[Bug network/22479] getaddrinfo: check_pf caches "ipv6 seen, ipv4 not seen" and any DNS lookup fails until restarting the process

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=22479

--- Comment #14 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Frederick Eaton from comment #13)
> I see, thanks Florian... so the kernel requires applications to enumerate
> all interfaces at once, when they just might be curious about one?
> 
> How long does this enumeration take, milliseconds? Microseconds?

With many interfaces and frequent changes leading to partial/aborted dumps,
hundreds of milliseconds are possible.  This could happen during system
startup, for example.

Cache experiation is difficult.  We could the kernel folks to update a counter
whenever the interface configuration changes, but such changes could be rather
frequent if this includes changes in adjacency information.

I want to add an option to /etc/gai.conf which tells getaddrinfo to use a
routing lookup in AI_ADDRCONFIG mode and include the address only if the kernel
knows how to route the address.  It's not quite what AI_ADDRCONFIG means today,
hence the need for configuration, but I think this has approach would have
other benefits besides solving the caching issue.

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