netdev - [PATCH net-next v6 04/10] ipv6: Set HBH and DestOpt limits to 2

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Message-ID: <20260203004853.94438-5-tom@herbertland.com>
Date: Mon,  2 Feb 2026 16:48:47 -0800
From: Tom Herbert <tom@...bertland.com>
To: davem@...emloft.net,
	kuba@...nel.org,
	netdev@...r.kernel.org,
	justin.iurman@...ege.be,
	willemdebruijn.kernel@...il.com
Cc: Tom Herbert <tom@...bertland.com>,
	Justin Iurman <justin.iurman@...il.com>
Subject: [PATCH net-next v6 04/10] ipv6: Set HBH and DestOpt limits to 2

Set the default limits of non-padding Hop-by-Hop and Destination
options to 2. This means that if a packet contains more than two
non-padding options then it will be dropped. The previous limit
was 8, but that was too liberal considering that the stack only
support two Destination Options and the most Hop-by-Hop options
likely to ever be in the same packet are IOAM and JUMBO. The limit
can be increased via sysctl for private use and experimentation.

Signed-off-by: Tom Herbert <tom@...bertland.com>
Reviewed-by: Justin Iurman <justin.iurman@...il.com>
---
 include/net/ipv6.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index cf2203ff2bfd..85d89b27a92c 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -86,9 +86,12 @@ struct ip_tunnel_info;
  * silently discarded.
  */
 
-/* Default limits for Hop-by-Hop and Destination options */
-#define IP6_DEFAULT_MAX_DST_OPTS_CNT	 8
-#define IP6_DEFAULT_MAX_HBH_OPTS_CNT	 8
+/* Default limits for Hop-by-Hop and Destination non-padding options. The
+ * default value for both is 2. This sets a limit at two non-padding options
+ * (see sysctl documentation)
+ */
+#define IP6_DEFAULT_MAX_DST_OPTS_CNT	 2
+#define IP6_DEFAULT_MAX_HBH_OPTS_CNT	 2
 #define IP6_DEFAULT_MAX_DST_OPTS_LEN	 INT_MAX /* No limit */
 #define IP6_DEFAULT_MAX_HBH_OPTS_LEN	 INT_MAX /* No limit */
 
-- 
2.43.0