diff --git a/metropolis/pkg/logtree/proto/logtree.proto b/metropolis/pkg/logtree/proto/logtree.proto
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+++ b/metropolis/pkg/logtree/proto/logtree.proto
@@ -0,0 +1,59 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+package metropolis.pkg.logtree.proto;
+option go_package = "source.monogon.dev/metropolis/pkg/logtree/proto";
+
+import "google/protobuf/timestamp.proto";
+
+// Severity level corresponding to //metropolis/pkg/logtree.Severity.
+enum LeveledLogSeverity {
+  INVALID = 0;
+  INFO = 1;
+  WARNING = 2;
+  ERROR = 3;
+  FATAL = 4;
+}
+
+// LogEntry corresponding to logtree.LogEntry in //metropolis/pkg/logtree.
+message LogEntry {
+  // A leveled log entry emitted from a compatible system, eg. Metorpolis code
+  // or a klog-parsed line.
+  message Leveled {
+    repeated string lines = 1;
+    google.protobuf.Timestamp timestamp = 2;
+    LeveledLogSeverity severity = 3;
+    // Source of the error, expressed as file:line.
+    string location = 4;
+  }
+  // Raw log entry, captured from an external system without parting. Might
+  // contain some timestamp/level/origin information embedded in data. Data
+  // contained within should be treated as unsanitized external data.
+  message Raw {
+    string data = 1;
+    // Original length of line, set if data was truncated.
+    int64 original_length = 2;
+  }
+
+  // Origin DN (Distinguished Name), a unique identifier which is provided by
+  // the supervisor system.
+  string dn = 1;
+  oneof kind {
+    Leveled leveled = 2;
+    Raw raw = 3;
+  }
+}