Squid ipv6 preffer

https://wiki.squid-cache.org/Features/IPv6

acl to_ipv6 dst ipv6

# Magic entry. Place first in your config. This makes sure Squid has the IP available.
http_access deny to_ipv6 !all

tcp_outgoing_address 10.255.0.1 !to_ipv6
tcp_outgoing_address dead:beef::1 to_ipv6