[asterisk-users] IAX port

jg webaccounts173 at jgoettgens.de
Tue Feb 10 03:02:16 CST 2015

> I get an occasional similar problem, we have Mikrotik firewalls and from tcpdump monitoring on 
> the asterisk boxes I can see that the firewall (unbidden) has changed the IAX port. Usually a 
> firewall reset and sometimes PBX reset combination fixes it.
> Its odd as its only one direction, occurs rarely and with no obvious driver. So IAX is happy 
> in one direction but not the other. And I can see packets in the unhappy point arriving on the 
> wrong port.
> I couldn't fix it without kicking the router/firewall so I would say its a router problem in 
> the Destination NAT process.
> Cheers Duncan
>> Port is changed when NAT is applied from LAN to WAN.
>> While UDP session is maintained as ESTABLISHED, that port should not change.
>> If your peer changes constantly of session port could be UDP session
>> is too short in NAT table on routers.
>> You can try setting qualify=1000 (which is in ms. Default is 2000),
>> and see if peer keeps same port.
>> Regards.
voip-info.org also has an entry about general NAT related issues, which could be relevant here

I do not seem to have problems with Netgear firewalls, but other firewalls show this effect. So 
far it happened only on a single side, such that calls work from the other side. I already 
checked the open ports with nc/ncat/netcat as UDP sender and receiver on the other end. The 
ports are open, even when the arbitrary ports are used by Asterisk.

I'll need to read a bit more and evaluate my pcap traces and possibly ask the router vendors.

Thank you for your efforts.


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