TCP listener remains occupied due to absence of read() and shutdown()?

Adriaan

I have this strange experience.

In Holland, I am testing NB-IOT and CAT M1 SOS buttons, that work 100% on a Chinese platform, but as soon as I point the button to my clients Windows Azure server or my own Linux server, it keeps the TCP listener occuped. I guess it’s because the TCP connection is not properly closed, but I am not an expert here.

I have tcpdumped my ass off, here is an strace of the processes:

On my Linux, my listener is started like this:

strace netcat -k -l

Then in a normal sequence, I get this (strace output):

     0.000640 accept4(3, …
    20.274326 poll([{fd=0, events=POLLIN}, …
     0.003220 read(4, "MT;6;**TEST WEBSRV …
     0.000537 read(4, "", 16309)        = 0
     0.000304 shutdown(4, SHUT_RD)      = 0
     0.000282 write(1, "MT;6;**TEST WEBSRV …
     0.000354 close(4)                  = 0

This is the problem causing one:

     0.000519 accept4(3, …
    12.447784 poll([{fd=0, events=POLLIN}, …
     0.092840 read(4, "MT;6;862785045411272;…
     0.000411 poll([{fd=0, events=POLLIN}, …
     0.000518 write(1, "MT;6;862785045411272;…
     0.000341 poll([{fd=0, events=POLLIN}, {fd=4, events=0}, {fd=4, events=POLLIN}, {fd=1, events=0}], 4, -1

–> And the it waits forever, keeping my TCP listener occupied.

I see the latter is not doing read() and shutdown(). I think that has to do with the button. Or can it be the T-Mobile network?

I don’t get it. Maybe someone here?

Thanks, Adriaan

Adriaan

@Adriaan

Well, at least I can reproduce the issue. On the sending side on Linux, the -N flag does the thing.

echo "Hello there" | nc machine.domain <port> -N -v 2>&1

When connecting to a remote host, and using strace -r -f netcat -k -l <port> , setting the -N flag results in

 0.000596 read(4, "", 16308)        = 0
 0.000354 shutdown(4, SHUT_RD)      = 0

Without the -N flag, listener remains listening, and hence not closing the connection. This is the last line then:

0.000404 poll([{fd=0, events=POLLIN}, {fd=4, events=0}, {fd=4, events=POLLIN}, {fd=1, events=0}], 4, -1

Next step is that I am going to look if I can change something on the listener side as well.

Greetz,

Adriaan