Simple port forward with iptables...

[fizgig]

I'd like to forward udp packets with port 4569 from my gateway ubuntu server (PUBLICIP) to my asterisk server inside (LOCALIP).

Here are the rules I've done:

$IPT -t nat -A PREROUTING -p udp --dport 4569 -j DNAT --to LOCALIP
$IPT -A FORWARD -p upd --dport 4569 -j ACCEPT
$IPT -t nat -A POSTROUTING -p udp -s LOCALIP -j SNAT --to PUBLICIP

I run tcpdump on my gateway computer and also on my asterisk server. When the keep-alive heartbeat packet comes from a computer on the outside, I see it hit the ubuntu gateway but not the asterisk server. If my asterisk server generates the heartbeat packet, I see it on both ends.

Also, forwarding is enabled under /proc/sys/net/ipv4/ip_forward

Anyone know why the ubuntu gateway isn't forwarding the packet on to my asterisk server?

[Doug S]

I think that this rule

$IPT -t nat -A POSTROUTING -p udp -s LOCALIP -j SNAT --to PUBLICIP

needs to specify an interface. Specifically your external interface.

$IPT -t nat -A POSTROUTING -o EXTERNAL_INTERFACE -p udp -s LOCALIP -j SNAT --to PUBLICIP

Similiarly for your PREROUTING rule. This:

$IPT -t nat -A PREROUTING -p udp --dport 4569 -j DNAT --to LOCALIP

should be this:

$IPT -t nat -A PREROUTING -i EXTERNAL_INTERFACE -p udp --dport 4569 -j DNAT --to LOCALIP

[fizgig]

Well, it turns out the server needed to be rebooted. I guess you can only flush iptables so many times...

Thanks for your help!