Using glibc, why does my gethostbyname fail after I/DHCP has changed the DNS server?

Question

If our server (running on a device) starts before a DHCP lease had been acquired then it can never connect using a hostname.

If that happens it can find hosts by IP address but not by DNS.

I initially thought that the Curl DNS cache was at fault as the curl connections failed. But I used CURLOPT_DNS_CACHE_TIMEOUT to prevent curl from caching address but connections still failed.

Paraphernalia

May I dredge up this ancient question by asking if there's any other solution? The trouble is that res_init() is much less portable than the libcurl API. On some Curl-supporting platforms it doesn't exist; on many others it's not thread-safe. – Staff

This issue was finally resolved in glibc 2.26 back in late 2017. It was incorporated into RHE and CentOS in version 7.5: sourceware.org/bugzilla/show_bug.cgi?id=984 – Macaco

Answer 1

It turns out that glibc gethostbyname_r won't automatically reload it's configuration if that configuration changes. You have to manually call res_init. See bug report below.

Note: Neither the man page for gethostbyname_r nor for rer_init mentioned this limitation.

My solution is very specific. It works for our long running server but it is not my ideal solution.

I have a function that checks the mtime of the /etc/resolv.conf against the last known mtime (0 for DNE). If the two mtime differ then I call res_init. This function is called on program startup and then periodically to optionally reload the configuration.

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The glibc bug report

libc caches resolv.conf forever

...

That's what res_init() is for, call it.

Paraphernalia

This issue was finally resolved in glibc 2.26 back in late 2017. It was incorporated into RHE and CentOS in version 7.5: sourceware.org/bugzilla/show_bug.cgi?id=984 – Macaco