[PATCH] netifd: add support of GRE tunnel ignore-df option

[Stefan Hellermann]

I have an easier patch in my private repo, maybe it's enough without a 
new configuration option? I'm using it in production for a gretap link 
which is bridged on both sides to MTU 1500 ethernet links.

See https://bugzilla.kernel.org/show_bug.cgi?id=14837, which seems to 
say you always have to use both options: ignore-df nopmtudisc

Signed-off-by: Stefan Hellermann <stefan at the2masters.de>

--- a/system-linux.c
+++ b/system-linux.c
@@ -3496,6 +3496,7 @@ static int system_add_gre_tunnel(const c
              ttl = 64;

          nla_put_u8(nlm, IFLA_GRE_PMTUDISC, set_df ? 1 : 0);
+        nla_put_u8(nlm, IFLA_GRE_IGNORE_DF, set_df ? 0 : 1);

          nla_put_u8(nlm, IFLA_GRE_TOS, tos);
      }

Am 17.04.23 um 11:29 schrieb Denis Kalashnikov:
> This is useful for GRE TAP tunnel when tunnel is added to a br-lan bridge.
> In this case you need to create it with "nopmtudisc ignore-df". Otherwise
> large IP-packets with DF=1 (TCP-data, large pings) will be silently dropped
> (since DF=1 but stack failed to send ICMP "need fragmentation" back). But with
> "ignore-df" packets with DF=1 will be fragmented.
>
> Signed-off-by: Denis Kalashnikov <denis281089 at gmail.com>
> ---
>   system-linux.c | 5 +++++
>   system.c       | 1 +
>   system.h       | 1 +
>   3 files changed, 7 insertions(+)
>
> diff --git a/system-linux.c b/system-linux.c
> index e4041fb..4397460 100644
> --- a/system-linux.c
> +++ b/system-linux.c
> @@ -3500,6 +3500,11 @@ static int system_add_gre_tunnel(const char *name, const char *kind,
>   
>   		nla_put_u8(nlm, IFLA_GRE_PMTUDISC, set_df ? 1 : 0);
>   
> +		if ((cur = tb[TUNNEL_ATTR_IGNORE_DF])) {
> +			nla_put_u8(nlm, IFLA_GRE_IGNORE_DF,
> +				blobmsg_get_bool(cur));
> +		}
> +
>   		nla_put_u8(nlm, IFLA_GRE_TOS, tos);
>   	}
>   
> diff --git a/system.c b/system.c
> index 32597c1..e773245 100644
> --- a/system.c
> +++ b/system.c
> @@ -21,6 +21,7 @@ static const struct blobmsg_policy tunnel_attrs[__TUNNEL_ATTR_MAX] = {
>   	[TUNNEL_ATTR_REMOTE] = { .name = "remote", .type = BLOBMSG_TYPE_STRING },
>   	[TUNNEL_ATTR_MTU] = { .name = "mtu", .type = BLOBMSG_TYPE_INT32 },
>   	[TUNNEL_ATTR_DF] = { .name = "df", .type = BLOBMSG_TYPE_BOOL },
> +	[TUNNEL_ATTR_IGNORE_DF] = { .name = "ignore-df", .type = BLOBMSG_TYPE_BOOL },
>   	[TUNNEL_ATTR_TTL] = { .name = "ttl", .type = BLOBMSG_TYPE_INT32 },
>   	[TUNNEL_ATTR_TOS] = { .name = "tos", .type = BLOBMSG_TYPE_STRING },
>   	[TUNNEL_ATTR_LINK] = { .name = "link", .type = BLOBMSG_TYPE_STRING },
> diff --git a/system.h b/system.h
> index 1f7037d..a7a713d 100644
> --- a/system.h
> +++ b/system.h
> @@ -29,6 +29,7 @@ enum tunnel_param {
>   	TUNNEL_ATTR_LOCAL,
>   	TUNNEL_ATTR_MTU,
>   	TUNNEL_ATTR_DF,
> +	TUNNEL_ATTR_IGNORE_DF,
>   	TUNNEL_ATTR_TTL,
>   	TUNNEL_ATTR_TOS,
>   	TUNNEL_ATTR_LINK,