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

[Philip Prindeville]

I'm not sure you want it to be unconditional.

When I wrote the option for netlink and iproute2, it was for some very specialized scenarios.


> On Apr 24, 2023, at 3:14 AM, Stefan Hellermann <stefan at the2masters.de> wrote:
> 
> 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,
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