Upper-case letters in a hostname pattern
medium
Browsers hand the PAC a lower-cased hostname. A pattern such as "Intranet.Corp.Example" never matches in Chromium or Firefox. Where engines do not lower-case (WinHTTP), the comparison is case-sensitive anyway, so patterns must be lower case and host should be lower-cased explicitly.
Why it matters
URL parsing in Chromium and Firefox follows the URL Standard, which lower-cases ASCII
domain names, so host arrives lower-cased and the pattern must be lower-case to match.
WinHTTP is reported to pass the hostname as typed, so there the same upper-case pattern
matches only if the user typed it that way. The robust idiom is to lower-case both sides:
host = host.toLowerCase(); at the top of the function and lower-case literals everywhere
(PAC-K009). Mixed-case literals are therefore always wrong.
How to fix
Write all hostname literals in lower case and add host = host.toLowerCase(); at the start of the function.
Examples
Bad
function FindProxyForURL(url, host) {
if (dnsDomainIs(host, ".Corp.Example")) {
return "DIRECT";
}
return "PROXY proxy.corp.example:8080";
}
Good
function FindProxyForURL(url, host) {
host = host.toLowerCase();
if (dnsDomainIs(host, ".corp.example")) {
return "DIRECT";
}
return "PROXY proxy.corp.example:8080";
}
Engine behaviour
| Engine | Behaviour | Source |
|---|---|---|
| chromium | host is passed lower-cased (URL canonicalisation). | doc, unverified |
| firefox | host is passed lower-cased (URL canonicalisation). | doc, unverified |
| pacparser | Passes host as given; dnsDomainIs(host, ".example.com") does not match "Example.com". | lab, verified 2026-05-20 |
| winhttp | Reported not to lower-case host; pattern comparison is case-sensitive. | expert, unverified |