Currently, __mkroute_output overrules the MTU value configured for broadcast routes.
This buggy behaviour can be reproduced with:
ip link set dev eth1 mtu 9000 ip route del broadcast 192.168.0.255 dev eth1 proto kernel scope link src 192.168.0.2 ip route add broadcast 192.168.0.255 dev eth1 proto kernel scope link src 192.168.0.2 mtu 1500
The maximum packet size should be 1500, but it is actually 8000:
ping -b 192.168.0.255 -s 8000
Fix __mkroute_output to allow MTU values to be configured for for broadcast routes (to support a mixed-MTU local-area-network).
Signed-off-by: Oscar Maes oscmaes92@gmail.com --- net/ipv4/route.c | 1 - 1 file changed, 1 deletion(-)
diff --git a/net/ipv4/route.c b/net/ipv4/route.c index fccb05fb3..a2a3b6482 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -2585,7 +2585,6 @@ static struct rtable *__mkroute_output(const struct fib_result *res, do_cache = true; if (type == RTN_BROADCAST) { flags |= RTCF_BROADCAST | RTCF_LOCAL; - fi = NULL; } else if (type == RTN_MULTICAST) { flags |= RTCF_MULTICAST | RTCF_LOCAL; if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr,
Hi,
Thanks for your patch.
FYI: kernel test robot notices the stable kernel rule is not satisfied.
The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#opti...
Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree. Subject: [PATCH net] net: ipv4: fix incorrect MTU in broadcast routes Link: https://lore.kernel.org/stable/20250702141515.9414-1-oscmaes92%40gmail.com
On Wed, Jul 2, 2025 at 7:15 AM Oscar Maes oscmaes92@gmail.com wrote:
Currently, __mkroute_output overrules the MTU value configured for broadcast routes.
This buggy behaviour can be reproduced with:
ip link set dev eth1 mtu 9000 ip route del broadcast 192.168.0.255 dev eth1 proto kernel scope link src 192.168.0.2 ip route add broadcast 192.168.0.255 dev eth1 proto kernel scope link src 192.168.0.2 mtu 1500
The maximum packet size should be 1500, but it is actually 8000:
ping -b 192.168.0.255 -s 8000
Looks sane to me, but could you add a test in tools/testing/selftests/net ?
Fix __mkroute_output to allow MTU values to be configured for for broadcast routes (to support a mixed-MTU local-area-network).
Signed-off-by: Oscar Maes oscmaes92@gmail.com
net/ipv4/route.c | 1 - 1 file changed, 1 deletion(-)
diff --git a/net/ipv4/route.c b/net/ipv4/route.c index fccb05fb3..a2a3b6482 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -2585,7 +2585,6 @@ static struct rtable *__mkroute_output(const struct fib_result *res, do_cache = true; if (type == RTN_BROADCAST) { flags |= RTCF_BROADCAST | RTCF_LOCAL;
fi = NULL; } else if (type == RTN_MULTICAST) { flags |= RTCF_MULTICAST | RTCF_LOCAL; if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr,-- 2.39.5
linux-stable-mirror@lists.linaro.org
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Eric Dumazet -
kernel test robot -
Oscar Maes