Apt supports a subset of the DNS SRV server records protocol as described in RFC 2782 for service discovery.
Before connecting to the requested server APT will send a SRV
record request of the form _$protocol._tcp._$host, e.g.
_http._tcp.ftp.debian.org or _http._tcp.security.debian.org.
If the server sends SRV records
as a reply APT will use those to connect to the server(s) in
this reply. It will honor the priority field in the reply.
However it does not implement the weight algorithm as described
in RFC 2782. It will use an equal weight for each server of the
same priority.
If connecting to a server fails APT will retry with the next one and remove the server from the list of valid servers for this session.