"malformed option dhcp.tcode" every so often on Linux router

psycroptic · 08-08-2014, 11:31 PM

Is an Archlinux PC. dhclient has recently occasionally (once every couple of days) been throwing up this error in logs:

Code:

Aug 08 09:54:42 Gateway dhclient[334]: parse_option_buffer: malformed option dhcp.tcode (code 101): option length exceeds option buffer length.

seems like this is a malformed packet from my ISPs DHCP server, as dhclient is only used on the WAN interface of this router.

Ser Olmy · 08-08-2014, 11:51 PM

It would seem that dhclient and the DHCP service used by your ISP have a difference of opinion with regards to what actually constitutes option 101.

Option 101 is supposed to be a POSIX timezone string and can be of variable length (specified by a single-byte length indicator). It's defined by RFC 4833.

You could capture the packet with tcpdump and see what the problem is. Could be something as simple as the server erroneously sending a null-terminated string.

psycroptic · 08-10-2014, 04:42 PM

word. my ISP is Comcast. it doesn't seem like it's causing me any problems, just a minor blip in my logs. if i get interested enough i might packet trace it.