[Serusers] Error 483 response to ACK and BYE

[SIP]

In response to a BYE and an ACK from some (not all) servers that send to 
our proxy, our server's spitting back a 483 'Too many hops' reply.

It doesn't happen with all peers, and it certainly doesn't happen 
locally...  but it does happen with some, and I'm not entirely sure as 
to why.

The 483 block at the beginning is normal:


if (msg:len >  max_len ) {
       sl_send_reply("513", "Message too big");
       break;
};



Exchange looks like this:

U 198.65.166.131:5060 -> 63.64.65.66:5060
BYE sip:1101XXXXXXX at 63.64.65.66:5060 SIP/2.0.
Record-Route: <sip:198.65.166.131;ftag=gpp0q468oh;lr>.
Via: SIP/2.0/UDP 198.65.166.131;branch=z9hG4bK9b05.b75fbe14.0.
Via: SIP/2.0/UDP 
192.168.1.51:2054;received=63.XX.XX.XX;branch=z9hG4bK-wdtud2nffq88;rport=2054.
Route: <sip:63.64.65.66;ftag=gpp0q468oh;lr=on>.
From: "User One" <sip:1747XXXXXXX at their.proxy.server:5060>;tag=gpp0q468oh.
To: <sip:1101XXXXXXX at their.proxy.server;user=phone>;tag=as7fbbcb66.
Call-ID: 3c267038e7ef-586eg1emcfsi at snom190.
CSeq: 2 BYE.
Max-Forwards: 16.
Contact: <sip:1747XXXXXXX at 192.168.1.51:2054;line=oxd9zlst;nat=yes>.
User-Agent: snom190/3.60x.
Content-Length: 0.
RemoteIP: 63.XX.XX.XX.
P-hint: rr-enforced.
P-NATed-URI: YES (1).
P-RTP-Proxy: YES (1).


U 63.64.65.66:5060 -> 198.65.166.131:5060
SIP/2.0 483 Too Many Hops.
Via: SIP/2.0/UDP 198.65.166.131;branch=z9hG4bK9b05.b75fbe14.0.
Via: SIP/2.0/UDP 
192.168.1.51:2054;received=63.XX.XX.XX;branch=z9hG4bK-wdtud2nffq88;rport=2054.
From: "User One" <sip:1747XXXXXXX at their.proxy.server:5060>;tag=gpp0q468oh.
To: <sip:1101XXXXXXX at their.proxy.server;user=phone>;tag=as7fbbcb66.
Call-ID: 3c267038e7ef-586eg1emcfsi at snom190.
CSeq: 2 BYE.
Server: Sip EXpress router (0.9.6 (i386/linux)).
Content-Length: 0.
Warning: 392 63.64.65.66:5060 "Noisy feedback tells:  pid=3488 
req_src_ip=63.64.65.66 req_src_port=5060 
in_uri=sip:1101XXXXXXX at 63.64.65.66:5060 
out_uri=sip:1101XXXXXXX at 63.64.65.66:5060 via_cnt==18".



What would be causing this? Is it because the IP address is being used 
in the URI to us as opposed to the domain (I tried adding the IP to the 
domain table and to an alias line alternately, but it didn't fix things) ?

It's irksome in that it only happens with certain peers and not others, 
so there's something in the way we're handling messages from them that's 
not right or different, but since we handle all incoming the same way, 
I'm at a loss as to why it works with some but not with others.

N.