[Bug 1916541] Re: smtp "fatal: unknown service: smtp/tcp", probably after libc upgrade

[Max Bowsher]

I've just been bitten by this.

It seems this was broken by
https://sourceware.org/git/?p=glibc.git;a=commit;h=429029a73ec2dba7f808f69ec8b9e3d84e13e804

and it has recently been realised that was a bad idea:
https://sourceware.org/git/?p=glibc.git;a=commit;h=58673149f37389495c098421085ffdb468b3f7ad

https://sourceware.org/bugzilla/show_bug.cgi?id=27389

It looks like hirsute is going to need a backport of this

** Bug watch added: Sourceware.org Bugzilla #27389
   https://sourceware.org/bugzilla/show_bug.cgi?id=27389

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https://bugs.launchpad.net/bugs/1916541

Title:
  smtp "fatal: unknown service: smtp/tcp", probably after libc upgrade

Status in glibc package in Ubuntu:
  Confirmed
Status in postfix package in Ubuntu:
  Confirmed

Bug description:
  On hirsute, on Jan 20, postfix stopped sending outgoing mails with the
  error

  Feb 22 23:35:18 pccross postfix/smtp[9582]: fatal: unknown service:
  smtp/tcp

  There were no changes in the system other than daily upgrades.

  Strace shows that `smtp` process (while chrooted), after successful
  open of the file in the queue, tried to use NSS, ran `fstat()` against
  `/etc/nsswitch.conf`, that succeeded (presumably using the copy in the
  /var/spool/postfix/etc/ directory), but there was no subsequent
  `open()`, and the next syscall is `sendto()` of the error message to
  syslog.

  There was a big upgrade between the last successful and the first
  unsuccessful email delivery, and among other things libc was updated
  from 2.32-0ubuntu6 to 2.33-0ubuntu2. I suspect that there was some
  change in the NSS in libc that resulted in the failure of
  `getservbyname()`.

  Is anybody else observing this problem?

  ProblemType: Bug
  DistroRelease: Ubuntu 21.04
  Package: postfix 3.5.6-1
  ProcVersionSignature: Ubuntu 5.10.0-14.15-generic 5.10.11
  Uname: Linux 5.10.0-14-generic x86_64
  ApportVersion: 2.20.11-0ubuntu59
  Architecture: amd64
  CasperMD5CheckResult: unknown
  CurrentDesktop: X-Cinnamon
  Date: Tue Feb 23 00:02:13 2021
  EtcMailname: pccross.average.org
  Hostname: pccross
  PostconfMydomain: localdomain
  PostconfMyhostname: pccross.localdomain
  PostconfMyorigin: /etc/mailname
  ResolvConf:
   # Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
   #     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN

   nameserver 127.0.0.1
   search average.org
  SourcePackage: postfix
  UpgradeStatus: No upgrade log present (probably fresh install)

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