Problem
I installed Squid 3.5.20 on a fresh copy of CentOS 7 but the web proxy was not working when I pointed a browser to it.
Troubleshooting
I could telnet locally to port 3128 but when I tried from another device on the network I was unable to connect, despite opening the port on the firewall.
Running a netstat command I was able to see that port 3128 was utilizing ipv6 only:
# netstat -an
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN
tcp 0 64 10.64.208.100:22 10.64.214.254:51700 ESTABLISHED
tcp6 0 0 :::22 :::* LISTEN
tcp6 0 0 :::3128 :::* LISTEN
tcp6 0 0 ::1:25 :::* LISTEN
udp 0 0 0.0.0.0:45496 0.0.0.0:*
udp 0 0 127.0.0.1:323 0.0.0.0:*
udp6 0 0 :::33526 :::*
udp6 0 0 ::1:323 :::*
raw6 0 0 :::58 :::* 7
Resolution
Edit squid.conf and change the http_port 3128 to http_port 0.0.0.0:3128
# vi /etc/squid/squid.conf
eg
#http_port 3128
http_port 0.0.0.0:3128