How to handle Proxy PAC configuration with Qt

  • qt • javascript • pac • proxy • qtscript • webkit • configuration • qtnetwork
  • 937 words

This article was originally written on another blog. That blog was never officially published, and it will probably never be. It was written on the 3rd of June 2010, but is still "somewhat important" and relevant.

Since the post was published, Richard Moore @linux…> has done a commit in the Qt examples directory that resamples very closely my code below.

So, I decided to repost it here.

Qt and Proxy Configuration

What’s great of Qt, is that it comes with a very rich set of libraries to cover almost everything you can think of. As a Qt developer, you should always double check the Documentation first, to see if “it’s already there”, before jumping into coding. It saves tons of time.

For us a Mobile Developer like me, coming from a Symbian background, Qt makes me feel “guilty” (stuff gets too easy) and “lazy” (I have to squeeze my neurons less). But I also get more productive.

There are (very small) shortcomings though. Qt provides QNetworkProxyFactory to get the proxy configuration from the System Configuration, but it works only on Windows and, partially, on Mac. Other platforms are not handled for now. That’s unfortunate, given that we mainly work with Qt on Symbian and, in my current case, Maemo 5.

Automatic Proxy Configuration on Mac OS X Automatic Proxy Configuration on Mac OS X

What could be done on Linux

Of course this can be worked out: in the case of Maemo, being it based on Linux, I will probably just have to find a way to interface with the gconftool or make use of QProcess::systemEnvironment(). But what if you are in a LAN where the Proxy configuration is distributed across different offices using a PAC file? Well, you need to work by hand.

A PAC file is a Javascript that looks like this:

function FindProxyForURL(url, host) {
   // our local URLs from the domains below don't need a proxy:
   if (shExpMatch(url,"**"))                  {return "DIRECT";}
   if (shExpMatch(url, "**/*"))               {return "DIRECT";}

   // URLs within this network are accessed through
   // port 8080 on
   if (isInNet(host, "",  ""))    {
      return "PROXY";

   // All other requests go through port 8080 of
   // should that fail to respond, go directly to the WWW:
   return "PROXY; DIRECT";

Working by hand looks scary, because, like in our case, a script can be huge and complex. A RegExp is a very bad idea, and start from “square one” with LEX and YACC would feel dumb… and an overkill.

QtScript to the rescue

Fortunately, Qt provides a lot of help. The QtNetwork and QtScript (now based on WebKit’s JavascriptCore) have what it takes to solve the problem. What I show here is a usage of the QtScript Engine, that takes the PAC file and executes it to extract the right configuration.

#include <QtCore/QCoreApplication>
#include <QScriptEngine>
#include <QIODevice>
#include <QFile>
#include <QDebug>
#include <QScriptValue>
#include <QHostInfo>
#include <QHostAddress>
#include <QNetworkInterface>

// Functions to implement are here:
// -
// -

QScriptValue dnsResolve(QScriptContext *context, QScriptEngine *engine) {
   QString toResolve = context->argument(0).toString();
   qDebug() << "dnsResolve - toResolve: " << toResolve;

   QHostInfo info = QHostInfo::fromName(toResolve);
   if ( info.error() == QHostInfo::NoError ) {
      return info.addresses().first().toString(); //< Pick the first one
   } else {
      return QString("Host not found");

QScriptValue isPlainHostName(QScriptContext *context, QScriptEngine *engine) {
   QString hostname = context->argument(0).toString();
   qDebug() << "isPlainHostName - hostname: " << hostname;

   return hostname.contains('.') ? false : true;

QScriptValue isInNet(QScriptContext *context, QScriptEngine *engine) {
   QString host = context->argument(0).toString();
   QHostAddress hostAddress;
   QPair<QHostAddress, int> subnet;
   QString pattern = context->argument(1).toString();
   QString mask = context->argument(2).toString();
   qDebug() << "isInNet - host: " << host;
   qDebug() << "isInNet - pattern: " << pattern;
   qDebug() << "isInNet - mask: " << mask;

   QHostInfo info = QHostInfo::fromName(host);
   if ( info.error() == QHostInfo::NoError ) {
      hostAddress = info.addresses().first();
      subnet = QHostAddress::parseSubnet(QString("%1/%2").arg(pattern,mask));
      qDebug() << "Host "<< host << " is in Subnet " << QString("%1/%2").arg(pattern,mask) << "? " << hostAddress.isInSubnet(subnet);
      return hostAddress.isInSubnet(subnet);
   } else {
      return false;

QScriptValue myIpAddress(QScriptContext *context, QScriptEngine *engine) {

   foreach ( QHostAddress address, QNetworkInterface::allAddresses() ) {
      if( !address.isNull()
         && address != QHostAddress::Null
         && address != QHostAddress::LocalHost
         && address != QHostAddress::LocalHostIPv6
         && address != QHostAddress::Broadcast
         && address != QHostAddress::Any
         && address != QHostAddress::AnyIPv6
         ) {
         qDebug() << "myIpAddress - " << address.toString();
         return address.toString(); //< Pick the First one

int main(int argc, char *argv[])
   QCoreApplication a(argc, argv);

   QScriptEngine engine;
   QString str;

   // Read the PAC file in a String
   QFile scriptFile(":/proxy-config.pac");
   if ( !|QIODevice::Text) ) {
      return EXIT_FAILURE;
   str = scriptFile.readAll();

   // Inject the extra JS function as explained above
   QScriptValue dnsResolveFunction = engine.newFunction(dnsResolve);
   engine.globalObject().setProperty("dnsResolve", dnsResolveFunction);
   QScriptValue isPlainHostNameFunction = engine.newFunction(isPlainHostName);
   engine.globalObject().setProperty("isPlainHostName", isPlainHostNameFunction);
   QScriptValue isInNetFunction = engine.newFunction(isInNet);
   engine.globalObject().setProperty("isInNet", isInNetFunction);
   QScriptValue myIpAddressFunction = engine.newFunction(myIpAddress);
   engine.globalObject().setProperty("myIpAddress", myIpAddressFunction);

   // Evalute the PAC file
   if ( engine.canEvaluate(str) ) {
      QScriptValue ctor = engine.evaluate("FindProxyForURL(\"\", \"\")");
      if ( !ctor.isError() ) {
         qDebug() << ctor.toString();
         return EXIT_SUCCESS;

   qDebug() << "Error Occurred";
   return EXIT_FAILURE;

As you can see, what I had to add to the QtScript module was just the implementation of some of the core functions that a PAC interpreter has to offer. Those are not part of the standard Javascript, but QtScript makes it super-easy to inject what is missing.

Contributing back?

Maybe I’ll should see if the Trolls are interested in getting something like that added to Qt. Maybe a method like

QNetworkProxyFactory::proxyForPac(const QIODevice &pacFile)

could be added to the current QNetworkProxyFactory.

What do you think?

PS If you are asking yourself if there is a BUG/FEATURE-REQUEST on the Qt Official Bugzilla, here is what you can currently find:

and both are marked as “P3: Somewhat important”.

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