Support rtable on OpenBSD

[lgv5]

OpenBSD supports multiple routing tables. The current code doesn't account for that and instead always request the route for routing table 0. If the process is run in another rtable, then this operation will get stuck. getrtable isn't part of libc crate, but part of OpenBSD's libc. The cast is safe: the syscall never fails, and the actual values are between 0 and 255.

Additional details can be found in:

• https://man.openbsd.org/rtable.4 for the mechanism in question
• https://man.openbsd.org/route.4#The_Routing_Socket for the fields in struct rt_msghdr, which includes the field that the patches set and isn't present in other BSDs
• https://man.openbsd.org/getrtable.2 for the syscall for obtaining the process' rtable

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.80%. Comparing base (e7daabb) to head (42bcc02).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   86.76%   86.80%   +0.04%     
==========================================
  Files           5        5              
  Lines         559      561       +2     
  Branches      559      561       +2     
==========================================
+ Hits          485      487       +2     
  Misses         48       48              
  Partials       26       26              

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[larseggert]

LGTM, but a merge would need to wait until the OpenBSD rust package is at 1.82, so we can run CI.

[lgv5]

That won't happen until 7.7, which is usually around April. On -current, rustc is at 1.85.0. Sadly, vmactions/openbsd-vm doesn't seem to support running -current.

[larseggert]

I also started #103 but no dice.

[larseggert]

Thank you for your contribution!