[PATCH v2] udhcpc: ignore zero-length DHCP options

[Denys Vlasenko]

Applied, thanks.

On Tue, Mar 16, 2021 at 7:52 AM Russell Senior
<russell at personaltelco.net> wrote:
>
>
> Discovered that the DHCP server on a TrendNet router (unknown model)
> provides a zero-length option 12 (Host Name) in the DHCP ACK message. This
> has the effect of causing udhcpc to drop the rest of the options, including
> option 51 (IP Address Lease Time), 3 (Router), and 6 (Domain Name Server),
> most importantly leaving the OpenWrt device with no default gateway.
>
> The TrendNet behavior violates RFC 2132, which in Section 3.14 declares that
> option 12 has a miniumum length of 1 octet. It is perhaps not a cosmic
> coincidence that I found this behavior on Pi Day.
>
> This patch allows zero length options without bailing out, by simply
> skipping them.
>
> v2 changelog:
> * advance the optionptr by two bytes, not one;
> * add a message to warn about the rfc violation;
>
> Signed-off-by: Russell Senior <russell at personaltelco.net>
> ---
>  networking/udhcp/common.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/networking/udhcp/common.c b/networking/udhcp/common.c
> index 4bc719001..a16fd85d0 100644
> --- a/networking/udhcp/common.c
> +++ b/networking/udhcp/common.c
> @@ -277,8 +277,13 @@ uint8_t* FAST_FUNC udhcp_scan_options(struct dhcp_packet *packet, struct dhcp_sc
>                         goto complain; /* complain and return NULL */
>                 len = 2 + scan_state->optionptr[OPT_LEN];
>                 scan_state->rem -= len;
> -               /* So far no valid option with length 0 known. */
> -               if (scan_state->rem < 0 || scan_state->optionptr[OPT_LEN] == 0)
> +               /* skip any options with zero length */
> +               if (scan_state->optionptr[OPT_LEN] == 0) {
> +                       scan_state->optionptr += 2;
> +                       bb_simple_error_msg("warning: zero length DHCP option violates rfc2132, skipping");
> +                       continue;
> +               }
> +               if (scan_state->rem < 0)
>                         goto complain; /* complain and return NULL */
>
>                 if (scan_state->optionptr[OPT_CODE] == DHCP_OPTION_OVERLOAD) {
> --
> 2.30.1
>
>
> --
> Russell Senior, President
> russell at personaltelco.net
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