Squid 3.1 and higer
Squid 3.1 supports IPv6. Squid 3.3 supports the
RFC 6555 Happy
eyeballs algorithm, which deals with broken IPv6 implementations.
Happy eyeballs is supposed to prefer IPv6. Unfortunately, Squid 5 and higher
prefer IPv4.
A fix: Let Squid 5 and higher prefer IPv6
Squid redirector with RBL support
Wikipedia: Squid (software)