Debugging Squid PAC on Chrome

I’m tearing my hair out trying to get our Squid proxy to play nice with WebSockets, so one of the things I decided to try was to use a PAC file to get browsers to skip the proxy for websocket URLs. Should be easy right? The URL for Ripple’s websocket server, for example, happens to be wss://

So a proxy.pac should be quite trivial, no?

if (shExpMatch(url, "wss://*")) {
    return "DIRECT";

(Note that in the above example, only “secure” websockets will be skipped as non-SSL websockets use the ws:// scheme)

The problem is it doesn’t fucking work. It’s also non-trivial to diagnose any problems with a PAC file on Chrome, but I figured out a nifty little trick. First, embed a Javascript alert() in the PAC:

if (shExpMatch(url, "wss://*")) {
    return "DIRECT";

Now if you open up chrome://net-internals/proxy-service#events in another tab and reload your page, you’ll see in the event log that the alert is indeed showing up (filter by PAC_JAVASCRIPT_ALERT to thin out the herd a bit). It seems as though this is getting fired, so what’s going on?

If I move the alert(url); outside of the if block, then check the logs, it appears upon a cursory check that Chrome evaluates the proxy once for the wss:// URL, and then, because WebSockets are HTTP[S] at their core, does it again for an https:// version of the URL.

t=1368594864610 [st=0]  PAC_JAVASCRIPT_ALERT
                         --> message = "wss://"
t=1368594864610 [st=0]  PAC_JAVASCRIPT_ALERT
                         --> message = ""

Obviously, the second attempt at working out which proxy to use is the one that has control, so it fails to avoid the proxy for wss:// connections after all.

I can specify a list of websockets in the PAC file and Chrome behaves as expected, but I don’t particularly want to add a new websocket URL to the PAC every time something breaks (particularly because there appears to be no sure-fire way to reload the PAC).

Clearly this is going to take some more diagnosing.

Update: Filtering the event log by wss:// and then inspecting individual SOCKET_STREAM events, you can see all the alert()s in one convenient place.

Update #2: I tried on IRC, and posted in the Chrome Product Forums, both to no avail at this time. :(

Red Lion VIC 3371, Australia fwaggle



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