Dovecot LDAP host - hostip resolution and alternative

[Andrey Fesenko]

dovecot-2.2.10-5.el7, CentOS Linux release 7.2.1511 (Core)

Users stored in LDAP, backend name set his hostname (need for TLS on backend)

auth: Debug: passdb out: PASS 1 user=mail proxy=yes host=backend
starttls=yes hostip=IPv6-public

how algorithm choice IP for hostip, and whether it is possible to
implement it repeated request?

Now I'm understand the following algorithm, DNS IPv6, /etc/hosts IPv4
single not alternative resolution.

If my backend have only local address this FAIL