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Wed, 02 Jul 2025 07:15:42 -0700 (PDT) Received: from localhost.localdomain ([45.128.133.218]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4538a4215dbsm198343375e9.35.2025.07.02.07.15.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 02 Jul 2025 07:15:41 -0700 (PDT) From: Oscar Maes To: netdev@vger.kernel.org Cc: davem@davemloft.net, dsahern@kernel.org, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, stable@vger.kernel.org, linux-kernel@vger.kernel.org, Oscar Maes Subject: [PATCH net] net: ipv4: fix incorrect MTU in broadcast routes Date: Wed, 2 Jul 2025 16:15:15 +0200 Message-Id: <20250702141515.9414-1-oscmaes92@gmail.com> X-Mailer: git-send-email 2.39.5 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable 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=C2=A0broadcast 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 1= 92.168.0.2 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 --- 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 f= ib_result *res, do_cache =3D true; if (type =3D=3D RTN_BROADCAST) { flags |=3D RTCF_BROADCAST | RTCF_LOCAL; - fi =3D NULL; } else if (type =3D=3D RTN_MULTICAST) { flags |=3D RTCF_MULTICAST | RTCF_LOCAL; if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr, --=20 2.39.5