Add spec for proxy support (microsoft#4152)

Adding spec for microsoft#190

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doc/specs/#190 - Proxy Support.md

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+---
+author: Flor Chacon @florelis
+created on: 2024-02-07
+last updated: 2024-02-07
+issue id: 190
+---
+
+# Proxy support
+
+For [#190](https://github.com/microsoft/winget-cli/issues/190)
+
+## Abstract
+
+This spec describes a feature to specify a proxy for winget to use when connecting to the internet.
+
+## Solution Design
+
+A new command line argument will be added to specify a proxy to use during a particular invocation of winget.
+This functionality will first need to be enabled through an admin setting, similar to local manifests or hash override.
+
+An option to set a default proxy to use on every flow will be added, but it will require administrator permissions to be set.
+
+New Group Policy will also be added for IT admins to control the use of proxies.
+The policies will be similar to those we already have for sources, so that a specific proxy can be required or only a predefined set of proxies can be allowed.
+
+Proxies will not be used for the configuration features for now.
+Proxy settings in winget will not affect the behavior of installers themselves, so an installation may still generate traffic outside of the proxy.
+
+The proxy will be used to download the installers and access the sources.
+
+Since the APIs used for Delivery Optimization and MSIX deployment do not provide a way to specify a custom proxy, if a proxy is specified, we will change the use of those APIs to accomodate the proxy.
+For Delivery Optimization, a proxy will force use of WinINet instead.
+For MSIX deployment, the packages will be fully downloaded before deploying; this will require more network traffic than a streaming install of only the required bits.
+
+## UI/UX Design
+
+We will add a command line argument taking the URI to the proxy.
+A separate argument will be available to disable the use of proxy if there is a default set.
+Both of these arguments will be disabled by default and require admin privileges to enable.
+
+```
+> winget settings --enable ProxyCommandLineArgument
+> winget install Contoso.App --proxy https://127.0.0.1:2345
+> winget install Contoso.App --no-proxy
+```
+
+To configure the default proxy, new `set` and `reset` subcommands will be added to the `settings` command.
+This will require admin privileges and does not require `ProxyCommandLineArgument` to be enabled.
+
+```
+> winget settings set DefaultProxy https://127.0.0.1:2345
+> winget settings reset DefaultProxy
+```
+
+The current default proxy will be added to the output `winget --info`.
+
+## Capabilities
+
+### Accessibility
+
+This should have no direct impact on accessibility.
+
+### Security
+
+There is a possibility of an attacker using a malicious proxy to tamper with the data received from the source, or with the contents of the installer file.
+This is not much different from the risks of using a public network.
+The following mitigating factors will be in place:
+* (New) The ability to set a default proxy will be restricted to administrators, to prevent attackers from adding a proxy without the user realizing.
+* (New) A Group Policy will be available to block the use of proxies, require the use of a specific proxy, or limit them to an approved list.
+* Pre-indexed sources need to be signed, and the publisher is required to match during source update.
+  When initially adding the source, administrator privileges are already required to limit misuse.
+* Pre-indexed sources include manifest hashes in the local database, to ensure that the manifest downloaded later is as expected.
+* For the Microsoft Store source, we use certificate pinning to ensure we are talking to the right server.
+* When communicating with REST sources, the certificate used by the source for HTTPS needs to match the domain.
+* Manifests include a hash of the installer that is validated before executing it.
+  The ability to ignore installer hash mismatches is disabled by default, and enabling it requires administrator privileges.
+
+### Compatibility
+
+No breaking changes to existing behavior.
+
+### Performance, Power, and Efficiency
+
+There should not be any notable performance changes.
+
+## Potential Issues
+
+A faulty or misconfigured proxy could impact most of winget's functionality, but it can be worked around by disabling the use of the proxy.
+
+## Future considerations
+
+Things we may want to consider in the future:
+* Extend support for proxies to the Configuration feature
+* Add proxy support to the COM API
+* Add support for proxies that require authentication
+* Add the ability for admins to set multiple allowed proxies that a user can use
+* Add the ability to specify a different default proxy for each source
+* Use proxies with Delivery Optimization. This requires changes to the Delivery Optimization APIs.