Recording echoes RTP packets back to the sender

[Orgad Shaneh]

Hi,

I'm using rtpengine for recording pcap files.

We send offer and answer command, both with sendonly.

The packets are recorded correctly, but are also bounced back to the sender SBC.

Is there a way to prevent this, and only record to pcap?

Log attached with log-level 7.

Thanks,

- Orgad

[Richard Fuchs]

On 16/11/2025 06.38, Orgad Shaneh wrote:
> I'm using rtpengine for recording pcap files.
>
> We send offer and answer command, both with sendonly.
>
> The packets are recorded correctly, but are also bounced back to the
> sender SBC.
>
> Is there a way to prevent this, and only record to pcap?

This looks like you're trying to use rtpengine as something like a
SIPREC SRS, right?

With rtpengine being primarily a media proxy, the usage of offer/answer
is normally used to a establish a call between the A-party and the
B-party. So anything received on the A leg is proxied through to the B
leg, and vice versa. Also in that scenario both sides being sendonly
doesn't really make sense. The output SDPs you get back from rtpengine
aren't answer SDPs, but rather rewritten versions of the input SDPs.

So I'm guessing you want to use it not as a proxy but rather as a media
sink. You can do that with offer/answer, but all your media sources
should be on one side (i.e. all offers), and the other side (all the
answers) should be something that accepts these offer SDPs and acts as
the media sink, so that rtpengine in between can still be a (pretend)
proxy. You can do that with dummy SDPs that use a dummy address/port as
media endpoint, for example.

Perhaps someone else here is using rtpengine in a similar way and can
share some more ideas.

There are extra flags you can try passing to rtpengine in these offers
and answers (e.g. `blackhole`) that may help to keep rtpengine from
trying to proxy the media.

A less hackish approach would be not to use offer/answer but rather use
"publish", as this exists for exactly that purpose (set up a receiver
from a one-way inbound media stream) and the output SDP you would get
from it is actually an answer SDP. But ISTR that somebody already tried
this for SIPREC purposes and it currently doesn't actually work to
record the media, but I could be wrong about that off the top of my head.

Cheers

[Orgad Shaneh]

Hi,

Yes, I'm using rtpengine for SRS.

media-echo=blackhole solved the issue. Thank you.