BOOTP/DHCP packets minimum size


DHCP relayer would (correctly) refuse too short DHCP packets*
*Issue:* According to the BOOTP and DHCP RFCs, all BOOTP/DHCP packets must be at least 300 bytes in size, and use padding to reach this size if the actual data is shorter. The DHCP relayer used to verify this size and drop packets *Problem:* Some DHCP clients/servers (notably, the Windows 2000 DHCP server) violate the RFCs and send packets shorter than 300 bytes. *Results:* Misbehaving clients would be unable to aquire DHCP leases through the DHCP relayer. Misbehaving servers would be unable to hand out DHCP leases through the DHCP relayer.

This means that the buildmsg.c patch should have the value of PAD_STOP set to a minimum value of 300. This parameter is controlling the size of the DHCP/BOOTP packet. Otherwise, there is a danger that firewalls, and routers configured as DHCP relay agents, may drop the dhcp messages as invalid. Some DHCP servers may refuse to respong to a discover or request that violates this rule.

Bill Moss
Professor, Mathematical Sciences
Clemson University

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