As you probably know you can’t achieve a true active/active configuration but you can get pretty close by utilizing a PAC file with the appropriate JavaScript. In my example script above I evaluate the fourth octet of the client IP address to make a decision regarding which proxy server to return. This helps provide some “load-balancing” between the clients but assures that the same client always queries the same proxy server – easier for troubleshooting and caching of site related information.

You can achieve redundancy by returning two proxy servers in the string. Internet Explore will try the first and if unable to connect will try the second.

return "PROXY proxy.example.com:8080; DIRECT";

In the example above I’m using proxy.example.com but if that does not respond the browser will attempt a direct connection.

Good Luck!

will my below wpad conf work?

BackupRoute=”DIRECT”;
UseDirectForLocal=true;
function MakeIPs(){
this[0]=”172.16.102.20″;
this[1]=”255.255.255.255″;
this[2]=”172.25.61.50″;
this[3]=”255.255.255.255″;
this[4]=”10.10.10.100″;
}
DirectNames=new MakeNames();
cDirectNames=24;
cNodes=1;
function MakeProxies(){
this[0]=new Node(“172.16.1.236:8080″,0,1.000000);
}
Proxies = new MakeProxies();
function Node(name, hash, load){
this.name = name;
this.hash = hash;
this.load = load;
this.score = 0;
return this;
}
function FindProxyForURL(url, host){
var urlhash, ibest, bestscore, list, i, j;
var DCVIP1 = “PROXY 172.16.1.236″;
var DRVIP1 = “PROXY 172.16.20.222″;

if (UseDirectForLocal && isPlainHostName(host))
return “DIRECT”;
if (cDirectNames > 0)
for (i = 0; i 0)
for (i = 0; i < cDirectIPs; i += 2)
if (isInNet(host, DirectIPs[i], DirectIPs[i+1]))
return "DIRECT";
urlhash = HashString(url);
for (i = 0; i < cNodes; i++)
Proxies[i].score = Proxies[i].load * Scramble(MakeInt(urlhash ^ Proxies[i].hash));

if ( (ipSubs[3] % 2) == 0 )
{
return DCVIP1 + ":8080";
} else
{
return DRVIP1 + ":8080";
}

list = "";
for (j = 0; j < cNodes; j++) {
for (bestscore = -1, i = 0; i bestscore) {
bestscore = Proxies[i].score;
ibest = i;
}
}
Proxies[ibest].score = -1;
list = list + “PROXY ” + Proxies[ibest].name + “; “;
}
list = list + BackupRoute;
return list;
}
function HashString(url){
var h = 0;
var slashes = 0;
for (var i = 0; i < url.length; i++) {
var c = url.charAt(i);
if (c == '/')
slashes++;
if (slashes < 3)
c = c.toLowerCase();
h += (((h & 0x1fff) <> 13) & 0x7ffff)) + CharToAscii(c);
h = MakeInt(h);
}
return h;
}
function Scramble(h){
h += ((h & 0xffff) * 0×1965) + ((((h >> 16) & 0xffff) * 0×1965) << 16) + (((h & 0xffff) * 0×6253) << 16);
h = MakeInt(h);
h += (((h & 0x7ff) <> 11) & 0x1fffff));
return MakeInt(h);
}
var Chars =” !\”#$%&\’()*+,-./0123456789:;?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~??????????????????????????????????????????????????????????????????a??????????????????????? “;
function CharToAscii(c){
return Chars.indexOf(c) + 32;
}
function MakeInt(x){
if (x = 4294967296) {
return x – 4294967296;
}
return x;
}

you wrote “If you wanted geographic redundancy you could alter the “return” statement like so;
“PROXY 10.1.127.62:8080; 10.2.127.62:8080″
In this case the browser will attempt to contact the proxy server at 10.1.127.62:8080 and if the browser is unable to contact that proxy it will then try the proxy server at 10.2.127.62:8080. You’ll need to make sure that you include the (semicolon) “;” between each proxy server.”

How does the browser check that theres no proxy on 10.1.127.62. Does it check how long the proxy is unavaible or is it possible to check errors like http404 if the proxy is unavaiable ?

thanks
tom

There are a number of ways you could host the PAC file in a high availability fashion. You could placed the PAC file on two servers in a server farm that are front-ended by a Layer 7 load balancing switch. You could placed the PAC file on a Windows server that belongs to a Microsoft Cluster.

In my case I’m currently hosting the PAC file on my Intranet website. Since the Intranet is already in a high-availability configuration (two redundant Nortel Application Switch 2216s) it didn’t cost me anything. If you already have a similar solution you might want to host the PAC file within that solution and save yourself the effort and cost of building out a dedicated solution.

Good Luck!