Re: [PATCH v11] dhcp, dhcpv6: Add hostname and client fqdn ops

[Stefano Brivio <sbrivio@redhat.com>]

On Fri,  7 Feb 2025 12:36:55 +0100
Enrique Llorente <ellorent@redhat.com> wrote:

> Both DHCPv4 and DHCPv6 has the capability to pass the hostname to
> clients, the DHCPv4 uses option 12 (hostname) while the DHCPv6 uses option 39
> (client fqdn), for some virt deployments like kubevirt is expected to
> have the VirtualMachine name as the guest hostname.
> 
> This change add the following arguments:
>  - -H --hostname NAME to configure the hostname DHCPv4 option(12)
>  - --fqdn NAME to configure client fqdn option for both DHCPv4(81) and
>    DHCPv6(39)
> 
> Signed-off-by: Enrique Llorente <ellorent@redhat.com>

I tried to break it but this time my little hammer broke instead...

Applied, thanks!

I noticed one (pre-existing) issue:

> +/* Total option size (excluding end option) is 576 (RFC 2131), minus
> + * offset of options (268), minus end option and its length (2).
> + */
> +#define OPT_MAX		306
> +
>  /**
>   * dhcp_init() - Initialise DHCP options
>   */
> @@ -122,7 +127,7 @@ struct msg {
>  	uint8_t sname[64];
>  	uint8_t file[128];
>  	uint32_t magic;
> -	uint8_t o[308];
> +	uint8_t o[OPT_MAX + 2 /* End option and its length */ ];

...actually, option 255 is special in that it takes just one byte,
because it has no length byte.

I just assumed things from my memory without re-reading the RFC, sorry
for that. That's RFC 2131, section 3.2.

It's different from option 80, Rapid Commit, RFC 4039, which takes two
bytes (zero length, and one length byte).

So we should change the two code lines above, and drop my stupid:

	m->o[offset++] = 0;

It's harmless because padding is harmless (option 0, also without
length byte, taking one byte), but not needed, and we could stuff one
extra byte in the response.

-- 
Stefano