Document listening on port 0 assigns a random unused port #24116

1. 
   thonkle commented at 9:57 pm on January 20, 2022: none

   If port 0 is supplied, the Linux kernel will select an available port. Reference: https://github.com/torvalds/linux/blob/2c271fe77d52a0555161926c232cd5bc07178b39/net/ipv4/inet_connection_sock.c#L358

   This is also true for Windows: https://docs.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-bind#remarks

   Since this behavior does not appear in manpages for bind(), listen(), etc. it is not reasonable to expect node operators to know this behavior exists, so it’s best to document it where they will find it: in the help text.

2. Document listening on port 0 assigns a random unused port c88dec60a8
3. 
   DrahtBot commented at 7:58 am on January 21, 2022: member

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Conflicts

   Reviewers, this pull request conflicts with the following ones:

   • #23542 (net: open p2p connections to nodes that listen on non-default ports by vasild)

   If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

4. Successahead approved
5. luke-jr changes_requested
6. 
   luke-jr commented at 4:20 am on January 29, 2022: member

   Since it’s not documented by the APIs, it’s undefined behaviour, and we shouldn’t document it either (unless we explicitly add code to support it).
7. 
   ghost commented at 3:16 pm on February 14, 2022: none

   This is also true for Windows: https://docs.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-bind#remarks

   I could not reproduce this on Win 10 based on below logs for bitcoind -port=0

   02022-02-14T15:13:26Z Bound to 127.0.0.1:18334
   12022-02-14T15:13:26Z Bound to [::]:0
   22022-02-14T15:13:26Z Bound to 0.0.0.0:0
   

8. 
   thonkle commented at 6:05 pm on February 14, 2022: none

   This is also true for Windows: https://docs.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-bind#remarks

   I could not reproduce this on Win 10 based on below logs for bitcoind -port=0

   02022-02-14T15:13:26Z Bound to 127.0.0.1:18334
   12022-02-14T15:13:26Z Bound to [::]:0
   22022-02-14T15:13:26Z Bound to 0.0.0.0:0
   

   The log does not report the actual listening port when 0 is selected. Try sudo netstat -pln | grep bitcoin to see what port is being used.

9. 
   sipa commented at 7:19 pm on February 14, 2022: member

   I think we should just abort initialization if port 0 is requested. It’s not actually something you can listen on, and the apparently default behavior of selecting a random port is undesirable for us.
10. 
    luke-jr commented at 7:22 pm on February 14, 2022: member

    The log does not report the actual listening port when 0 is selected.

    IMO that means it isn’t working as described by this PR.

    Do we even recognise the assigned port for addr publishing purposes?

    I think we should just abort initialization if port 0 is requested. It’s not actually something you can listen on, and the apparently default behavior of selecting a random port is undesirable for us.

    I agree. It seems code would be needed to support a random port, and I don’t see justification for the added complexity that entails. Better to just make it an error.

11. 
    ghost commented at 8:26 pm on February 14, 2022: none

    I think we should just abort initialization if port 0 is requested. It’s not actually something you can listen on, and the apparently default behavior of selecting a random port is undesirable for us.

    I agree. It seems code would be needed to support a random port, and I don’t see justification for the added complexity that entails. Better to just make it an error.

    Agree

12. 
    thonkle commented at 11:39 pm on February 14, 2022: none

    Closing in favor of a new PR to do bounds checking on -port: #24344 Port 0 is not the only case that gives unexpected behavior, so we can fix all of them at once.
13. thonkle closed this on Feb 14, 2022

    ---

14. fanquake referenced this in commit 5fc3939850 on Oct 12, 2022
15. DrahtBot locked this on Feb 14, 2023

Contributors
thonkle DrahtBot Successahead luke-jr ghost sipa