Implement prevent-proxy-connections

This adds the `prevent-proxy-connections` server property and implements it into valence.

Note that if the server is in offline mode, this changes nothing.

By default, proxy connections are allowed.

Fixes #95

src/config.rs

@@ -95,6 +95,20 @@ pub trait Config: Sized + Send + Sync + UnwindSafe + RefUnwindSafe + 'static {
         true
     }
 
+    /// Called once at startup to get the "prevent-proxy-connections" option,
+    /// which determines if client IP validation should take place.
+    ///
+    /// When prevent_proxy_connections is enabled, clients can no longer log-in
+    /// if they connected to the yggdrasil server using a different IP.
+    ///
+    /// # Default Implementation
+    /// Proxy connections are allowed.
+    ///
+    /// Returns `false`.
+    fn prevent_proxy_connections(&self) -> bool {
+        false
+    }
+
     /// Called once at startup to get the capacity of the buffer used to
     /// hold incoming packets.
     ///
@@ -225,7 +239,11 @@ pub trait Config: Sized + Send + Sync + UnwindSafe + RefUnwindSafe + 'static {
         auth_digest: &str,
         player_ip: &IpAddr,
     ) -> String {
+        if self.prevent_proxy_connections() {
             format!("https://sessionserver.mojang.com/session/minecraft/hasJoined?username={username}&serverId={auth_digest}&ip={player_ip}")
+        } else {
+            format!("https://sessionserver.mojang.com/session/minecraft/hasJoined?username={username}&serverId={auth_digest}")
+        }
     }
 
     /// Called after the server is created, but prior to accepting connections

src/server.rs

@@ -46,6 +46,7 @@ use crate::protocol::packets::s2c::play::S2cPlayPacket;
 use crate::protocol::packets::s2c::status::{PingResponse, StatusResponse};
 use crate::protocol::packets::Property;
 use crate::protocol::{BoundedArray, BoundedString, VarInt};
+use crate::text::Text;
 use crate::util::valid_username;
 use crate::world::Worlds;
 use crate::{ident, Ticks, PROTOCOL_VERSION, VERSION_NAME};
@@ -716,11 +717,17 @@ async fn handle_login<C: Config>(
         );
         let resp = server.0.http_client.get(url).send().await?;
 
-        let status = resp.status();
-        ensure!(
-            status.is_success(),
-            "session server GET request failed: {status}"
-        );
+        match resp.status().as_u16() {
+            200 => (),
+            204 => {
+                let reason = Text::translate("multiplayer.disconnect.unverified_username");
+                c.enc.write_packet(&DisconnectLogin { reason }).await?;
+                bail!("Could not verify username");
+            }
+            status => {
+                bail!("session server GET request failed: {status}");
+            }
+        }
 
         let data: AuthResponse = resp.json().await?;