This directive causes Squid to bind all UDP sockets to a specific interface address. The IP address must correspond to one of the system’s network interfaces. This directive affects the DNS (when using the internal implementation), ICP, HTCP, and SNMP sockets.
If your system has just one IP address, you probably shouldn’t use this directive.
If you set udp_outgoing_address to one of your other network interface addresses, Squid can receive UDP datagrams on that interface as well.
udp_outgoing_address, icp_port, htcp_port, snmp_port