If Squid proxy is configured on a server that have no IPv6 connectivity, and trying to reach an IPv6 compatible site, the squid will try to reach the site using IPv6 and will fail without even trying IPv4.
We can resolve this problem by disabling IPv6 in squid.conf.
Step1: Open squid configuration file
Step2: Add the below line the end of this file
tcp_outgoing_address 0.0.0.0 all
Step3: Restart squid
That's all... No all sites will load via proxy...