net: ipv4: fix incorrect MTU in broadcast routes

[PATCH net] net: ipv4: fix incorrect MTU in broadcast routes

Posted by Oscar Maes 3 months, 1 week ago

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,
-- 
2.39.5

Re: [PATCH net] net: ipv4: fix incorrect MTU in broadcast routes

Posted by Eric Dumazet 3 months, 1 week ago

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
>