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

[Russell Senior]

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