diff --git a/wire/wire_io.c b/wire/wire_io.c
index 4e58b9da7..3c2e1b9ea 100644
--- a/wire/wire_io.c
+++ b/wire/wire_io.c
@@ -34,7 +34,7 @@ static int do_read_wire_header(int fd, struct io_plan_arg *arg)
 
 	/* Length bytes read?  Set up for normal read of data. */
 	if (arg->u2.s == INSIDE_HEADER_BIT + HEADER_LEN) {
-		arg->u2.s = *(wire_len_t *)p;
+		arg->u2.s = wirelen_to_cpu(*(wire_len_t *)p);
 		if (arg->u2.s >= INSIDE_HEADER_BIT) {
 			errno = E2BIG;
 			return -1;
@@ -88,7 +88,7 @@ static int do_write_wire_header(int fd, struct io_plan_arg *arg)
 {
 	ssize_t ret;
 	size_t len = arg->u2.s & ~INSIDE_HEADER_BIT;
-	wire_len_t hdr = tal_count(arg->u1.const_vp);
+	wire_len_t hdr = cpu_to_wirelen(tal_count(arg->u1.const_vp));
 
 	ret = write(fd, (char *)&hdr + len, HEADER_LEN - len);
 	if (ret <= 0)
diff --git a/wire/wire_io.h b/wire/wire_io.h
index b381ece5e..9bff19000 100644
--- a/wire/wire_io.h
+++ b/wire/wire_io.h
@@ -2,12 +2,15 @@
 #define LIGHTNING_WIRE_WIRE_IO_H
 #include "config.h"
 #include <ccan/io/io.h>
+#include <ccan/endian/endian.h>
 #include <ccan/short_types/short_types.h>
 
 /* We don't allow > 64M msgs: enough for 483 64k failure msgs. */
 #define WIRE_LEN_LIMIT (1 << 26)
 
-typedef u32 wire_len_t;
+typedef be32 wire_len_t;
+#define wirelen_to_cpu be32_to_cpu
+#define cpu_to_wirelen cpu_to_be32
 
 /* Read message into *data, allocating off ctx. */
 struct io_plan *io_read_wire_(struct io_conn *conn,
diff --git a/wire/wire_sync.c b/wire/wire_sync.c
index 0dc65af7b..a04b98be5 100644
--- a/wire/wire_sync.c
+++ b/wire/wire_sync.c
@@ -7,12 +7,12 @@
 
 bool wire_sync_write(int fd, const void *msg TAKES)
 {
-	wire_len_t len = tal_len(msg);
+	wire_len_t hdr = cpu_to_wirelen(tal_len(msg));
 	bool ret;
 
 	assert(tal_len(msg) < WIRE_LEN_LIMIT);
-	ret = write_all(fd, &len, sizeof(len))
-		&& write_all(fd, msg, len);
+	ret = write_all(fd, &hdr, sizeof(hdr))
+		&& write_all(fd, msg, tal_count(msg));
 
 	if (taken(msg))
 		tal_free(msg);
@@ -26,12 +26,12 @@ u8 *wire_sync_read(const tal_t *ctx, int fd)
 
 	if (!read_all(fd, &len, sizeof(len)))
 		return NULL;
-	if (len >= WIRE_LEN_LIMIT) {
+	if (wirelen_to_cpu(len) >= WIRE_LEN_LIMIT) {
 		errno = E2BIG;
 		return NULL;
 	}
-	msg = tal_arr(ctx, u8, len);
-	if (!read_all(fd, msg, len))
+	msg = tal_arr(ctx, u8, wirelen_to_cpu(len));
+	if (!read_all(fd, msg, wirelen_to_cpu(len)))
 		return tal_free(msg);
 	return msg;
 }