Redhat Enterprise Linux 5 / CentOS 5 monitor and track TCP connections on the network (eth0)

Q. How do I track and monitor connection for eth1 public network interface under Redhat Enterprise Linux (RHEL) 5 server?

A.You can use netstat command or tcptrack command. Both command can show established TCP connection and provides the ability to monitor the same.
netstat command

netstat command prints information about the Linux networking subsystem. It also works under UNIX and *BSD oses. It can display network connections, routing tables, interface statistics, masquerade connections, and multicast memberships etc.
netstat command to display established connections

Type the command as follows:
$ netstat -nat
Output:

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 127.0.0.1:2208 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:52459 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:10000 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:8080 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:1521 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:53 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:3128 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:31323 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:2207 0.0.0.0:* LISTEN
tcp 0 0 192.168.1.100:59917 74.86.48.98:291 ESTABLISHED
tcp 0 0 127.0.0.1:3128 127.0.0.1:49413 TIME_WAIT
tcp 0 0 127.0.1.1:54624 127.0.1.1:1521 ESTABLISHED
tcp 0 0 127.0.1.1:1521 127.0.1.1:54624 ESTABLISHED
tcp 0 0 192.168.1.100:55914 74.125.19.147:80 ESTABLISHED
tcp 0 0 127.0.0.1:3128 127.0.0.1:42471 TIME_WAIT
tcp 0 0 192.168.1.100:56357 74.86.48.98:993 ESTABLISHED
tcp 0 0 192.168.1.100:56350 74.86.48.98:993 ESTABLISHED
tcp6 0 0 :::53 :::* LISTEN
tcp6 0 0 :::22 :::* LISTEN

To display client / server ESTABLISHED connections only:
$ netstat -nat | grep 'ESTABLISHED'
tcptrack command

tcptrack command displays the status of TCP connections that it sees on a given network interface. tcptrack monitors their state and displays information such as state, source/destination addresses and bandwidth usage in a sorted, updated list very much like the top command.
Install tcptrack

Redhat (RHEL) / Fedora / CentOS user, download tcptract here. For example download RHEL 64 bit version:
# cd /tmp/
# wget http://dag.wieers.com/rpm/packages/tcptrack/tcptrack-1.1.5-1.2.el5.rf.x86_64.rpm
# rpm -ivh tcptrack-1.1.5-1.2.el5.rf.x86_64.rpm

Debian / Ubuntu Linux user use apt-get as follows:
$ sudo apt-get install tcptrack
How do I use tcptract to monitor and track TCP connections ?

tcptrack requires only one parameter to run i.e. the name of an interface such as eth0, eth1 etc. Use the -i flag followed by an interface name that you want tcptrack to monitor.
# tcptrack -i eth0
# tcptrack -i eth1

Redhat Enterprise Linux 5 / CentOS 5 monitor and track TCP connections on the network (eth0)
(tcptrack in action)

You can just monitor TCP port 25 (SMTP)
# tcptrack -i eth0 port 25

The next example will only show web traffic monitoring on port 80:
# tcptrack -i eth1 port 80

tcptrack can also take a pcap filter expression as an argument. The format of this filter expression is the same as that of tcpdump and other libpcap-based sniffers. The following example will only show connections from host 76.11.22.12:
# tcptrack -i eth0 src or dst 76.11.22.12