This directive specifies the source address for UDP messages that Squid sends. It affects DNS (when using the internal implementation), ICP, HTCP, and SNMP messages. The specified address must correspond to one of the system’s network interfaces. You should use this directive only if your system has multiple IP addresses.
The default value of 255.255.255.255 causes Squid to use the incoming address for sending, as well as receiving. In other words, rather than creating a separate UDP socket for sending, Squid sends and receives messages through a single socket.
If you use this directive, it must have a different value than udp_incoming_address. Squid can’t create two UDP sockets bound to the same IP address and port number.
udp_outgoing_address, icp_port, htcp_port, snmp_port