[PATCH net] ipv6: ignore link state in gateway check

[PATCH net] ipv6: ignore link state in gateway check

[Yaroslav Dudkov]

IPv6 route cannot be added in table when all
following conditions are true:
- gateway is not link-local address
- ignore_routes_with_linkdown=1 on device
- nexthop has no carrier
- gateway is not present or covered by any route in table already
Also multipath routes addition is invalidated by kernel even
if only one of added nexthops satisfies all above conditions.

Reproducer:

  ip netns add t
  ip -n t link add va type veth peer name vb
  ip -n t link set va up
  ip -n t addr add fd00::1/64 dev va nodad
  ip netns exec t sysctl -w \
    net.ipv6.conf.va.ignore_routes_with_linkdown=1
  ip -n t -6 route add 2001:db8::/64 via fd00::2 dev va
  # added
  ip -n t -6 route add 2001:db8::/64 table 100 via fd00::2 dev va
  # RTNETLINK answers: No route to host
  ip -n t -6 route show table all
  ip netns del t

Route should be added because:
- RT6_LOOKUP_F_IGNORE_LINKSTATE in commit d5d32e4b7668
("net: ipv6: Do not consider link state for nexthop validation")
was added for this
- the same route is valid in main table (when no specifying table id
while adding route)
- fib6_nh_init() initializes nexthops with no carrier
- IPv4 already does this (fib_check_nh_v4_gw() ignores link state
using the FIB_LOOKUP_IGNORE_LINKSTATE flag)

Before commit b2c709cce682 ("ipv6: Convert gateway validation to
use fib6_info") rt6_lookup() was used for checking if nexthop is
valid, rt6_lookup called rt6_device_match() which does not check the
link state. After change rt6_lookup() to fib6_lookup()
find_match() under fib6_lookup() started to invalidate linkdown
devices, because RT6_LOOKUP_F_IGNORE_LINKSTATE is not passed to it.

Adding RT6_LOOKUP_F_IGNORE_LINKSTATE to flags for ip6_route_check_nh()
solves the problem. RT6_LOOKUP_F_IGNORE_LINKSTATE is already used
inside the first search ip6_nh_lookup_table() and fixes the
bug in second search fib6_lookup(). Also RT6_LOOKUP_F_IGNORE_LINKSTATE
isn't read inside fib6_select_path().

Fixes: b2c709cce682 ("ipv6: Convert gateway validation to use fib6_info")
Signed-off-by: Yaroslav Dudkov <aroslavdudkov622@gmail.com>
---
IPv4 example of this situation:
  ip netns add t
  ip -n t link add va type veth peer name vb
  ip -n t link set va up
  ip -n t addr add 192.168.0.1/24 dev va
  ip netns exec t sysctl -w \
    net.ipv4.conf.va.ignore_routes_with_linkdown=1
  ip -n t route add 192.0.2.0/24 table 100 via 192.168.0.2 dev va
  # added
  ip -n t route show table all
  ip netns del t

Also when adding RT6_LOOKUP_F_IGNORE_LINKSTATE to flags of
ip6_route_check_nh() makes adding this flag inside of
ip6_nh_lookup_table() unnecessary for this call, making another call in
ip6_route_check_nh_onlink() with flags=0 the only place where it is
needed.
---
 net/ipv6/route.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 94e5ef1c4f83..95d7b5b10e51 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3426,7 +3426,7 @@ static int ip6_route_check_nh(struct net *net,
 {
 	const struct in6_addr *gw_addr = &cfg->fc_gateway;
 	struct net_device *dev = _dev ? *_dev : NULL;
-	int flags = RT6_LOOKUP_F_IFACE;
+	int flags = RT6_LOOKUP_F_IFACE | RT6_LOOKUP_F_IGNORE_LINKSTATE;
 	struct fib6_result res = {};
 	int err = -EHOSTUNREACH;
 
-- 
2.54.0

Re: [PATCH net] ipv6: ignore link state in gateway check

[Ido Schimmel]

On Wed, Oct 28, 2026 at 03:26:33PM +0000, Yaroslav Dudkov wrote:
> IPv6 route cannot be added in table when all
> following conditions are true:
> - gateway is not link-local address
> - ignore_routes_with_linkdown=1 on device
> - nexthop has no carrier
> - gateway is not present or covered by any route in table already
> Also multipath routes addition is invalidated by kernel even
> if only one of added nexthops satisfies all above conditions.
> 
> Reproducer:
> 
>   ip netns add t
>   ip -n t link add va type veth peer name vb
>   ip -n t link set va up
>   ip -n t addr add fd00::1/64 dev va nodad
>   ip netns exec t sysctl -w \
>     net.ipv6.conf.va.ignore_routes_with_linkdown=1
>   ip -n t -6 route add 2001:db8::/64 via fd00::2 dev va
>   # added
>   ip -n t -6 route add 2001:db8::/64 table 100 via fd00::2 dev va
>   # RTNETLINK answers: No route to host
>   ip -n t -6 route show table all
>   ip netns del t
> 
> Route should be added because:
> - RT6_LOOKUP_F_IGNORE_LINKSTATE in commit d5d32e4b7668
> ("net: ipv6: Do not consider link state for nexthop validation")
> was added for this
> - the same route is valid in main table (when no specifying table id
> while adding route)
> - fib6_nh_init() initializes nexthops with no carrier
> - IPv4 already does this (fib_check_nh_v4_gw() ignores link state
> using the FIB_LOOKUP_IGNORE_LINKSTATE flag)
> 
> Before commit b2c709cce682 ("ipv6: Convert gateway validation to
> use fib6_info") rt6_lookup() was used for checking if nexthop is
> valid, rt6_lookup called rt6_device_match() which does not check the
> link state. After change rt6_lookup() to fib6_lookup()
> find_match() under fib6_lookup() started to invalidate linkdown
> devices, because RT6_LOOKUP_F_IGNORE_LINKSTATE is not passed to it.
> 
> Adding RT6_LOOKUP_F_IGNORE_LINKSTATE to flags for ip6_route_check_nh()
> solves the problem. RT6_LOOKUP_F_IGNORE_LINKSTATE is already used
> inside the first search ip6_nh_lookup_table() and fixes the
> bug in second search fib6_lookup(). Also RT6_LOOKUP_F_IGNORE_LINKSTATE
> isn't read inside fib6_select_path().
> 
> Fixes: b2c709cce682 ("ipv6: Convert gateway validation to use fib6_info")
> Signed-off-by: Yaroslav Dudkov <aroslavdudkov622@gmail.com>
> ---
> IPv4 example of this situation:
>   ip netns add t
>   ip -n t link add va type veth peer name vb
>   ip -n t link set va up
>   ip -n t addr add 192.168.0.1/24 dev va
>   ip netns exec t sysctl -w \
>     net.ipv4.conf.va.ignore_routes_with_linkdown=1
>   ip -n t route add 192.0.2.0/24 table 100 via 192.168.0.2 dev va
>   # added
>   ip -n t route show table all
>   ip netns del t

Please turn this into a selftest so that it doesn't regress in the
future. Something like [1] (vibe coded).

> 
> Also when adding RT6_LOOKUP_F_IGNORE_LINKSTATE to flags of
> ip6_route_check_nh() makes adding this flag inside of
> ip6_nh_lookup_table() unnecessary for this call, making another call in
> ip6_route_check_nh_onlink() with flags=0 the only place where it is
> needed.
> ---
>  net/ipv6/route.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index 94e5ef1c4f83..95d7b5b10e51 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -3426,7 +3426,7 @@ static int ip6_route_check_nh(struct net *net,
>  {
>  	const struct in6_addr *gw_addr = &cfg->fc_gateway;
>  	struct net_device *dev = _dev ? *_dev : NULL;
> -	int flags = RT6_LOOKUP_F_IFACE;
> +	int flags = RT6_LOOKUP_F_IFACE | RT6_LOOKUP_F_IGNORE_LINKSTATE;

Maintain reverse xmas tree:

https://docs.kernel.org/next/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs

>  	struct fib6_result res = {};
>  	int err = -EHOSTUNREACH;
>  
> -- 
> 2.54.0
> 

[1]
diff --git a/tools/testing/selftests/net/fib_tests.sh b/tools/testing/selftests/net/fib_tests.sh
index 7c47d7fe250d..76f950eaef52 100755
--- a/tools/testing/selftests/net/fib_tests.sh
+++ b/tools/testing/selftests/net/fib_tests.sh
@@ -406,10 +406,48 @@ fib_carrier_unicast_test()
 	cleanup
 }
 
+fib_carrier_gw_test()
+{
+	echo
+	echo "Gateway validation with carrier down"
+
+	setup
+
+	set -e
+	$NS_EXEC sysctl -qw net.ipv4.conf.dummy0.ignore_routes_with_linkdown=1
+	$NS_EXEC sysctl -qw net.ipv6.conf.dummy0.ignore_routes_with_linkdown=1
+	$IP link set dev dummy0 carrier off
+	sleep 1
+	set +e
+
+	$IP route add 203.0.113.0/25 via 198.51.100.2 dev dummy0
+	log_test $? 0 "IPv4 route in main table"
+	$IP -6 route add 2001:db8:10::/64 via 2001:db8:1::2 dev dummy0
+	log_test $? 0 "IPv6 route in main table"
+
+	$IP route add 203.0.113.0/25 table 100 via 198.51.100.2 dev dummy0
+	log_test $? 0 "IPv4 route in non-main table"
+	$IP -6 route add 2001:db8:10::/64 table 100 via 2001:db8:1::2 dev dummy0
+	log_test $? 0 "IPv6 route in non-main table"
+
+	$IP route add 203.0.113.128/25 table 100 via 198.51.100.2
+	log_test $? 0 "IPv4 route in non-main table without device"
+	$IP -6 route add 2001:db8:11::/64 table 100 via 2001:db8:1::2
+	log_test $? 0 "IPv6 route in non-main table without device"
+
+	$IP route show table 100 203.0.113.0/25 | grep -q "linkdown"
+	log_test $? 0 "IPv4 linkdown flag set"
+	$IP -6 route show table 100 2001:db8:10::/64 | grep -q "linkdown"
+	log_test $? 0 "IPv6 linkdown flag set"
+
+	cleanup
+}
+
 fib_carrier_test()
 {
 	fib_carrier_local_test
 	fib_carrier_unicast_test
+	fib_carrier_gw_test
 }
 
 fib_rp_filter_test()