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dockerdocker-composelsof

Docker port forwarding create two processes

From my Docker container I forward the port 8545 as below

ports:
      - '127.0.0.1:8545:8545'

And after run the container, if I run lsof -i :8545 it shows me two processes with the same PID.

COMMAND  PID  USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
vpnkit  6576 Admin   27u  IPv4 0xfdda6e6d5013cf77      0t0  TCP localhost:8545 (LISTEN)
vpnkit  6576 Admin   28u  IPv6 0xfdda6e6d4509106f      0t0  TCP localhost:8545 (LISTEN)

Why there are two processes running?

Solution

  • There are no two different processes as you can see under the PID column. The reason lsof lists more than one entry is because the tool shows you which process has handles to some file descriptors and in your case the single process has two of them, because it is bound on both sockets - IPv4 and IPv6 (as seen under the TYPE column).

    You can restrict the output by using the -i parameter twice:

    lsof -i 4 -i :8545

    This filters on both, port and socket which should give the expected output. For your original question: From a docker perspective, everything is fine.