diff --git a/games/factorio/modproxy/proto/modproxy.proto b/games/factorio/modproxy/proto/modproxy.proto
new file mode 100644
index 0000000..a4121b2
--- /dev/null
+++ b/games/factorio/modproxy/proto/modproxy.proto
@@ -0,0 +1,43 @@
+syntax = "proto3";
+package modproxy;
+option go_package = "code.hackerspace.pl/hscloud/games/factorio/modproxy/proto";
+
+service ModProxy {
+    rpc Mirror(MirrorRequest) returns (MirrorResponse);
+    rpc Download(DownloadRequest) returns (stream DownloadResponse);
+}
+
+message MirrorRequest {
+    // Authentication details to access the mod portal.
+    string username = 1;
+    string token = 2;
+}
+
+message MirrorResponse {
+    repeated string mods_okay = 1;
+    map<string, string> mods_errors = 2;
+}
+
+message DownloadRequest {
+    string mod_name = 1;
+    string file_sha1 = 2;
+}
+
+message DownloadResponse {
+    enum Status {
+        STATUS_INVALID = 0;
+        STATUS_OKAY = 1;
+        STATUS_NOT_AVAILABLE = 2;
+    };
+    Status status = 1;
+    bytes chunk = 2;
+}
+
+// Configuration for client (in text proto, so singular names in repeated fields)
+message ClientConfig {
+    message Mod {
+        string name = 1;
+        string version = 2;
+    }
+    repeated Mod mod = 1;
+}