diff --git a/tools/testing/selftests/net/mctp/Makefile b/tools/testing/selftests/net/mctp/Makefile
new file mode 100644
index 000000000000..baee43d5b048
--- /dev/null
+++ b/tools/testing/selftests/net/mctp/Makefile
@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: GPL-2.0
+
+CFLAGS =  -Wall -Wl,--no-as-needed -O2 -g
+CFLAGS += -I../../../../../usr/include/
+
+TEST_PROGS := mctp-tun
+include ../../lib.mk
+
+mctp-tun: LDLIBS += -lpthread
+
+all: $(TEST_PROGS)
diff --git a/tools/testing/selftests/net/mctp/config b/tools/testing/selftests/net/mctp/config
new file mode 100644
index 000000000000..92c0b8c79ac7
--- /dev/null
+++ b/tools/testing/selftests/net/mctp/config
@@ -0,0 +1,2 @@
+CONFIG_TUN=y
+CONFIG_MCTP=y
diff --git a/tools/testing/selftests/net/mctp/mctp-defs.h b/tools/testing/selftests/net/mctp/mctp-defs.h
new file mode 100644
index 000000000000..fcb1f5250ca9
--- /dev/null
+++ b/tools/testing/selftests/net/mctp/mctp-defs.h
@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Protocol numbers are not exposed through uapi */
+#ifndef AF_MCTP
+#define AF_MCTP 45
+#endif
diff --git a/tools/testing/selftests/net/mctp/mctp-tun.c b/tools/testing/selftests/net/mctp/mctp-tun.c
new file mode 100644
index 000000000000..ce289b10d23f
--- /dev/null
+++ b/tools/testing/selftests/net/mctp/mctp-tun.c
@@ -0,0 +1,422 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2021,2022 Code Construct
+ */
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/ioctl.h>
+#include <sys/random.h>
+#include <linux/if.h>
+#include <linux/if_tun.h>
+#include <linux/mctp.h>
+#include <pthread.h>
+#include <fcntl.h>
+#include <poll.h>
+#include <assert.h>
+
+#include "mctp-defs.h"
+
+#include "../../kselftest_harness.h"
+
+static const int BUF_SIZE = 70000;
+
+static void *tun_start(void *arg);
+static int create_tun(struct __test_metadata *_metadata,
+	const char *name);
+
+FIXTURE(TUNPAIR) {
+	pthread_t pth;
+	int fd[2];
+	volatile bool running;
+	bool skip;
+};
+
+const size_t default_mtu = 70;
+const size_t default_msglen = 4;
+
+FIXTURE_VARIANT(TUNPAIR) {
+	/* some defaults above */
+	size_t msglen;
+	size_t mtu;
+	bool listen_addr_any;
+	bool listen_net_any;
+	bool expire_key;
+	bool duplicate_reply;
+};
+
+FIXTURE_VARIANT_ADD(TUNPAIR, len4) {
+	.listen_addr_any = true,
+	.listen_net_any = false,
+};
+
+FIXTURE_VARIANT_ADD(TUNPAIR, len300) {
+	.msglen = 300,
+	.mtu = 500,
+	.listen_addr_any = true,
+};
+
+/* With fragmentation */
+FIXTURE_VARIANT_ADD(TUNPAIR, len300frag) {
+	.msglen = 300,
+	.listen_addr_any = true,
+};
+
+FIXTURE_VARIANT_ADD(TUNPAIR, net_any) {
+	.listen_addr_any = true,
+	.listen_net_any = true,
+};
+
+FIXTURE_VARIANT_ADD(TUNPAIR, addr_specific) {
+	.listen_net_any = true,
+};
+
+FIXTURE_VARIANT_ADD(TUNPAIR, expire_key) {
+	.expire_key = true,
+};
+
+FIXTURE_VARIANT_ADD(TUNPAIR, duplicate_reply) {
+	.duplicate_reply = true,
+};
+
+FIXTURE_SETUP(TUNPAIR)
+{
+	int rc;
+	char str[1000];
+	int mtu;
+
+	memset(self, 0, sizeof(*self));
+
+	/* Check requirements first */
+	/* TODO: cap_get() instead? */
+	if (getuid() != 0) {
+		self->skip = 1;
+		TH_LOG("Not root, skipping");
+		return;
+	}
+
+	rc = system("sh -c \"which mctp 2> /dev/null > /dev/null\"");
+	if (rc != 0) {
+		self->skip = 1;
+		TH_LOG("No mctp utility installed, skipping");
+		return;
+	}
+
+	rc = socket(AF_MCTP, SOCK_DGRAM, 0);
+	if (rc == -1 && errno == EAFNOSUPPORT) {
+		self->skip = 1;
+		TH_LOG("No AF_MCTP support, skipping");
+		return;
+	}
+	close(rc);
+
+	/* Pair of netdevs */
+	self->fd[0] = create_tun(_metadata, "mctptun1");
+	self->fd[1] = create_tun(_metadata, "mctptun2");
+
+	mtu = variant->mtu ?: default_mtu;
+
+	/* Forward packets between devices */
+	self->running = true;
+	rc = pthread_create(&self->pth, NULL, tun_start, self);
+	ASSERT_EQ(rc, 0);
+
+	/* Set up devices. All in one command to avoid
+	 * slow shell invocations
+	 */
+	rc = snprintf(str, sizeof(str),
+		/* net needs to be set before routes */
+		"mctp link set mctptun1 up net 31 mtu %d && "
+		"mctp link set mctptun2 up net 32 mtu %d && "
+		"mctp addr add 131 dev mctptun1 && "
+		"mctp addr add 132 dev mctptun2 && "
+		"mctp route add 132 via mctptun1 && "
+		"mctp route add 131 via mctptun2 && "
+		"true",
+		mtu, mtu);
+	ASSERT_LT(rc, sizeof(str));
+	rc = system(str);
+	ASSERT_EQ(rc, 0);
+}
+
+FIXTURE_TEARDOWN(TUNPAIR)
+{
+	int rc;
+
+	if (self->skip)
+		return;
+
+	self->running = false;
+	rc = pthread_join(self->pth, NULL);
+	ASSERT_EQ(rc, 0);
+	ASSERT_EQ(close(self->fd[0]), 0);
+	ASSERT_EQ(close(self->fd[1]), 0);
+
+	/* mctp addr/routes go away with devices */
+}
+
+/* Creates a socket, binds.
+ * Always returns success, asserts on failure
+ */
+static int mctp_bind(struct __test_metadata *_metadata,
+	int net, int eid, int type)
+{
+	struct sockaddr_mctp addr = {0};
+	int sd, rc;
+
+	sd = socket(AF_MCTP, SOCK_DGRAM, 0);
+	ASSERT_GE(sd, 0);
+	addr.smctp_family = AF_MCTP;
+	addr.smctp_network = net;
+	addr.smctp_addr.s_addr = eid;
+	addr.smctp_type = type;
+	addr.smctp_tag = MCTP_TAG_OWNER;
+	rc = bind(sd, (struct sockaddr *)&addr, sizeof(addr));
+	ASSERT_EQ(rc, 0);
+	return sd;
+}
+
+/* Creates a socket, sends on it.
+ * Always returns success, asserts on failure.
+ * Returns the socket sd.
+ */
+static int mctp_send(struct __test_metadata *_metadata,
+	int net, int eid, int type, void *buf, size_t len)
+{
+	struct sockaddr_mctp addr = {0};
+	ssize_t sent;
+	int sd;
+
+	addr.smctp_family = AF_MCTP;
+	addr.smctp_network = net;
+	addr.smctp_addr.s_addr = eid;
+	addr.smctp_type = type;
+	addr.smctp_tag = MCTP_TAG_OWNER;
+	sd = socket(AF_MCTP, SOCK_DGRAM, 0);
+	ASSERT_GE(sd, 0);
+	sent = sendto(sd, buf, len, 0,
+			(struct sockaddr *)&addr, sizeof(addr));
+	ASSERT_EQ(sent, len);
+	return sd;
+}
+
+/* Helper to test key expiry */
+static void test_expire_key(struct __test_metadata *_metadata,
+	int sock_listen, int sd, struct sockaddr_mctp *addr)
+{
+	struct pollfd pf = { .fd = sd, .events = POLLIN };
+	int val1 = 0x11223344;
+	ssize_t len;
+	int rc;
+
+	/* Wait for timeout, > mctp_key_lifetime = 6 secs */
+	TH_LOG("Waiting 7 seconds for key expiry...");
+	sleep(7);
+
+	addr->smctp_tag &= ~MCTP_TAG_OWNER;
+	len = sendto(sock_listen, &val1, sizeof(val1),
+		0, (struct sockaddr *)addr, sizeof(*addr));
+	ASSERT_EQ(len, sizeof(val1));
+
+	/* Wait 100ms */
+	rc = poll(&pf, 1, 100);
+	/* We should time out, packet is never received */
+	ASSERT_EQ(rc, 0);
+}
+
+/* Helper to test duplicate replies */
+static void test_duplicate_reply(struct __test_metadata *_metadata,
+	int sock_listen, int sd, struct sockaddr_mctp *addr)
+{
+	struct pollfd pf = { .fd = sd, .events = POLLIN };
+	int val1 = 0x11223344;
+	ssize_t len;
+	int rc;
+
+	/* Send first reply */
+	addr->smctp_tag &= ~MCTP_TAG_OWNER;
+	len = sendto(sock_listen, &val1, sizeof(val1),
+		0, (struct sockaddr *)addr, sizeof(*addr));
+	ASSERT_EQ(len, sizeof(val1));
+
+	/* Receive reply */
+	len = recvfrom(sd, NULL, 0, MSG_TRUNC, NULL, NULL);
+	EXPECT_EQ(len, sizeof(val1));
+
+	/* Try a second reply */
+	len = sendto(sock_listen, &val1, sizeof(val1),
+		0, (struct sockaddr *)addr, sizeof(*addr));
+	ASSERT_EQ(len, sizeof(val1));
+
+	/* Wait 100ms */
+	rc = poll(&pf, 1, 100);
+	/* We should time out, packet is never received */
+	ASSERT_EQ(rc, 0);
+}
+
+TEST_F(TUNPAIR, roundtrip)
+{
+	const size_t msglen = variant->msglen ?: default_msglen;
+	struct sockaddr_mctp addr = {0};
+	int sock_listen, sd;
+	void *buf1, *buf2;
+	socklen_t addrlen;
+	int net, eid;
+	ssize_t len;
+
+	if (self->skip)
+		SKIP(return, "Skipping");
+
+	buf1 = malloc(msglen);
+	buf2 = malloc(msglen);
+
+	/* Listen on mctptun2 (eid 132 net 32) */
+	if (variant->listen_net_any)
+		net = MCTP_NET_ANY;
+	else
+		net = 32;
+
+	if (variant->listen_addr_any)
+		eid = MCTP_ADDR_ANY;
+	else
+		eid = 132;
+
+	sock_listen = mctp_bind(_metadata, net, eid, 1);
+
+	/* Send mctptun1 -> mctptun2 */
+	getrandom(buf1, msglen, 0);
+	sd = mctp_send(_metadata, 31, 132, 1, buf1, msglen);
+
+	/* Receive it */
+	addrlen = sizeof(addr);
+	len = recvfrom(sock_listen, buf2, msglen, MSG_TRUNC,
+		(struct sockaddr *)&addr, &addrlen);
+	EXPECT_EQ(len, msglen);
+	EXPECT_EQ(memcmp(buf1, buf2, msglen), 0);
+	TH_LOG("addr family %d ", addr.smctp_family);
+
+	if (variant->expire_key) {
+		test_expire_key(_metadata, sock_listen, sd, &addr);
+	} else if (variant->duplicate_reply) {
+		test_duplicate_reply(_metadata, sock_listen, sd, &addr);
+	} else {
+		/* Reply mctptun2 -> mctptun1 */
+		getrandom(buf1, msglen, 0);
+		addr.smctp_tag &= ~MCTP_TAG_OWNER;
+		len = sendto(sock_listen, buf1, msglen,
+			0, (struct sockaddr *)&addr, sizeof(addr));
+		ASSERT_EQ(len, msglen);
+
+		/* Receive reply */
+		addrlen = sizeof(addr);
+		len = recvfrom(sd, buf2, msglen, MSG_TRUNC,
+			(struct sockaddr *)&addr, &addrlen);
+		EXPECT_EQ(len, msglen);
+		EXPECT_EQ(memcmp(buf1, buf2, msglen), 0);
+	}
+
+	close(sd);
+	close(sock_listen);
+	free(buf2);
+	free(buf1);
+}
+
+/* Returns file descriptor.
+ * Asserts on failure
+ */
+static int create_tun(struct __test_metadata *_metadata,
+	const char *name)
+{
+	struct ifreq ifr = {0};
+	int rc;
+	int fd;
+
+	fd = open("/dev/net/tun", O_RDWR);
+	ASSERT_GE(fd, 0);
+
+	assert(strlen(name) < IFNAMSIZ);
+	strcpy(ifr.ifr_name, name);
+	ifr.ifr_flags = IFF_TUN;
+
+	rc = ioctl(fd, TUNSETIFF, &ifr);
+	ASSERT_EQ(rc, 0) TH_LOG("tun ioctl failed: %s",
+		strerror(errno));
+	return fd;
+}
+
+static int atomic_write(int fd, void *buffer, size_t len)
+{
+	while (len) {
+		ssize_t wlen = write(fd, buffer, len);
+
+		if (wlen == -1 && errno == EINTR)
+			continue;
+
+		if (wlen < 0)
+			return wlen;
+
+		len -= wlen;
+		buffer += wlen;
+	}
+	return 0;
+}
+
+
+/* Thread to forward messages between FDs */
+static void *tun_start(void *arg)
+{
+	struct _test_data_TUNPAIR *fix = arg;
+	struct pollfd pf[2] = {
+		{ .fd = fix->fd[0], .events = POLLIN },
+		{ .fd = fix->fd[1], .events = POLLIN },
+	};
+	char *buffer = NULL;
+	ssize_t len;
+	int rc;
+
+	buffer = malloc(BUF_SIZE);
+	if (!buffer) {
+		fprintf(stderr, "malloc failed");
+		goto out;
+	}
+
+	while (fix->running) {
+		rc = poll(pf, 2, 100);
+		if (rc == 0 || (rc == -1 && errno == EINTR)) {
+			continue;
+		}
+		if (rc < 0) {
+			fprintf(stderr, "poll error: %s", strerror(errno));
+			goto out;
+		}
+
+		for (int i = 0; i < 2; i++) {
+			if (pf[i].revents & POLLIN) {
+				len = read(pf[i].fd, buffer, BUF_SIZE);
+				if (len == -1 && errno == EINTR) {
+					continue;
+				}
+				if (len == 0) {
+					fprintf(stderr, "EOF from tun");
+					goto out;
+				}
+				if (len < 0) {
+					fprintf(stderr, "error from tun read: %s",
+						strerror(errno));
+					goto out;
+				}
+
+				rc = atomic_write(pf[(i+1) % 2].fd, buffer, len);
+				if (rc < 0) {
+					fprintf(stderr, "error from tun write: %s",
+						strerror(errno));
+					goto out;
+				}
+			}
+		}
+	}
+out:
+	return NULL;
+}
+
+TEST_HARNESS_MAIN
