diff --git a/osbase/freeport/BUILD.bazel b/osbase/freeport/BUILD.bazel
new file mode 100644
index 0000000..b0fcd25
--- /dev/null
+++ b/osbase/freeport/BUILD.bazel
@@ -0,0 +1,8 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "freeport",
+    srcs = ["freeport.go"],
+    importpath = "source.monogon.dev/osbase/freeport",
+    visibility = ["//visibility:public"],
+)
diff --git a/osbase/freeport/freeport.go b/osbase/freeport/freeport.go
new file mode 100644
index 0000000..da52311
--- /dev/null
+++ b/osbase/freeport/freeport.go
@@ -0,0 +1,54 @@
+// 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.
+
+package freeport
+
+import (
+	"io"
+	"net"
+)
+
+// AllocateTCPPort allocates a TCP port on the looopback address, and starts a
+// temporary listener on it. That listener is returned to the caller alongside with
+// the allocated port number. The listener must be closed right before the port is
+// used by the caller. This naturally still leaves a race condition window where
+// that port number might be snatched up by some other process, but there doesn't
+// seem to be a better way to do this.
+func AllocateTCPPort() (uint16, io.Closer, error) {
+	addr, err := net.ResolveTCPAddr("tcp", "127.0.0.1:0")
+	if err != nil {
+		return 0, nil, err
+	}
+
+	l, err := net.ListenTCP("tcp", addr)
+	if err != nil {
+		return 0, nil, err
+	}
+	return uint16(l.Addr().(*net.TCPAddr).Port), l, nil
+}
+
+// MustConsume takes the result of AllocateTCPPort, closes the listener and returns
+// the allocated port. If anything goes wrong (port could not be allocated or
+// closed) it will panic.
+func MustConsume(port uint16, lis io.Closer, err error) int {
+	if err != nil {
+		panic(err)
+	}
+	if err := lis.Close(); err != nil {
+		panic(err)
+	}
+	return int(port)
+}