feat(switch/brocade): Implement client to interact with Brocade Switch

* Expose a high-level `brocade::init()` function to connect to a Brocade switch and automatically pick the best implementation based on its OS and version
* Implement a client for Brocade switches running on Network Operating System (NOS)
* Implement a client for older Brocade switches running on FastIron (partial implementation)

The architecture for the library is based on 3 layers:
1. The `BrocadeClient` trait to describe the available capabilities to
   interact with a Brocade switch. It is partly opinionated in order to
   offer higher level features to group multiple commands into a single
   function (e.g. create a port channel). Its implementations are
   basically just the commands to run on the switch and the functions to
   parse the output.
2. The `BrocadeShell` struct to make it easier to authenticate, send commands, and interact with the switch.
3. The `ssh` module to actually connect to the switch over SSH and execute the commands.

With time, we will add support for more Brocade switches and their various OS/versions. If needed, shared behavior could be extracted into a separate module to make it easier to add new implementations.

brocade/src/ssh.rs

@@ -0,0 +1,113 @@
+use std::borrow::Cow;
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use russh::client::Handler;
+use russh::kex::DH_G1_SHA1;
+use russh::kex::ECDH_SHA2_NISTP256;
+use russh_keys::key::SSH_RSA;
+
+use super::BrocadeOptions;
+use super::Error;
+
+#[derive(Default, Clone, Debug)]
+pub struct SshOptions {
+    pub preferred_algorithms: russh::Preferred,
+}
+
+impl SshOptions {
+    fn ecdhsa_sha2_nistp256() -> Self {
+        Self {
+            preferred_algorithms: russh::Preferred {
+                kex: Cow::Borrowed(&[ECDH_SHA2_NISTP256]),
+                key: Cow::Borrowed(&[SSH_RSA]),
+                ..Default::default()
+            },
+        }
+    }
+
+    fn legacy() -> Self {
+        Self {
+            preferred_algorithms: russh::Preferred {
+                kex: Cow::Borrowed(&[DH_G1_SHA1]),
+                key: Cow::Borrowed(&[SSH_RSA]),
+                ..Default::default()
+            },
+        }
+    }
+}
+
+pub struct Client;
+
+#[async_trait]
+impl Handler for Client {
+    type Error = Error;
+
+    async fn check_server_key(
+        &mut self,
+        _server_public_key: &russh_keys::key::PublicKey,
+    ) -> Result<bool, Self::Error> {
+        Ok(true)
+    }
+}
+
+pub async fn try_init_client(
+    username: &str,
+    password: &str,
+    ip: &std::net::IpAddr,
+    base_options: BrocadeOptions,
+) -> Result<BrocadeOptions, Error> {
+    let ssh_options = vec![
+        SshOptions::default(),
+        SshOptions::ecdhsa_sha2_nistp256(),
+        SshOptions::legacy(),
+    ];
+
+    for ssh in ssh_options {
+        let opts = BrocadeOptions {
+            ssh,
+            ..base_options.clone()
+        };
+        let client = create_client(*ip, 22, username, password, &opts).await;
+
+        match client {
+            Ok(_) => {
+                return Ok(opts);
+            }
+            Err(e) => match e {
+                Error::NetworkError(e) => {
+                    if e.contains("No common key exchange algorithm") {
+                        continue;
+                    } else {
+                        return Err(Error::NetworkError(e));
+                    }
+                }
+                _ => return Err(e),
+            },
+        }
+    }
+
+    Err(Error::NetworkError(
+        "Could not establish ssh connection: wrong key exchange algorithm)".to_string(),
+    ))
+}
+
+pub async fn create_client(
+    ip: std::net::IpAddr,
+    port: u16,
+    username: &str,
+    password: &str,
+    options: &BrocadeOptions,
+) -> Result<russh::client::Handle<Client>, Error> {
+    let config = russh::client::Config {
+        preferred: options.ssh.preferred_algorithms.clone(),
+        ..Default::default()
+    };
+    let mut client = russh::client::connect(Arc::new(config), (ip, port), Client {}).await?;
+    if !client.authenticate_password(username, password).await? {
+        return Err(Error::AuthenticationError(
+            "ssh authentication failed".to_string(),
+        ));
+    }
+    Ok(client)
+}