[Spce-user] UA NAT identification

Andreas Granig agranig at sipwise.com
Wed Jul 24 09:53:55 EDT 2013

Hi John,

On 07/24/2013 12:54 PM, John Murray wrote:
> How does SPCE recognise a UA is behind a NAT device? Can I force it to
> use NAT?

It checks for RFC1918 addresses in various parts of the SIP message, and 
it checks whether these addresses are different from the one where the 
message was received from.

I don't think we have a preference to control the behaviour on SIP level 
(only for media).

> I have a cisco handset and a Xlite client which register correctly and
> call can be made in both directions.
> The device registrations on the OSS show as ‘NAT yes’
> I also have an Avaya IP Office PBX behind another NAT router (Draytek
> 2830 with SIP ALG off) that registers correctly and can place outbound
> calls but device registrations show as ‘NAT no’ so call to the device
> are send to the private address.

You can check with ngrep-sip what the phone sends in REGISTER and 
INVITE. Probably the phone itself already does some NAT detection via STUN?

> Registrations from the Avaya come from the public address but contain a
> VIA header with the private address which matches registrations from the
> cisco.

Then either the phone or the router does some black magic (and as 
always, does it incorrectly).

> Also the private address of the Avaya is rather than rfc1918
> but I guess that shouldn’t matter.

Well, that check is one part of the overall NAT procedure.


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