Snort:Intrusion Detection System

Snort is a lightweight intrusion detection system that can log packets coming across your network. This
program can be used on smaller networks but on larger ones, with Gigabit Ethernet, snort can become
unreliable. Snort doesn't require that you recompile your kernel or add any software or hardware to your
existing distribution but it does require that you have root privileges.

Now we'll take a look at some basic rules to see how they work. The basic format of a rule is as follows:
function protocol source_ip/mask source_port -> destination_ip/mask destination_port options
A rules file will contain many lines of rules and it can look a little complicated at first. Here’s an example.
# look for stealth port scans/sweeps
alert tcp any any -> $HOME_NET any (msg:"SYN FIN Scan"; flags: SF;)
alert tcp any any -> $HOME_NET any (msg:"FIN Scan"; flags: F;)
alert tcp any any -> $HOME_NET any (msg:"NULL Scan"; flags: 0;)
alert tcp any any -> $HOME_NET any (msg:"XMAS Scan"; flags: FPU;)
alert tcp any any -> $HOME_NET any (msg:"Full XMAS Scan"; flags: SRAFPU;)
alert tcp any any -> $HOME_NET any (msg:"URG Scan"; flags: U;)
alert tcp any any -> $HOME_NET any (msg:"URG FIN Scan"; flags: FU;)
alert tcp any any -> $HOME_NET any (msg:"PUSH FIN Scan"; flags: FP;)
alert tcp any any -> $HOME_NET any (msg:"URG PUSH Scan"; flags: PU;)
alert tcp any any -> $HOME_NET any (flags: A; ack: 0; msg:"NMAP TCP ping!";)
# IMAP buffer overflow
alert tcp any any -> $HOME_NET 143 (msg:"IMAP Buffer Overflow!"; content:"|E8C0FF FFFF|"; flags: PA;)
# x86 named buffer overflow
alert tcp any any -> $HOME_NET 53 (msg:"named Buffer Overflow!"; content:"|CD80E8D7 FFFF FF|"; flags: PA;)

Now lets try to make our own rule and test it out....

Step1: Making rule:

Step2: Now run snort with this rule and try to telnet to this machine to generate alert messages.

You can use many options with snort to get better console display.To check out other option just type snort --help in terminal.

Output:
check out the log directory which we have defined with -l option.