Hi kamailio community,

I have an issue with a Kamailio 5.7. It's listening both in TCP and
UDP.  In my scenario requests arrive from devices on TCP, but I want to
forward to the next hops on UDP.  I am avoiding using any type of DNS
resolution; since I am always forwarding to predefined next hops I am
using the dispatcher module (defined with the IP addresses and
transport=udp) or I wrote config files using t_relay_to_udp or
t_relay_to with a udp:  followed by IP address. I never set up FQDNS
only IP addresses and in all of them I explicitly mention UDP.

In all of these scenarios I have tried Kamailio insists in trying to use
TCP  with the next hop and failing because the next_hop is only UDP. I
guess because the message arrived using TCP Kamilio does that but I find
the behavior very confusing.

I nailed down that in my situation its the tm module function
prepare_new_uac (in  file src/modules/tm/t_fwd.c line 119) being the one
that missbehaves. The documentation of the function says literraly :

"* t->uac[branch].request.dst will be filled if next_hop !=0 with the result
  * of the DNS resolution (next_hop, fproto and fsocket).
  * If next_hop is 0 all the dst members except the send_flags are read-only
  * (send_flags it's updated) and are supposed to be pre-filled."

I found out that even when next_hop is 0  the function changes the
t->uac[branch].request.dst  proto, socket etc.  its there that the
kamailio takes the wrong decision, until that function is called within
add_auc,  the destination proto or the fproto etc is always 1 (UDP)
which is what I am trying to force from the config file or the
dispatcher definition (I tried both ways).  But after calling that
function then it comes as a 2 (TCP)

The problematic function is a monster of 500 lines and I would like to
avoid having to understand it. Since I think the scenario is not so
unusual I just want to ask if maybe I am missing something that I should
do to avoid the Kamailio to select TCP and in order to have the tm
module respecting my preference for UDP (either with dispatcher module
and the transport param or with t_relay_to or t_relay_to_udp  I don't
care the way).

Any hints are welcome.

Best regards

alberto

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