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Fri, 17 Oct 2025 07:37:37 +0000 Date: Fri, 17 Oct 2025 06:56:16 +0000 (UTC) From: Household Cang To: 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" Message-ID: <21658780.3286902.1760684176107@mail.yahoo.com> In-Reply-To: References: <20231218162326.173127-1-romain.gantois@bootlin.com> <0351C5C2-FEE2-4AED-84C8-9DCACCE4ED0A@aol.com> <20231222123023.voxoxfcckxsz2vce@skbuf> Subject: Proxy ARP NetNS Awareness Precedence: bulk X-Mailing-List: netdev@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 X-Mailer: WebService/1.1.24562 AolMailNorrin Last light the Linux Librechat was focused on digging the kernel code surro= unding /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=C2=A0arp_net_init() in arp.c. And I am currentl= y stuck on this line 1497 proc_create_net("arp", 0444, net->proc_net, &arp_seq_ops, =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0siz= eof(struct neigh_seq_state)) It is unknown whether this function creates a "view" to the ARP neighbor ta= ble such that each netns has a different view to the neighbor table, OR eac= h netns maintains its own neighbor table. Either way, the implication is wh= ether proxy_arp enabled by sysctl is restricted to the current netns. If proxy_arp is retricted to the current netns, then the Debian documentati= on on wireless bridge-less pseudo-bridge=C2=A0https://wiki.debian.org/Bridg= eNetworkConnections 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 c= annot work across namespaces, or the reasons for the behavior. >From the arp_filter option description, this line "IP addresses are owned b= y the complete host on Linux, not by=C2=A0particular interfaces." is a sing= le-netns statement, but highly suggests that arp operations are per namespa= ce. When a virtual ethernet pair is created between two netns, parprouted users= pace program can relay arps across namespaces, but the kernel proxy_arp can= not. Thank you for any insights and feel free to forward to subject matter exper= ts. I really want to get to the bottom of this. Lucas