netdev - [PATCHv4 net-next 09/10] docs: bridge: add netfilter doc

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Date: Fri,  1 Dec 2023 16:19:49 +0800
From: Hangbin Liu <liuhangbin@...il.com>
To: netdev@...r.kernel.org
Cc: "David S . Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Ido Schimmel <idosch@...sch.org>,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	Roopa Prabhu <roopa@...dia.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Florian Westphal <fw@...len.de>,
	Andrew Lunn <andrew@...n.ch>,
	Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <olteanv@...il.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Marc Muehlfeld <mmuehlfe@...hat.com>,
	Hangbin Liu <liuhangbin@...il.com>,
	Florian Fainelli <florian.fainelli@...adcom.com>
Subject: [PATCHv4 net-next 09/10] docs: bridge: add netfilter doc

Add netfilter part for bridge document.

Reviewed-by: Florian Westphal <fw@...len.de>
Reviewed-by: Florian Fainelli <florian.fainelli@...adcom.com>
Acked-by: Nikolay Aleksandrov <razor@...ckwall.org>
Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
---
 Documentation/networking/bridge.rst | 36 +++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/Documentation/networking/bridge.rst b/Documentation/networking/bridge.rst
index e96af89cd061..39ff8d126a04 100644
--- a/Documentation/networking/bridge.rst
+++ b/Documentation/networking/bridge.rst
@@ -251,6 +251,42 @@ kernel.
 
 Please see the :ref:`switchdev` document for more details.
 
+Netfilter
+=========
+
+The bridge netfilter module is a legacy feature that allows to filter bridged
+packets with iptables and ip6tables. Its use is discouraged. Users should
+consider using nftables for packet filtering.
+
+The older ebtables tool is more feature-limited compared to nftables, but
+just like nftables it doesn't need this module either to function.
+
+The br_netfilter module intercepts packets entering the bridge, performs
+minimal sanity tests on ipv4 and ipv6 packets and then pretends that
+these packets are being routed, not bridged. br_netfilter then calls
+the ip and ipv6 netfilter hooks from the bridge layer, i.e. ip(6)tables
+rulesets will also see these packets.
+
+br_netfilter is also the reason for the iptables *physdev* match:
+This match is the only way to reliably tell routed and bridged packets
+apart in an iptables ruleset.
+
+Note that ebtables and nftables will work fine without the br_netfilter module.
+iptables/ip6tables/arptables do not work for bridged traffic because they
+plug in the routing stack. nftables rules in ip/ip6/inet/arp families won't
+see traffic that is forwarded by a bridge either, but that's very much how it
+should be.
+
+Historically the feature set of ebtables was very limited (it still is),
+this module was added to pretend packets are routed and invoke the ipv4/ipv6
+netfilter hooks from the bridge so users had access to the more feature-rich
+iptables matching capabilities (including conntrack). nftables doesn't have
+this limitation, pretty much all features work regardless of the protocol family.
+
+So, br_netfilter is only needed if users, for some reason, need to use
+ip(6)tables to filter packets forwarded by the bridge, or NAT bridged
+traffic. For pure link layer filtering, this module isn't needed.
+
 FAQ
 ===
 
-- 
2.41.0