server: Allow setting an alternative substituter endpoint

server/src/config.rs

@@ -58,6 +58,15 @@ pub struct Config {
     #[serde(rename = "api-endpoint")]
     pub api_endpoint: Option<String>,
 
+    /// The canonical Nix Binary Cache endpoint of this server.
+    ///
+    /// This is usually the same as `api_endpoint` but can be configured
+    /// to a different value.
+    ///
+    /// If unconfigured, it's assumed to be the same as `api_endpoint`.
+    #[serde(rename = "substituter-endpoint")]
+    pub substituter_endpoint: Option<String>,
+
     /// Whether to soft-delete caches.
     ///
     /// If this is enabled, caches are soft-deleted instead of actually

server/src/lib.rs

@@ -75,6 +75,9 @@ struct RequestStateInner {
     /// The canonical API endpoint.
     api_endpoint: Option<String>,
 
+    /// The canonical substituter endpoint.
+    substituter_endpoint: Option<String>,
+
     /// The potentially-invalid Host header supplied by the client.
     host: String,
 
@@ -172,7 +175,11 @@ impl RequestStateInner {
     /// The binary cache endpoint may live on another host than
     /// the canonical API endpoint.
     fn substituter_endpoint(&self, cache: CacheName) -> ServerResult<String> {
-        Ok(format!("{}{}", self.api_endpoint()?, cache.as_str()))
+        if let Some(substituter_endpoint) = &self.substituter_endpoint {
+            Ok(format!("{}{}", substituter_endpoint, cache.as_str()))
+        } else {
+            Ok(format!("{}{}", self.api_endpoint()?, cache.as_str()))
+        }
     }
 
     /// Indicates whether the cache the client is interacting with is public.

server/src/middleware.rs

@@ -31,6 +31,7 @@ pub async fn init_request_state<B>(
     let req_state = Arc::new(RequestStateInner {
         auth: AuthState::new(),
         api_endpoint: state.config.api_endpoint.to_owned(),
+        substituter_endpoint: state.config.substituter_endpoint.to_owned(),
         host,
         client_claims_https,
         public_cache: AtomicBool::new(false),