TCPdump
TCPdump is the most used network sniffer/analyzer for UNIX. TCPTrace analyzes the dump file format generated by TCPdump and other applications.
Hping
Hping is a command-line oriented TCP/IP packet assembler/analyzer, kind of like the “ping” program (but with a lot of extensions).
DNSiff
DNSiff is a collection of tools for network auditing and penetration testing. dsniff, filesnarf, mailsnarf, msgsnarf, urlsnarf, and webspy passively monitor a network for interesting data (passwords, e-mail, files, etc.).
Snort
Snort is an open source network intrusion detection system, capable of performing real-time traffic analysis and packet logging on IP networks. GFI Languard
GFI Languard Network Security Scanner (N.S.S.) automatically scans your entire network, IP by IP, and plays the devil's advocate alerting you to security vulnerabilities.
Ettercap
Ettercap is a multipurpose sniffer/interceptor/logger for switched LAN. It supports active and passive dissection of many protocols (even ciphered ones)and includes many feature for network and host analysis.
Nikto
Nikto is an Open Source (GPL) web server scanner which performs comprehensive tests against web servers for multiple items, including over 2500 potentially dangerous files/CGIs, versions on over 375 servers, and version specific problems on over 230 servers.
OpenSSH
OpenSSH is a FREE version of the SSH protocol suite of network connectivity tools, which encrypts all traffic (including passwords) to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks.
Kismet
Kismet is an 802.11 wireless network sniffer - this is different from a normal network sniffer (such as Ethereal or tcpdump) because it separates and identifies different wireless networks in the area.
NetFilter
NetFilter and iptables are the framework inside the Linux 2.4.x kernel which enables packet filtering, network address translation (NAT) and other packetmangling.
John the Ripper
John the Ripper is a fast password cracker, currently available for many flavors of Unix. IP Filter
IP Filter is a software package that can be used to provide network address translation (NAT) or firewall services.
fport
fport identifys all open TCP/IP and UDP ports and maps them to the owning application.
OpenPGP
OpenPGP is a non-proprietary protocol for encrypting email using public key cryptography. It is based on PGP as originally developed by Phil Zimmermann.
SAINT
SAINT network vulnerability assessment scanner detects vulnerabilities in your network's security before they can be exploited etc.
Thanks! Enjoy…