A Proxy Auto-Configuration (PAC) file is a JavaScript function that determines whether web browser requests (HTTP, HTTPS, and FTP) go directly to the destination or are forwarded to a web proxy server. The JavaScript function contained in the PAC file defines the function:
function FindProxyForURL(url, host) {
// ...
} function FindProxyForURL(url, host)
urlhttps:// URLs are stripped. In Chrome (versions 52 to 73), you can disable this by setting PacHttpsUrlStrippingEnabled to false in policy or by launching with the --unsafe-pac-url command-line flag (in Chrome 74, only the flag works, and from 75 onward, there is no way to disable path-stripping; as of Chrome 81, path-stripping does not apply to HTTP URLs, but there is interest in changing this behavior to match HTTPS); in Firefox, the preference is network.proxy.autoconfig_url.include_path.host:// and the first : or / after that. The port number is not included in this parameter. It can be extracted from the URL when necessary.Returns a string describing the configuration. The format of this string is defined in return value format below.
DIRECTPROXY host:portSOCKS host:portRecent versions of Firefox support as well:
HTTP host:portHTTPS host:portSOCKS4 host:portSOCKS5 host:portIf there are multiple semicolon-separated settings, the left-most setting will be used, until Firefox fails to establish the connection to the proxy. In that case, the next value will be used, etc.
The browser will automatically retry a previously unresponsive proxy after 30 minutes. Additional attempts will continue beginning at one hour, always adding 30 minutes to the elapsed time between attempts.
If all proxies are down, and there was no DIRECT option specified, the browser will ask if proxies should be temporarily ignored, and direct connections attempted. After 20 minutes, the browser will ask if proxies should be retried, asking again after an additional 40 minutes. Queries will continue, always adding 20 minutes to the elapsed time between queries.
PROXY w3proxy.netscape.com:8080; PROXY mozilla.netscape.com:8081PROXY w3proxy.netscape.com:8080; PROXY mozilla.netscape.com:8081; DIRECTPROXY w3proxy.netscape.com:8080; SOCKS socks:1080The auto-config file should be saved to a file with a .pac filename extension:
proxy.pac
And the MIME type should be set to:
application/x-ns-proxy-autoconfig
Next, you should configure your server to map the .pac filename extension to the MIME type.
Notes:
These functions can be used in building the PAC file:
ProxyConfig.bindings
Note: pactester (part of the pacparser package) was used to test the following syntax examples.
proxy.pac
pactester -p ~/pacparser-master/tests/proxy.pac -u http://www.mozilla.org (passes the host parameter www.mozilla.org and the url parameter http://www.mozilla.org)isPlainHostName(host)
True if and only if there is no domain name in the hostname (no dots).
isPlainHostName("www.mozilla.org") // false
isPlainHostName("www") // true
dnsDomainIs()dnsDomainIs(host, domain)
Returns true if and only if the domain of hostname matches.
dnsDomainIs("www.mozilla.org", ".mozilla.org") // true
dnsDomainIs("www", ".mozilla.org") // false
localHostOrDomainIs(host, hostdom)
Is true if the hostname matches exactly the specified hostname, or if there is no domain name part in the hostname, but the unqualified hostname matches.
localHostOrDomainIs("www.mozilla.org" , "www.mozilla.org") // true (exact match)
localHostOrDomainIs("www" , "www.mozilla.org") // true (hostname match, domain not specified)
localHostOrDomainIs("www.google.com" , "www.mozilla.org") // false (domain name mismatch)
localHostOrDomainIs("home.mozilla.org", "www.mozilla.org") // false (hostname mismatch) isResolvable(host)
Tries to resolve the hostname. Returns true if succeeds.
isResolvable("www.mozilla.org") // true
isInNet(host, pattern, mask)
True if and only if the IP address of the host matches the specified IP address pattern.
Pattern and mask specification is done the same way as for SOCKS configuration.
function alert_eval(str) { alert(str + ' is ' + eval(str)) }
function FindProxyForURL(url, host) {
alert_eval('isInNet(host, "63.245.213.24", "255.255.255.255")')
// "PAC-alert: isInNet(host, "63.245.213.24", "255.255.255.255") is true"
}
dnsResolve(host)
Resolves the given DNS hostname into an IP address, and returns it in the dot-separated format as a string.
dnsResolve("www.mozilla.org"); // returns the string "104.16.41.2" convert_addr(ipaddr)
Concatenates the four dot-separated bytes into one 4-byte word and converts it to decimal.
convert_addr("104.16.41.2"); // returns the decimal number 1745889538 myIpAddress()
(none)
Returns the server IP address of the machine Firefox is running on, as a string in the dot-separated integer format.
myIpAddress() returns the same IP address as the server address returned by nslookup localhoston a Linux machine. It does not return the public IP address.
myIpAddress() //returns the string "127.0.1.1" if you were running Firefox on that localhost
dnsDomainLevels(host)
Returns the number (integer) of DNS domain levels (number of dots) in the hostname.
dnsDomainLevels("www"); // 0
dnsDomainLevels("mozilla.org"); // 1
dnsDomainLevels("www.mozilla.org"); // 2
shExpMatch(str, shexp)
Returns true if the string matches the specified shell expression.
Note that the patterns are shell glob expressions, not regular expressions. * and ? are always supported, while [characters] and [^characters] are supported by some implmentations including Firefox. This is mainly because the expression is translated to a RegExp via subsitution of [.*?]. For a reliable way to use these RegExp syntaxes, just use RegExp instead.
shExpMatch("http://home.netscape.com/people/ari/index.html" , "*/ari/*"); // returns true
shExpMatch("http://home.netscape.com/people/montulli/index.html", "*/ari/*"); // returns false weekdayRange(wd1, wd2, [gmt])
Note: (Before Firefox 49) wd1 must be less than wd2 if you want the function to evaluate these parameters as a range. See the warning below.
"SUN"|"MON"|"TUE"|"WED"|"THU"|"FRI"|"SAT"
Only the first parameter is mandatory. Either the second, the third, or both may be left out.
If only one parameter is present, the function returns a value of true on the weekday that the parameter represents. If the string "GMT" is specified as a second parameter, times are taken to be in GMT. Otherwise, they are assumed to be in the local timezone.
If both wd1 and wd1 are defined, the condition is true if the current weekday is in between those two ordered weekdays. Bounds are inclusive, but the bounds are ordered. If the "GMT" parameter is specified, times are taken to be in GMT. Otherwise, the local timezone is used.
The order of the days matters; Before Firefox 49, weekdayRange("SUN", "SAT") will always evaluate to true. Now weekdayRange("WED", "SUN") will only evaluate true if the current day is Wednesday or Sunday.
weekdayRange("MON", "FRI"); // returns true Monday through Friday (local timezone)
weekdayRange("MON", "FRI", "GMT"); // returns true Monday through Friday (GMT timezone)
weekdayRange("SAT"); // returns true on Saturdays local time
weekdayRange("SAT", "GMT"); // returns true on Saturdays GMT time
weekdayRange("FRI", "MON"); // returns true Friday and Monday only (note, order does matter!) dateRange(<day> | <month> | <year>, [gmt]) // ambiguity is resolved by assuming year is greater than 31 dateRange(<day1>, <day2>, [gmt]) dateRange(<month1>, <month2>, [gmt]) dateRange(<year1>, <year2>, [gmt]) dateRange(<day1>, <month1>, <day2>, <month2>, [gmt]) dateRange(<month1>, <year1>, <month2>, <year2>, [gmt]) dateRange(<day1>, <month1>, <year1>, <day2>, <month2>, <year2>, [gmt])
Note: (Before Firefox 49) day1 must be less than day2, month1 must be less than month2, and year1 must be less than year2 if you want the function to evaluate these parameters as a range. See the warning below.
1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31
"JAN"|"FEB"|"MAR"|"APR"|"MAY"|"JUN"|"JUL"|"AUG"|"SEP"|"OCT"|"NOV"|"DEC"
If only a single value is specified (from each category: day, month, year), the function returns a true value only on days that match that specification. If both values are specified, the result is true between those times, including bounds, but the bounds are ordered.
The order of the days, months, and years matter; Before Firefox 49, dateRange("JAN", "DEC") will always evaluate to true. Now dateRange("DEC", "JAN") will only evaluate true if the current month is December or January.
dateRange(1); // returns true on the first day of each month, local timezone
dateRange(1, "GMT") // returns true on the first day of each month, GMT timezone
dateRange(1, 15); // returns true on the first half of each month
dateRange(24, "DEC"); // returns true on 24th of December each year
dateRange("JAN", "MAR"); // returns true on the first quarter of the year
dateRange(1, "JUN", 15, "AUG");
// returns true from June 1st until August 15th, each year
// (including June 1st and August 15th)
dateRange(1, "JUN", 1995, 15, "AUG", 1995);
// returns true from June 1st, 1995, until August 15th, same year
dateRange("OCT", 1995, "MAR", 1996);
// returns true from October 1995 until March 1996
// (including the entire month of October 1995 and March 1996)
dateRange(1995);
// returns true during the entire year of 1995
dateRange(1995, 1997);
// returns true from beginning of year 1995 until the end of year 1997 // The full range of expansions is analogous to dateRange. timeRange(<hour1>, <min1>, <sec1>, <hour2>, <min2>, <sec2>, [gmt])
Note: (Before Firefox 49) the category hour1, min1, sec1 must be less than the category hour2, min2, sec2 if you want the function to evaluate these parameters as a range. See the warning below.
If only a single value is specified (from each category: hour, minute, second), the function returns a true value only at times that match that specification. If both values are specified, the result is true between those times, including bounds, but the bounds are ordered.
The order of the hour, minute, second matter; Before Firefox 49, timeRange(0, 23) will always evaluate to true. Now timeRange(23, 0) will only evaluate true if the current hour is 23:00 or midnight.
timerange(12); // returns true from noon to 1pm timerange(12, 13); // returns true from noon to 1pm timerange(12, "GMT"); // returns true from noon to 1pm, in GMT timezone timerange(9, 17); // returns true from 9am to 5pm timerange(8, 30, 17, 00); // returns true from 8:30am to 5:00pm timerange(0, 0, 0, 0, 0, 30); // returns true between midnight and 30 seconds past midnight
Note: Since all of the examples that follow are very specific, they have not been tested.
All hosts which aren't fully qualified, or the ones that are in local domain, will be connected to directly. Everything else will go through w3proxy.mozilla.org:8080. If the proxy goes down, connections become direct automatically:
function FindProxyForURL(url, host) {
if (isPlainHostName(host) || dnsDomainIs(host, ".mozilla.org")) {
return "DIRECT";
} else {
return "PROXY w3proxy.mozilla.org:8080; DIRECT";
}
} Note: This is the simplest and most efficient autoconfig file for cases where there's only one proxy.
If there are hosts (such as the main Web server) that belong to the local domain but are outside the firewall and are only reachable through the proxy server, those exceptions can be handled using the localHostOrDomainIs() function:
function FindProxyForURL(url, host) {
if (
(isPlainHostName(host) || dnsDomainIs(host, ".mozilla.org")) &&
!localHostOrDomainIs(host, "www.mozilla.org") &&
!localHostOrDoaminIs(host, "merchant.mozilla.org")
) {
return "DIRECT";
} else {
return "PROXY w3proxy.mozilla.org:8080; DIRECT";
}
} The above example will use the proxy for everything except local hosts in the mozilla.org domain, with the further exception that hosts www.mozilla.org and merchant.mozilla.org will go through the proxy.
Note the order of the above exceptions for efficiency: localHostOrDomainIs() functions only get executed for URLs that are in local domain, not for every URL. Be careful to note the parentheses around the or expression before the and expression to achieve the above-mentioned efficient behaviour.
This example will work in an environment where the internal DNS server is set up so that it can only resolve internal host names, and the goal is to use a proxy only for hosts that aren't resolvable:
function FindProxyForURL(url, host) {
if (isResolvable(host))
return "DIRECT";
else
return "PROXY proxy.mydomain.com:8080";
} The above requires consulting the DNS every time; it can be grouped intelligently with other rules so that DNS is consulted only if other rules do not yield a result:
function FindProxyForURL(url, host) {
if (
isPlainHostName(host) ||
dnsDomainIs(host, ".mydomain.com") ||
isResolvable(host)
) {
return "DIRECT";
} else {
return "PROXY proxy.mydomain.com:8080";
}
} In this example all of the hosts in a given subnet are connected-to directly, others are connected through the proxy:
function FindProxyForURL(url, host) {
if (isInNet(host, "198.95.0.0", "255.255.0.0"))
return "DIRECT";
else
return "PROXY proxy.mydomain.com:8080";
} Again, use of the DNS server in the above can be minimized by adding redundant rules in the beginning:
function FindProxyForURL(url, host) {
if (
isPlainHostName(host) ||
dnsDomainIs(host, ".mydomain.com") ||
isInNet(host, "198.95.0.0", "255.255.0.0")
) {
return "DIRECT";
} else {
return "PROXY proxy.mydomain.com:8080";
}
} This example is more sophisticated. There are four (4) proxy servers; one of them is a hot stand-by for all of the other ones, so if any of the remaining three goes down the fourth one will take over. Furthermore, the three remaining proxy servers share the load based on URL patterns, which makes their caching more effective (there is only one copy of any document on the three servers - as opposed to one copy on each of them). The load is distributed like this:
| Proxy | Purpose |
|---|---|
| #1 | .com domain |
| #2 | .edu domain |
| #3 | all other domains |
| #4 | hot stand-by |
All local accesses are desired to be direct. All proxy servers run on the port 8080 (they don't need to, you can just change your port but remember to modify your configuations on both side). Note how strings can be concatenated with the + operator in JavaScript.
function FindProxyForURL(url, host) {
if (isPlainHostName(host) || dnsDomainIs(host, ".mydomain.com"))
return "DIRECT";
else if (shExpMatch(host, "*.com"))
return "PROXY proxy1.mydomain.com:8080; " +
"PROXY proxy4.mydomain.com:8080";
else if (shExpMatch(host, "*.edu"))
return "PROXY proxy2.mydomain.com:8080; " +
"PROXY proxy4.mydomain.com:8080";
else
return "PROXY proxy3.mydomain.com:8080; " +
"PROXY proxy4.mydomain.com:8080";
} Most of the standard JavaScript functionality is available for use in the FindProxyForURL() function. As an example, to set different proxies based on the protocol the startsWith() function can be used:
function FindProxyForURL(url, host) {
if (url.startsWith("http:"))
return "PROXY http-proxy.mydomain.com:8080";
else if (url.startsWith("ftp:"))
return "PROXY ftp-proxy.mydomain.com:8080";
else if (url.startsWith(“gopher:"))
return "PROXY gopher-proxy.mydomain.com:8080";
else if (url.startsWith("https:") || url.startsWith("snews:"))
return "PROXY security-proxy.mydomain.com:8080";
else
return "DIRECT";
} Note: The same can be accomplished using the shExpMatch() function described earlier.
For example:
// ...
if (shExpMatch(url, "http:*")) {
return "PROXY http-proxy.mydomain.com:8080";
}
// ... The autoconfig file can be output by a CGI script. This is useful, for example, when making the autoconfig file act differently based on the client IP address (the REMOTE_ADDR environment variable in CGI).
Usage of isInNet(), isResolvable() and dnsResolve() functions should be carefully considered, as they require the DNS server to be consulted. All the other autoconfig-related functions are mere string-matching functions that don't require the use of a DNS server. If a proxy is used, the proxy will perform its DNS lookup which would double the impact on the DNS server. Most of the time these functions are not necessary to achieve the desired result.
Proxy auto-config was introduced into Netscape Navigator 2.0 in the late 1990s, at the same time when JavaScript was introduced. Open-sourcing Netscape eventually lead to Firefox itself.
The most "original" implementation of PAC and its JavaScript libraries is, therefore, nsProxyAutoConfig.js found in early versions of Firefox. These utilities are found in many other open-source systems including Chromium. Firefox later integrated the file into ProxyAutoConfig.cpp as a C++ string literal. To extract it into its own file, it suffices to copy the chunk into JavaScript with a console.log directive to print it.
Microsoft in general made its own implementation. There used to be some problems with their libraries, but most are resolved by now. They have defined some new "Ex" suffixed functions around the address handling parts to support IPv6. The feature is supported by Chromium, but not yet by Firefox (bugzilla #558253).
© 2005–2020 Mozilla and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Proxy_servers_and_tunneling/Proxy_Auto-Configuration_(PAC)_file