mjhjmtu

I'm using linux mint and I want to block all incoming connections on port 5210 except 3 IPs. I've searched and went through a lot of threads, and found only results allowing just ranges of LAN IPs, and I cannot find anything related to allowing exactly 3 different IPs that are not in the LAN.

How should I do this or what should I search for?

Allow the three, reject/drop the rest. With iptables from the command line:

iptables -A INPUT -p tcp --dport 5210 --source "$addr1" -j ACCEPT
iptables -A INPUT -p tcp --dport 5210 --source "$addr2" -j ACCEPT
iptables -A INPUT -p tcp --dport 5210 --source "$addr3" -j ACCEPT
iptables -A INPUT -p tcp --dport 5210 -j REJECT

For, e.g. addr2, the first rule does not match, and it's ignored, while the second rule matches and accepts the packet.

Or, make a chain that does nothing for the three addresses and rejects the rest, then accept or do any further processing in the upper level:

iptables -N p5210
iptables -A p5210 --source "$addr1" -j RETURN
iptables -A p5210 --source "$addr2" -j RETURN
iptables -A p5210 --source "$addr3" -j RETURN
iptables -A p5210 -j REJECT

iptables -A INPUT -p tcp --dport 5210 -j p5210
# add whatever further limitations you want
iptables -A INPUT -p tcp --dport 5210 -j ACCEPT 

Of course, putting the addresses in a variable and using a loop to run the same command for all of them is also an option:

#!/bin/bash
allowed_addresses=(1.2.3.4 4.5.6.7 7.8.9.0)
for addr in "$allowed_addresses[@]" ; do 
 iptables -A INPUT -p tcp --dport 5210 --source "$addr" -j ACCEPT
done