PAC file is large
low
Above roughly 50 KB a PAC starts to cost measurable time per connection; above 1 MB engines refuse to load it. Long host lists usually belong on the proxy, not in the file every client downloads and executes.
Why it matters
The PAC is downloaded by every client and run for every connection. Thousands of
if (shExpMatch(...)) lines are evaluated sequentially until one matches; for default-route
traffic that is all of them. Very large files also hit hard limits: Chromium rejects PAC
scripts above 1 MiB. Hostname allow-lists with hundreds of entries are a sign that proxy
policy (categories, bypass lists) is being maintained in the PAC. Serve the file compressed
(PAC-D006) as a stop-gap; move policy to the proxy as the fix.
How to fix
Move long bypass lists to the proxy; group remaining rules by domain suffix; enable gzip on the PAC server.
Examples
Bad
// imagine 3000 lines of the following
function FindProxyForURL(url, host) {
if (host == "host0001.corp.example") { return "DIRECT"; }
if (host == "host0002.corp.example") { return "DIRECT"; }
if (host == "host0003.corp.example") { return "DIRECT"; }
return "PROXY proxy.corp.example:8080";
}
Good
function FindProxyForURL(url, host) {
if (dnsDomainIs(host, ".corp.example")) {
return "DIRECT";
}
return "PROXY proxy.corp.example:8080";
}
Engine behaviour
| Engine | Behaviour | Source |
|---|---|---|
| chromium | PAC scripts larger than 1 MiB are rejected by the fetcher. | expert, unverified |