[OpenSIPS-Users] Weird behaviour when replying to an OPTIONS with a Cseq of 0

Karolis Pabijanskas k.pabijanskas at gmail.com
Wed Jul 6 18:44:58 CEST 2016

Hi List,

We seem to be hitting a strange behaviour when we get an OPTIONS ping with
a Cseq of 0. (latest 1.11 branch).

Our routing script contains this at the very beginning to decline OPTIONS
route {
    if (is_method("OPTIONS")) {
        sl_send_reply("501", "Method not allowed");
   ## blah...

If we send this OPTIONS request:

2016-07-06 17:12:05 +0100 : CLIENT_IP:5061 -> OPENSIPS_IP:5060

OPTIONS sip:200 at HOSTNAME:5060 SIP/2.0 Via: SIP/2.0/UDP
;branch=z9hG4bK-895-1-0 From: sipp <sip:100 at>;tag=1 To: <sip:200@
HOSTNAME:5060> Call-ID: 1-895 at CSeq: 0 OPTIONS Contact: sip:100 at 1 Max-Forwards: 100 Content-Length: 0

There is no reply from OpenSIPS. Interestingly, siptrace module is also
running and saving captures in Homer. Homer, actually, is getting a copy of
the generated reply:
2016-07-06 17:12:05 +0100 : OPENSIPS_IP:5060 -> CLIENT_IP:5061
SIP/2.0 501 Method not allowed Via: SIP/2.0/UDP;received=
*CLIENT_IP*;branch=z9hG4bK-895-1-0 From: sipp <sip:100 at>;tag=1 To:
<sip:200 at HOSTNAME:5060>;tag=06a366df8881a48001f15f72f7138d9f.7522 Call-ID: 1
-895 at CSeq: 0 OPTIONS Server: User Agent String Content-Length: 0

But running a tcpdump on the OpenSIPS host reveals that no actual packet is
ever sent to the client. Debug shows:
Jul  6 17:40:25 HOSTNAME /sbin/opensips[48357]: ERROR:core:udp_send:
sendto(sock,0x7f7867aee470,324,0,0x7fff77d00090,16): Operation not
Jul  6 17:40:25 HOSTNAME /sbin/opensips[48357]: ERROR:sl:msg_send: udp_send

OpenSIPS is running as root.

Switching Cseq to 1 in that original OPTIONS message works. But in this
particular case we have no control over the Cseq of the host we are being
pinged from, and need to reply. According to the RFC, Cseq should be a
32bit unsigned integer, so 0 should not be an issue.

Any ideas?

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