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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: 8bit 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 --- 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