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Q. How do I find out which service is listening on a specific port? How do I find out what program is listening on a specific TCP Port?
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Restrict ssh access using tcpd (TCPWrapper)

tcpd is use to access control facility for internet services. The tcpd program can be set up to monitor incoming requests for telnet, finger, ftp, exec, rsh, rlogin, tftp, sshd and other services that have a one-to-one mapping onto executable files. Your sshd server must be configuring (compiled with) to support tcpd.

You can find out tcpd (tcpwrapper) support easily with following command:

# strings $(which sshd)| grep libwrap

libwrap.so.0
libwrap refuse returns

If you get output libwrap (as above) then you can use tcpd as follows. Open file /etc/hosts.deny in text editor. This file list of hosts/IPs that are not allowed to access the system. In your case you will block sshd (port 22). Let us say you would like to deny access to IPs 202.54.1.20 64.66.44.22 64.66.44.25.
# vi /etc/hosts.deny Add/append following line to file:sshd: 202.54.1.20 64.66.44.22 64.66.44.25Save and exit to shell prompt. Next make sure your rules are correct with the following command:
# tcpdchk -v
Output:

Using network configuration file: /etc/inetd.conf
>>> Rule /etc/hosts.deny line 20:
daemons:  sshd
clients:  202.54.1.20 64.66.44.22 64.66.44.25
access:   denied

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