I restarted php on affected servers, problem didn't come back yet.
What I saw in strace / tcpdump is a dns request with rrtype 0x5100 in the
question.
There have been problems with libc6 updates before and there is this
debconf thingy "Services that need to be restarted". This indicates a
known problem with libc6 updates. php-fpm should be added to this list
of services...
I don't fully understand how exactly the bug "works". My working theory goes
like this:
php-fpm master process has lots of shared libs mmaped into memory. Those are
not affected by a libc6 upgrade that changes libs on the file system.
When php-fpm forks a worker process, it inherits all the maps.
But the worker process does some actual work and loads (diff
/proc/{$master,$worker}/maps) /lib/x86_64-linux-gnu/libnss_dns-2.23.so which is
the *new* version and not ABI compatible with the already loaded libc6.
If my theory holds, restarting the affected process will indeed fix the issue.
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https://bugs.launchpad.net/bugs/1674733
Title:
php_network_getaddresses since last update
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