (65,535 TCP and 65,535 UDP).Nmap, by default, only scans
1,000 of the most commonly used ports.

In the table below, We're showing some of options that you require to perform port scanning.

Feature

Option

Do a quick scan

-F

Scanning a specific port

-p [port]

Scanning port through a name

-p [name]

Scanning Ports by Protocol

-p U:[UDP ports],T:[TCP ports]

Scan All Ports

-p “*”

Scan Top Ports

--top-ports [number]

Perform port scanning consecutive

-r

Table 4 - Port Scanning Options

For use this options We're showing some of the options require special settings.
Do a quick scan
The -F option instructs Nmap to perform a scan of only the 100 most commonly used ports.
Usage syntax: nmap -F [target]
$ nmap -F 10.10.1.44

Nmap scans the top 1000 commonly used ports by default. The -F option reduces that number to
100.

Scanning port through a name
The -p option can be used to scan ports by name.
Usage syntax: nmap -p [port name(s)] [target]
$ nmap -p smtp,http 10.10.1.44

Scanning Ports by Protocol
Specifying a T: or U: prefix with the -p option allows you to search for a specific port and protocol
combination.
Usage syntax: nmap -p U:[UDP ports],T:[TCP ports] [target]
# nmap -sU -sT -p U:53,T:25 10.10.1.44

Scan Top Ports
The --top-ports option is used to scan the specified number of top ranked ports.
Usage syntax: nmap --top-ports [number] [target]

# nmap --top-ports 10 10.10.1.41

Operating System and Service Detection
One of Nmap’s features is its ability to detect operating systems and services on remote systems.
This feature analyzes responses from scanned targets and attempts to identify the host’s operating
system and installed services.

In the table below,We're showing some of options that you require to perform Operating System
and Service Detection.
Feature

Option
-O

Operating System Detection

Trying to guess the unknown operating system

--osscan-guess
-sV

Service Version Detection
Perform a RPC Scan

--version-trace

Troubleshooting Version Scans

-sR

Table 5-options to perform Operation System and Service Detection
Use these options as well as other options are. for example :

Operating System Detection
The -O parameter enables Nmap’s operating system detection feature.
Usage syntax: nmap -O [target]
# nmap -O 10.10.1.48

Attempt to Guess an Unknown Operating System
If Nmap is unable to accurately identify the OS, you can force it to guess by using the --osscan-guess
option.
Usage syntax: nmap -O --osscan-guess [target]
# nmap -O --osscan-guess 10.10.1.11

Evading Firewalls
A firewall is a device or set of devices that is configured to permit or deny network transmissions.
Firewalls are used to control traffic and limit specific activity. Firewalls can be implemented in either
hardware or software, or a combination of both.firewall prevent unauthorized Internet users into
private networks and block some attack.

Figure 5-How to work the firewall

Firewalls and IDS designed to prevent tools like Nmap. Nmap includes a number of features
designed to circumvent these defenses.
Feature

Option
-f

Fragment Packets

--mtu

Specify a Specific MTU
Use a Decoy

-D

Idle Zombie Scan

-sI

To specify the source port to
manually

--source-port

Append Random Data

--data-length

Randomize Target Scan Order

--randomize-hosts

Spoof MAC Address

--spoof-mac

Send Bad Checksums

--badsum

Table 6 Some options to escape the firewall
6-

We quickly show how to use these options.
Fragment Packets
The -f option is used to fragment probes into 8
f
8-byte packets.
Usage syntax: nmap -f [target]
f
# nmap -f 10.10.1.48

Specify a Specific MTU
Usage syntax: nmap --mtu [number] [target]
# nmap --mtu 16 10.10.1.48

In the above example, the --mtu 16 argument instructs Nmap to use tiny 16-byte packets for the
scan.

Use a Decoy
Usage syntax: nmap -D [decoy1,decoy2,etc|RND:number] [target]
# nmap -D RND:10 10.10.1.48

In the above example nmap -D RND:10 instructs Nmap to generate 10 random decoys.

Idle Zombie Scan
Nmap uses idlescan to gather port information using another station on the network instead of the
Nmap station. For more information about Zombie go to http://en.wikipedia.org/wiki/Idle_scan.
Usage syntax: nmap -sI [zombie host] [target]
# nmap -sI 10.10.1.41 10.10.1.252

In this example 10.10.1.41 is the zombie and 10.10.1.252 is the target system.

To specify the source port to manually
Usage syntax: nmap --source-port [port] [target]
# nmap --source-port 53 scanme.insecure.org

Append Random Data
Usage syntax: nmap --data-length [number] [target]
# nmap --data-length 25 10.10.1.252

In the above example 25 additional bytes are added to all packets sent to the target.

Randomize Target Scan Order
Usage syntax: nmap --randomize-hosts [targets]
$ nmap --randomize-hosts 10.10.1.100-254

Spoof MAC Address
Usage syntax: nmap --spoof-mac [vendor|MAC|0] [target]
# nmap -sT -PN --spoof-mac 0 192.168.1.1

The --spoof-mac option Has the following parameters :

Argument

Function

0 (zero)

Generates a random MAC address

Specific MAC Address

Uses the specified MAC address

Vendor Name

Generates a MAC address from the
specified vendor (such as Apple, Dell,
3Com, etc)

Table 7- Some of options to Spoof MAC Address

Send Bad Checksums
Usage syntax: nmap --badsum [target]
# nmap --badsum 10.10.1.41

Only a system that poorly configuration, would a respond to a packet with a bad checksum.

Output Options
Nmap Offers Some option for generate formatted output. You can save your output in Text, Xml and
single line grepable file.

In the table below,We're showing some of options that you require to Generate your Desired
output.
Feature

Option

Save Output to a Text File

-oN

Save Output to a XML File

-oX

Grepable Output

-oG

Output All Supported File Types

-oA

133t Output

-oS

Table 8- some of options to Generate your Desired output

All Features to use as they are, So we just give an example showing.
Save Output to a Text File
For save output as a text file we use –oN Option.
Usage syntax: nmap -oN [scan.txt] [target]

$ nmap -oN scan.txt 10.10.1.1

Other features are like each other but in Output All Supported File Types option ,you don't need to
specify Extensions.
This property is used as follows :
Usage syntax: nmap -oA [filename] [target]
$ nmap -oA scans 10.10.1.1

Another option is 133t Output, it is an alternative alphabet for the English language that used on the
internet, It output only for joking. In below you can see example of this option.
Usage syntax: nmap -oS [scan.txt] [target]
$ nmap -oS scan.txt 10.10.1.1
$ cat scan.txt
StaRtING NMap 5.00 ( htTp://nmap.oRg ) aT 2009-08-13 15:45 CDT
!nt3r3St|ng pOrts 0n 10.10.1.1:
n0t $h0wn: 998 cl0$3d p0rt$
P0RT
$TATE seRV!CE
80/tcp
Open
hTtp
443/tcp
0pen
https
Nmap DOnE: 1 Ip addresz (1 host up) $canned iN 0.48 $3c0nds

For more information about 133t(leet) go to http://en.wikipedia.org/wiki/133t

Remotely scan.
Nmap have a version that run online and you can scanning your target from remotely. Visit
http://nmap-online.com/ and enter your ip address for scanning and select your scan type then click
scan now button, Scanning results later will be displayed.

References:
http://en.wikipedia.org/wiki/Port_scanner
http://en.wikipedia.org/wiki/Vulnerability_scanner
http://en.wikipedia.org/wiki/Firewall_computing
http://nmap.org

Nmap® Cookbook The fat-free guide to network scanning
Nmap in the Enterprise: Your Guide to Network Scanning
Secrets of Network Cartography: A Comprehensive Guide to Nmap Second Edition, Revision 2
CEH Official Certified Ethical Hacker Review Guide
About The author :
My Name is Mohsen Mostafa Jokar, I'm a student in software Engineering .I'm working with
GNULinux and Programming in it. I'm interested in Network and security issues and like to examine
them. I'm writer too.

Nmap

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