ares_parse_mx_reply(3) on Linux
The text d86643846e68, as Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie), Debian 7 (Wheezy), Debian 6.0 (Squeeze) shipped it.
Parse a reply to a DNS query of type MX