DHCPNAK after neutron-dhcp-agent restart

[Andrew Woodward]

(backport of https://bugs.launchpad.net/neutron/+bug/1345947)

After rolling out a configuration change, we restarted neutron-dhcp-agent service, and then dnsmasq logs start flooding: DHCPNAK ... lease not found.
DHCPNAK is replied by dnsmasq for all DHCPREQUEST renews from all VMs. However the MAC and IP pairs exist in host files.
The log flooding increases when more and more VMs start renewing and they keep retrying until IP expire and send DHCPDISCOVER and reinit the IP.
The log flooding gradually disappears when the VMs IP expire and send DHCPDISCOVER, to which dnsmasq respond DHCPOFFER properly.

Analysis:
I noticed that option --leasefile-ro is used in dnsmasq command when started by neutron dhcp-agent. According to dnsmasq manual, this option should be used together with --dhcp-script to customize the lease database. However, the option --dhcp-script was removed when fixing bug 1202392.
Because of this, dnsmasq will not save lease information in persistent storage, and when it is restarted, lease information is lost.

Solution:
Simply replace --leasefile-ro by --dhcp-leasefile=<path to dhcp runtime files>/lease would solve the problem. (patch attached)

[Sergey Kolekonov (skolekonov)]

The fix for this bug is available in upstream stable/kilo branch and has been already backported to MOS.
The commit is https://github.com/openstack/neutron/commit/c07687c985703609ccc80f0151a2a40e2036a3f4

[Oleg Bondarev (obondarev)]

Was it reproduced on MOS 7.0?

[Vitaly Sedelnik (vsedelnik)]

Andrew - please confirm this issues affects MOS 7.0 and provide exact steps to reproduce. Note, that per comment #1 the backport you are requesting is already in the 7.0 branch.

[Andrew Woodward (xarses)]

@Vitaly,

If the patch is already in 7.0 then it's resolved for 7. Status was New in order for someone to check

(It was reproduced in 6 so added confirmed there and new for 7)

[Denis Meltsaykin (dmeltsaykin)]

Andrew, was the bug reproduced in 6.1 release or 6.0? 6.1 contains already the fix you've mentioned.

[Alexey Khivin (akhivin)]

already merged into 6.1
https://review.fuel-infra.org/#/c/7057/

[Denis Meltsaykin (dmeltsaykin)]

Andrew, could you clarify is this a customer-found bug and what are affected releases?

[Eugene Nikanorov (enikanorov)]

6.1+ are not affected since we are providing lease database to each dhcp server (dnsmasq).