[Serusers] rewriting the outgoing URI to its alias

[Greger V. Teigre]

I see a lot of confusion on the different user related ids in SIP. Let 
me try to explain (and answer your question, Andy, at the end of the 
post...)

Let's start at the user agent(UA) side:
--------------------
AOR: Address of record is the sip:myuser at domain.com address that you are 
known as, just like your email address.  Many user agents don't ask for 
AOR, but builds it from other parameters you need to set
Username: The username is normally the user (before @) of AOR)
Realm: Often the domain portion of the AOR, thus AOR = sip:username at realm
Friendly name/displayname: "My name"

Out of this info, the From header is constructed:
From: "My name" <sip:username at realm>
---
Entirely separate (but sometimes confused in user agents):
Authentication username: The user used for authentication
Authentication realm: The realm to authenticate within
Very often the authentication realm is implicitly assumed to be the same 
as the AOR realm
---
Then, the user agent will create the Contact header. The Contact header 
should be the public contact address of your current location. Thus: 
Contact: sip:username at mypublicip:5060
If the user agent is behind a NAT, the mypublicip will be a private address.
---
A sidenote: Unless registration server is explicitly specified, the 
realm in the AOR will be used for looking up the SIP registration server 
using DNS SRV or A lookups. You should avoid putting the FQDN of your 
SIP server in the realm.
Also, you may in some user agents specify outbound proxy. This is the 
proxy where the user agent will send INVITEs (and other outbound messages).
-------------------

And on the SER side:
-------------------
Authentication user/realm are used to do Digest authentication, but are 
then forgotten (i.e. not stored).
The AOR is registered in the location table, together with the Contact 
header, as well as the source ip and port (if different from Contact). 
The fix_nated_register() function handles this setting of the so-called 
received parameter.

So, to the routing:
- Messages that need to be routed (i.e. do not have Route headers) will 
have a Request URI; the first line and the part after the message type: 
INVITE sip:username at domain.com
- It is by changing this request URI, you do routing.  The t_relay() 
command uses the URI to forward the message correctly
- Before forwarding to a user agent, you want the request uri to be the 
same as the stored Contact header for the AOR you are looking up. If 
not, the user agent may reply with a 404 User not found
- The From header is NOT used for routing, and for backwards RFC 
compatibility, you should not change the From header as some UAs will 
use the content of the From header to match a dialog (however, if your 
UAs from experience still work, there should not be a problem doing it, 
it's just not RFC-compliant and may pop up and kick your butt later ;-)
- The AOR in location table is used for looking up incoming messages if 
you can find a direct match between the Request-URI in the incoming 
message and the stored AOR. If not, you can use the aliases table (and 
lookup("aliases")) to match the Request-URI with something in the 
aliases table, that again will map to the AOR in the location table
---
So keeping this (fairly simple) concept in focus: Routing (regardless of 
LCR, avps or whatever) should focus on finding the correct Request-URI 
before you call t_relay().

BUT, there are ways of "messing" up this...
---
- There are several commands in SER you can use to override the 
Request-URI (forward_*). They should be avoided, unless you have a valid 
reason for having a Request-URI in the message you are forwarding that 
is NOT resolvable (either IP address or DNS name or DNS SRV/A 
resolvable) to the party you are forwarding to. The reason can be if you 
want the R-URI to contain the AOR and then forward the message to a 
server handling voicemail
- The dst_uri parameter (implicitly set by lookup) will tell t_relay() 
to send the message to dst_uri instead of the Request-URI. lookup() will 
set this when it finds that the Contact stored for the AOR also has an 
associated received ip:port (because the user agent was NATed)

In general trying all sorts of tricks o fix-up things the way you need 
it may not be so smart. Stick to the basics and question yourself: Is 
this something I really want to do?
----------------------

To Andy, you want the aliases table to map to the AOR (as registered by 
the UA in the REGISTER command). You do lookup("aliases") to resolve 
your DID into an AOR. Then, later you can do lookup("location") to map 
the AOR to the location of the UA (i.e. Contact/dst_uri). Then your 
Request-URI will be correct.

g-)

Andy Thomas wrote:
>
> Does anyone know how I would do this-
>
>  
>
> For all users who have PSTN access, they are assigned a number in the 
> MySQL alias table which matches a PSTN DDI number.
>
> e.g. user 8000 has an alias of 2071231234, so on an incoming call the 
> lookup("aliases") function correctly matches the DDI to the user.
>
>  
>
> I want my ser.cfg to rewrite the user on an outgoing call, if a number 
> exists in the alias table for that user
>
> Obviously, the rewriteuser function will be used, but what do I put in 
> after that?
>
> I have tried rewriteuser (lookup ("aliases"))  but that doesn't work
>
>  
>
> Can anyone help?
>
>  
>
> ------------------------------------------------------------------------
>
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