Proxy-failover@mozilla.com.xpi What does it do?

Add-ons addons.mozilla.org
#1

Hi,

In the last week I’m seeing proxy-failover@mozilla.com.xpi added to recent FF releases.
It is currently being blocked with application whitelisting software.

There appears to be no information on it, and Google shows no useful results.

Is it a safe addon and is there any explanation of what it does?

Alternatively is there a link to its github page or other place where I can look for info?

thanks,
Ryan

#2

Looks like it comes from here: https://ftp.mozilla.org/pub/system-addons/proxy-failover/proxy-failover@mozilla.com-1.0.0-signed.xpi and it implements failover rules for system requests over malfunctioning proxies. The exact rules are laid out at the top of api.jsif you extract that .xpi file:

/**
 * ProxyMonitor monitors system and protected requests for failures
 * due to bad or unavailable proxy configurations.
 * 
 * In a system with multiple layers of proxy configuration, if there is a 
 * failing proxy we try to remove just that confuration from the chain.  However if 
 * we get too many failures, we'll make a direct connection the top "proxy".
 * 
 * 1. Any proxied system request without a direct failover will have one added.
 * 
 * 2. If a proxied system request fails, the proxy configuration in use will
 * be disabled.  On later requests, disabled proxies are removed from the proxy chain.
 * Disabled proxy configurations remain disabled for 48 hours to allow any necessary
 * requests to operate for a period of time. When disabled proxies are used as a
 * failover to a direct request (step 3 or 4 below), the proxy can be detected
 * as functional and be re-enabled despite not having reached the 48 hours.
 * 
 * 3. If too many proxy configurations got disabled, we make a direct config first
 * with failover to all other proxy configurations (essentially skipping step 2).
 * This state remains for 48 hours before retrying without "direct".
 * 
 * 4. If we've removed all proxies we make a direct config first and failover to 
 * the other proxy configurations, similar to step 3.
 * 
 * 5. Starting with Fx92, we will only disable proxy configurations provided by
 * extensions.  Prior to 92, we could not definitively identify extensions from
 * the proxyInfo instance.
 * 
 * If we've disabled proxies, we continue to watch the requests for failures in
 * "direct" connection mode.  If we continue to fail with direct connections,
 * we fall back to allowing proxies again.
 */
#3

Thanks @idk seems reasonable, I’ll update the app whitelisting to allow the file. many thanks

#4

No problem. I’m not a Mozilla employee or anything but I extracted the code and looked over the contents and it all seems perfectly normal. The thing I pulled apart doesn’t seem to be hiding anything or doing anything weird. The whole thing is less than 600 lines total and it’s just Javascript.