dhcpd problem on virtual machines

[Alex Zeffertt]

Alex Zeffertt wrote:
> All,
> 
> I've discovered an issue with running dhcpd in one virtual machine and 
> dhclient in another virtual machine on the same physical host.  I saw 
> this problem on a Xen host with paravirtualised linux guests.  The 
> problem is that the REQUEST sent by the client when half the lease has 
> expired is ignored by the server.
> 
> 
> I'll describe what happens step by step:
> 
>  1.  dhclient sends a DISCOVER over an AF_PACKET socket and gets an 
> OFFER back
>  2.  dhclient sends a REQUEST over the AF_PACKET socket and gets an ACK 
> back
>  3.  half the lease period passes
>  4.  dhclient sends a REQUEST over an *AF_INET* socket
>  5.  The sending network device advertises checksum offload so linux 
> only checksums the UDP pseudo header before passing the packet to the 
> driver.
>  6.  The receiving network device sets the packet's ip_summed flag to 
> CHECKSUM_PARTIAL.  This tells linux that there is no need to verify the 
> UDP checksum before passing to userspace, but that it must be completed 
> by hardware or software if the packet is forwarded.
>  7.  dhcpd receives the packet over an AF_PACKET socket, finds the 
> checksum is invalid, and silently discards it.
> 
> 
> There are several possible solutions to this, and I'm keen to find out 
> which of these - if any - seems the best to you:
> 
>  i)   Always use AF_PACKET in dhclient
>  ii)  Use AF_INET in dhcpd for receiving packets instead of AF_PACKET (*)
>  iii) Don't verify the checksum in dhcpd if the hardware supports 
> checksum offload
> 

I'm attaching a patch which does (i) above.  I'm a dhcp-hacker n00bie so I'd be 
interested to know if you think this fix would break anything, or if there is a 
better way to resolve this issue.

Regards,

Alex
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