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([2a01:e0a:b41:c160:6a1d:efff:fe52:1959]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-47152959b55sm141457455e9.6.2025.10.20.02.58.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Oct 2025 02:58:13 -0700 (PDT) Message-ID: <61394377-195d-455c-b782-93f7f9c3072e@6wind.com> Date: Mon, 20 Oct 2025 11:58:12 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Reply-To: nicolas.dichtel@6wind.com Subject: Re: Proxy ARP NetNS Awareness To: Household Cang , Vladimir Oltean , Lucas Pereira , Romain Gantois , "David S. Miller" Cc: Alexandre Torgue , Jose Abreu , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Maxime Coquelin , Miquel Raynal , Maxime Chevallier , Sylvain Girard , Pascal EBERHARD , Richard Tresidder , "netdev@vger.kernel.org" References: <20231218162326.173127-1-romain.gantois@bootlin.com> <0351C5C2-FEE2-4AED-84C8-9DCACCE4ED0A@aol.com> <20231222123023.voxoxfcckxsz2vce@skbuf> <21658780.3286902.1760684176107@mail.yahoo.com> From: Nicolas Dichtel Content-Language: en-US Organization: 6WIND In-Reply-To: <21658780.3286902.1760684176107@mail.yahoo.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Le 17/10/2025 à 08:56, Household Cang a écrit : > Last light the Linux Librechat was focused on digging the kernel code surrounding /net/ipv4/arp.c and /net/core/net_namespace.c to answer whether the proxy_arp feature enabled by sysctl is namespace aware. > > After many hours of tracing from namespace-generating unshare --net command all the way to the kernel net_namespace.c gave us some clues that the main ns and new ns converged at arp_net_init() in arp.c. And I am currently stuck on this line 1497 > > proc_create_net("arp", 0444, net->proc_net, &arp_seq_ops, >             sizeof(struct neigh_seq_state)) > > It is unknown whether this function creates a "view" to the ARP neighbor table such that each netns has a different view to the neighbor table, OR each netns maintains its own neighbor table. Either way, the implication is whether proxy_arp enabled by sysctl is restricted to the current netns. Each netns maintains its own neighbor tables. The sysctl proxy_arp is per netns. Nicolas > > If proxy_arp is retricted to the current netns, then the Debian documentation on wireless bridge-less pseudo-bridge https://wiki.debian.org/BridgeNetworkConnections may be wrong in insinuating that the proxy_arp feature in the modern kernel can replace parprouted userspace program. > > The current documentation with sysctl net-related options are really vague in terms of netns interaction. arp_ignore, arp_announce, arp_filter did not do a good job of disambiguating whether any of these arp features can or cannot work across namespaces, or the reasons for the behavior. > > From the arp_filter option description, this line "IP addresses are owned by the complete host on Linux, not by particular interfaces." is a single-netns statement, but highly suggests that arp operations are per namespace. > > When a virtual ethernet pair is created between two netns, parprouted userspace program can relay arps across namespaces, but the kernel proxy_arp cannot. > > Thank you for any insights and feel free to forward to subject matter experts. > I really want to get to the bottom of this. > > Lucas >