udp-splice

an userspace udp splicer written in Go. Receive packets and forward them to multiple destinations.

Keywords: UDP, Multiplexer, Multiplicator, Networking

Why / Problem

In case a UDP client sends to a server (receiver) there are many ways to send the same data to another / multiple destinations.

• use the broadcast address
• use IPv4 multicast
• iptables with mangle + -j tee and some nat rules
• socat + tee
• if the second receiver is running on the same host, libpcap could be used to intercept (not recommended)
• patch the client

How / Solution

[ peer0 (tx/rx) ] <-------> [ udp-splice ] <--------> [ peer1 (tx/rx) ]
                                   |
                                   | \
                                   |  \
                                   V   V
                          [ peer2 rx ] .. [ peerN (rx) ]

Here is another one, implemented in userspace with go that does exactly what is described above. The main advantage is that you don't need to mess with your machines setup (iptables) or touch the client itself. If your connection between peerA and peerB already works, you just need to point peerA to the address udp-splice is listening on which inturn then forwards the traffic to peerB. peer1 can also send traffic back to udp-slice which will forward it to peer0 - this is basically NAT. If configured, the traffic will also be forwarded to peer2 through peerN. This connection however is transmit-only from the perspective of udp-splice. Obviously the major downside of this setup is that udp-splice presents a single point of failure for the connection.

What / Usage

Start udp-splice:

./udp-splice --listen localhost:10000 --destination localhost:10001,localhost:10002

Start peer1

socat - UDP4-RECVFROM:10001,fork

Start peer2

socat - UDP4-RECVFROM:10002,fork

Send some traffic to udp-splice

echo  "udp-splice" | socat - UDP4-SENDTO:localhost:10000

Toggle forwarding

To further optimize resources it is possible to disable forwarding to peer2 through peerN if this is not needed.

Simply send SIGUSR1 to the running instance using kill -SIGUSR1 $PID_OF_UDP_SPLICE.

To re-enable it, repeat the same command.

Copyright & License

MIT (see LICENSE)

2021 Maximilian Güntner code@mguentner.de