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Keep an Eye on Your Linux Systems with Netstat Print E-mail
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Source: enterprisenetworkingplanet.com - Posted by Vincenzo Ciaglia   
Documentation Two of the fundamental aspects of Linux system security and troubleshooting are knowing what services are running, and what connections and services are available. We're all familiar with ps for viewing active services. netstat goes a couple of steps further, and displays all available connections, services, and their status. . . . Two of the fundamental aspects of Linux system security and troubleshooting are knowing what services are running, and what connections and services are available. We're all familiar with ps for viewing active services. netstat goes a couple of steps further, and displays all available connections, services, and their status. It shows one type of service that ps does not: services run from inetd or xinetd, because inetd/xinetd start them up on demand. If the service is available but not active, such as telnet, all you see in ps is either inetd or xinetd:

$ ps ax | grep -E 'telnet|inetd'

520 ? Ss 0:00 /usr/sbin/inetd

But netstat shows telnet sitting idly, waiting for a connection:

$ netstat --inet -a | grep telnet

tcp 0 0 *:telnet *:* LISTEN

This netstat invocation shows all activity:

$ netstat -a
Active Internet connections (servers and established)

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 *:telnet *:* LISTEN

tcp 0 0 *:ipp *:* LISTEN

tcp 0 0 *:smtp *:* LISTEN

tcp 0 0 192.168.1.5:32851 nest.anthill.echid:ircd
ESTABLISHED

udp 0 0 *:ipp *:*

Active UNIX domain sockets (servers and established)

Proto RefCnt Flags Type State I-Node Path

unix 2 [ ACC ] STREAM LISTENING 1065 /tmp/ksocket-carla/klaunchertDCh2b.slave-socket

unix 2 [ ACC ] STREAM LISTENING 1002 /tmp/ssh-OoMGfFm666/agent.666

unix 2 [ ACC ] STREAM LISTENING 819 private/smtp

Your total output will probably run to a couple hundred lines. (A fun and quick way to count lines of output is netstat -a | wc -l.) You can ignore everything under "Active UNIX domain sockets." Those are local inter-process communications, not network connections. To avoid displaying them at all, do this:

Read this full article at enterprisenetworkingplanet.com

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