Documentation for prefer-ip4, Issue #165.

doc/example.conf.in

@@ -70,6 +70,9 @@ server:
 	# Set this to yes to prefer ipv6 upstream servers over ipv4.
 	# prefer-ip6: no
 
+	# Prefer ipv4 upstream servers, even if ipv6 is available.
+	# prefer-ip4: no
+
 	# number of ports to allocate per thread, determines the size of the
 	# port range that can be open simultaneously.  About double the
 	# num-queries-per-thread, or, use as many as the OS will allow you.

doc/unbound.conf.5.in

@@ -381,6 +381,13 @@ IPv6 to the internet nameservers.  With this option you can disable the
 ipv6 transport for sending DNS traffic, it does not impact the contents of
 the DNS traffic, which may have ip4 and ip6 addresses in it.
 .TP
+.B prefer\-ip4: \fI<yes or no>
+If enabled, prefer IPv4 transport for sending DNS queries to internet
+nameservers. Default is no.  Useful if the IPv6 netblock the server has,
+the entire /64 of that is not owned by one operator and the reputation of
+the netblock /64 is an issue, using IPv4 then uses the IPv4 filters that
+the upstream servers have.
+.TP
 .B prefer\-ip6: \fI<yes or no>
 If enabled, prefer IPv6 transport for sending DNS queries to internet
 nameservers. Default is no.