netdev - [PATCH net-next] net: bridge: do not send arp replies if src and target hw addr is the same

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Message-ID: <20240104142501.81092-1-nbd@nbd.name>
Date: Thu,  4 Jan 2024 15:25:01 +0100
From: Felix Fietkau <nbd@....name>
To: netdev@...r.kernel.org
Subject: [PATCH net-next] net: bridge: do not send arp replies if src and target hw addr is the same

There are broken devices in the wild that handle duplicate IP address
detection by sending out ARP requests for the IP that they received from a
DHCP server and refuse the address if they get a reply.
When proxyarp is enabled, they would go into a loop of requesting an address
and then NAKing it again.

Link: https://github.com/openwrt/openwrt/issues/14309
Signed-off-by: Felix Fietkau <nbd@....name>
---
 net/bridge/br_arp_nd_proxy.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/bridge/br_arp_nd_proxy.c b/net/bridge/br_arp_nd_proxy.c
index c7869a286df4..3a2770938374 100644
--- a/net/bridge/br_arp_nd_proxy.c
+++ b/net/bridge/br_arp_nd_proxy.c
@@ -204,7 +204,10 @@ void br_do_proxy_suppress_arp(struct sk_buff *skb, struct net_bridge *br,
 			if ((p && (p->flags & BR_PROXYARP)) ||
 			    (f->dst && (f->dst->flags & BR_PROXYARP_WIFI)) ||
 			    br_is_neigh_suppress_enabled(f->dst, vid)) {
-				if (!vid)
+				replied = true;
+				if (!memcmp(n->ha, sha, dev->addr_len))
+					replied = false;
+				else if (!vid)
 					br_arp_send(br, p, skb->dev, sip, tip,
 						    sha, n->ha, sha, 0, 0);
 				else
@@ -212,7 +215,6 @@ void br_do_proxy_suppress_arp(struct sk_buff *skb, struct net_bridge *br,
 						    sha, n->ha, sha,
 						    skb->vlan_proto,
 						    skb_vlan_tag_get(skb));
-				replied = true;
 			}
 
 			/* If we have replied or as long as we know the
-- 
2.43.0