Re: [PATCH] wg-quick: Android: Use ip link to set MTU

[Harsh Shandilya <me@msfjarvis.dev>]

Hey Adam,

On 2021-01-03 05:59, Adam Irr wrote:
> The ndc command used didn't work on my Nvidia Shield (rooted with a
> custom-built kernel). The ip link command does work. I have only
> tested this on an Nvidia Shield (Android TV - Version 9.0 Pie).
> 
> I couldn't find a lot of documentation on ndc so it's possible my
> device/kernel is misconfigured. If you suspect this is the case, let
> me know.

You couldn't find documentation because there isn't any, it's 
effectively "private API" and Google has broken it time and again. It is 
rather curious that you're running into this issue though, since we've 
had the NDC code in place since before Android 9 came out and went 
through breaking changes in every cycle without these specific 
invocations failing. The last ndc related problem we had was with the 
Android 11 release, where a similar fix[1] was implemented.

I've Cc'ed Jason for reviewing this change, but I'm decidedly curious as 
to why just the Nvidia SHIELD seems to need it.

> 
> Signed-off-by: Adam Irr <adam.irr@outlook.com>
> ---
>  src/wg-quick/android.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/src/wg-quick/android.c b/src/wg-quick/android.c
> index 326efa9..941c7b8 100644
> --- a/src/wg-quick/android.c
> +++ b/src/wg-quick/android.c
> @@ -995,7 +995,7 @@ static void set_mtu(const char *iface, unsigned int 
> mtu)
>         int endpoint_mtu, next_mtu;
> 
>         if (mtu) {
> -               cndc("interface setmtu %s %u", iface, mtu);
> +               cmd("ip link set dev %s mtu %d", iface, mtu);
>                 return;
>         }
> 
> @@ -1016,7 +1016,7 @@ static void set_mtu(const char *iface, unsigned 
> int mtu)
>                         endpoint_mtu = next_mtu;
>         }
> 
> -       cndc("interface setmtu %s %d", iface, endpoint_mtu - 80);
> +       cmd("ip link set dev %s mtu %d", iface, endpoint_mtu - 80);
>  }
> 
>  static void add_route(const char *iface, unsigned int netid, const 
> char *route)
> --
> 2.25.1

Cheers,
Harsh

1: 
https://git.zx2c4.com/wireguard-tools/commit/?id=9a0d65e2afbd6e418c883e0de8555b4fac29f1d5