PROXY entry without a port
medium
"PROXY proxy.corp.example" names no port. Chromium assumes 80; other engines are reported to reject the entry, and a rejected sole entry means a direct connection. Always write host:port.
Why it matters
The PAC return grammar is PROXY host:port. Chromium fills in a per-scheme default (80 for
PROXY, 443 for HTTPS, 1080 for SOCKS), so the entry works there if the proxy happens to
listen on that port, which enterprise proxies on 8080, 3128 or 9090 do not. Firefox and
WinHTTP are reported to discard the unported entry; if nothing valid remains, the request
goes DIRECT. pacparser returns the string verbatim (lab, 2026-05-20), so desk tests do not
reveal the problem. This is mechanism (c) of the “silent DIRECT” family.
How to fix
Write the port explicitly, e.g. "PROXY proxy.corp.example:8080".
Examples
Bad
function FindProxyForURL(url, host) {
return "PROXY proxy.corp.example";
}
Good
function FindProxyForURL(url, host) {
return "PROXY proxy.corp.example:8080";
}
Engine behaviour
| Engine | Behaviour | Source |
|---|---|---|
| chromium | Missing port is replaced by the scheme default (PROXY 80, HTTPS 443, SOCKS/SOCKS4/SOCKS5 1080; ProxyServer::GetDefaultPortForScheme). "host:" with an empty port is rejected as an invalid entry. | code, verified 2026-10-04 · ref |
| firefox | Reported to reject the entry; needs lab confirmation. | expert, unverified |
| pacparser | Returns the unported string unchanged; no validation. | lab, verified 2026-05-20 |
| winhttp | Reported to reject the entry; needs lab confirmation. | expert, unverified |