[Serusers] SER routing fails with two levels of alias indirection w/o hosts entry pointing SIP domain to itself...

[Jim Burwell]

Hi,

A client of mine has encountered a problem with SER.  When he has 
aliases which point to other aliases, SER fails to route a call UNLESS 
the SIP domain of the callee has an /etc/hosts entry associated with it 
(or, presumably, a DNS A record for the domain, or a DNS SRV record for 
SIP inside that domain).

For example:

    ser.cfg has line:  alias="domain.com"
    Callee is "alias1 at domain.com"
    SER user alias "alias1 at domain.com" -> "sip:alias2 at domain.com" ->
    "sip:user at domain.com"
    SER usrloc entry exists for "user at domain.com" ->
    "sip:user@[IP-of-user's-phone]"
    /etc/hosts file has entry in the form of: 
        1.2.3.4 siprouter siprouter.domain.com domain.com
    CALL WORKS

    However, change /etc/host file entry to: 
        1.2.3.4 siprouter siprouter.domain.com
    (remove alias for whole domain)
    CALL FAILS

    Other, more simple SER user aliases WORK in BOTH scenereos: 
        SER user alias "somealias at domain.com" -> "sip:someuser at domain.com"
        SER usrloc exists for "someuser at domain.com" ->
    "sip:someuser@[IP-of-user's-phone]"
    CALL WORKS
    This is whether the "domain.com" /etc/hosts alias exists or not.

So, SER appears to NEED to resolve an IP address for the SIP domain to 
do two (and above) levels of alias indirection.  It's almost as if the 
resolution of aliases isn't recursive or something (seeing how 
single-level aliases work, and it doesn't try to resolve an IP to "call 
out" to another SIP router). 

Note, that "ser.cfg" DOES have a line alias="domain.com", so SER should 
know that it is responsible for this SIP domain. 

It appears that SER is trying to make a network call to this SIP domain 
before it even begins processing the ser.cfg script, where it would 
match the if(uri == myself) condition because of the "alias=" lines.  
Perhaps a heuristic needs to be put in somewhere to check the condition 
"callee_domain == myself" before SER goes through the trouble of trying 
to resolve the callee domain when it will just point back at itself ?

This problem reminds me of the one I ran into earlier with the 
t_uac_dlg() FIFO call used by the click-to-dial scripts.  This also had 
the problem where SER would try to resolve its own SIP domain before 
checking if SIP_DOMAIN == myself.

The quick and dirty fix was to add a /etc/hosts alias (as illustrated 
above), but this has the unwanted effect of breaking sendmail mail 
routing, requiring an ugly rewriting rule hack to "fix" the problem 
(sendmail thinks myself == mail domain, and recipients w/ @domain.com 
addresses are local users). 

The "good" fix is to use a DNS SRV entry (at least I hope this would fix 
both SER problems mentioned here).

But, should SER really be doing these name resolution calls in 
situations like this ?  Isn't this inefficient ?  It seems like a bug to 
me :-).  It seems to me that SER should pay attention to the "alias=" 
lines in all phases of calee routing, with the exception of when the 
calee's SIP address is resolved down to userloc entry pointing it to 
another SIP device.

Perhaps my concept of the purpose/scope of a SIP domain is wrong ?  I 
assume that a SIP server, or group of servers with identical 
configurations/databases (for load balancing) is "authoritative" for a 
domain, and unless an alias or usrloc entry directs the call to a 
different SIP UA or router via IP or a different SIP [sub]domain for 
further processing, it should presume it has all information to resolve 
this call fully itself, and recurse onto itself to resolve any names 
inside its own domain.  Basically, the DNS model for domain authority.

- Jim

-- 
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|         Jim Burwell - Sr. Systems/Network/Security Engineer, JSBC         |
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