I have a piece of software that transmits data over a UDP connection. However in our system we can transmit data only over a serial port. I was thinking of making a bridge between a UDP and serial using socat. However I'm not familiar with it very well. Tried going over a few tutorials but they were not helpful. Also tried reading the manual, but it wasn't helpful as well.

I found an example here, and adapted it to my needs. Here is what I have on my embedded system (it has a static IP)

$ socat -v udp4-datagram: \

and here is what I have on my host machine (has a static IP as well)

$ socat -v udp4-datagram: \

You would have to excuse me, but I'm illiterate in socat and networking in general.

Several comments and questions.

  1. What does open:/dev/ttyUSB0,raw,nonblock,waitlock=/tmp/s0.locak,echo=0,b115200,crnl mean? Where would I find description of it? I understand /dev/ttyUSB0 and b115200, but what is the rest?
  2. On an embedded system ( I ran a netcat server:

    $ nc -ul 54321

    and on the host machine I sent a UDP packet:

    $ echo -n "hello" | nc -4u -w1 54321

    but nothing happened. What am I missing?

1 Answer 1


echo server setup

I used socat to set up a UDP echo server like this:

echo server
$ socat UDP4-LISTEN:54321,fork EXEC:cat
echo client
$ echo -n "hello" | nc -4u -w1 54321

tcpdump tip

When debugging this, I like to monitor the network, so will run this to monitor the 'echo server'

$ tcpdump -i any -vnn port 54321
tcpdump: listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
22:33:21.280053 IP (tos 0x0, ttl 64, id 8684, offset 0, flags [none], proto UDP (17), length 33) > UDP, length 5
22:33:21.282140 IP (tos 0x0, ttl 64, id 40761, offset 0, flags [DF], proto UDP (17), length 33) > UDP, length 5

NOTE: This above tip can be heavily modified to debug anything that's going on between your socat/nc server and its clients.

Making sense of socat arguments

I too find the configuration of socat exhausting at times. But like anything else, it's easiest to grasp if you break it up into smaller bits.

$ socat -v udp4-datagram: \

So the first 2 switches should be somewhat obvious:

  • -v - verbose
  • udp4-datagram: - UDP listening on port 54321
  • open:/dev/ttyUSB0 - sets up sending intput/output to your USB #0 device
  • raw - raw mode, passes input/output unprocessed
  • nonblock - opens/uses files in nonblocking mode
  • waitlock=/tmp/s0.locak- creates a waitlock file - guards against multiple socat's
  • echo=0 - disables local echo on server
  • b115200 - sets the baud rate to 115,200 bits
  • crnl - converts newlines from \n to \r\n

Incidentally if you look in the socat man page, there are many examples. Also full descriptions of the above switches are there as well.


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