As per project's README:
> Recent systems can resolve IPv6 host names using getaddrinfo(). This
> primitive is not present in all libcs and does not work in all of
> them either. Support in glibc was broken before 2.3. Some embedded
> libs may not properly work either, thus, support is disabled by
> default, meaning that some host names which only resolve as IPv6
> addresses will not resolve and configs might emit an error during
> parsing. If you know that your OS libc has reliable support for
> getaddrinfo(), you can add USE_GETADDRINFO=1 on the make command
> line to enable it. This is the recommended option for most Linux
> distro packagers since it's working fine on all recent mainstream
> distros. It is automatically enabled on Solaris 8 and above, as it's
> known to work.
Without this option, it is not possible for HAProxy to solve IPv6-only
names. This option is enabled in Debian builds without any notable
adverse effect.
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