[pve-devel] [PATCH pve-network] dnsmasq: configure static range for each subnet

[DERUMIER, Alexandre]

the debug log without my patch:

vm with 12:45:db:3a:04:97  got  192.168.2.10

Nov 15 21:33:31 formationkvm3 dnsmasq-dhcp[796025]: DHCPDISCOVER(vnetpve) 192.168.2.10 12:45:db:3a:04:97
Nov 15 21:33:31 formationkvm3 dnsmasq-dhcp[796025]: DHCPOFFER(vnetpve) 192.168.2.10 12:45:db:3a:04:97
Nov 15 21:33:31 formationkvm3 dnsmasq-dhcp[796025]: DHCPREQUEST(vnetpve) 192.168.2.10 12:45:db:3a:04:97
Nov 15 21:33:31 formationkvm3 dnsmasq-dhcp[796025]: DHCPACK(vnetpve) 192.168.2.10 12:45:db:3a:04:97 testovn1
Nov 15 21:34:47 formationkvm3 systemd[1]: Reloading dnsmasq at simpve.service - dnsmasq (simpve) - A lightweight DHCP and caching DNS server...

remove nic, assign another mac 12:45:db:3a:04:97 with 192.168.2.10

vm start, generate ether file + reload

Nov 15 21:34:47 formationkvm3 dnsmasq[796025]: cleared cache
Nov 15 21:34:47 formationkvm3 dnsmasq-dhcp[796025]: read /etc/dnsmasq.d/simpve/ethers
Nov 15 21:34:47 formationkvm3 systemd[1]: Reloaded dnsmasq at simpve.service - dnsmasq (simpve) - A lightweight DHCP and caching DNS server.

vm do the dhcp request and the server refuse because 192.168.2.10  is still leased to 12:45:db:3a:04:97

Nov 15 21:35:05 formationkvm3 dnsmasq-dhcp[796025]: not using configured address 192.168.2.10 because it is leased to 12:45:db:3a:04:97
Nov 15 21:35:05 formationkvm3 dnsmasq-dhcp[796025]: DHCPDISCOVER(vnetpve) 192.168.2.10 12:45:10:22:fb:fd no address available
Nov 15 21:35:09 formationkvm3 dnsmasq-dhcp[796025]: not using configured address 192.168.2.10 because it is leased to 12:45:db:3a:04:97
Nov 15 21:35:09 formationkvm3 dnsmasq-dhcp[796025]: DHCPDISCOVER(vnetpve) 192.168.2.10 12:45:10:22:fb:fd no address available
Nov 15 21:35:15 formationkvm3 dnsmasq-dhcp[796025]: not using configured address 192.168.2.10 because it is leased to 12:45:db:3a:04:97
Nov 15 21:35:15 formationkvm3 dnsmasq-dhcp[796025]: DHCPDISCOVER(vnetpve) 192.168.2.10 12:45:10:22:fb:fd no address available
Nov 15 21:35:26 formationkvm3 dnsmasq-dhcp[796025]: not using configured address 192.168.2.10 because it is leased to 12:45:db:3a:04:97
Nov 15 21:35:26 formationkvm3 dnsmasq-dhcp[796025]: DHCPDISCOVER(vnetpve) 192.168.2.10 12:45:10:22:fb:fd no address available


-------- Message initial --------
De: Stefan Lendl <s.lendl at proxmox.com>
À: "DERUMIER, Alexandre" <alexandre.derumier at groupe-cyllene.com>
Cc: pve-devel at lists.proxmox.com <pve-devel at lists.proxmox.com>
Objet: Re: [pve-devel] [PATCH pve-network] dnsmasq: configure static range for each subnet
Date: 16/11/2023 13:53:45

"DERUMIER, Alexandre" <alexandre.derumier at groupe-cyllene.com> writes:


I can reproduce easily 100%:


- create a nic with mac:xx:xx:xx:xY  ip: 192.168.0.10
- start vm. (the ether file is filed + reload)
- the guest do a dhcp request, the dnsmasq respond  a store the lease
in /var/lib/misc/zone.lease

- delete the nic


- add a new nic in same vm or another vm,  free found ip is
192.168.0.10  (because it was removed)


- start the vm (the ether file is upgrade with the new ip mac + reload)

- the guest do a dhcp request: the dnsmasq can't respond (with my last
patch) or give a dynamic ip in the range (with current implementation)
because it's still see his lease file the old mac:ip assocation


I experimented with several approaches with dnsmasq leases.
I cannot reproduce your example because it works in my examples.
My procedure:

dnsmasq config:
dhcp-range=set:DHCPNAT-10.1.0.0-16,10.1.0.0,static,255.255.0.0,infinite

VM 108 net1: MAC: bc:24:11:ad:0e:2e

qm set 108 --delete net1

dnsmasq lease file still contains the lease for MAC bc:24:11:ad:0e:2e

qm set 108 --net1 model=virtio,bridge=dhcpnat

- ethers file gets updated to new mac: BC:24:11:51:10:AD
- soon after dnsmasq lease gets updated to the new lease as well!
- correct IP assigned in the VM

I also tried:
- ip link set down > ip link set upi
- reboot
- force Stop the VM

So far, *this all works!*


I also tried with a short dhcp lease in dnsmasq. With this
configuration, the new IP will even propagate to the VM and set
correctly after IPAM update.

In my tests I used 30s but something like 5 or 10min should be fine as well.

dhcp-range=set:DHCPNAT-10.1.0.0-16,10.1.0.0,static,255.255.0.0,30

The VM is polling every ~60s as seen on the wire with tcpdump:

tcpdump -i dhcpnat -n port 67 or port 68

After I manually update the ethers file and `systemctl *reload*
dnsmasq`, it will respond with the new IP.

dnsmasq is running *locally only* so any DHCP queries are limited to the
local bridge.

The biggest problem and for me the reason I think it's not a feasible
solution, is that dnsmasq becomes a single point of failure.
If dnsmasq is offline, all of the VMs will have *NO IP*.