2018-01-23 Avoiding the linux statefull firewall for some traffic 4 weeks ago
I was setting up a linux based firewall on a busy ntp server and to make sure everything worked as designed I added the usual:iptables -A INPUT -j ACCEPT --protocol all -m state --state ESTABLISHED,RELATEDAnd after less than half an hour the system log started filling withnf_conntrack: table full, dropping packet nf_conntrack: table full, dropping packet nf_conntrack: table full, dropping packet nf_conntrack: table full, dropping packetIt is indeed a busy server. The solution is to exclude all the ntp traffic from the stateful firewall. Which means I have to allow all kinds of ntp traffic (outgoing and incoming) by itself. The specific ruleset:iptables -t raw -A PREROUTING --protocol udp --dport 123 -j NOTRACK iptables -t raw -A OUTPUT --protocol udp --sport 123 -j NOTRACK iptables -A INPUT -j ACCEPT --protocol udp --destination-port 123I also made sure the rules for the ntp traffic are the first rules. Traffic at this server is somewhat over 1000 ntp request per second. So the counters of the NOTRACK rules go fast.# iptables -t raw -L -v Chain PREROUTING (policy ACCEPT 1652K packets, 126M bytes) pkts bytes target prot opt in out source destination 9635K 732M CT udp -- any any anywhere anywhere udp dpt:ntp NOTRACK 1650K 125M CT udp -- any any anywhere anywhere udp dpt:ntp NOTRACK Chain OUTPUT (policy ACCEPT 1522K packets, 117M bytes) pkts bytes target prot opt in out source destination 9029K 686M CT udp -- any any anywhere anywhere udp spt:ntp NOTRACK 1520K 116M CT udp -- any any anywhere anywhere udp spt:ntp NOTRACKBut no packets are dropped, which is good as this server is supposed to be under a constant DDoS.