How to filter SSH at the firewall.
Posted by admin on 11 September 2009 11:12 PM
If you only need remote access from one IP address (say from work to your home server), then consider filtering connections at your firewall by either adding a firewall rule on your router or in iptables to limit access on port 22 to only that specific IP address. For example, in iptables this could be achieved with the following type of rule:|
iptables -A INPUT -p tcp -s 126.96.36.199 --dport 22 -j ACCEPT
SSH also natively supports TCP wrappers and access to the ssh service may be similarly controlled using hosts.allow and hosts.deny.
If you are unable to limit source IP addresses, and must open the ssh port globally, then iptables can still help prevent brute-force attacks by logging and blocking repeated attempts to login from the same IP address. For example,
iptables -A INPUT -p tcp --dport 22 -m recent --set --name ssh --rsource
iptables -A INPUT -p tcp --dport 22 -m recent ! --rcheck --seconds 60 --hitcount 4 --name ssh --rsource -j ACCEPT
The first rule records the IP address of each attempt to access port 22 using the recent module. The second rule checks to see if that IP address has attempted to connect 4 or more times within the last 60 seconds, and if not then the packet is accepted. Note this rule would require a default policy of DROP on the input chain.
Don't forget to change the port as appropriate if you are running ssh on a non-standard port. Where possible, filtering at the firewall is an extremely effective method of securing access to an ssh server.