Here important moment about "sips" URI supports.
That requires use of rfc5630 <>

3.1.3 <>.  Using TLS
with SIP Instead of SIPS

   Because a SIPS URI implies that requests sent to the resource
   identified by it be sent over each SIP hop over TLS, SIPS URIs are
   not suitable for "best-effort TLS": they are only suitable for "TLS-
   only" requests.  This is recognized in Section 26.2.2 of [RFC3261]

3.2 <>.  Detection of
Hop-by-Hop Security

   The presence of a SIPS Request-URI does not necessarily indicate that
   the request was sent securely on each hop.  So how does a UAS know if
   SIPS was used for the entire request path to secure the request end-
   to-end?  Effectively, the UAS cannot know for sure.  However,
   [RFC3261], Section 26.4.4
<>, recommends how a
UAS can make some checks
   to validate the security.

rfc3261 <>

   Ensuring that TLS will be used for all of the request segments up to
   the target domain is somewhat complex.  It is possible that
   cryptographically authenticated proxy servers along the way that are
   non-compliant or compromised may choose to disregard the forwarding
   rules associated with SIPS (and the general forwarding rules in
   Section 16.6 <>).
Such malicious intermediaries could, for example,
   retarget a request from a SIPS URI to a SIP URI in an attempt to
   downgrade security.

   To address these concerns, it is RECOMMENDED that recipients of a
   request whose Request-URI contains a SIP or SIPS URI inspect the To
   header field value to see if it contains a SIPS URI (though note that
   it does not constitute a breach of security if this URI has the same
   scheme but is not equivalent to the URI in the To header field).
   Although clients may choose to populate the Request-URI and To header
   field of a request differently, when SIPS is used this disparity
   could be interpreted as a possible security violation, and the
   request could consequently be rejected by its recipient.  Recipients
   MAY also inspect the Via header chain in order to double-check
   whether or not TLS was used for the entire request path until the
   local administrative domain was reached.  S/MIME may also be used by
   the originating UAC to help ensure that the original form of the To
   header field is carried end-to-end.

IF this recommendation is applied, then you not able to establish SIPS
call. Because TCP or TLS proto will be used in socket or in via headers.

On Thu, Oct 10, 2019 at 1:46 AM Joel Serrano <> wrote:

> Hey David,
> No, I was testing using the GSLB in GCP (Google) platform, but I believe
> the exact same scenario would either work (or not) on AWS, as at the end
> the key here is taking advantage of proxy protocol and SSL offloading, and
> both these cloud providers have that option.
> I'm going to give a try using force socket, maybe that's the way to go,
> I'll report back what I find out.
> Thanks!
> Joel.
> On Wed, Oct 9, 2019 at 3:37 PM David Villasmil <
>> wrote:
>> Out of curiosity, is this on AWS?
>> On Wed, 9 Oct 2019 at 23:33, David Villasmil <
>>> wrote:
>>> It looks pretty clear to me. I think whenever kamailio “sees” that
>>> protocol specification it will switch to that. Have you tried forcing the
>>> socket? Just wondering whether that would work.
>>> On Wed, 9 Oct 2019 at 20:33, Joel Serrano <> wrote:
>>>> Hi everyone,
>>>> I was giving a try to setup Kamailio with a Cloud TCP load balancer in
>>>> front, taking advantage of the newly added proxy protocol compatibility and
>>>> my initial tests went very well.
>>>> Flow: client -> (tcp) -> load balancer -> (tcp) -> Kamailio TCP socket
>>>> I then did another quick test and enabled TLS, also with good results:
>>>> Flow: client -> (tls) -> load balancer -> (tls) -> Kamailio TLS socket
>>>> So far so good, proxy protocol works as expected.
>>>> I wanted to go one step further and see if I could somehow offload SSL
>>>> operations at the load balancer level, and leave kamailio handling plain
>>>> tcp.
>>>> Flow: client -> (tls) -> load balancer -> (tcp) -> Kamailio TCP socket
>>>> This partially worked, and before I start digging into what I have to
>>>> do to get it completely working, I'd like to know if anyone already has a
>>>> similar setup, or even if Kamailio is able to handle such a scenario, the
>>>> reason I'm asking is because of the headers, etc.
>>>> In this last scenario, I receive in a TCP socket, a request with TLS
>>>> headers all over the place..
>>>> INVITE SIP/2.0
>>>> Via: SIP/2.0/TLS;branch=z9hG4bK.KmUpamn5P;rport
>>>> From: ...
>>>> To: ...
>>>> CSeq: 21 INVITE
>>>> Call-ID: -j1QSnam9o
>>>> Max-Forwards: 70
>>>> Route: <;lr>
>>>> Supported: replaces, outbound
>>>> Content-Type: application/sdp
>>>> Content-Length: 436
>>>> Contact: 
>>>> <sip:linphone@A.B.C.D:60717;transport=tls>;+sip.instance="<urn:uuid:fabcb441-a348-49a7-948d-72448d6840eb>"
>>>> I then forward this request via UDP to subsequent proxies for further
>>>> processing, on the replies, my payload information back to the client
>>>> should be TLS, although sent via a TCP socket..
>>>> Is this something that will not work by design? Is there any hack I can
>>>> take advantage of?
>>>> The goal would be for Kamailio to handle TLS headers via TCP socket, as
>>>> the client expects TLS information, but the actual traffic should go in
>>>> plan TCP, and the load balancer will take care of re-encrypting before
>>>> replying to the client.
>>>> Any ideas/suggestions/comments?
>>>> I hope this email is understandable, I find it complicated to detail
>>>> the exact problem, feel free to ask any questions if you don't understand
>>>> anything.
>>>> Thanks,
>>>> Joel.
>>>> _______________________________________________
>>>> Kamailio (SER) - Users Mailing List
>>> --
>>> Regards,
>>> David Villasmil
>>> email:
>>> phone: +34669448337
>> --
>> Regards,
>> David Villasmil
>> email:
>> phone: +34669448337
>> _______________________________________________
>> Kamailio (SER) - Users Mailing List
> _______________________________________________
> Kamailio (SER) - Users Mailing List
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