I have a Linux router (Debian 6.x) where i forward some ports to internal services. Some tcp ports (like 80, 22...) are OK. I have one Application listening on port 54277udp. No return is coming from this app, i only get Data on this port. Router: cat /proc/sys/net/ipv4/conf/all/rp_filter = 1 cat /proc/sys/net/ipv4/conf/eth0/forwarding = 1 cat /proc/sys/net/ipv4/conf/ppp0/forwarding = 1 $IPTABLES -t nat -I PREROUTING -p udp -i ppp0 --dport 54277 -j DNAT --to-destination $SRV_IP:54277 $IPTABLES -I FORWARD -p udp -d $SRV_IP --dport 54277 -j ACCEPT Also MASQUERADING internal traffic to ppp0(internet) is active & working. Default Policy INPUT&OUTPUT&FORWARD is DROP What is strange, when i do: tcpdump -p -vvvv -i ppp0 port 54277 I get a lot of traffic: 18:35:43.646133 IP (tos 0x0, ttl 57, id 0, offset 0, flags [DF], proto UDP (17), length 57) source.ip > own.external.ip..54277: [udp sum ok] UDP, length 29 18:35:43.652301 IP (tos 0x0, ttl 57, id 0, offset 0, flags [DF], proto UDP (17), length 57) source.ip > own.external.ip..54277: [udp sum ok] UDP, length 29 18:35:43.653324 IP (tos 0x0, ttl 57, id 0, offset 0, flags [DF], proto UDP (17), length 57) source.ip > own.external.ip..54277: [udp sum ok] UDP, length 29 18:35:43.655795 IP (tos 0x0, ttl 57, id 0, offset 0, flags [DF], proto UDP (17), length 57) source.ip > own.external.ip..54277: [udp sum ok] UDP, length 29 18:35:43.656727 IP (tos 0x0, ttl 57, id 0, offset 0, flags [DF], proto UDP (17), length 57) source.ip > own.external.ip..54277: [udp sum ok] UDP, length 29 18:35:43.659719 IP (tos 0x0, ttl 57, id 0, offset 0, flags [DF], proto UDP (17), length 57) source.ip > own.external.ip..54277: [udp sum ok] UDP, length 29 tcpdump -p -i eth0 port 54277 (on the same machine, the router) i get much less traffic. also on the destination $SRV_IP there are only a few packets coming in, but not all. INTERNAL SERVER: 19:15:30.039663 IP source.ip.52394 > 192.168.215.4.54277: UDP, length 16 19:15:30.276112 IP source.ip.52394 > 192.168.215.4.54277: UDP, length 16 19:15:30.726048 IP source.ip.52394 > 192.168.215.4.54277: UDP, length 16 So some udp ports are "ignored/dropped" ? Any idea what could be wrong? Edit: This is strange: The Forward rule has data packets, but the PREROUTING rule has 0 packets... iptables -nvL -t filter |grep 54277 Chain FORWARD (policy DROP 0 packets, 0 bytes) 168 8401 ACCEPT udp -- * * 0.0.0.0/0 192.168.215.4 state NEW,RELATED,ESTABLISHED udp dpt:54277 iptables -nvL -t nat |grep 54277 Chain PREROUTING (policy ACCEPT 405 packets, 24360 bytes) 0 0 DNAT udp -- ppp0 * 0.0.0.0/0 my.external.ip udp dpt:54277 state NEW,RELATED,ESTABLISHED to:192.168.215.4