Chrome Downloading Proxy Script Message

Posted : admin On 8/23/2021
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This document establishes basic proxy terminology and describes Chrome-specific proxy behaviors.

Have you noticed Chrome showing ‘Downloading Proxy Script’ message at the bottom when connecting to a website in the browser? If the message appears frequently and taking more time to load sites in the browser, then you should know, this is not the general behavior of Chrome browser, the good news is you can disable or fix that issue, here is how that can be done. The message “downloading proxy script” may be displayed by Google Chrome. If you see it frequently, you may want to know why the message is displayed by the browser and what you can do about it. That is why you’re seeing the message ‘ downloading proxy script ’ on the bottom-left of your browser. Because your PC has to go via a proxy and to launch web pages can often be a slow process. For that reason, you will need to access your settings on your Google Chrome browser to speed up the loading process.

Proxy server identifiers

A proxy server is an intermediary used for network requests. A proxy server can be described by its address, along with the proxy scheme that should be used to communicate with it.

This can be written as a string using either the “PAC format” or the “URI format”.

The PAC format is how one names a proxy server in Proxy auto-config scripts. For example:

Script
  • PROXY foo:2138
  • SOCKS5 foo:1080
  • DIRECT

The “URI format” instead encodes the information as a URL. For example:

  • foo:2138
  • http://foo:2138
  • socks5://foo:1080
  • direct://

The port number is optional in both formats. When omitted, a per-scheme default is used.

See the Proxy server schemes section for details on what schemes Chrome supports, and how to write them in the PAC and URI formats.

Most UI surfaces in Chrome (including command lines and policy) expect URI formatted proxy server identifiers. However outside of Chrome, proxy servers are generally identified less precisely by just an address -- the proxy scheme is assumed based on context.

In Windows' proxy settings there are host and port fields for the “HTTP”, “Secure”, “FTP”, and “SOCKS” proxy. With the exception of “SOCKS”, those are all identifiers for insecure HTTP proxy servers (proxy scheme is assumed as HTTP).

Proxy resolution

Proxying in Chrome is done at the URL level.

When the browser is asked to fetch a URL, it needs to decide which IP endpoint to send the request to. This can be either a proxy server, or the target host.

Chrome stuck on downloading proxy

This is called proxy resolution. The input to proxy resolution is a URL, and the output is an ordered list of proxy server identifiers.

What proxies to use can be described using either:

  • Manual proxy settings - proxy resolution is defined using a declarative set of rules. These rules are expressed as a mapping from URL scheme to proxy server identifier(s), and a list of proxy bypass rules for when to go DIRECT instead of using the mapped proxy.

  • PAC script - proxy resolution is defined using a JavaScript program, that is invoked whenever fetching a URL to get the list of proxy server identifiers to use.

  • Auto-detect - the WPAD protocol is used to probe the network (using DHCP/DNS) and possibly discover the URL of a PAC script.

Proxy server schemes

When using an explicit proxy in the browser, multiple layers of the network request are impacted, depending on the scheme that is used. Some implications of the proxy scheme are:

  • Is communication to the proxy done over a secure channel?
  • Is name resolution (ex: DNS) done client side, or proxy side?
  • What authentication schemes to the proxy server are supported?
  • What network traffic can be sent through the proxy?

Chrome supports these proxy server schemes:

DIRECT proxy scheme

  • Default port: N/A (neither host nor port are applicable)
  • Example identifier (PAC): DIRECT
  • Example identifier (URI): direct://

This is a pseudo proxy scheme that indicates instead of using a proxy we are sending the request directly to the target server.

It is imprecise to call this a “proxy server”, but it is a convenient abstraction.

HTTP proxy scheme

  • Default port: 80
  • Example identifier (PAC): PROXY proxy:8080, proxy (non-standard; don't use)
  • Example identifiers (URI): http://proxy:8080, proxy:8080 (can omit scheme)

Generally when one refers to a “proxy server” or “web proxy”, they are talking about an HTTP proxy.

When using an HTTP proxy in Chrome, name resolution is always deferred to the proxy. HTTP proxies can proxy http://, https://, ws:// and wss:// URLs. (Chrome's FTP support is deprecated, and HTTP proxies cannot proxy ftp:// anymore)

Communication to HTTP proxy servers is insecure, meaning proxied http:// requests are sent in the clear. When proxying https:// requests through an HTTP proxy, the TLS exchange is forwarded through the proxy using the CONNECT method, so end-to-end encryption is not broken. However when establishing the tunnel, the hostname of the target URL is sent to the proxy server in the clear.

HTTP proxies in Chrome support the same HTTP authentiation schemes as for target servers: Basic, Digest, Negotiate, NTLM.

HTTPS proxy scheme

  • Default port: 443
  • Example identifier (PAC): HTTPS proxy:8080
  • Example identifier (URI): https://proxy:8080

This works like an HTTP proxy, except the communication to the proxy server is protected by TLS, and may negotiate HTTP/2 (but not QUIC).

Because the connection to the proxy server is secure, https:// requests sent through the proxy are not sent in the clear as with an HTTP proxy. Similarly, since CONNECT requests are sent over a protected channel, the hostnames for proxied https:// URLs is also not revealed.

In addition to the usual HTTP authentication methods, HTTPS proxies also support client certificates.

HTTPS proxies using HTTP/2 can offer better performance in Chrome than a regular HTTP proxy due to higher connection limits (HTTP/1.1 proxies in Chrome are limited to 32 simultaneous connections across all domains).

Chrome, Firefox, and Opera support HTTPS proxies; however, most older HTTP stacks do not.

Specifying an HTTPS proxy is generally not possible through system proxy settings. Instead, one must use either a PAC script or a Chrome proxy setting (command line, extension, or policy).

See the dev.chromium.org document on secure web proxies for tips on how to run and test against an HTTPS proxy.

SOCKSv4 proxy scheme

  • Default port: 1080
  • Example identifiers (PAC): SOCKS4 proxy:8080, SOCKS proxy:8080
  • Example identifier (URI): socks4://proxy:8080

SOCKSv4 is a simple transport layer proxy that wraps a TCP socket. Its use is transparent to the rest of the protocol stack; after an initial handshake when connecting the TCP socket (to the proxy), the rest of the loading stack is unchanged.

No proxy authentication methods are supported for SOCKSv4.

When using a SOCKSv4 proxy, name resolution for target hosts is always done client side, and moreover must resolve to an IPv4 address (SOCKSv4 encodes target address as 4 octets, so IPv6 targets are not possible).

There are extensions to SOCKSv4 that allow for proxy side name resolution, and IPv6, namely SOCKSv4a. However Chrome does not allow configuring, or falling back to v4a.

A better alternative is to just use the newer version of the protocol, SOCKSv5 (which is still 20+ years old).

SOCKSv5 proxy scheme

  • Default port: 1080
  • Example identifier (PAC): SOCKS5 proxy:8080
  • Example identifiers (URI): socks://proxy:8080, socks5://proxy:8080

SOCKSv5 is a transport layer proxy that wraps a TCP socket, and allows for name resolution to be deferred to the proxy.

In Chrome when a proxy's scheme is set to SOCKSv5, name resolution is always done proxy side (even though the protocol allows for client side as well). In Firefox client side vs proxy side name resolution can be configured with network.proxy.socks_remote_dns; Chrome has no equivalent option and will always use proxy side resolution.

No authentication methods are supported for SOCKSv5 in Chrome (although some do exist for the protocol).

A handy way to create a SOCKSv5 proxy is with ssh -D, which can be used to tunnel web traffic to a remote host over SSH.

In Chrome SOCKSv5 is only used to proxy TCP-based URL requests. It cannot be used to relay UDP traffic.

QUIC proxy scheme

  • Default (UDP) port: 443
  • Example identifier (PAC): QUIC proxy:8080
  • Example identifier (URI): quic://proxy:8080

A QUIC proxy uses QUIC (UDP) as the underlying transport, but otherwise behaves as an HTTP proxy. It has similar properties to an HTTPS proxy, in that the connection to the proxy server is secure, and connection limits are less restrictive.

Support for QUIC proxies in Chrome is currently experimental and not ready for production use. In particular, sending https:// and wss:// URLs through a QUIC proxy is disabled by default.

Another caveat is that QUIC does not currently support client certificates since it does not use a TLS handshake. This may change in future versions.

Manual proxy settings

The simplest way to configure proxy resolution is by providing a static list of rules comprised of:

  1. A mapping of URL schemes to proxy server identifiers.
  2. A list of proxy bypass rules

We refer to this mode of configuration as “manual proxy settings”.

Manual proxy settings can succinctly describe setups like:

  • Use proxy http://foo:8080 for all requests
  • Use proxy http://foo:8080 for all requests except those to a google.com subdomain.
  • Use proxy http://foo:8080 for all https:// requests, and proxy socsk5://mysocks:90 for everything else

Although manual proxy settings are a ubiquituous way to configure proxies across platforms, there is no standard representation or feature set.

Chrome‘s manual proxy settings most closely resembles that of WinInet. But it also supports idioms from other platforms -- for instance KDE’s notion of reversing the bypass list, or Gnome's interpretation of bypass patterns as suffix matches.

When defining manual proxy settings in Chrome, we specify three (possibly empty) lists of proxy server identifiers.

  • proxies for HTTP - A list of proxy server identifiers to use for http:// requests, if non-empty.
  • proxies for HTTPS - A list of proxy server identifiers to use for https:// requests, if non-empty.
  • other proxies - A list of proxy server identifiers to use for everything else (whatever isn't matched by the other two lists)

There are a lot of ways to end up with manual proxy settings in Chrome (discussed in other sections).

The following examples will use the command line method. Launching Chrome with --proxy-server=XXX (and optionally --proxy-bypass-list=YYY)

Example: To use proxy http://foo:8080 for all requests we can launch Chrome with --proxy-server='http://foo:8080'. This translates to:

  • proxies for HTTP - empty
  • proxies for HTTPS - empty
  • other proxies - http://foo:8080

With the above configuration, if the proxy server was unreachable all requests would fail with ERR_PROXY_CONNECTION_FAILED. To address this we could add a fallback to DIRECT by launching using --proxy-server='http://foo:8080,direct://' (note the comma separated list). This command line means:

  • proxies for HTTP - empty
  • proxies for HTTPS - empty
  • other proxies - http://foo:8080, direct://

If instead we wanted to proxy only http:// URLs through the HTTPS proxy https://foo:443, and have everything else use the SOCKSv5 proxy socks5://mysocks:1080 we could launch Chrome with --proxy-server='http=https://foo:443;socks=socks5://mysocks:1080'. This now expands to:

  • proxies for HTTP - https://foo:443
  • proxies for HTTPS - empty
  • other proxies - socks5://mysocks:1080

The command line above uses WinInet's proxy map format, with some additional features:

  • Instead of naming proxy servers by just a hostname:port, you can use Chrome‘s URI format for proxy server identifiers. In other words, you can prefix the proxy scheme so it doesn’t default to HTTP.
  • The socks= mapping is understood more broadly as “other proxies”. The subsequent proxy list can include proxies of any scheme, however if the scheme is omitted it will be understood as SOCKSv4 rather than HTTP.

Mapping WebSockets URLs to a proxy

Manual proxy settings don't have mappings for ws:// or wss:// URLs.

Selecting a proxy for these URL schemes is a bit different from other URL schemes. The algorithm that Chrome uses is:

  • If “other proxies” is non-empty use it
  • If “proxies for HTTPS” is non-empty use it
  • Otherwise use “proxies for HTTP”

This is per the recommendation in section 4.1.3 of RFC 6455.

It is possible to route ws:// and wss:// separately using a PAC script.

Proxy credentials in manual proxy settings

Most platforms' manual proxy settings allow specifying a cleartext username/password for proxy sign in. Chrome does not implement this, and will not use any credentials embedded in the proxy settings.

Proxy authentication will instead go through the ordinary flow to find credentials.

Proxy bypass rules

In addition to specifying three lists of proxy server identifiers, Chrome's manual proxy settings lets you specify a list of “proxy bypass rules”.

This ruleset determines whether a given URL should skip use of a proxy all together, even when a proxy is otherwise defined for it.

This concept is also known by names like “exception list”, “exclusion list” or “no proxy list”.

Proxy bypass rules can be written as an ordered list of strings. Ordering generally doesn't matter, but may when using subtractive rules.

When manual proxy settings are specified from the command line, the --proxy-bypass-list='RULES' switch can be used, where RULES is a semicolon or comma separated list of bypass rules.

Following are the string constructions for the bypass rules that Chrome supports. They can be used when defining a Chrome manual proxy settings from command line flags, extensions, or policy.

When using system proxy settings, one should use the platform‘s rule format and not Chrome’s.

Bypass rule: Hostname

Matches a hostname using a wildcard pattern, and an optional scheme and port restriction.

Examples:

  • foobar.com - Matches URL of any scheme and port, whose normalized host is foobar.com
  • *foobar.com - Matches URL of any scheme and port, whose normalized host ends with foobar.com (for instance blahfoobar.com and foo.foobar.com).
  • *.org:443 - Matches URLs of any scheme, using port 443 and whose top level domain is .org
  • https://x.*.y.com:99 - Matches https:// URLs on port 99 whose normalized hostname matches x.*.y.com

Bypass rule: Subdomain

Hostname patterns that start with a dot are special cased to mean a subdomain matches. .foo.com is effectively another way of writing *.foo.com.

Examples:

  • .google.com - Matches calendar.google.com and foo.bar.google.com, but not google.com.
  • http://.google.com - Matches only http:// URLs that are a subdomain of google.com.

Bypass rule: IP literal

Matches URLs that are IP address literals, and optional scheme and port restrictions. This is a special case of hostname matching that takes into account IP literal canonicalization. For example the rules [0:0:0::1] and [::1] are equivalent (both represent the same IPv6 address).

Examples:

  • 127.0.0.1
  • http://127.0.0.1
  • [::1] - Matches any URL to the IPv6 loopback address.
  • [0:0::1] - Same as above
  • http://[::1]:99 - Matches any http:// URL to the IPv6 loopback on port 99

Bypass rule: IPv4 address range

Matches any URL whose hostname is an IPv4 literal, and falls between the given address range.

Note this only applies to URLs that are IP literals.

Examples:

  • 192.168.1.1/16

Bypass rule: IPv6 address range

Chrome Downloading Proxy Script Message

Matches any URL that is an IPv6 literal that falls between the given range. Note that IPv6 literals must not be bracketed.

Note this only applies to URLs that are IP literals.

Examples:

  • fefe:13::abc/33
  • [fefe::]/40 -- WRONG! IPv6 literals must not be bracketed.

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Bypass rule: Simple hostnames

Matches hostnames without a period in them, and that are not IP literals. This is a naive string search -- meaning that periods appearing anywhere count (including trailing dots!).

This rule corresponds to the “Exclude simple hostnames” checkbox on macOS and the “Don't use proxy server for local (intranet) addresses” on Windows.

The rule name comes from WinInet, and can easily be confused with the concept of localhost. However the two concepts are completely orthogonal. In practice one wouldn't add rules to bypass localhost, as it is already done implicitly.

Downloading

Bypass rule: Subtract implicit rules

Subtracts the implicit proxy bypass rules (localhost and link local addresses). This is generally only needed for test setups. Beware of the security implications to proxying localhost.

Whereas regular bypass rules instruct the browser about URLs that should not use the proxy, this rule has the opposite effect and tells the browser to instead use the proxy.

Ordering may matter when using a subtractive rule, as rules will be evaluated in a left-to-right order. <-loopback>;127.0.0.1 has a subtly different effect than 127.0.0.1;<-loopback>.

Meaning of IP address range bypass rules

The IP address range bypass rules in manual proxy settings applies only to URL literals. This is not what one would intuitively expect.

Example:

Say we have have configured a proxy for all requests, but added a bypass rule for 192.168.0.0.1/16. If we now navigate to http://foo (which resolves to 192.168.1.5 in our setup) will the browser connect directly (bypass proxy) because we have indicated a bypass rule that includes this IP?

It will go through the proxy.

The bypass rule in this case is not applicable, since the browser never actually does a name resolution for foo. Proxy resolution happens before name resolution, and depending on what proxy scheme is subsequently chosen, client side name resolution may never be performed.

The usefulness of IP range proxy bypass rules is rather limited, as they only apply to requests whose URL was explicitly an IP literal.

If proxy decisions need to be made based on the resolved IP address(es) of a URL's hostname, one must use a PAC script.

Implicit bypass rules

Requests to certain hosts will not be sent through a proxy, and will instead be sent directly.

We call these the implicit bypass rules. The implicit bypass rules match URLs whose host portion is either a localhost name or a link-local IP literal. Essentially it matches:

The complete rules are slightly more complicated. For instance on Windows we will also recognize loopback.

This concept of implicit proxy bypass rules is consistent with the platform-level proxy support on Windows and macOS (albeit with some differences due to their implementation quirks - see compatibility notes in net::ProxyBypassRules::MatchesImplicitRules)

Why apply implicit proxy bypass rules in the first place? Certainly there are considerations around ergonomics and user expectation, but the bigger problem is security. Since the web platform treats localhost as a secure origin, the ability to proxy it grants extra powers. This is especially problematic when proxy settings are externally controllable, as when using PAC scripts.

Historical support in Chrome:

  • Prior to M71 there were no implicit proxy bypass rules, except if using --winhttp-proxy-resolver.
  • In M71 Chrome applied implicit proxy bypass rules to PAC scripts
  • In M72 Chrome generalized the implicit proxy bypass rules to manually configured proxies

Overriding the implicit bypass rules

If you want traffic to localhost to be sent through a proxy despite the security concerns, it can be done by adding the special proxy bypass rule <-loopback>. This has the effect of subtracting the implicit rules.

For instance, launch Chrome with the command line flag:

Note that there currently is no mechanism to disable the implicit proxy bypass rules when using a PAC script. Proxy bypass lists only apply to manual settings, so the technique above cannot be used to let PAC scripts decide the proxy for localhost URLs.

Evaluating proxy lists (proxy fallback)

Proxy resolution results in a list of proxy server identifiers to use for a given request, not just a single proxy server identifier.

For instance, consider this PAC script:

What proxy will Chrome use for connections to www.example.com, given that we have a choice of three separate proxy server identifiers to choose from {http://proxy1:80, https://proxy2:443, socks5://proxy3:1080}?

Initially, Chrome will try the proxies in order. This means first attempting the request through http://proxy1:80. If that “fails”, the request is next attempted through https://proxy2:443. Lastly if that fails, the request is attempted through socks5://proxy3:1080.

This process is referred to as proxy fallback. What constitutes a “failure” is described later.

Proxy fallback is stateful. The actual order of proxy attempts made be Chrome is influenced by the past responsiveness of proxy servers.

Let's say we request http://www.example.com/. Per the PAC script this resolves to a list of three proxy server identifiers:

{http://proxy1:80, https://proxy2:443, socks5://proxy3:1080}

Chrome will first attempt to issue the request through these proxies in the left-to-right order.

Let's say that the attempt through http://proxy1:80 fails, but then the attempt through https://proxy2:443 succeeds. Chrome will mark http://proxy1:80 as bad for the next 5 minutes. Being marked as bad means that http://proxy1:80 is de-prioritized with respect to other proxy server identifiers (including direct://) that are not marked as bad.

That means the next time http://www.example.com/ is requested, the effective order for proxies to attempt will be:

{https://proxy2:443, socks5://proxy3:1080, http://proxy1:80}

Conceptually, bad proxies are moved to the end of the list, rather than being removed from consideration all together.

What constitutes a “failure” when it comes to triggering proxy fallback depends on the proxy type. Generally speaking, only connection level failures are deemed eligible for proxy fallback. This includes:

  • Failure resolving the proxy server's DNS
  • Failure connecting a TCP socket to the proxy server

(There are some caveats for how HTTPS and QUIC proxies count failures for fallback)

Prior to M67, Chrome would consider failures establishing a CONNECT tunnel as an error eligible for proxy fallback. This policy resulted in problems for deployments whose HTTP proxies intentionally failed certain https:// requests, since that necessitates inducing a failure during the CONNECT tunnel establishment. The problem would occur when a working proxy fallback option like DIRECT was given, since the failing proxy would then be marked as bad.

Currently there are no options to configure proxy fallback (including disabling the caching of bad proxies). Future versions of Chrome may remove caching of bad proxies to make fallback predictable.

To investigate issues relating to proxy fallback, one can collect a NetLog dump using chrome://net-export/. These logs can then be loaded with the NetLog viewer.

There are a few things of interest in the logs:

  • The “Proxy” tab will show which proxies (if any) were marked as bad at the time the capture ended.
  • The “Events” tab notes what the resolved proxy list was, and what the re-ordered proxy list was after taking into account bad proxies.
  • The “Events” tab notes when a proxy is marked as bad and why (provided the event occurred while capturing was enabled).

When debugging issues with bad proxies, it is also useful to reset Chrome's cache of bad proxies. This can be done by clicking the “Clear bad proxies” button on chrome://net-internals/#proxy. Note the UI will not give feedback that the bad proxies were cleared, however capturing a new NetLog dump can confirm it was cleared.

Arguments passed to FindProxyForURL() in PAC scripts

PAC scripts in Chrome are expected to define a JavaScript function FindProxyForURL.

The historical signature for this function is:

Download Proxy Script Chrome Slow

Scripts can expect to be called with string arguments url and host such that:

  • url is a sanitized version of the request's URL
  • host is the unbracketed host portion of the origin.

Sanitization of the URL means that the path, query, fragment, and identity portions of the URL are stripped. Effectively url will be limited to a scheme://host:port/ style URL

Examples of how FindProxyForURL() will be called:

Stripping the path and query from the url is a departure from the original Netscape implementation of PAC. It was introduced in Chrome 52 for security reasons.

There is currently no option to turn off sanitization of URLs passed to PAC scripts (removed in Chrome 75).

The sanitization of http:// URLs currently has a different policy, and does not strip query and path portions of the URL. That said, users are advised not to depend on reading the query/path portion of any URL type, since future versions of Chrome may deprecate that capability in favor of a consistent policy.

Resolving client's IP address within a PAC script using myIpAddress()

PAC scripts can invoke myIpAddress() to obtain the client's IP address. This function returns a single IP literal, or '127.0.0.1' on failure.

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This API is inherently ambiguous when used on multi-homed hosts, as such hosts can have multiple IP addresses and yet the browser can pick just one to return.

Chrome's algorithm for myIpAddress() favors returning the IP that would be used if we were to connect to the public internet, by executing the following ordered steps and short-circuiting once the first candidate IP is found:

  1. Select the IP of an interface that can route to public Internet:
    • Probe for route to 8.8.8.8.
    • Probe for route to 2001:4860:4860::8888.
  2. Select an IP by doing a DNS resolve of the machine's hostname:
    • Select the first IPv4 result if there is one.
    • Select the first IP result if there is one.
  3. Select the IP of an interface that can route to private IP space:
    • Probe for route to 10.0.0.0.
    • Probe for route to 172.16.0.0.
    • Probe for route to 192.168.0.0.
    • Probe for route to FC00::.

Note that when searching for candidate IP addresses, link-local and loopback addresses are skipped over. Link-local or loopback address will only be returned as a last resort when no other IP address was found by following these steps.

This sequence of steps explicitly favors IPv4 over IPv6 results, to match Internet Explorer's IPv6 support.

Historical note: Prior to M72, Chrome's implementation of myIpAddress() was effectively just getaddrinfo(gethostname). This is now step 2 of the heuristic.

Resolving client's IP address within a PAC script using myIpAddressEx()

Chrome supports the Microsoft PAC extensionmyIpAddressEx().

This is like myIpAddress(), but instead of returning a single IP address, it can return multiple IP addresses. It returns a string containing a semi-colon separated list of addresses. On failure it returns an empty string to indicate no results (whereas myIpAddress() returns 127.0.0.1).

There are some differences with Chrome's implementation:

  • In Chrome the function is unconditionally defined, whereas in Internet Explorer one must have used the FindProxyForURLEx entrypoint.
  • Chrome does not necessarily enumerate all of the host's network interfaces
  • Chrome does not return link-local or loopback addresses (except if no other addresses were found).

Downloading Proxy Script Disable

The algorithm that Chrome uses is nearly identical to that of myIpAddress() described earlier, but in certain cases may return multiple IPs.

  1. Select all the IPs of interfaces that can route to public Internet:
    • Probe for route to 8.8.8.8.
    • Probe for route to 2001:4860:4860::8888.
    • If any IPs were found, return them, and finish.
  2. Select an IP by doing a DNS resolve of the machine's hostname:
    • If any IPs were found, return them, and finish.
  3. Select the IP of an interface that can route to private IP space:
    • Probe for route to 10.0.0.0.
    • Probe for route to 172.16.0.0.
    • Probe for route to 192.168.0.0.
    • Probe for route to FC00::.
    • If any IPs were found, return them, and finish.

Note that short-circuiting happens whenever steps 1-3 find a candidate IP. So for example if at least one IP address was discovered by checking routes to public Internet, only those IPs will be returned, and steps 2-3 will not run.

myIpAddress() / myIpAddressEx() and multi-homed hosts

myIpAddress() is a poor API for hosts that have multiple IP addresses, as it can only return a single IP, which may or may not be the one you wanted. Both myIpAddress() and myIpAddressEx() favor returning the IP for the interface that would be used to route to the public internet.

As an API, myIpAddressEx() offers more flexibility since it can return multiple IP addresses. However Chrome‘s implementation restricts which IPs a PAC script can see due to privacy concerns. So using myIpAddressEx() is not as powerful as enumerating all the host’s IPs, and may not address all use-cases.

A more reliable strategy for PAC scripts to check which network(s) a user is on is to probe test domains using dnsResolve() / dnsResolveEx().

Moreover, note that Chrome does not support the Firefox-specific pacUseMultihomedDNS option, so adding that global to a PAC script has no special side-effect in Chrome. Whereas in Firefox it reconfigures myIpAddress() to be dependent on the target URL that FindProxyForURL() was called with.

Android quirks

Proxy resolving via PAC works differently on Android than other desktop Chrome platforms:

  • Android Chrome uses the same Chromium PAC resolver, however does not run it out-of-process as on Desktop Chrome. This architectural difference is due to the higher process cost on Android, and means Android Chrome is more susceptible to malicious PAC scripts. The other consequence is that Android Chrome can have distinct regressions from Desktop Chrome as the service setup is quite different (and most browser_tests are not run on Android either).

  • WebView does not use Chrome's PAC resolver. Instead Android WebView uses the Android system‘s PAC resolver, which is less optimized and uses an old build of V8. When the system is configured to use PAC, Android WebView’s net code will see the proxy settings as being a single HTTP proxy on localhost. The system localhost proxy will in turn evaluate the PAC script and forward the HTTP request on to the resolved proxy. This translation has a number of effects, including what proxy schemes are supported, the maximum connection limits, how proxy fallback works, and overall performance (the current Android PAC evaluator blocks on DNS).

  • Android system log messages for PacProcessor are not related to Chrome or its PAC evaluator. Rather, these are log messages generated by the Android system's PAC implementation. This confusion can arise when users add alert() to debug PAC script logic, and then refer to output in logcat to try and diagnose a resolving issue in Android Chrome.

Downloading PAC scripts

When a network context is configured to use a PAC script, proxy resolution will stall while downloading the PAC script.

Fetches for PAC URLs are initiated by the network stack, and behave differently from ordinary web visible requests:

  • Must complete within 30 seconds.
  • Must complete with an HTTP response code of exactly 200.
  • Must have an uncompressed body smaller than 1 MB.
  • Do not follow ordinary HTTP caching semantics.
  • Are never fetched through a proxy
  • Are not visible to the WebRequest extension API, or to service workers.
  • Do not support HTTP authentication (ambient authentication may work, but cannot prompt UI for credentials).
  • Do not support client certificates (including AutoSelectCertificateForUrls)
  • Do not support auxiliary certificate network fetches (will only used cached OCSP, AIA, and CRL responses during certificate verification).

Caching of successful PAC fetches

PAC URLs are always fetched from the network, and never from the HTTP cache. After a PAC URL is successfully fetched, its contents (which are used to create a long-lived Java Script context) will be assumed to be fresh until either:

  • The network changes (IP address changes, DNS configuration changes)
  • The response becomes older than 12 hours
  • A user explicitly invalidates PAC through chrome://net-internals#proxy

Once considered stale, the PAC URL will be re-fetched the next time proxy resolution is requested.

Fallback for failed PAC fetches

When the proxy settings are configured to use a PAC URL, and that PAC URL cannot be fetched, proxy resolution will fallback to the next option, which is often DIRECT:

  • If using system proxy settings, and the platform supports fallback to manual proxy settings (e.g. Windows), the specified manual proxy servers will be used after the PAC fetch fails.
  • If using Chrome's proxy settings, and the PAC script was marked as mandatory, fallback to DIRECT is not permitted. Subsequent network requests will fail proxy resolution and complete with ERR_MANDATORY_PROXY_CONFIGURATION_FAILED.
  • Otherwise proxy resolution will silently fall back to DIRECT.

Recovering from failed PAC fetches

When fetching an explicitly configured PAC URL fails, the browser will try to re-fetch it:

  • In exactly 8 seconds
  • 32 seconds after that
  • 2 minutes after that
  • Every 4 hours thereafter

This background polling of the PAC URL is only initiated in response to an incoming proxy resolution request, so it will not trigger work when the browser is otherwise idle.

Similarly to successful fetches, the PAC URL will be also be re-fetched whenever the network changes, the proxy settings change, or it was manually invalidated via chrome://net-internals#proxy.

Text encoding

Note that UTF-8 is not the default interpretation of PAC response bodies.

The priority for encoding is determined in this order:

  1. The charset property of the HTTP response's Content-Type
  2. Any BOM at the start of response body
  3. Otherwise defaults to ISO-8859-1.

When setting the Content-Type, servers should prefer using a mime type of application/x-ns-proxy-autoconfig or application/x-javascript-config. However in practice, Chrome does not enforce the mime type.

Capturing a Net Log for debugging proxy resolution issues

Issues in proxy resolution are best investigated using a Net Log.

A good starting point is to follow the general instructions for net-export, and while the Net Log is being captured perform these steps:

  1. Reproduce the failure (ex: load a URL that fails)
  2. If you can reproduce a success, do so (ex: load a different URL that succeeds).
  3. In a new tab, navigate to chrome://net-internals/#proxy and click both buttons (“Re-apply settings” and “Clear bad proxies”).
  4. Repeat step (1)
  5. Stop the Net Log and save the file.

The resulting Net Log should have enough information to diagnose common problems. It can be attached to a bug report, or explored using the Net Log Viewer. See the next section for some tips on analyzing it.

Analyzing Net Logs for proxy issues

Load saved Net Logs using Net Log Viewer.

Proxy overview tab

Start by getting a big-picture view of the proxy settings by clicking to the “Proxy” tab on the left. This summarizes the proxy settings at the time the capture ended.

  • Does the original proxy settings match expectation? The proxy settings might be coming from:

    • Managed Chrome policy (chrome://policy)
    • Command line flags (ex: --proxy-server)
    • (per-profile) Chrome extensions (ex: chrome.proxy)
    • (per-network) System proxy settings
  • Was proxy autodetect (WPAD) specified? In this case the final URL probed will be reflected by the difference between the “Effective” and “Original” settings.

  • Internally, proxy settings are per-NetworkContext. The proxy overview tab shows settings for a particular NetworkContext, namely the one associated with the Profile used to navigate to chrome://net-export. For instance if the net-export was initiated from an Incognito window, it may show different proxy settings here than a net-export capture initiated by a non-Incognito window. When the net-export was triggered from command line (--log-net-log) no particular NetworkContext is associated with the capture and hence no proxy settings will be shown in this overview.

  • Were any proxies marked as bad?

Import tab

Skim through the Import tab and look for relevant command line flags and active field trials. A find-in-page for proxy is a good starting point. Be on the lookout for --winhttp-proxy-resolver which has known problems.

Events tab

To deep dive into proxy resolution, switch to the Events tab.

You can start by filtering on type:URL_REQUEST to see all the top level requests, and then keep click through the dependency links to trace the proxy resolution steps and outcome.

The most relevant events have either PROXY_, PAC_, or WPAD_ in their names. You can also try filtering for each of those.

Documentation on specific events is available in net_log_event_type_list.h.

Network change events can also be key to understanding proxy issues. After switching networks (ex VPN), the effective proxy settings, as well as content of any PAC scripts/auto-detect can change.

Web Proxy Auto-Discovery (WPAD)

When configured to use WPAD (aka “autotmaticaly detect proxy settings”), Chrome will prioritize:

  1. DHCP-based WPAD (option 252)
  2. DNS-based WPAD

These are tried in order, however DHCP-based WPAD is only supported for Chrome on Windows and Chrome on Chrome OS.

WPAD is the system default for many home and Enterprise users.

Chrome on macOS support for DHCP-based WPAD

Chrome on macOS does not support DHCP-based WPAD when configured to use “autodetect”.

However, macOS might perform DHCP-based WPAD and embed this discovered PAC URL as part of the system proxy settings. So effectively when Chrome is configured to “use system proxy settings” it may behave as if it supports DHCP-based WPAD.

Dangers of DNS-based WPAD and DNS search suffix list

DNS-based WPAD involves probing for the non-FQDN wpad. This means WPAD‘s performance and security is directly tied to the user’s DNS search suffix list.

When resolving wpad, the host's DNS resolver will complete the hostname using each of the suffixes in the search list:

  1. If the suffix list is long this process can very slow, as it triggers a cascade of NXDOMAIN.
  2. If the suffix list includes domains outside of the administrative domain, WPAD may select an attacker controlled PAC server, and can subsequently funnel the user's traffic through a proxy server of their choice. The evolution of TLDs further increases this risk, since what were previously private suffixes used by an enterprise can become publicly registerable. See also WPAD Name Collision Vulnerability
Downloading proxy script disable

--winhttp-proxy-resolver command line switch

Passing the --winhttp-proxy-resolver command line argument instructs Chrome to use the system libraries for one narrow part of proxy resolution: evaluating a given PAC script.

Use of this flag is NOT a supported mode, and has known problems: It can break Chrome extensions (chrome.proxy API), the interpretation of Proxy policies, hurt performance, and doesn't ensure full fidelity interpretation of system proxy settings.

Another oddity of this switch is that it actually gets interpreted with a smilar meaning on other platforms (macOS), despite its Windows-specific naming.

This flag was historically exposed for debugging, and to mitigate unresolved policy differences in PAC execution. In the future this switch will be removed.

Although Chrome would like full fidelity with Windows proxy settings, there are limits to those integrations. Dependencies like NRPT for proxy resolution necessitate using Windows proxy resolution libraries directly instead of Chrome's. We hope these less common use cases will be fully addressed by this feature

Sometimes you try everything you can to speed up Chrome but it’s still invariably slooow.

So here’s the thing: the next time your browser crawls, glance in the bottom left edge of the browser. Does it say: “Downloading Proxy Script?” If so, here’s a quick fix to make Chrome stop the shenanigans and just load your websites.

The “Downloading proxy script” zinger is often a corollary of how your PC is configured to connect to the internet. A proxy is the middle man you use to get out to the interwebs.

Some IT departments use proxies to filter data, control content or improve performance. Others use it as a way to connect to the corporate network. Whatever the case may be, you can often resolve the problem by disabling Automatically Detect Settings in your Internet Options.

Let’s do this.

Press the Windows logo key and enter this phrase:

Now click the Connections tab and choose the LAN settings button nestled in the bottom corner of the window.

Uncheck Automatically detect settings and click all the OK buttons to close out the windows.

Now the next time you kick open Chrome it should load without any issues. Let me know if that didn’t work for you in the comments.

Posted in Google Chrome, Web Browsers, Windows, Windows 10, Windows 7, Windows 8, Windows 8.1, Windows Vista, Windows XP Tagged with: Browser, Tricks