Document HTTP PROXY for dist-source rust-changelog

Author: foresterre

book/src/SUMMARY.md

@@ -5,6 +5,7 @@
     - [Installation](./getting-started/installation.md)
     - [Quick Start](./getting-started/quick-start.md)
     - [Cargo Workspace](./getting-started/cargo-workspace.md)
+    - [HTTP PROXY](./getting-started/rust-releases-proxy.md)
 - [Releases](releases/index.md)
     - [v0.15 to v0.16](releases/v0.15_v0.16_highlights.md)
     - [v0.15 to v0.16 JSON](releases/v0.15_v0.16_json.md)

book/src/getting-started/index.md

@@ -2,4 +2,5 @@
 
 * [Installation](installation.md)
 * [Quick Start](quick-start.md)
-* [Cargo Workspace](cargo-workspace.md)
+* [Cargo Workspace](cargo-workspace.md)
+* [HTTP PROXY](rust-releases-proxy.md)

book/src/getting-started/rust-releases-proxy.md

@@ -0,0 +1,51 @@
+### Rust Releases: HTTP PROXY
+
+`cargo-msrv` depends on the [rust-releases](https://github.com/foresterre/rust-releases/) crate to determine which Rust versions exist. This is a necessary evil for
+ the `cargo msrv find` and `cargo msrv verify` subcommands.
+ 
+To fetch an index of known Rust releases, it accesses the network. By default, the Rust GitHub repository is used to determine
+which stable releases and toolchains are available. As an alternative, this data can also be fetched from the Rust AWS S3
+distribution bucket.
+
+The source can be set with the `--release-source <source>` flag. The possible values are respectively `rust-changelog` and `rust-dist`,
+for the Rust GitHub repository and the Rust AWS S3 distribution bucket. For example: `cargo msrv find --release-source rust-changelog`.
+
+
+
+#### Release source: `rust-changelog`
+
+[rust-releases](https://github.com/foresterre/rust-releases/) uses [ureq](https://crates.io/crates/ureq) as HTTP client
+for the `rust-changelog` source. From `cargo-msrv 0.17.1` (and [rust-releases 0.29.0](https://github.com/foresterre/rust-releases/releases/tag/v0.29.0)
+respectively), `ureq` has been configured to support configuring a network proxy from the environment.
+
+The environment variable, `ureq` uses [are](https://docs.rs/ureq/2.11.0/src/ureq/proxy.rs.html#87-92):
+ 
+- `ALL_PROXY` or `all_proxy` or,
+- `HTTPS_PROXY` or `https_proxy` or,
+- `HTTP_PROXY` or `http_proxy`
+
+The environment variable can be configured as follows:
+
+`<protocol>://<user>:<password>@<host>:port`, where all parts except host are optional.
+
+The `<protocol>` must be one of: `http` (`socks4`, `socks4a` and `socks5` are currently not enabled). The default is `http`.
+
+The default `<port>` is 80 when the `<protocol>` is `http` .
+
+Examples: 
+- `localhost`
+- `http://127.0.0.1:8080`
+
+
+#### Release source: `rust-dist`
+
+TODO: Not configured specifically by cargo-msrv, but could be the case.
+
+The following crates are used for the `rust-dist` source:
+
+- [aws-config](https://crates.io/crates/aws-config)
+- [aws-sdk-s3](https://crates.io/crates/aws-sdk-s3)
+
+Probably also relevant as transitive dependencies are:
+
+- [aws-smithy-http](https://crates.io/crates/aws-smithy-http)