netdev - Re: [PATCH] net: ipconfig: Allow DNS to be overwritten by DHCPACK

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Date: Wed, 3 May 2023 12:13:39 +0200
From: Martin Wetterwald <martin@...terwald.eu>
To: davem@...emloft.net, dsahern@...nel.org
Cc: netdev@...r.kernel.org
Subject: Re: [PATCH] net: ipconfig: Allow DNS to be overwritten by DHCPACK

On Wed, May 3, 2023 at 12:06 PM Martin Wetterwald <martin@...terwald.eu> wrote:
> diff --git a/net/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
> index e90bc0aa85c7..c125095453da 100644
> --- a/net/ipv4/ipconfig.c
> +++ b/net/ipv4/ipconfig.c
> @@ -937,9 +937,11 @@ static void __init ic_do_bootp_ext(u8 *ext)
>          servers= *ext/4;
>          if (servers > CONF_NAMESERVERS_MAX)
>              servers = CONF_NAMESERVERS_MAX;
> -        for (i = 0; i < servers; i++) {
> -            if (ic_nameservers[i] == NONE)
> +        for (i = 0; i < CONF_NAMESERVERS_MAX; i++) {
> +            if (i < servers)
>                  memcpy(&ic_nameservers[i], ext+1+4*i, 4);
> +            else
> +                ic_nameservers[i] = NONE;
>          }
>          break;
>      case 12:    /* Host name */
> --

I reset the whole DNS array every time because, in case we have a DHCPOFFER
containing more DNS servers than the DHCPACK, we probably don't want to have a
"mix" between DNS servers from the DHCPOFFER and from the DHCPACK at the same
time.
But this could break users having a DHCP server sending only DNS information in
the DHCPOFFER and nothing in the DHCPACK.

Do such kind of DHCP server implementation exist?
Do you see a better way than resetting the whole array every time?