diff --git a/conn/bind_std.go b/conn/bind_std.go
index cc5cf23..428e528 100644
--- a/conn/bind_std.go
+++ b/conn/bind_std.go
@@ -136,6 +136,11 @@ func listenNet(network string, port int) (*net.UDPConn, int, error) {
 	return conn.(*net.UDPConn), uaddr.Port, nil
 }
 
+// errEADDRINUSE is syscall.EADDRINUSE, boxed into an interface once
+// in erraddrinuse.go on almost all platforms. For other platforms,
+// it's at least non-nil.
+var errEADDRINUSE error = errors.New("")
+
 func (s *StdNetBind) Open(uport uint16) ([]ReceiveFunc, uint16, error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
@@ -162,7 +167,7 @@ again:
 
 	// Listen on the same port as we're using for ipv4.
 	v6conn, port, err = listenNet("udp6", port)
-	if uport == 0 && errors.Is(err, syscall.EADDRINUSE) && tries < 100 {
+	if uport == 0 && errors.Is(err, errEADDRINUSE) && tries < 100 {
 		v4conn.Close()
 		tries++
 		goto again
diff --git a/conn/controlfns_unix.go b/conn/controlfns_unix.go
index 91692c0..5cc4d98 100644
--- a/conn/controlfns_unix.go
+++ b/conn/controlfns_unix.go
@@ -1,4 +1,4 @@
-//go:build !windows && !linux && !wasm
+//go:build !windows && !linux && !wasm && !plan9
 
 /* SPDX-License-Identifier: MIT
  *
diff --git a/conn/erraddrinuse.go b/conn/erraddrinuse.go
new file mode 100644
index 0000000..751660e
--- /dev/null
+++ b/conn/erraddrinuse.go
@@ -0,0 +1,14 @@
+//go:build !plan9
+
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2017-2023 WireGuard LLC. All Rights Reserved.
+ */
+
+package conn
+
+import "syscall"
+
+func init() {
+	errEADDRINUSE = syscall.EADDRINUSE
+}
diff --git a/ipc/uapi_wasm.go b/ipc/uapi_fake.go
similarity index 72%
rename from ipc/uapi_wasm.go
rename to ipc/uapi_fake.go
index fa84684..e68863d 100644
--- a/ipc/uapi_wasm.go
+++ b/ipc/uapi_fake.go
@@ -1,3 +1,5 @@
+//go:build wasm || plan9
+
 /* SPDX-License-Identifier: MIT
  *
  * Copyright (C) 2017-2023 WireGuard LLC. All Rights Reserved.
@@ -5,7 +7,7 @@
 
 package ipc
 
-// Made up sentinel error codes for {js,wasip1}/wasm.
+// Made up sentinel error codes for {js,wasip1}/wasm, and plan9.
 const (
 	IpcErrorIO        = 1
 	IpcErrorInvalid   = 2
diff --git a/rwcancel/rwcancel.go b/rwcancel/rwcancel.go
index e397c0e..dd649d4 100644
--- a/rwcancel/rwcancel.go
+++ b/rwcancel/rwcancel.go
@@ -1,4 +1,4 @@
-//go:build !windows && !wasm
+//go:build !windows && !wasm && !plan9
 
 /* SPDX-License-Identifier: MIT
  *
diff --git a/rwcancel/rwcancel_stub.go b/rwcancel/rwcancel_stub.go
index 2a98b2b..4623801 100644
--- a/rwcancel/rwcancel_stub.go
+++ b/rwcancel/rwcancel_stub.go
@@ -1,4 +1,4 @@
-//go:build windows || wasm
+//go:build windows || wasm || plan9
 
 // SPDX-License-Identifier: MIT