PAC-K005 · socks-version-keyword

SOCKS keywords mean different versions on different engines

low · Compatibility across engines

The PAC format only defines "SOCKS host:port" and does not say which protocol version. Chromium treats plain SOCKS as SOCKS4 and also accepts SOCKS4 and SOCKS5; Firefox is reported to treat SOCKS as SOCKS5; WinHTTP support for the versioned keywords is unverified.

Why it matters

A SOCKS return is ambiguous by design: the original format predates the SOCKS4/SOCKS5 distinction. Chromium resolves the ambiguity towards SOCKS4 “for compatibility” (code, 2026-10-04) and understands the explicit SOCKS4 and SOCKS5 keywords. Firefox is reported to speak SOCKS5 for plain SOCKS. An entry with a keyword an engine does not know is dropped (PAC-E011). The result is that the same return string may use different protocol versions on different browsers, or no proxy at all. SOCKS proxies are rare in enterprise web routing; where one is needed, confirm the keyword and version on every target engine and keep a PROXY fallback. Draft until the Firefox and WinHTTP behaviour is confirmed in the lab.

How to fix

Prefer an HTTP proxy (PROXY host:port); if SOCKS is required, verify keyword and version per target engine and add a PROXY fallback.

Examples

function FindProxyForURL(url, host) {
  return "SOCKS5 socks.corp.example:1080";
}

function FindProxyForURL(url, host) {
  return "PROXY proxy.corp.example:8080; SOCKS socks.corp.example:1080";
}

Engine behaviour

Source: code = read in the engine's source, doc = vendor documentation, lab = observed in a lab run, expert = practitioner knowledge, not yet verified.

Related rules

• Return value is not a valid proxy string PAC-E011
• HTTPS proxy keyword without a PROXY fallback PAC-K006

References

• https://source.chromium.org/chromium/chromium/src/+/main:net/base/proxy_string_util.cc
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Proxy_servers_and_tunneling/Proxy_Auto-Configuration_PAC_file#return_value_format