[Serusers] Outbound proxy

[Greger V. Teigre]

Iqbal,
not a dumb question. outbound proxy is a bit difficult concept to grasp.
Klaus had an excellent post, see below.

I'm not really sure why registering doesn't work when outbound proxy is not 
set, it should. Outbound proxy is only used for outgoing calls. Without an 
outbound proxy, the client will try to do DNS lookups to resolve a call 
outside the domain, i.e. your sip-server is sip.domain1.com and you make a 
call to somebody at domain2.com.  With an outbound proxy it will always be sent 
to there, without it will be resolved by the client and a call attempted to 
domain2.com's SIP server.
For your other questions, see Klaus' post below.

g-)
-------------------------
First, the definition from RFC 3261:


> Outbound Proxy: A proxy that receives requests from a client, even
>         though it may not be the server resolved by the Request-URI.
>         Typically, a UA is manually configured with an outbound proxy,
>         or can learn about one through auto-configuration protocols.

Typically, SIP messages are sent to the SIP proxy which is responsible
for this domain. that means if the request URI is sip:klaus at iptel.org,
the meassage will be sent to the SIP proxy of iptel.org.

In same cases, it is requiered that the SIP messages passes a certain
SIP proxy. E.g. in enterprise environments with certain security
policies all SIP messages will be controlled by the company's SIP proxy
before they will leave the corporate network. Another scenario is NAT
traversal. If the iptel.org's SIP proxy wouldn't do NAT traversal, you
can insert a certain SIP proxy into the signaling path, which is
responsible for NAT traversal.

In this cases, this additional SIP proxies will be called Outboundproxy.
If a SIP UA is configured with an outboundproxy, all SIP requests will
be sent to the outboundproxy instead of sending it directly to the SIP
proxy indicated in the request URI.

In certain cases (NAT travesal, security ...) the outboundproxy also
rewrites the SDP in the SIP messages to direct audio to an RTP proxy.

Iqbal wrote:
> Hi
>
> I have a dumb question, it conecrns NAT and ser. I am using xlite and
> grandstream 486 to test with, SER has been working great in all
> situations, however I have realised that if I dont put an entry into
> outbound proxy, then REGISTER does not work if behind a NAT. When I
> put an entry into there it works, I am using mediaproxy which is
> running on the same server as ser.
>
> The question is what is the outbound proxy settings for in the UA, I
> assume it is/was either for NAT, or when the register server is
> separate from the proxy I guess.
>
> If its for the NAT then shouldnt my mediaproxy ser combo take care of
> it, since I am dealing with most NAT options at the SER/MP end.
>
> Iqbal
>
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