Ordered Proxy Failover in a PAC File

A semicolon-separated PAC result supplies alternative routes in order:

return "PROXY proxy-a.example:8080; PROXY proxy-b.example:8080";

Use the actual HTTP endpoints required by the policy. For public-destination tests where public proxies are permitted, the HTTP proxy list supplies candidates. The browser can try B when appropriate after A fails. Traffic does not pass through A and then B. A chain requires separate support at the client or proxy servers; curl's SOCKS-to-HTTP chain is one supported arrangement.

Script availability and proxy availability

The client must first obtain a usable PAC script. A second proxy in its return value cannot help a browser that failed to download the script and has no usable cached copy.

After evaluation, the browser chooses among the returned routes. The PAC reference defines the ordered format; the browser determines which failures trigger fallback and how long it remembers a failed candidate. An HTTP error received through a functioning proxy does not automatically trigger another route.

A direct alternative

Adding ; DIRECT permits a route without either proxy. That can suit a roaming policy, but conflicts with a requirement that the affected traffic always use an approved exit.

For an outage report, record whether the script was retrieved, its result for the affected URL, and the alternative the browser attempted. Those observations locate the failure among script delivery, evaluation, and proxy connection.