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48.2.8. Verifying Which Ports Are Listening
After configuring network services, it is important to pay attention to which ports are actually listening on the system's network interfaces. Any open ports can be evidence of an intrusion.
There are two basic approaches for listing the ports that are listening on the network. The less reliable approach is to query the network stack using commands such as
netstat -an or lsof -i. This method is less reliable since these programs do not connect to the machine from the network, but rather check to see what is running on the system. For this reason, these applications are frequent targets for replacement by attackers. Crackers attempt to cover their tracks if they open unauthorized network ports by replacing netstat and lsof with their own, modified versions.
A more reliable way to check which ports are listening on the network is to use a port scanner such as
nmap.
The following command issued from the console determines which ports are listening for TCP connections from the network:
nmap -sT -O localhost
The output of this command appears as follows:
Starting nmap 3.55 ( http://www.insecure.org/nmap/ ) at 2004-09-24 13:49 EDT Interesting ports on localhost.localdomain (127.0.0.1): (The 1653 ports scanned but not shown below are in state: closed) PORT STATE SERVICE 22/tcp open ssh 25/tcp open smtp 111/tcp open rpcbind 113/tcp open auth 631/tcp open ipp 834/tcp open unknown 2601/tcp open zebra 32774/tcp open sometimes-rpc11 Device type: general purpose Running: Linux 2.4.X|2.5.X|2.6.X OS details: Linux 2.5.25 - 2.6.3 or Gentoo 1.2 Linux 2.4.19 rc1-rc7) Uptime 12.857 days (since Sat Sep 11 17:16:20 2004) Nmap run completed -- 1 IP address (1 host up) scanned in 5.190 seconds
This output shows the system is running
portmap due to the presence of the sunrpc service. However, there is also a mystery service on port 834. To check if the port is associated with the official list of known services, type:
cat /etc/services | grep 834
This command returns no output. This indicates that while the port is in the reserved range (meaning 0 through 1023) and requires root access to open, it is not associated with a known service.
Next, check for information about the port using
netstat or lsof. To check for port 834 using netstat, use the following command:
netstat -anp | grep 834
The command returns the following output:
tcp 0 0 0.0.0.0:834 0.0.0.0:* LISTEN 653/ypbind
The presence of the open port in
netstat is reassuring because a cracker opening a port surreptitiously on a hacked system is not likely to allow it to be revealed through this command. Also, the [p] option reveals the process ID (PID) of the service that opened the port. In this case, the open port belongs to ypbind (NIS), which is an RPC service handled in conjunction with the portmap service.
The
lsof command reveals similar information to netstat since it is also capable of linking open ports to services:
lsof -i | grep 834
The relevant portion of the output from this command follows:
ypbind 653 0 7u IPv4 1319 TCP *:834 (LISTEN) ypbind 655 0 7u IPv4 1319 TCP *:834 (LISTEN) ypbind 656 0 7u IPv4 1319 TCP *:834 (LISTEN) ypbind 657 0 7u IPv4 1319 TCP *:834 (LISTEN)
These tools reveal a great deal about the status of the services running on a machine. These tools are flexible and can provide a wealth of information about network services and configuration. Refer to the man pages for
lsof, netstat, nmap, and services for more information.
