[PATCH net-next v3 0/2] ipv6: update NUD_FAILED neighbors from NA messages

[PATCH net-next v3 0/2] ipv6: update NUD_FAILED neighbors from NA messages

[Lawrence Lee]

Neighbor Advertisements for existing NUD_FAILED entries are currently
ignored. This can leave a first-hop router with an unusable neighbor after
the host becomes reachable and announces itself, particularly when
forwarding is offloaded away from the kernel and traffic destined to the
FAILED neighbor is unable to trigger the kernel's normal neighbor
resolution process.

Allowing neighbor entries to be updated from NAs already received by the
kernel is preferable since it creates less work compared to periodically
probing FAILED neighbors (e.g. using NTF_EXT_MANAGED).

Extend accept_untracked_na to treat FAILED entries like absent entries.
Require a target link-layer address, IPv6 forwarding, and acceptance by the
sysctl, including its source-prefix check in mode 2. Recover the entry only
to STALE, including for solicited NAs, to maintain parity with handling of
non-existent neighbor cache entries.

Patch #1 implements the behavior and updates RFC 9131-related comments and
accept_untracked_na documentation.

Patch #2 extends ndisc_unsolicited_na_test.sh to cover FAILED-entry
recovery and each acceptance gate, including in-prefix and out-of-prefix
mode 2.

Thanks to Ido Schimmel for feedback on the original RFC.

---
v3:
- Cite RFC 4861 section 7.3.3 when describing FAILED entries.
- Simplify the FAILED-entry path as suggested by Ido Schimmel.
- Drop the NDISC callback and 6LoWPAN changes. The existing 6LoWPAN
  override-only behavior also applies to INCOMPLETE entries and is left
  unchanged.
- Restore the existing rt6_clean_tohost() source-address behavior. The
  source-versus-target behavior applies to all neighbor states and is left
  unchanged.
- Verify the link-layer address learned during recovery in selftests.
- Keep the NA source, target, and neighbor key fixed while varying the
  router prefix in the mode 2 selftest cases.
- Arm packet capture in selftest before starting host DAD and wait for
  capture readiness and receive completion.

v2:
- Full implementation of logic to update FAILED entries to STALE.
- Update the RFC 9131-related comments and sysctl documentation.
- Add selftests to verify state changes only when the accept_untracked_na
  setting is enabled.

v1 (RFC):
https://lore.kernel.org/r/20260813233344.445265-1-lfqlee314@gmail.com

Testing:
- x86_64 allyesconfig build with W=1
- Sparse on net/ipv6/ndisc.c
- tools/testing/selftests/net/ndisc_unsolicited_na_test.sh: 14 passed
- Local NIPA checks: all 45 checks passed
- Sashiko local review completed

Lawrence Lee (2):
  ipv6: update NUD_FAILED neighbors from NA messages
  selftests: net: test untracked NA recovery of FAILED neighbors

 Documentation/networking/ip-sysctl.rst        |  28 +--
 net/ipv6/ndisc.c                              |  47 +++--
 .../net/ndisc_unsolicited_na_test.sh          | 195 ++++++++++++++----
 3 files changed, 201 insertions(+), 69 deletions(-)


base-commit: 4bb9710c6a68d35207f123aef55dcd50e7195ec5
-- 
2.43.0

[PATCH net-next v3 1/2] ipv6: update NUD_FAILED neighbors from NA messages

[Lawrence Lee]

Transition a FAILED neighbor entry to STALE upon receipt of an NA
message on routers when accept_untracked_na is enabled. This extends the
RFC 9131 accept_untracked_na behavior so that FAILED entries are treated
the same as non-existent entries.

RFC 4861 section 7.3.3 says that an entry should be deleted when address
resolution fails. Linux instead retains the entry in NUD_FAILED, so
treating it as untracked is consistent with the protocol model.

Trying to resolve FAILED neighbors via periodic probing (e.g. using
NTF_EXT_MANAGED) is more work compared to this approach which uses
information in NAs that the kernel may already be receiving. Note that
because this behavior in IPv6 is dependent on the accept_untracked_na
sysctl setting, this approach is more conservative than IPv4 which
transitions FAILED neighbors to STALE by default upon receiving GARPs.

Link: https://lore.kernel.org/r/20260813233344.445265-1-lfqlee314@gmail.com
Assisted-by: LLM Sashiko sparse
Signed-off-by: Lawrence Lee <lfqlee314@gmail.com>
---
The existing 6LoWPAN override-only handling also applies to INCOMPLETE
entries and is intentionally left unchanged. The existing
rt6_clean_tohost() source-versus-target behavior applies to all neighbor
states and is also left unchanged.

 Documentation/networking/ip-sysctl.rst | 28 ++++++++-------
 net/ipv6/ndisc.c                       | 47 ++++++++++++++------------
 2 files changed, 41 insertions(+), 34 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index f7af0286341c..685c84cf543d 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -3225,18 +3225,19 @@ drop_unsolicited_na - BOOLEAN
 	Default: 0 (disabled).
 
 accept_untracked_na - INTEGER
-	Define behavior for accepting neighbor advertisements from devices that
-	are absent in the neighbor cache:
+	Define behavior for accepting neighbor advertisements for IPv6 addresses
+	that are absent from the neighbor cache or whose entries are in FAILED
+	state:
 
-	- 0 - (default) Do not accept unsolicited and untracked neighbor
-	  advertisements.
+	- 0 - (default) Do not create new neighbor cache entries or update
+	  FAILED entries from neighbor advertisements.
 
-	- 1 - Add a new neighbor cache entry in STALE state for routers on
-	  receiving a neighbor advertisement (either solicited or unsolicited)
-	  with target link-layer address option specified if no neighbor entry
-	  is already present for the advertised IPv6 address. Without this knob,
-	  NAs received for untracked addresses (absent in neighbor cache) are
-	  silently ignored.
+	- 1 - For routers, add a new neighbor cache entry or update an existing
+	  FAILED entry to STALE upon receiving a neighbor advertisement (either
+	  solicited or unsolicited) with the target link-layer address option
+	  specified. Without this knob, NAs received for untracked addresses
+	  (absent from the neighbor cache or in FAILED state) are silently
+	  ignored.
 
 	  This is as per router-side behavior documented in RFC9131.
 
@@ -3251,9 +3252,10 @@ accept_untracked_na - INTEGER
 	  used in conjunction with the ndisc_notify setting on the host to
 	  satisfy this prerequisite.
 
-	- 2 - Extend option (1) to add a new neighbor cache entry only if the
-	  source IP address is in the same subnet as an address configured on
-	  the interface that received the neighbor advertisement.
+	- 2 - Extend option (1) to add a new neighbor cache entry or update a
+	  FAILED entry only if the source IP address is in the same subnet as
+	  an address configured on the interface that received the neighbor
+	  advertisement.
 
 enhanced_dad - BOOLEAN
 	Include a nonce option in the IPv6 neighbor solicitation messages used for
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 90cd5d852569..12d85d7f8234 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -973,13 +973,13 @@ static enum skb_drop_reason ndisc_recv_ns(struct sk_buff *skb)
 static int accept_untracked_na(struct inet6_dev *idev, struct in6_addr *saddr)
 {
 	switch (READ_ONCE(idev->cnf.accept_untracked_na)) {
-	case 0: /* Don't accept untracked na (absent in neighbor cache) */
+	case 0: /* Don't accept untracked NA (absent or FAILED) */
 		return 0;
-	case 1: /* Create new entries from na if currently untracked */
+	case 1: /* Create new or update FAILED entries from NA */
 		return 1;
-	case 2: /* Create new entries from untracked na only if saddr is in the
+	case 2: /* Create new or update FAILED entries only if saddr is in the
 		 * same subnet as an address configured on the interface that
-		 * received the na
+		 * received the NA
 		 */
 		return !!ipv6_chk_prefix(saddr, idev->dev);
 	default:
@@ -1067,34 +1067,39 @@ static enum skb_drop_reason ndisc_recv_na(struct sk_buff *skb)
 	neigh = neigh_lookup(tbl, &msg->target, dev);
 
 	/* RFC 9131 updates original Neighbour Discovery RFC 4861.
-	 * NAs with Target LL Address option without a corresponding
-	 * entry in the neighbour cache can now create a STALE neighbour
-	 * cache entry on routers.
+	 * NAs with Target LL Address option can now create a STALE neighbor
+	 * cache entry on routers if the NA does not have a corresponding entry
+	 * in the neighbour cache or has a corresponding FAILED entry.
 	 *
-	 *   entry accept  fwding  solicited        behaviour
-	 * ------- ------  ------  ---------    ----------------------
-	 * present      X       X         0     Set state to STALE
-	 * present      X       X         1     Set state to REACHABLE
-	 *  absent      0       X         X     Do nothing
-	 *  absent      1       0         X     Do nothing
-	 *  absent      1       1         X     Add a new STALE entry
+	 *       entry accept  fwding  solicited        behaviour
+	 * ----------- ------  ------  ---------    ----------------------
+	 *  non-FAILED      X       X         0     Set state to STALE
+	 *  non-FAILED      X       X         1     Set state to REACHABLE
+	 *      FAILED      0       X         X     Do nothing
+	 *      FAILED      1       0         X     Do nothing
+	 *      FAILED      1       1         X     Set state to STALE
+	 *      absent      0       X         X     Do nothing
+	 *      absent      1       0         X     Do nothing
+	 *      absent      1       1         X     Add a new STALE entry
 	 *
 	 * Note that we don't do a (daddr == all-routers-mcast) check.
 	 */
 	new_state = msg->icmph.icmp6_solicited ? NUD_REACHABLE : NUD_STALE;
-	if (!neigh && lladdr && idev && READ_ONCE(idev->cnf.forwarding)) {
-		if (accept_untracked_na(idev, saddr)) {
-			neigh = neigh_create(tbl, &msg->target, dev);
-			new_state = NUD_STALE;
+	if (!neigh || (READ_ONCE(neigh->nud_state) & NUD_FAILED)) {
+		if (!lladdr || !idev || !READ_ONCE(idev->cnf.forwarding) ||
+		    !accept_untracked_na(idev, saddr)) {
+			if (neigh)
+				neigh_release(neigh);
+			return reason;
 		}
+		if (!neigh)
+			neigh = neigh_create(tbl, &msg->target, dev);
+		new_state = NUD_STALE;
 	}
 
 	if (neigh && !IS_ERR(neigh)) {
 		u8 old_flags = neigh->flags;
 
-		if (READ_ONCE(neigh->nud_state) & NUD_FAILED)
-			goto out;
-
 		/*
 		 * Don't update the neighbor cache entry on a proxy NA from
 		 * ourselves because either the proxied node is off link or it
-- 
2.43.0

[PATCH net-next v3 2/2] selftests: net: test untracked NA recovery of FAILED neighbors

[Lawrence Lee]

Seed a FAILED neighbor before triggering an unsolicited NA. Verify that
the entry transitions to STALE and learns the advertised link-layer
address only when drop_unsolicited_na is disabled, accept_untracked_na
mode 1 or in-prefix mode 2 is enabled, and IPv6 forwarding is enabled.
Verify that disabling each gate keeps the entry in FAILED without a
link-layer address.

Mark the seed externally learned and disable carrier-based eviction so
the entry cannot be garbage-collected before the NA arrives. Require the
marker after processing to prove that a recovered entry was updated in
place instead of deleted and recreated.

Keep the NA source, target, and neighbor key fixed for the mode 2 cases.
Vary the prefix configured on the router to exercise the source-prefix
check without changing the entry under test.

Arm packet capture before bringing up the host interface and starting
DAD. Wait for the capture to become ready and for tcpdump to exit so the
NA cannot be missed and receive processing is complete before checking
the neighbor entry.

Link: https://lore.kernel.org/r/20260813233344.445265-1-lfqlee314@gmail.com
Assisted-by: LLM Sashiko sparse
Signed-off-by: Lawrence Lee <lfqlee314@gmail.com>
---
 .../net/ndisc_unsolicited_na_test.sh          | 195 ++++++++++++++----
 1 file changed, 160 insertions(+), 35 deletions(-)

diff --git a/tools/testing/selftests/net/ndisc_unsolicited_na_test.sh b/tools/testing/selftests/net/ndisc_unsolicited_na_test.sh
index 5db69dad0cfc..ba9e670b5149 100755
--- a/tools/testing/selftests/net/ndisc_unsolicited_na_test.sh
+++ b/tools/testing/selftests/net/ndisc_unsolicited_na_test.sh
@@ -3,12 +3,18 @@
 
 # This test is for the accept_untracked_na feature to
 # enable RFC9131 behaviour. The following is the test-matrix.
-# drop   accept  fwding                   behaviour
-# ----   ------  ------  ----------------------------------------------
-#    1        X       X  Don't update NC
-#    0        0       X  Don't update NC
-#    0        1       0  Don't update NC
-#    0        1       1  Add a STALE NC entry
+# state   drop   accept  fwding                   behaviour
+# ------  ----   ------  ------  ----------------------------------------------
+# absent     1        X       X  Don't update NC
+# absent     0        0       X  Don't update NC
+# absent     0        1       0  Don't update NC
+# absent     0        1       1  Add a STALE NC entry
+# failed     1        X       X  Keep the NC entry in FAILED state
+# failed     0        0       X  Keep the NC entry in FAILED state
+# failed     0        1       0  Keep the NC entry in FAILED state
+# failed     0        1       1  Update the NC entry to STALE
+# failed     0        2       1  Update the NC entry to STALE if in-network
+# failed     0        2       1  Keep the NC entry FAILED if out-of-network
 
 source lib.sh
 ret=0
@@ -19,14 +25,18 @@ PAUSE=no
 HOST_INTF="veth-host"
 ROUTER_INTF="veth-router"
 
-ROUTER_ADDR="2000:20::1"
+ROUTER_ADDR_IN_NETWORK="2000:20::1"
+ROUTER_ADDR_OUT_OF_NETWORK="2000:21::1"
+ROUTER_ADDR="${ROUTER_ADDR_IN_NETWORK}"
 HOST_ADDR="2000:20::2"
+HOST_LLADDR="02:00:00:00:00:02"
 SUBNET_WIDTH=64
 ROUTER_ADDR_WITH_MASK="${ROUTER_ADDR}/${SUBNET_WIDTH}"
 HOST_ADDR_WITH_MASK="${HOST_ADDR}/${SUBNET_WIDTH}"
 
 tcpdump_stdout=
 tcpdump_stderr=
+tcpdump_pid=
 
 log_test()
 {
@@ -75,6 +85,7 @@ setup()
 
 	${IP_ROUTER} link add ${ROUTER_INTF} type veth \
                 peer name ${HOST_INTF} netns ${HOST_NS}
+	${IP_HOST} link set dev "${HOST_INTF}" address "${HOST_LLADDR}"
 
 	# Enable IPv6 on both router and host, and configure static addresses.
 	# The router here is the DUT
@@ -88,6 +99,8 @@ setup()
                 ${ROUTER_CONF}.drop_unsolicited_na=${drop_unsolicited_na}
 	${IP_ROUTER_EXEC} sysctl -qw \
                 ${ROUTER_CONF}.accept_untracked_na=${accept_untracked_na}
+	${IP_ROUTER_EXEC} sysctl -qw \
+                ${ROUTER_CONF}.ndisc_evict_nocarrier=0
 	${IP_ROUTER_EXEC} sysctl -qw ${ROUTER_CONF}.disable_ipv6=0
 	${IP_ROUTER} addr add ${ROUTER_ADDR_WITH_MASK} dev ${ROUTER_INTF}
 
@@ -102,24 +115,39 @@ setup()
 }
 
 start_tcpdump() {
-	set -e
-	tcpdump_stdout=`mktemp`
-	tcpdump_stderr=`mktemp`
+	tcpdump_stdout=$(mktemp) || return 1
+	tcpdump_stderr=$(mktemp) || return 1
 	${IP_ROUTER_EXEC} timeout 15s \
                 tcpdump --immediate-mode -tpni ${ROUTER_INTF} -c 1 \
                 "icmp6 && icmp6[0] == 136 && src ${HOST_ADDR}" \
-                > ${tcpdump_stdout} 2> /dev/null
-	set +e
+                > "${tcpdump_stdout}" 2> "${tcpdump_stderr}" &
+	tcpdump_pid=$!
+
+	slowwait 5 grep -q "listening on ${ROUTER_INTF}" "${tcpdump_stderr}"
+}
+
+wait_tcpdump()
+{
+	local rc
+
+	wait "${tcpdump_pid}"
+	rc=$?
+	tcpdump_pid=
+
+	return "${rc}"
 }
 
 cleanup_tcpdump()
 {
-	set -e
-	[[ ! -z  ${tcpdump_stdout} ]] && rm -f ${tcpdump_stdout}
-	[[ ! -z  ${tcpdump_stderr} ]] && rm -f ${tcpdump_stderr}
+	if [ -n "${tcpdump_pid}" ]; then
+		kill "${tcpdump_pid}" 2> /dev/null
+		wait "${tcpdump_pid}" 2> /dev/null
+	fi
+	[ -n "${tcpdump_stdout}" ] && rm -f "${tcpdump_stdout}"
+	[ -n "${tcpdump_stderr}" ] && rm -f "${tcpdump_stderr}"
 	tcpdump_stdout=
 	tcpdump_stderr=
-	set +e
+	tcpdump_pid=
 }
 
 cleanup()
@@ -129,58 +157,145 @@ cleanup()
 	ip netns del ${ROUTER_NS}
 }
 
-link_up() {
-	set -e
+router_link_up()
+{
 	${IP_ROUTER} link set dev ${ROUTER_INTF} up
+}
+
+host_link_up()
+{
 	${IP_HOST} link set dev ${HOST_INTF} up
-	set +e
 }
 
 verify_ndisc() {
 	local drop_unsolicited_na=$1
 	local accept_untracked_na=$2
 	local forwarding=$3
+	local initial_state=${4:-absent}
+	local same_subnet=${5:-1}
+	local expected_lladdr
+	local neigh_show_output
+	local expected_state
+
+	if [ "${drop_unsolicited_na}" -eq 0 ] &&
+			[ "${forwarding}" -eq 1 ]; then
+		case "${accept_untracked_na}" in
+		1)
+			expected_state=STALE
+			expected_lladdr="${HOST_LLADDR}"
+			;;
+		2)
+			if [ "${same_subnet}" -eq 1 ]; then
+				expected_state=STALE
+				expected_lladdr="${HOST_LLADDR}"
+			fi
+			;;
+		esac
+	fi
+	if [ -z "${expected_state}" ] &&
+			[ "${initial_state}" = "failed" ]; then
+		expected_state=FAILED
+	fi
 
-	neigh_show_output=$(${IP_ROUTER} neigh show \
-                to ${HOST_ADDR} dev ${ROUTER_INTF} nud stale)
-	if [ ${drop_unsolicited_na} -eq 0 ] && \
-			[ ${accept_untracked_na} -eq 1 ] && \
-			[ ${forwarding} -eq 1 ]; then
-		# Neighbour entry expected to be present for 011 case
-		[[ ${neigh_show_output} ]]
+	if [ -n "${expected_state}" ]; then
+		neigh_show_output=$(${IP_ROUTER} neigh show \
+			to "${HOST_ADDR}" dev "${ROUTER_INTF}")
+		if [[ " ${neigh_show_output} " != \
+				*" ${expected_state} "* ]]; then
+			return 1
+		fi
+		if [ -n "${expected_lladdr}" ] &&
+				[[ " ${neigh_show_output} " != \
+				*" lladdr ${expected_lladdr} "* ]]; then
+			return 1
+		fi
+		if [[ "${expected_state}" == "FAILED" &&
+		      "${neigh_show_output}" == *"lladdr"* ]]; then
+			return 1
+		fi
+		if [ "${initial_state}" = "failed" ]; then
+			[[ "${neigh_show_output}" == *"extern_learn"* ]]
+		fi
 	else
-		# Neighbour entry expected to be absent for all other cases
+		neigh_show_output=$(${IP_ROUTER} neigh show \
+			to "${HOST_ADDR}" dev "${ROUTER_INTF}")
 		[[ -z ${neigh_show_output} ]]
 	fi
 }
 
 test_unsolicited_na_common()
 {
+	local same_subnet=${5:-1}
+	local neigh_show_output
+
+	if [ "${same_subnet}" -eq 1 ]; then
+		ROUTER_ADDR="${ROUTER_ADDR_IN_NETWORK}"
+	else
+		ROUTER_ADDR="${ROUTER_ADDR_OUT_OF_NETWORK}"
+	fi
+	ROUTER_ADDR_WITH_MASK="${ROUTER_ADDR}/${SUBNET_WIDTH}"
+
 	# Setup the test bed, but keep links down
-	setup $1 $2 $3
+	setup "$1" "$2" "$3"
+
+	if [ "${4:-absent}" = "failed" ]; then
+		if ! ${IP_ROUTER} neigh replace "${HOST_ADDR}" \
+				dev "${ROUTER_INTF}" \
+				nud failed extern_learn; then
+			echo "Unable to create NUD_FAILED neighbor entry"
+			return 1
+		fi
+		neigh_show_output=$(${IP_ROUTER} neigh show \
+			to "${HOST_ADDR}" dev "${ROUTER_INTF}")
+		if [[ " ${neigh_show_output} " != *" FAILED "* ]]; then
+			echo "Unable to verify NUD_FAILED neighbor entry"
+			return 1
+		fi
+		if [[ "${neigh_show_output}" != *"extern_learn"* ]]; then
+			echo "Neighbor entry is not externally learned"
+			return 1
+		fi
+	fi
 
-	# Bring the link up, wait for the NA,
-	# and add a delay to ensure neighbour processing is done.
-	link_up
-	start_tcpdump
+	# Arm the capture before bringing up the host and starting DAD.
+	router_link_up || return 1
+	start_tcpdump || return 1
+	host_link_up || return 1
+
+	# Closing tcpdump's packet socket calls synchronize_net(), so waiting
+	# for it also waits for receive processing of the captured NA.
+	wait_tcpdump || return 1
 
 	# Verify the neighbour table
-	verify_ndisc $1 $2 $3
+	verify_ndisc "$1" "$2" "$3" "$4" "${same_subnet}"
 
 }
 
 test_unsolicited_na_combination() {
-	test_unsolicited_na_common $1 $2 $3
+	local initial_state=${4:-absent}
+	local same_subnet=${5:-1}
+	local rc
+
+	test_unsolicited_na_common "$1" "$2" "$3" "${initial_state}" \
+		"${same_subnet}"
+	rc=$?
 	test_msg=("test_unsolicited_na: "
 		"drop_unsolicited_na=$1 "
 		"accept_untracked_na=$2 "
 		"forwarding=$3")
-	log_test $? 0 "${test_msg[*]}"
+	if [ "${initial_state}" = "failed" ]; then
+		test_msg+=("initial_state=failed")
+	fi
+	if [ "$2" -eq 2 ]; then
+		test_msg+=("same_subnet=${same_subnet}")
+	fi
+	log_test "${rc}" 0 "${test_msg[*]}"
 	cleanup
 }
 
 test_unsolicited_na_combinations() {
 	# Args: drop_unsolicited_na accept_untracked_na forwarding
+	#       [initial_state] [same_subnet]
 
 	# Expect entry
 	test_unsolicited_na_combination 0 1 1
@@ -193,6 +308,16 @@ test_unsolicited_na_combinations() {
 	test_unsolicited_na_combination 1 0 1
 	test_unsolicited_na_combination 1 1 0
 	test_unsolicited_na_combination 1 1 1
+
+	# Expect FAILED entry to become STALE
+	test_unsolicited_na_combination 0 1 1 failed
+	test_unsolicited_na_combination 0 2 1 failed 1
+
+	# Expect FAILED entry to remain FAILED
+	test_unsolicited_na_combination 0 0 1 failed
+	test_unsolicited_na_combination 0 1 0 failed
+	test_unsolicited_na_combination 1 1 1 failed
+	test_unsolicited_na_combination 0 2 1 failed 0
 }
 
 ###############################################################################
-- 
2.43.0