Runtime option to enable IPv4 preference over IPv6

[ivanka2012]

Is there any flag to make OpenSMTPD try resolving IPv4 addresses first instead of IPv6 addresses?

GMail seems to strongly prefer emails coming from IPv4 addresses, and the option to have IPv4 preference has been locked away a long time ago by a compile time flag (#182).

[clarfonthey]

Isn't this something you can configure via resolved directly? I tried looking locally for my own config but couldn't quite remember where it was located, and I think that in my case at least, I'm relying on systemd-specific configs to prefer v6 resolution over v4.

[ivanka2012]

I configured the resolver to prefer IPv4 using /etc/gai.conf on Linux, but OpenSMTPD still sent emails from IPv6.

[omar-polo]

Sorry for the long delay.

OpenSMTPD uses its own DNS resolver, which is a port of OpenBSD' asr, so I don't think it would honour /etc/gai.conf. We have a (hidden) configure-time knob to change the preference, see

OpenSMTPD/openbsd-compat/libasr/asr.c

Lines 541 to 552 in e2703a1

	/*
	* opensmtpd-portable knob to prefer IPv6 over IPv4.
	*
	* See: https://github.com/poolpOrg/OpenSMTPD/issues/182
	*/
	#ifndef ASR_IPV4_BEFORE_IPV6
	ac->ac_family[0] = AF_INET6;
	ac->ac_family[1] = AF_INET;
	#else
	ac->ac_family[0] = AF_INET;
	ac->ac_family[1] = AF_INET6;
	#endif

but it is a build-item knob, it's not possible to change it via a configuration option.

(although I think we should, by default, mirror OpenBSD and have ipv4 before ipv6)