[Openstack-operators] [Grizzly] Problems with DHCP Agents

Samuel Winchenbach swinchen at gmail.com
Tue Nov 26 14:18:04 UTC 2013


I will give it a shot.  Does it matter that it is inside the namespace?   I
noticed this rule outside of all the namespaces:

Chain POSTROUTING (policy ACCEPT 166K packets, 50M bytes)
 pkts bytes target     prot opt in     out     source
destination
 176M  224G nova-api-POSTROUTING  all  --  *      *       0.0.0.0/0
   0.0.0.0/0
    0     0 CHECKSUM   udp  --  *      virbr0  0.0.0.0/0
0.0.0.0/0            udp dpt:68 CHECKSUM fill

Thanks!



On Mon, Nov 25, 2013 at 10:43 PM, Lorin Hochstein
<lorin at nimbisservices.com>wrote:

>
>
> On Mon, Nov 25, 2013 at 7:27 PM, Samuel Winchenbach <swinchen at gmail.com>wrote:
>
>> Hi All,
>>
>> So I am experiencing a rather frustrating problem.  I have three nodes
>> running quantum dhcp agent (for high availability) and nova-compute.  I
>> found that certain VMs do not get an address if all three DHCP agents are
>> running.   Here is the output of dhcpdump on the tap interface to one of
>> the VMs:
>>
>> * WITHOUT test2 (worked - using test1 and test3)
>> http://pastie.org/pastes/8508325/text
>>
>>
>> * WITH test2 (did not work - using test1, test2, test3)
>> http://pastie.org/pastes/8508340/text
>>
>> The log files look fine, no errors in dnsmasq.log or dhcp-agent.log
>>
>> Here is what _seems_ to be in common when things do not work:
>> * the guest vm is using udhcpc as the DHCP client
>> * It is possible this only occurs when the DHCP agent is running on the
>> same node as the VM.
>>
>>
> If it's only failing when the DHCP server is on the same host as VM, my
> first guess would be the infamous DHCP checksum issue.
>
> Try adding this rule to the DHCP network namespace on each node that has a
> DHCP agent (change <uuid> to the appropriate id):
>
> ip netns exec qdhcp-<uuid> iptables -A POSTROUTING -t mangle -p udp
> --dport 68 -j CHECKSUM --checksum-fill
>
>
> This is happens if your machine is configured for hardware offloading of
> the UDP checksum calculations, and your DHCP packets don't get proper
> checksums if they don't cross a physical NIC. Certain DHCP clients barf if
> the checksum is invalid.
>
>
> Lorin
> --
> Lorin Hochstein
> Lead Architect - Cloud Services
> Nimbis Services, Inc.
> www.nimbisservices.com
>
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