Firefox’s Local Proxy Exception Is About Short Names

For internal names such as build303 and build909, Firefox’s <local> proxy exception matches hostnames without a dot. It is a useful shortcut when the network intentionally uses short names.

<local>, example.internal

Put these entries in the manual configuration’s No proxy for field. The first covers plain names. The second covers example.internal and its subdomains at a domain boundary.

The same computer can have two kinds of name

http://build303/ matches the local-name exception. http://build303.office.example/ contains dots and needs the intended hostname or domain exception. Both can resolve to the same machine; their spelling still gives the bypass matcher different inputs.

Likewise, a dotless public name can satisfy the syntactic test. “Local” here does not mean Firefox has proved the computer is physically on the LAN.

A prefix is not a domain rule

A shell-style entry such as build-* is not a general hostname-prefix rule. Use explicit hostnames or the actual domain boundary when those express the policy. A PAC script can implement more complex decisions when the network needs them.

Current hostname matching respects a subdomain boundary: example.internal does not cover an unrelated name merely ending in the same characters. An IP subnet entry is another kind of match and does not make Firefox resolve every short name before deciding its route.