dhcrelay '-A' command line option does nothing

[Evan Hunt]

Thanks very much for the report, Mark.  As it happens I'm working on
dhcrelay at the moment for other reasons, so your timing is excellent.

On Tue, May 06, 2008 at 11:06:13AM +1200, mark mckinstry wrote:
> The '-A' command line option for dhcrelay does not work. According to dhcrelay(8)  it is supposed to set the maximum size to which a DHCP request packet can grow after dhcrelay inserts option 82, before forwarding the packet to the server - but the code to achieve this is missing. In the code the '-A' option does set the value of global variable dhcp_max_agent_option_packet_length, but this variable is then never used! Currently dhcrelay will insert option 82 only if there is sufficient option padding, otherwise it drops the packet! - it never increases the size of the packet.
> 
> Some DHCP clients seem to insert sufficient option padding to allow option 82 insertion (e.g. ISC dhclient), but others don't (e.g. Windows XP), and this is a problem for us.
> 
> We are currently using dhcrelay 3.0.4, but comparing it to 4.0.0 I can see no relevant code changes, so presumably the issue still exists. I searched the forum archives, but could find nothing relevant. I'm surprised nobody has noticed this before!
> 
> We have done our own fix for this - see patch below - but we'd prefer not to have to maintain such patches, so could ISC please implement a fix in the next release?
> 
> 
> --- dhcp-3.0.4/relay/dhcrelay.c	2008-04-22 10:02:20.000000000 +1200
> +++ dhcp-3.0.4/relay/dhcrelay.c.new	2008-04-22 10:05:02.000000000 +1200
> @@ -873,6 +873,17 @@
>  	if (memcmp(packet->options, DHCP_OPTIONS_COOKIE, 4))
>  		return length;
>  
> +  /* ATL addition - for support when the option 82 cannot be inserted to the DHCP message, 
> +   * if there is not enough paddings after the option field in the DHCP message.
> +   * Change the DHCP msg length. It is actually implementing the -A option setting that 
> +   * was missing in the code.
> +   */
> +  if ( length != dhcp_max_agent_option_packet_length )
> +  {
> +    length = dhcp_max_agent_option_packet_length;
> +  }
> +  /* ATL addition end */
> +
>  	max = ((u_int8_t *)packet) + length;
>  
>  	/* Commence processing after the cookie. */
> 
> 
> Thanks
> Mark
> 
> ----------------------------
> Mark McKinstry
> Senior Software Engineer
> Allied Telesis Labs
> Christchurch, New Zealand
> 
> 
> 

-- 
Evan Hunt -- evan_hunt at isc.org
Internet Systems Consortium, Inc.