create a UDP to serial bridge with socat

Question

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:172.24.176.207:54321 \
   open:/dev/ttyS0,raw,nonblock,waitlock=/tmp/s0.locak,echo=0,b115200,crnl

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

$ socat -v udp4-datagram:172.24.176.116:54321 \
   open:/dev/ttyUSB0,raw,nonblock,waitlock=/tmp/s0.locak,echo=0,b115200,crnl

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 (172.24.176.207) I ran a netcat server:

   $ nc -ul 172.24.176.207 54321
   

   and on the host machine I sent a UDP packet:

   $ echo -n "hello" | nc -4u -w1 172.24.176.207 54321
   

   but nothing happened. What am I missing?

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 192.168.56.101 54321
hello

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)
    192.168.56.1.62439 > 192.168.56.101.54321: UDP, length 5
22:33:21.282140 IP (tos 0x0, ttl 64, id 40761, offset 0, flags [DF], proto UDP (17), length 33)
    192.168.56.101.54321 > 192.168.56.1.62439: 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:172.24.176.116:54321 \
   open:/dev/ttyUSB0,raw,nonblock,waitlock=/tmp/s0.locak,echo=0,b115200,crnl

So the first 2 switches should be somewhat obvious:

• -v - verbose
• udp4-datagram:172.24.176.207:54321 - 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.

References

• socat man page