[asterisk-users] chan_sip sending from wrong source address when multiple interfaces are used

[Kevin P. Fleming]

On 07/12/2012 09:19 AM, Benny Amorsen wrote:
> "Kevin P. Fleming" <kpfleming at digium.com> writes:
>
>> That's quite interesting; can you describe a scenario where this occurs?
>
> Imagine you have a server with two interfaces, eth0 with 192.168.1.1/24
> and eth1 with 10.0.2.1/24. Further imagine that you wish to be able to
> move phones between the networks without changing the SIP server
> address, so you set 192.168.1.1 as the SIP server no matter which
> network they happen to be on.
>
> Now the phones which happen to be connected to eth1 will send a request
> to 192.168.1.1. If Asterisk is bound to 0.0.0.0, the reply will come
> from 10.0.2.1. This could be solved if Asterisk did a connect() to the
> socket and use the same socket for answering. That would tell the system
> IP stack that this is in fact a connection, and so the system would
> ensure that the reply source IP would be correct.

I must be missing something. If a phone sends a UDP packet to 
192.168.1.1, how does that get routed to (arrive at) the 10.0.2.1 
interface on the Asterisk server? The only way I can imagine that 
happening is if a router in between the phone and the server has been 
told that 192.168.1.0/24 is reachable *through* 10.0.2.1, which seems 
like a bizarre way to construct a network. Getting replies from Asterisk 
*back* to the phone would also require the IP stack on the Asterisk 
server to route those replies back over the 10.0.2.0/24 interface 
instead of the 192.168.1.0/24, which doesn't make any sense either.

chan_sip does have the ability to use connect()-ed sockets for dialogs 
now, since that is required for TCP, TLS and WebSocket support. It 
wouldn't be a huge leap to use them for UDP as well, if that was beneficial.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming at digium.com | SIP: kpfleming at digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at www.digium.com & www.asterisk.org