Store the generated host key on the disk

Instead of generating one on every connection. This avoids having to
either remove the host key again after every test or having to tell the
ssh client to not store host fingerprints.

Author: bjorn3

.gitignore

@@ -1 +1,2 @@
 /target
+ssh_host_ed25519_key

src/main.rs

@@ -1,5 +1,9 @@
 use core::net::{Ipv4Addr, SocketAddr};
-use std::sync::Arc;
+use std::{
+    fs::{self, File},
+    io::{self, Write},
+    sync::Arc,
+};
 
 use aws_lc_rs::signature::Ed25519KeyPair;
 use clap::Parser;
@@ -16,6 +20,25 @@ async fn main() -> anyhow::Result<()> {
 
     let args = Args::parse();
 
+    let host_key = Arc::new(if args.generate_host_key {
+        match File::create_new(&args.host_key_file) {
+            Ok(mut host_key_file) => {
+                let Ok(host_key) = Ed25519KeyPair::generate() else {
+                    anyhow::bail!("failed to generate host key");
+                };
+                // FIXME ensure the host key is only readable by the ssh server user
+                host_key_file.write_all(host_key.to_pkcs8v1()?.as_ref())?;
+                host_key
+            }
+            Err(err) if err.kind() == io::ErrorKind::AlreadyExists => {
+                anyhow::bail!("host key file `{}` already exists", &args.host_key_file);
+            }
+            Err(err) => return Err(err.into()),
+        }
+    } else {
+        Ed25519KeyPair::from_pkcs8(&fs::read(args.host_key_file)?)?
+    });
+
     let listener = match (ListenFd::from_env().take_tcp_listener(0)?, args.port) {
         (Some(listener), None) => {
             listener.set_nonblocking(true)?;
@@ -30,11 +53,6 @@ async fn main() -> anyhow::Result<()> {
     };
     info!(addr = %listener.local_addr()?, "listening for connections");
 
-    let Ok(host_key) = Ed25519KeyPair::generate() else {
-        anyhow::bail!("failed to generate host key");
-    };
-    let host_key = Arc::new(host_key);
-
     loop {
         match listener.accept().await {
             Ok((stream, addr)) => {
@@ -54,4 +72,8 @@ async fn main() -> anyhow::Result<()> {
 struct Args {
     #[clap(short, long)]
     port: Option<u16>,
+    #[clap(long, default_value = "ssh_host_ed25519_key")]
+    host_key_file: String,
+    #[clap(long)]
+    generate_host_key: bool,
 }