I'm trying to implement a TCP listener that accepts connections and then simply drops all of its input (it's for a test harness).

Right now, I'm using socat - tcp-listen:2003,fork,reuseaddr, but that prints the input to stdout. I don't want that.

I can't redirect the output to /dev/null, because I'm doing this in the alpine/socat docker container, and it's not actually using a shell, so redirection doesn't work.

If I try to use socat /dev/null tcp-listen:2003,fork,reuseaddr, then any connection is dropped immediately, presumably because socat can't read from /dev/null.

What's the best way to implement a TCP listener that simply drops everything on the floor?

socat /dev/null,ignoreeof tcp-listen:2003,fork,reuseaddr

Best is a tricky criterion, but one can observe that Docker is as necessary for this as Microsoft Excel is. ☺

We already know a Unix program that reads its input until EOF and effectively discards it:

cat > /dev/null

Turning this into a TCP server is a very simple exercise. It is what UCSPI-TCP is all about. It takes programs that use standard I/O and wraps a simple framework around them that makes them into TCP servers. One can substitute something else for cat and change the service protocol without having to worry about the transport. One can similarly substitute UCSPI-LOCAL tools for UCSPI-TCP tools, and change the transport, without having to do a thing to cat.

There are several UCSPI-TCP toolsets. Daniel J. Bernstein wrote one. Laurent Bercot wrote one. Gerrit Pape wrote one. Adam Sampson wrote one. I wrote one. Moreover, one can mix and match tools from multiple toolsets, this stuff being intentionally readily composable. One can use my fdredir or Laurent Bercot's redirfd, or even spawn a non-interactive sh. (The former two have a lower overhead than a full-blown sh, and for the nosh tools fdredir is even a built-in so it further avoids the overhead of chaining through an external command.)

So there are several ways of doing this:

1. Bernstein tcpserver (original or djbwares; no IPv6 support)
   
   
   1. tcpserver -R -H -l 0 0.0.0.0 2003 fdredir --write 1 /dev/null cat
   
   
   2. tcpserver -R -H -l 0 0.0.0.0 2003 redirfd -w 1 /dev/null cat
   
   
   3. tcpserver -R -H -l 0 0.0.0.0 2003 sh -c 'exec > /dev/null cat'
   
   


2. Hoffman tcpserver (does both IPv4 and IPv6; has original Bernstein defaults for DNS and INFO lookups that need to be turned off)
   
   
   1. tcpserver -R -H -l 0 ::0 2003 fdredir --write 1 /dev/null cat
   
   
   2. tcpserver -R -H -l 0 ::0 2003 redirfd -w 1 /dev/null cat
   
   
   3. tcpserver -R -H -l 0 ::0 2003 sh -c 'exec > /dev/null cat'
   
   


3. my tcpserver shim (does both IPv4 and IPv6 but has to run separate instances for each; has more modern defaults)
   
   
   1. tcpserver ::0 2003 fdredir --write 1 /dev/null cat
   
   
   2. tcpserver ::0 2003 redirfd -w 1 /dev/null cat
   
   
   3. tcpserver ::0 2003 sh -c 'exec > /dev/null cat'
   
   


4. my UCSPI-TCP tools (does both IPv4 and IPv6, in one on Linux and FreeBSD)
   
   
   1. tcp-socket-listen --combine4and6 ::0 2003 tcp-socket-accept fdredir --write 1 /dev/null cat
   
   
   2. tcp-socket-listen --combine4and6 ::0 2003 tcp-socket-accept redirfd -w 1 /dev/null cat
   
   
   3. tcp-socket-listen --combine4and6 ::0 2003 tcp-socket-accept sh -c 'exec > /dev/null cat'
   
   


5. Bercot s6-tcpserver6 (substitute s6-tcpserver4 for IPv4)
   
   
   1. s6-tcpserver6 ::0 2003 fdredir --write 1 /dev/null cat
   
   
   2. s6-tcpserver6 ::0 2003 redirfd -w 1 /dev/null cat
   
   
   3. s6-tcpserver6 ::0 2003 sh -c 'exec > /dev/null cat'
   
   


6. Bercot s6-networking tools (substitute s6-tcpserver4 for IPv4)
   
   
   1. s6-tcpserver6-socketbinder -B ::0 2003 s6-tcpserver6d fdredir --write 1 /dev/null cat
   
   
   2. s6-tcpserver6-socketbinder -B ::0 2003 s6-tcpserver6d redirfd -w 1 /dev/null cat
   
   
   3. s6-tcpserver6-socketbinder -B ::0 2003 s6-tcpserver6d sh -c 'exec > /dev/null cat'
   
   


7. Pape tcpsvd (no IPv6 support)
   
   
   1. tcpsvd 0.0.0.0 2003 fdredir --write 1 /dev/null cat
   
   
   2. tcpsvd 0.0.0.0 2003 redirfd -w 1 /dev/null cat
   
   
   3. tcpsvd 0.0.0.0 2003 sh -c 'exec > /dev/null cat'
   
   


8. Sampson onenetd (does both IPv4 and IPv6 but has to run separate instances for each)
   
   
   1. onenetd -6 ::0 2003 fdredir --write 1 /dev/null cat
   
   
   2. onenetd -6 ::0 2003 redirfd -w 1 /dev/null cat
   
   
   3. onenetd -6 ::0 2003 sh -c 'exec > /dev/null cat'
   
   

Enjoy substituting

hexdump -C 1>&2

fdmove -c 1 2 hexdump -C

Further reading

• Protocol:
  
  
  • Jonathan de Boyne Pollard (2016). The gen on the UNIX Client-Server Program Interface. Frequently Given Answers.
  
  
  • Daniel J. Bernstein (1996). UNIX Client-Server Program Interface. cr.yp.to.
  
  


• toolsets:
  
  
  • Daniel J. Bernstein. ucspi-tcp. cr.yp.to.
  
  
  • 
    s6-networking. Laurent Bercot. skarnet.org.
  
  
  • 
    execline. Laurent Bercot. skarnet.org.
  
  
  • Jonathan de Boyne Pollard (2019). nosh. Softwares.
  
  
  • Jonathan de Boyne Pollard (2019). djbwares. Softwares.
  
  
  • 
    ipsvd. Gerrit Pape. smarden.org.
  
  
  • 
    onenetd. Adam Sampson. offog.org.
  
  


• reference manuals:
  
  
  • Daniel J. Bernstein. The tcpserver program. ucspi-tcp.
  
  
  • Erwin Hoffmann. tcpserver. ucspi-tcp6. fehcom.de.
  
  
  • 
    s6-tcpserver4. Laurent Bercot. s6-networking. skarnet.org.
  
  
  • 
    s6-tcpserver6. Laurent Bercot. s6-networking. skarnet.org.
  
  
  • 
    fdmove. Laurent Bercot. execline. skarnet.org.
  
  
  • 
    redirfd. Laurent Bercot. execline. skarnet.org.
  
  
  • 
    tcpsvd. ipsvd. Gerrit Pape. smarden.org.
  
  
  • Jonathan de Boyne Pollard (2019). tcpserver. djbwares. Softwares.
  
  
  • Jonathan de Boyne Pollard (2019). fdredir. nosh Guide. Softwares.
  
  
  • Jonathan de Boyne Pollard (2019). fdmove. nosh Guide. Softwares.
  
  
  • Jonathan de Boyne Pollard (2019). tcp-socket-listen. nosh Guide. Softwares.
  
  
  • Jonathan de Boyne Pollard (2019). tcp-socket-accept. nosh Guide. Softwares.
  
  
  • Jonathan de Boyne Pollard (2019). tcpserver. nosh Guide. Softwares.