[Serusers] Need way to specify SOURCE IP address on packets of SIP messages being emitted by SER

[Frank Durda IV]

I have a problem that I thought SER could solve but now that I
need it, the method isn't obvious.

First, as an example, let's say I have three independent
organizations running Asterisk boxes, which send me
the SIP and RTP audio for calls.  Let's call them

Company A       Asterisk box at 31.1.2.3
Company B       Asterisk box at 42.4.5.6
Company C       Asterisk box at 53.7.8.9

So they are one side of the SER box all communicating SIP to a single
interface.   On the other side is a TDM PSTN switch capable of
accepting SIP calls.  The PSTN switch has exactly one IP address,
which we will call 101.2.3.4.

Inside the PSTN switch, for calls originating in TDM headed to SIP,
a "Route List" directs calls for a given trunk group (everything is
visualized in TDM terms even if the terms don't really apply) to a
specific IP address, where the SIP INVITE should go for this call.

In the case of a SIP to TDM call, when an INVITE comes in, the PSTN
switch looks through that list of Routes that point to IP addresses
and when it finds an IP address match, it knows which trunk group to
associate with this call for billing.  This also acts as a simple
friend/foe mechanism, because if there is no route pointing at your
SIP source, it rejects your INVITES.

So, if Company A, B and C were communicating directly with the PSTN
switch (no SER), you would simply need three trunk groups where
each IP address pointed directly to the Asterisk boxes at those
companies, like this:

TGN     SIP IP
1111 -> 31.1.2.3
2222 -> 42.4.5.6
3333 -> 53.7.8.9

and all would be well.  However, when you insert a SER box between
those the Asterisk boxes belonging to those companies and the
PSTN switch, problems arise.

I originally decided to address the issue by establishing two groups
of three IP addresses, one set that the Asterisk servers would
individually send their INVITEs to, and three that the PSTN switch
could send its INVITES to.  SER already supports the idea of
listening on numerous IP addresses, so that was easy (although the
documentation for "listen" and "alias" is weak and a code review was
needed to determine what alias actually did, something other than
what the "alias" option of an ifconfig command does...).

Anyway, we have a setup like this:

                         interface    interface      interface
                         em1 on SER   em0 on SER     sig0 on PSTN
                         10.1.0.0/16  10.2.0.0/16    101.2.3.0/28

Company A 31.1.2.3  ---- 10.1.2.3 SER 10.2.0.1 ----  101.2.3.4

Company B 42.4.5.6  ---- 10.1.5.6 SER 10.2.0.2 ----  101.2.3.4

Company C 53.7.8.9  ---- 10.1.8.9 SER 10.2.0.3 ----  101.2.3.4

Inside the ser.cfg, it is simple enough to rewrite the "host address"
(where you want the packet to go) to reflect the new destination
once that is determined, and for TDM to SIP calls work okay from the
perspective of the PSTN switch, but then it really never cared whether
the SER box was there or not.  As long as the PSTN switch knows who to bill,
it is vaguely happy.  To accomplish that, the routes inside the PSTN
switch were altered to point to the SIP IP addresses that
the PSTN switch would have contact with, as in:

TGN     SIP IP
1111 -> 10.2.0.1
2222 -> 10.2.0.2
3333 -> 10.2.0.3

(Changing the IPs in the PSTN switch turns out to be a non-trivial
 operation.)


The more serious problem arises with a call going SIP to TDM, because
when the SER box sends a packet on to the PSTN box at 101.2.3.4,
it needs to also re-write the SOURCE IP of the packet to reflect the
appropriate IP address on the em0 interface, not just the
first IP address that is discovered on the interface.
What it does right now is send everything out under the first IP
address listed on that interface, usually 10.2.0.1, so calls
fail to set up properly as the replies go back to company A even
if coming from company B or C.

Is there a function in SER that lets you set the right address to use
on outgoing packets?   As all of the IP addresses that would be used
are in fact ones set on that interface, it should be just a
question of setting the right one, but I haven't found a function for
ser.cfg or loadable module that clearly states that it will do this.

So in summary, when Company B sends an INVITE to 10.1.5.6
from their 42.4.5.6 system, I need to have SER send an invite to
101.2.3.4 from 10.2.0.2 and never any other address that happens
to be on that same interface.  And when the reply to the INVITE comes
to 10.2.0.2 from 101.2.3.4, I need SER to forward it on to 42.4.5.6
from 10.1.5.6.

Note that while I am showing an example with three companies, there
will be dozens, even hundreds of unique IPs involved here, hence
the enormous netblocks set aside for each interface on the SER box.

I'm certainly open to alternative methods to get SER to alter the
headers of the packets it forwards exactly as needed, but keep in
mind that I have basically zero control over how the PSTN switch
does things, and it has exactly one IP address (three actually, but
one does all the SIP work) that you send to and it originates from,
yet is keenly interested in where packets it receives came from
(for billing purposes) so you can't expect any help from the
PSTN switch side.

So is there a function callable from ser.cfg to force set the SOURCE
IP address on SIP messages on a message by message basis?
We are currently using ser-0.9.6.

Thanks for your assistance,

Frank Durda