Search code examples
macossocketsosx-elcapitan

Release UDP port used by dead process on OS X

I am on OS X 10.11.6 and trying to run a program that normally listens on UDP port 8008 upon startup.

This program normally also spawns a couple of helper child processes during its operation, but the port is bound by the parent process.

Unfortunately when exiting the program, sometimes the port remains open, even though the program (parent + children) no longer exist.

When this happens, if I try to run the program again it naturally fails with a EADDRINUSE error, and in these cases no matter what I try, the only solution I found was to reboot the machine.

I'm having a hard time believing that I cannot release the port without a reboot.

Here are some diagnostics that I ran so far (I ran all of these with and without sudo):

Find the process using port 8008 with lsof:

$ lsof -i -n -P | grep UDP | grep 8008

But surprisingly doesn't return any results.

However, I had more luck with netstat:

$ netstat -tulnvp udp | grep 8008
udp4  0  0  *.8008    *.*    196724   9216  47205   0

So, the port is indeed bound, and the culprit is pid 47205, however:

$ ps aux | grep 47205

Doesn't return anything. The same thing for PIDs 47206 and 47207 (most certainly the PIDs assigned to the children). I also tried other variations of the grep (program name, path, etc).

I also looked for any process reporting 47205 as its parent:

$ ps -axo pid,ppid,command | grep 47205

So the children processes are also clearly dead.

Not being able to kill anything, I tried to SIGHUP launchd in the hope that it might remove any zombie child processes:

$ sudo kill HUP 1
$ sudo kill -s HUP 1

But alas, netstat still shows the port bound.

Lastly, I tried to restart the loopback interface:

$ sudo ifconfig lo down
$ sudo ifconfig lo up

But again, to no effect.

I have waited several hours since the program last ran, so I'm pretty sure any timeout would have happened by now, but the port just won't get released.

Any ideas on how to force release the port without a reboot?

Edit:

  • The program in question is the electron-wrapped Patchwork.
  • This question originates from this github issue.
  • Although finding a solution/bugfix that prevents the issue from ocurring in the first place would be ideal, I'm also interested in ways to manually close that port from the terminal
Solution

  • It is indeed possible to close the port manually w/o restarting the machine. On various linux flavors this is usually done w/ GDB by issuing syscalls masquerading as a the process (for example close(fd) syscall on the sockets file descriptor).

    The process for that:

    • Open a UDP port: netcat -u 127.0.0.1 33333.
    • Check the UDP port: netstat -npu (u for UDP), which will give you the PID that occupies that port.
    • Run: lsof -np $pid for that PID to get the filedescriptor for the socket.
    • Then run GDB for that PID: sudo gdb -p 73599
    • When inside GDB run call close(file_descriptor)

    Example:

    COMMAND   PID  USER   FD   TYPE   DEVICE SIZE/OFF     NODE NAME
netcat  73599 ubunt  cwd    DIR  259,2     4096 13895497 /home/ubunt/Downloads
netcat  73599 ubunt  rtd    DIR  259,2     4096        2 /
netcat  73599 ubunt  txt    REG  259,2    31248 28835938 /bin/nc.openbsd
netcat  73599 ubunt  mem    REG  259,2    47600 23990813 /lib/x86_64-linux-gnu/libnss_files-2.23.so
netcat  73599 ubunt  mem    REG  259,2  1868984 23990714 /lib/x86_64-linux-gnu/libc-2.23.so
netcat  73599 ubunt  mem    REG  259,2   101200 23990866 /lib/x86_64-linux-gnu/libresolv-2.23.so
netcat  73599 ubunt  mem    REG  259,2    81040 23990710 /lib/x86_64-linux-gnu/libbsd.so.0.8.2
netcat  73599 ubunt  mem    REG  259,2   162632 23990686 /lib/x86_64-linux-gnu/ld-2.23.so
netcat  73599 ubunt    0u   CHR 136,19      0t0       22 /dev/pts/19
netcat  73599 ubunt    1u   CHR 136,19      0t0       22 /dev/pts/19
netcat  73599 ubunt    2u   CHR 136,19      0t0       22 /dev/pts/19
netcat  73599 ubunt    3u  IPv4 22142418    0t0      UDP 127.0.0.1:45255->127.0.0.1:33333

    Then GDB:

    $sudo gdb -p 73599
...
(gdb) call close(3u)
$1 = 0

    You will see that the port is no longer there:

    ubunt@ubunt-MS-7A94:~$ lsof -np 73599
COMMAND   PID  USER   FD   TYPE DEVICE SIZE/OFF     NODE NAME
netcat  73599 ubunt  cwd    DIR  259,2     4096 13895497 /home/ubunt/Downloads
netcat  73599 ubunt  rtd    DIR  259,2     4096        2 /
netcat  73599 ubunt  txt    REG  259,2    31248 28835938 /bin/nc.openbsd
netcat  73599 ubunt  mem    REG  259,2    47600 23990813 /lib/x86_64-linux-gnu/libnss_files-2.23.so
netcat  73599 ubunt  mem    REG  259,2  1868984 23990714 /lib/x86_64-linux-gnu/libc-2.23.so
netcat  73599 ubunt  mem    REG  259,2   101200 23990866 /lib/x86_64-linux-gnu/libresolv-2.23.so
netcat  73599 ubunt  mem    REG  259,2    81040 23990710 /lib/x86_64-linux-gnu/libbsd.so.0.8.2
netcat  73599 ubunt  mem    REG  259,2   162632 23990686 /lib/x86_64-linux-gnu/ld-2.23.so
netcat  73599 ubunt    0u   CHR 136,19      0t0       22 /dev/pts/19
netcat  73599 ubunt    1u   CHR 136,19      0t0       22 /dev/pts/19
netcat  73599 ubunt    2u   CHR 136,19      0t0       22 /dev/pts/19

    GDB is available for MacOS, so it should work for your case as well.