Add methods to Config to use a custom session server (#72)

* Add methods to Config to use a custom session server

Added new methods to the Config trait, that enables
implementors to use a custom session server host
or even a fully custom URL.

* Fix error in documentation

* Make format_session_server_url return a String

src/config.rs

@@ -1,7 +1,7 @@
 //! Configuration for the server.
 
 use std::borrow::Cow;
-use std::net::{Ipv4Addr, SocketAddr, SocketAddrV4};
+use std::net::{IpAddr, Ipv4Addr, SocketAddr, SocketAddrV4};
 use std::panic::{RefUnwindSafe, UnwindSafe};
 
 use async_trait::async_trait;
@@ -208,6 +208,26 @@ pub trait Config: Sized + Send + Sync + UnwindSafe + RefUnwindSafe + 'static {
         Ok(())
     }
 
+    /// Called upon (every) client connect (if online mode is enabled) to obtain
+    /// the full URL to use for session server requests. Defaults to
+    /// `https://sessionserver.mojang.com/session/minecraft/hasJoined?username=<username>&serverId=<auth-digest>&ip=<player-ip>`.
+    ///
+    /// It is assumed, that upon successful request, a structure matching the
+    /// description in the [wiki](https://wiki.vg/Protocol_Encryption#Server) was obtained.
+    /// Providing a URL that does not return such a structure will result in a
+    /// disconnect for every client that connects.
+    ///
+    /// The arguments are described in the linked wiki article.
+    fn format_session_server_url(
+        &self,
+        server: &SharedServer<Self>,
+        username: &str,
+        auth_digest: &str,
+        player_ip: &IpAddr,
+    ) -> String {
+        format!("https://sessionserver.mojang.com/session/minecraft/hasJoined?username={username}&serverId={auth_digest}&ip={player_ip}")
+    }
+
     /// Called after the server is created, but prior to accepting connections
     /// and entering the update loop.
     ///

src/server.rs

@@ -704,9 +704,15 @@ async fn handle_login<C: Config>(
             .chain(&server.0.public_key_der)
             .finalize();
 
-        let hex_hash = weird_hex_encoding(&hash);
+        let hex_hash = auth_digest(&hash);
 
-        let url = format!("https://sessionserver.mojang.com/session/minecraft/hasJoined?username={username}&serverId={hex_hash}&ip={}", remote_addr.ip());
+        let url = C::format_session_server_url(
+            server.config(),
+            server,
+            &username,
+            &hex_hash,
+            &remote_addr.ip(),
+        );
         let resp = server.0.http_client.get(url).send().await?;
 
         let status = resp.status();
@@ -823,7 +829,7 @@ async fn handle_play<C: Config>(
     Ok(())
 }
 
-fn weird_hex_encoding(bytes: &[u8]) -> String {
+fn auth_digest(bytes: &[u8]) -> String {
     BigInt::from_signed_bytes_be(bytes).to_str_radix(16)
 }
 
@@ -832,17 +838,17 @@ mod tests {
     use super::*;
 
     #[test]
-    fn weird_hex_encoding_correct() {
+    fn auth_digest_correct() {
         assert_eq!(
-            weird_hex_encoding(&Sha1::digest("Notch")),
+            auth_digest(&Sha1::digest("Notch")),
             "4ed1f46bbe04bc756bcb17c0c7ce3e4632f06a48"
         );
         assert_eq!(
-            weird_hex_encoding(&Sha1::digest("jeb_")),
+            auth_digest(&Sha1::digest("jeb_")),
             "-7c9d5b0044c130109a5d7b5fb5c317c02b4e28c1"
         );
         assert_eq!(
-            weird_hex_encoding(&Sha1::digest("simon")),
+            auth_digest(&Sha1::digest("simon")),
             "88e16a1019277b15d58faf0541e11910eb756f6"
         );
     }