Behavior with clients using "Broadcast Flag"

Nicolas C. dhcp at nryc.fr
Fri Oct 5 22:09:57 UTC 2012


Hi,

I'm using dhcpd version 4.1.1 (Debian Squeeze) and I have some Windows 
Seven clients doing some "discover" instead of "request". This is an 
example :

Sep 26 01:19:34 dhcpd: DHCPDISCOVER from 00:1c:c0:fc:f7:24 (VNDLOL65TEE) 
via 172.20.24.72
Sep 26 01:19:34 dhcpd: ICMP Echo reply while lease 172.20.24.86 valid.
Sep 26 01:19:34 dhcpd: if VNDLOL65TEE.domain.com IN TXT 
"31a956a7940aee9c9191f5c7770adf3ed5" rrset exists and 
VNDLOL65TEE.domain.com IN A 172.20.24.86 rrset exists delete 
VNDLOL65TEE.domain.com IN A 172.20.24.86: success.
Sep 26 01:19:34 dhcpd: if VNDLOL65TEE.domain.com IN A rrset doesn't 
exist and VNDLOL65TEE.domain.com IN AAAA rrset doesn't exist delete 
VNDLOL65TEE.domain.com IN TXT "31a956a7940aee9c9191f5c7770adf3ed5": success.
Sep 26 01:19:34 dhcpd: removed reverse map on 86.24.20.172.in-addr.arpa.
Sep 26 01:19:34 dhcpd: Abandoning IP address 172.20.24.86: pinged before 
offer

I think this behavior is caused by the DHCP "Broadcast flag" introduced 
by default by windows Vista (to be confirmed) :

http://blogs.technet.com/b/teamdhcp/archive/2009/02/12/dhcp-broadcast-flag-handling-in-windows-7.aspx

I'm trying to understand what's happening. The DHCP server :

  - receives a "DHCPDISCOVER", assumes that the client has no IP address 
but in fact the client is using its currently leased address (172.20.24.86).

  - finds a previous lease with the client hardware address, associated 
with the IP address 172.20.24.86

  - sends an ICMP message to the IP address before replying to the client

  - gets and ICMP response from the IP address and declares the lease 
invalid, cleans the DNS

  - assigns another IP address to the client (not displayed in the log).

Am I right?

Is there a workaround for this on the server-side, maybe on newer 
versions of dhcpd?

Off topic : we've been playing with the registry settings to reproduce / 
fix this problem and it's not working : the clients keep doing broadcast 
whatever the settings are and we cannot force a "fresh" install to do 
broadcat (!!).

Any help on this would be appreciated, the tech support is blaming the 
DHCP server :)

Regards,

Nicolas




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