Subject: Re: [PATCH net-next v5 4/7] ipv6: Set HBH and DestOpt limits to 2

From: Justin Iurman <[email-masked]>
Date: Tue, 27 Jan 2026 18:55:53 +0100

On 1/26/26 20:48, Tom Herbert wrote:
> Set the default limits of non-padding Hop-by-Hop and Destination
> options to 2. This means that if a packet contains more then 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 experimenation.
> 
> Signed-off-by: Tom Herbert <[email-masked]>
> ---
>   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 c7f597da01cd..31d270c8c2e4 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 documention)

Reported by AI:
s/documention/documentation

I guess there's no need to respin only for that?

> + */
> +#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 */
>   

Reviewed-by: Justin Iurman <[email-masked]>