Why is my secondary server trying sometimes to use a different source IP address for zone transfers?
There are several configuration options in named.conf that control which IPv4 and/or IPv6 source addresses are used for the SOA refresh queries and for the zone transfers themselves. See the Administrator Reference Manual for more details.
The well-known ones are these:
transfer-source - determines which local IPv4 address will be used as the source for both SOA refresh queries and zone transfer requests that need to be made to servers.
transfer-source-v6 - the same as transfer-source, but used when the destination is an IPv6 address.
Less well-known are:
alt-transfer-source - this provides an alternative local IPv4 source address to use if the first one fails.
alt-transfer-source-v6 - the same as alt-transfer-source, but used when the destination is an IPv6 address.
use-alt-transfer-source - defines whether or not named will fall back to using the alternative source addresses when the first attempt at a zone transfer fails.