KarRelay

Low-latency SRT fan-out relay with frame-accurate timecode injection. One binary, no dependencies to install, a web interface for everything.

Download 0.7.5 · Windows and Linux, 64-bit

This repository carries the description and the releases. KarRelay is not open source and its source is not published — see LICENSE. The binaries are free to download and run.

SRT (Secure Reliable Transport) is an open-source video transport protocol originally created and open-sourced by Haivision. SRT is a trademark of Haivision Systems Inc. KarRelay is not affiliated with, endorsed by, or sponsored by Haivision or the SRT Alliance.

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What it does

You have one SRT stream coming out of an encoder — vMix, OBS, ffmpeg, a hardware box — and you need several people to watch it. Most encoders will not send to more than one or two destinations, and asking them to is the fastest way to ruin the feed.

KarRelay sits in between. It takes the stream once and re-publishes it to as many subscribers as you like, each with its own buffering, so a viewer on a bad connection cannot slow down anyone else or stall the encoder.

  encoder ──SRT──▶  KarRelay  ──SRT──▶ viewer
                        │     ──SRT──▶ viewer
                        │     ──SRT──▶ recorder
                        │     ──SRT──▶ KarRelay at another site
                        └── web UI on :8484

Features

Transport

• SRT caller and listener input, per source — the relay dials out to your encoder, or waits for it to dial in.
• Fan-out with per-subscriber buffering. A slow viewer is dropped frames, never backpressure on the source.
• Stream-id whitelist per source, in and out.
• Push replication to a second KarRelay: one copy across the network instead of one per viewer, fanned out again at the far end, and a standby path if this relay becomes unreachable.
• SRT link encryption (passphrase, AES-128/192/256), per direction on a source and for a channel's subscribers. A peer with the wrong passphrase, or none, is rejected rather than quietly served in the clear.
• H.264 and H.265, passthrough — nothing is re-encoded, so there is no added latency and no quality loss.
• Live stats over a WebSocket: RTT, bitrate, loss, drops, viewer count, uptime, and the codec read out of the transport stream itself.

Switching between sources

One output can sit in front of several candidate sources and switch between them while people are watching. The cut happens at an IDR boundary, and the relay rewrites continuity counters and re-anchors the PCR, PTS and DTS so a receiver sees one continuous timeline — even when the two encoders run on clocks that were never synchronised.

Audio is spliced separately from video, because audio has no equivalent of an IDR to cut at. The outgoing source keeps providing audio for up to 100 ms while the new source's video is already on air, and the audio switch commits on the first clean frame boundary. No instant of audio is ever sent twice, and none is skipped.

Measured over ten switches on two outputs of one vMix instance: no gaps, no backward steps in the timeline, no duplicated audio, nothing audible. See the 0.5.0 notes for how that was arrived at — it took a while.

All candidate sources in one switchable output must share a codec: H.264 with H.264, or H.265 with H.265. Hardware decoders on the receiving side are set up for one codec at the first frame and cannot be changed mid-stream without dropping the session.

A second site

Viewers in more than one place do not each need a link back to the encoder. KarRelay pushes a single copy to a second KarRelay, which fans it out locally:

              ┌──SRT──▶ viewer
  encoder ──▶ │──SRT──▶ viewer        site A
   KarRelay   │
              └──SRT──▶ KarRelay ──┬──SRT──▶ viewer
                         site B    └──SRT──▶ viewer

Targets hang off a source or a switchable output — a switchable one sends the programme as cut, not one of its candidates. They are added and removed while everything is running, reconnect by themselves if the far relay restarts, and can carry their own passphrase. A target pointing back at this relay's own port is refused, because that feeds the stream into itself and nothing about the symptoms would say so.

Free on every licence.

Timecode for frame-accurate sync

KarRelay can inject a small SEI message carrying UTC microseconds into every I-frame. KarPlayer, the companion Android receiver, uses it to hold a constant, known lag — so several screens showing the same feed show the same frame.

Operation

• System tray app on Windows and desktop Linux: status icon, open the UI, quit. No console window.
• Headless build for servers and systemd.
• Web interface for sources, viewers, settings and licence state.
• Operator credentials on the interface and the API. A fresh install accepts admin / admin; change both on the About tab.
• Listen address — restrict the interface to one network, or to the relay machine itself, and the port disappears from everywhere else.
• TLS for the interface, one click, with a certificate generated for you. HTTPS and plain HTTP share the port, so KarPlayer keeps syncing its clock without needing the certificate.
• Rejection reasons surfaced both to the operator and to the subscriber that was turned away, so "it won't connect" has an answer.

Quick start

1. Download and unpack. There is nothing to install and no DLLs to place.
2. Run karrelay-tray.exe (Windows desktop) or ./karrelay (Linux).
3. Open http://localhost:8484/ and sign in with admin / admin.
4. Change the password on the About tab.
5. Sources → Add. Pick listener mode, give it an input port and an output port.

Point your encoder at the input port, point your viewers at the output port:

srt://<relay-host>:<output-port>?mode=caller

Try it with ffmpeg

ffmpeg -re -f lavfi -i testsrc=size=1280x720:rate=30 \
       -f lavfi -i sine=frequency=440 \
       -c:v libx264 -preset ultrafast -tune zerolatency -c:a aac \
       -f mpegts "srt://<relay-host>:9000?mode=caller&latency=120"

With a source on input port 9000 and output port 9001, any SRT receiver can then watch srt://<relay-host>:9001?mode=caller.

Free and paid

Everything above works without a licence key, within these limits:

	Free	Paid
Sources at once	1	unlimited
Viewers per source	3	unlimited
Switching between sources	—	✓
Timecode injection	—	✓
API access from outside the local network	—	✓
SRT link encryption, stream-id whitelist, statistics, H.265	✓	✓
Encrypted web interface (TLS)	✓	✓
Push replication to another KarRelay	✓	✓

Activation is checked once and cached locally, signed, so the relay keeps running with no network. A seven-day trial is available per machine from the licence panel in the UI.

Not built yet

Nothing in the table above is outstanding — push replication was the last of it, and arrived in 0.7.

Custom metadata injection, webhooks on events and interface branding have been mentioned in earlier plans. They are not built and are not planned; if they ever appear they will not change what is free and what is paid.

Security

Three things, none of them behind a paid tier, and worth the minute each takes:

1. Change the password. A fresh install accepts admin / admin so you can get in; until you change it on the About tab, so can anyone who can reach the port. The relay says so in its log on every start.
2. Turn on TLS, also on the About tab. Without it the password crosses the network in the clear. The certificate is generated for you and your browser will warn once because nobody signed it — compare the fingerprint it shows with the one in the UI, then accept it.
3. Narrow the listen address if the interface is only ever opened on the relay machine. That removes the port from every other network rather than defending it.

Separately, set a passphrase on your sources and channels if the path to your viewers is not one you control.

If you find something security-relevant, please open an issue saying only that you have, and I will follow up for the detail.

Platforms

	Status
Windows 10/11, 64-bit	released, tray and headless
Linux, x86-64	released, headless
Linux ARM64, macOS	build on request

Issues and requests

Bug reports and feature requests are welcome in Issues, source or no source. Please say which version, which platform, what the encoder is, and what the log said.

Licence

KarRelay is proprietary — see LICENSE.

The binary includes open-source components under their own licences, reproduced in full in THIRD-PARTY-NOTICES.md.

One of them is worth a word: the SRT implementation is datarhei/gosrt, pure Go, which is why KarRelay is a single binary with no libsrt to ship alongside. KarRelay currently builds against a fork carrying one fix, which has been proposed upstream.

Author

Alexander Karabatov · hellokarabatov@gmail.com