With the kernel's iptables completely empty (
iptables -F), this will do what you ask:
# iptables -A INPUT -p tcp --dport 22 -s 192.168.0.0/24 -j ACCEPT
# iptables -A INPUT -p tcp --dport 22 -s 127.0.0.0/8 -j ACCEPT
# iptables -A INPUT -p tcp --dport 22 -j DROP
This says that all LAN addresses are allowed to talk to TCP port 22, that localhost gets the same consideration (yes, 127.* not just 127.0.0.1), and packets from every other address not matching those first two rules get unceremoniously dropped into the bit bucket. You can use
REJECT instead of
DROP if you want an active rejection (TCP RST) instead of making TCP port 22 a black hole for packets.
If your LAN doesn't use the 192.168.0.* block, you will naturally need to change the IP and mask on the first line to match your LAN's IP scheme.
These commands may not do what you want if your firewall already has some rules configured. (Say
iptables -L as root to find out.) What frequently happens is that one of the existing rules grabs the packets you're trying to filter, so that appending new rules has no effect. While you can use
-I instead of
-A with the
iptables command to splice new rules into the middle of a chain instead of appending them, it's usually better to find out how the chains get populated on system boot and modify that process so your new rules always get installed in the correct order.
On RHEL type systems, the easiest way to modify firewall chains when ordering matters is to edit
/etc/sysconfig/iptables. The OS's GUI and TUI firewall tools are rather simplistic, so once you start adding more complex rules like this, it's better to go back to good old config files. Beware, once you start doing this, you risk losing your changes if you ever use the OS's firewall tools to modify the configuration, since it may not know how to deal with handcrafted rules like these.
Add something like this to that file:
-A RH-Firewall-1-INPUT -p tcp --dport 22 -s 192.168.0.0/24 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp --dport 22 -s 127.0.0.0/8 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp --dport 22 -j DROP
Where you add it is the tricky bit. If you find a line in that file talking about
--dport 22, simply replace it with the three lines above. Otherwise, it should probably go before the first existing line ending in
-j ACCEPT. Generally, you'll need to acquire some familiarity with the way iptables works, at which point the correct insertion point will be obvious.
Save that file, then say
service iptables restart to reload the firewall rules. Be sure to do this while logged into the console, in case you fat-finger the edits! You don't want to lock yourself out of your machine while logged in over SSH.
The similarity to the commands above is no coincidence. Most of this file consists of arguments to the
iptables command. The differences relative to the above are that the
iptables command is dropped and the
INPUT chain name becomes the special RHEL-specific
RH-Firewall-1-INPUT chain. (If you care to examine the file in more detail, you'll see earlier in the file where they've essentially renamed the
INPUT chain. Why? Couldn't say.)