let's look at these two iptables rules which are often used to allow outgoing DNS:
iptables -A OUTPUT -p udp --sport 1024:65535 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A INPUT -p udp --sport 53 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
My question is: How exactly should I understand the ESTABLISHED state in UDP? UDP is stateless.
Here is my intuition - I'd like to know, if or where this is incorrect:
The man page tells me this:
stateThis module, when combined with connection tracking, allows access to the connection tracking state for this packet. --state ...
So, iptables basically remembers the port number that was used for the outgoing packet (what else could it remember for a UDP packet?), and then allows the first incoming packet that is sent back within a short timeframe? An attacker would have to guess the port number (would that really be too hard?)
About avoiding conflicts:
The kernel keeps track of which ports are blocked (either by other services, or by previous outgoing UDP packets), so that these ports will not be used for new outgoing DNS packets within the timeframe? (What would happen, if I accidentally tried to start a service on that port within the timeframe - would that attempt be denied/blocked?)
Please find all errors in the above text :-).