Tuesday, July 5, 2011

How to allow only one ip accessing the server through ssh

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This post explains how to block all ips except one accessing your system through ssh. Here we are using iptables firewall for blocking and allowing  ips. These rules are tested on Centos linux and will work with other linux distros like redhat, fedora, etc.

Suppose you have your main server with ip 192.168.1.10
And you want to allow access from 192.168.1.4 only

Here is the rules :
Initial state [all accept]

root@test [~]# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
root@test [~]#
See the rules below.

Now writing rules :

root@test [~]# iptables -I INPUT -p tcp -s 192.168.1.4 --dport 22 -j ACCEPT
root@test [~]# iptables -I OUTPUT -p tcp -d 192.168.1.4 --sport 22 -j ACCEPT
root@test [~]# iptables -P INPUT DROP
root@test [~]# iptables -P OUTPUT DROP
root@test [~]# iptables -P FORWARD DROP
root@test [~]# iptables -L

After :
root@cpaneltest [~]# iptables -L
Chain INPUT (policy DROP)
target     prot opt source               destination
ACCEPT     tcp  --  192.168.1.4          anywhere            tcp dpt:ssh
Chain FORWARD (policy DROP)
target     prot opt source               destination
Chain OUTPUT (policy DROP)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             192.168.1.4         tcp spt:ssh

Thats it. now the server 192.168.1.10 will be only accessible through ssh from 192.168.1.4.
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