Add FAQ about using a proxy (#11933)

doc/03-cli.md

@@ -1232,20 +1232,12 @@ environment variable if you use Vagrant or VirtualBox and experience issues with
 being found during installation even though they should be present.
 
 ### http_proxy or HTTP_PROXY
+### HTTP_PROXY_REQUEST_FULLURI
+### HTTPS_PROXY_REQUEST_FULLURI
+### no_proxy or NO_PROXY
 
-If you are using Composer from behind an HTTP proxy, you can use the standard
-`http_proxy` or `HTTP_PROXY` env vars. Set it to the URL of your proxy.
-Many operating systems already set this variable for you.
-
-Using `http_proxy` (lowercased) or even defining both might be preferable since
-some tools like git or curl will only use the lower-cased `http_proxy` version.
-Alternatively you can also define the git proxy using
-`git config --global http.proxy <proxy url>`.
-
-If you are using Composer in a non-CLI context (i.e. integration into a CMS or
-similar use case), and need to support proxies, please provide the `CGI_HTTP_PROXY`
-environment variable instead. See [httpoxy.org](https://httpoxy.org/) for further
-details.
+See the [proxy documentation](faqs/how-to-use-composer-behind-a-proxy.md) for more details
+on how to use proxy env vars.
 
 ### COMPOSER_AUDIT_ABANDONED
 
@@ -1264,32 +1256,10 @@ in performance gains.
 Set to `4` or `6` to force IPv4 or IPv6 DNS resolution. This only works when the
 curl extension is used for downloads.
 
-### HTTP_PROXY_REQUEST_FULLURI
-
-If you use a proxy, but it does not support the request_fulluri flag, then you
-should set this env var to `false` or `0` to prevent Composer from setting the
-request_fulluri option.
-
-### HTTPS_PROXY_REQUEST_FULLURI
-
-If you use a proxy, but it does not support the request_fulluri flag for HTTPS
-requests, then you should set this env var to `false` or `0` to prevent Composer
-from setting the request_fulluri option.
-
 ### COMPOSER_SELF_UPDATE_TARGET
 
 If set, makes the self-update command write the new Composer phar file into that path instead of overwriting itself. Useful for updating Composer on a read-only filesystem.
 
-### no_proxy or NO_PROXY
-
-If you are behind a proxy and would like to disable it for certain domains, you
-can use the `no_proxy` or `NO_PROXY` env var. Set it to a comma separated list of
-domains the proxy should *not* be used for.
-
-The env var accepts domains, IP addresses, and IP address blocks in CIDR
-notation. You can restrict the filter to a particular port (e.g. `:80`). You
-can also set it to `*` to ignore the proxy for all HTTP requests.
-
 ### COMPOSER_DISABLE_NETWORK
 
 If set to `1`, disables network access (best effort). This can be used for debugging or

doc/faqs/how-to-use-composer-behind-a-proxy.md

@@ -0,0 +1,118 @@
+# How to use Composer behind a proxy
+
+Composer, like many other tools, uses environment variables to control the use of a proxy server and
+supports:
+
+- `http_proxy` - the proxy to use for HTTP requests
+- `https_proxy` - the proxy to use for HTTPS requests
+- `CGI_HTTP_PROXY` - the proxy to use for HTTP requests in a non-CLI context
+- `no_proxy` - domains that do not require a proxy
+
+These named variables are a convention, rather than an official standard, and their evolution and
+usage across different operating systems and tools is complex. Composer prefers the use of lowercase
+names, but accepts uppercase names where appropriate.
+
+## Usage
+
+Composer requires specific environment variables for HTTP and HTTPS requests. For example:
+
+```
+http_proxy=http://proxy.com:80
+https_proxy=http://proxy.com:80
+```
+
+Uppercase names can also be used.
+
+### Non-CLI usage
+
+Composer does not look for `http_proxy` or `HTTP_PROXY` in a non-CLI context. If you are running it
+this way (i.e. integration into a CMS or similar use case) you must use `CGI_HTTP_PROXY` for HTTP
+requests:
+
+```
+CGI_HTTP_PROXY=http://proxy.com:80
+https_proxy=http://proxy.com:80
+
+# cgi_http_proxy can also be used
+```
+
+> **Note:** CGI_HTTP_PROXY was introduced by Perl in 2001 to prevent request header manipulation and
+was popularized in 2016 when this vulnerability was widely reported: https://httpoxy.org
+
+## Syntax
+
+Use `scheme://host:port` as in the examples above. Although a missing scheme defaults to http and a
+missing port defaults to 80/443 for http/https schemes, other tools might require these values.
+
+The host can be specified as an IP address using dotted quad notation for IPv4, or enclosed in
+square brackets for IPv6.
+
+### Authorization
+
+Composer supports Basic authorization, using the `scheme://user:pass@host:port` syntax. Reserved url
+characters in either the user name or password must be percent-encoded. For example:
+
+```
+user:  me@company
+pass:  p@ssw$rd
+proxy: http://proxy.com:80
+
+# percent-encoded authorization
+me%40company:p%40ssw%24rd
+
+scheme://me%40company:p%40ssw%24rd@proxy.com:80
+```
+
+> **Note:** The user name and password components must be percent-encoded individually and then
+combined with the colon separator. The user name cannot contain a colon (even if percent-encoded),
+because the proxy will split the components on the first colon it finds.
+
+## HTTPS proxy servers
+
+Composer supports HTTPS proxy servers, where HTTPS is the scheme used to connect to the proxy, but
+only from PHP 7.3 with curl version 7.52.0 and above.
+
+```
+http_proxy=https://proxy.com:443
+https_proxy=https://proxy.com:443
+```
+
+## Bypassing the proxy for specific domains
+
+Use the `no_proxy` (or `NO_PROXY`) environment variable to set a comma-separated list of domains
+that the proxy should **not** be used for.
+
+```
+no_proxy=example.com
+# Bypasses the proxy for example.com and its sub-domains
+
+no_proxy=www.example.com
+# Bypasses the proxy for www.example.com and its sub-domains, but not for example.com
+```
+
+A domain can be restricted to a particular port (e.g. `:80`) and can also be specified as an IP
+address or an IP address block in CIDR notation.
+
+IPv6 addresses do not need to be enclosed in square brackets, like they are for
+http_proxy/https_proxy values, although this format is accepted.
+
+Setting the value to `*` will bypass the proxy for all requests.
+
+> **Note:** A leading dot in the domain name has no significance and is removed prior to processing.
+
+## Deprecated environment variables
+
+Composer originally provided `HTTP_PROXY_REQUEST_FULLURI` and `HTTPS_PROXY_REQUEST_FULLURI` to help
+mitigate issues with misbehaving proxies. These are no longer required or used.
+
+## Requirement changes
+
+Composer <2.8 used `http_proxy` for both HTTP and HTTPS requests if `https_proxy` was not set,
+but as of Composer 2.8.0 it requires [scheme-specific](#usage) environment variables.
+
+The reason for this change is to align Composer with current practice across other popular tools. To help
+with the transition, as of Composer 2.7.3 the original behaviour remains but a warning message is 
+shown instructing the user to add an `https_proxy` environment variable.
+
+To prevent the original behaviour during the transition period, set an empty environment variable
+(`https_proxy=`).

src/Composer/Console/Application.php

@@ -32,6 +32,7 @@ use Seld\JsonLint\ParsingException;
 use Composer\Command;
 use Composer\Composer;
 use Composer\Factory;
+use Composer\Downloader\TransportException;
 use Composer\IO\IOInterface;
 use Composer\IO\ConsoleIO;
 use Composer\Json\JsonValidationException;
@@ -404,6 +405,7 @@ class Application extends BaseApplication
                 $io->writeError('<warning>Composer now requires separate proxy environment variables for HTTP and HTTPS requests.</warning>');
                 $io->writeError('<warning>Please set `https_proxy` in addition to your existing proxy environment variables.</warning>');
                 $io->writeError('<warning>This fallback (and warning) will be removed in Composer 2.8.0.</warning>');
+                $io->writeError('<warning>https://getcomposer.org/doc/faqs/how-to-use-composer-behind-a-proxy.md</warning>');
             }
 
             return $result;
@@ -480,6 +482,11 @@ class Application extends BaseApplication
         }
         Silencer::restore();
 
+        if ($exception instanceof TransportException && str_contains($exception->getMessage(), 'Unable to use a proxy')) {
+            $io->writeError('<error>The following exception indicates your proxy is misconfigured</error>', true, IOInterface::QUIET);
+            $io->writeError('<error>Check https://getcomposer.org/doc/faqs/how-to-use-composer-behind-a-proxy.md for details</error>', true, IOInterface::QUIET);
+        }
+
         if (Platform::isWindows() && false !== strpos($exception->getMessage(), 'The system cannot find the path specified')) {
             $io->writeError('<error>The following exception may be caused by a stale entry in your cmd.exe AutoRun</error>', true, IOInterface::QUIET);
             $io->writeError('<error>Check https://getcomposer.org/doc/articles/troubleshooting.md#-the-system-cannot-find-the-path-specified-windows- for details</error>', true, IOInterface::QUIET);