How do I use tcpd on a Linux to restrict ssh access?
The 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 the following command:
# strings $(which sshd)| grep libwrap
Sample outputs:
libwrap.so.0 libwrap refuse returns
If you see libwrap as output (as shown above) then you can use tcpd as follows to monitor incoming ssh requests.
Configuration
Open or edit file /etc/hosts.deny using a text editor such as vi. This file lists hosts or IPs that are not allowed to access the system. In this example, you will block sshd server TCP port 22 for selected IPs.
Examples
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 the following line:
sshd: 202.54.1.20 64.66.44.22 64.66.44.25
Save and close the file. Next, make sure your rules are correct and run syntax check using the following command:
# tcpdchk -v
Sample outputs:
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
