[pve-devel] [WIP v2 cluster/network/manager/qemu-server/container 00/10] Add support for DHCP servers to SDN

DERUMIER, Alexandre alexandre.derumier at groupe-cyllene.com
Thu Oct 26 14:49:57 CEST 2023

Hi Stefan (Lendl),

I'm totally agreed with you, we should have persistent reservation,
at vm create/nic plug, nic delete, vm delete.

At least , for my usage with multiple cluster on different datacenters,
I really can wait to call ipam to api at each start (for scalability or
for security if ipam is down)

This also allow to simply do reservations in dnsmasq file without any
need to restart it. (AFAIK, openstack is using dnsmasq like this too)

I'm not sure if true dynamic ephemral ip , changing at each vm
stop/start is interesting for a server vm usage. (maybe for desktop
vmwhere you share a small pool of ip, but I personnaly don't known any
proxmox users using proxmox ve for this)

see my proposal here (with handle ephemeral && reserved, but it's even
easier with only reserved):


I think we could implement ipam call like:

create vm or add a new nic  --> 
qm create ... -net0

auto : search a free ip in ipam.  write the ip address in net0: ...,ip=
ip field  check if ip is free in ipam && register ip in ipam. write
the ip in ip field.

dynamic: write "ephemeral" in net0: ....,ip=ephemeral (This is a
dynamic ip registered at vm start, and release at vm stop)

vm start
- if ip=ephemeral, find && register a free ip in ipam, write it in vm
net0: ...,ip=[E] .   (maybe with a special flag [E] to
indicate it's ephemeral)
- read ip from vm config && inject in dhcp

if ip is ephemeral (netX: ip=[E]),  delete ip from ipam,
set ip=ephemeral in vm config

vm_destroy or nic remove/unplug
if netX: ...,ip=   ,  remove ip from ipam

nic update when vm is running:
if ip is defined : netX: ip=,  we don't allow bridge change
or ip change, as vm is not notified about theses changes, and still use
old ip.

We can allow nic hot-unplug && hotplug. (guest os will remove the ip on
nic removal, and will call dhcp again on nic hotplug)

nic hotplug with ip=auto:

--> add nic in pending state ----> find ip in ipam && write it in
pending ---> do the hotplug in qemu.

We need to handle the config revert to remove ip from ipam if the nic
hotplug is blocked in pending state(I never see this case until os
don't have pci_hotplug module loaded, but it's better to be carefull )


>>I am currently working on the SDN feature.  This is an initial review
>>the patch series and I am trying to make a strong case against
>>DHCP IP reservation.
>>The current state of the patch series invokes the IPAM on every VM/CT
>>start/stop to add or remove the IP from the IPAM.
>>This triggers the dnsmasq config generation on the specific host with
>>only the MAC/IP mapping of that particular host.

From reading the discussion of the v1 patch series I understand this
approach tries to implement the ephemeral IP reservation strategy. From
off-list conversations with Stefan Hanreich, I agree that having
ephemeral IP reservation coordinated by the IPAM requires us to
re-implement DHCP functionality in the IPAM and heavily rely on syncing
between the different services.

To maintain reliable sync we need to hook into many different places
where the IPAM need to be queried.  Any issues with the implementation
may lead to IPAM and DHCP local config state running out of sync
network issues duplicate multiple IPs.

Furthermore, every interaction with the IPAM requires a cluster-wide
lock on the IPAM. Having a central cluster-wide lock on every VM
start/stop/migrate will significantly limit parallel operations.  Event
starting two VMs in parallel will be limited by this central lock. At
boot trying to start many VMs (ideally as much in parallel as possible)
is limited by the central IPAM lock even further.

I argue that we shall not support ephemeral IPs altogether.
The alternative is to make all IPAM reservations persistent.

Using persistent IPs only reduces the interactions of VM/CTs with the
IPAM to a minimum of NIC joining a subnet and NIC leaving a subnet. I
deliberately not referring to VMs because a VM may be part of multiple
VNets or even multiple times in the same VNet (regardless if that is

Cases the IPAM needs to be involved:

- NIC with DHCP enabled VNet is added to VM config
- NIC with DHCP enabled VNet is removed from VM config
- NIC is assigned to another Bridge
  can be treated as individual leave + join events

Cases that are explicitly not covered but may be added if desired:

- Manually assign an IP address on a NIC
  will not be automatically visible in the IPAM
- Manually change the MAC on a NIC
  don't do that > you are on your own.
  Not handled > change in IPAM manually

Once an IP is reserved via IPAM, the dnsmasq config can be generated
stateless and idempotent from the pve IPAM and is identical on all
regardless if a VM/CT actually resides on that node or is running or
stopped.  This is especially useful for VM migration because the IP
stays consistent without spacial considering.

Snapshot/revert, backup/restore, suspend/hibernate/resume cases are
automatically covered because the IP will already be reserved for that

If the admin wants to change, the IP of a VM this can be done via the
IPAM API/UI which will have to be implemented separately.

A limitation of this approach vs dynamic IP reservation is that the IP
range on the subnet needs to be large enough to hold all IPs of all,
even stopped, VMs in that subnet. This is in contrast to default DHCP
functionality where only the number of actively running VMs is limited.
It should be enough to mention this in the docs.

I will further review the code an try to implement the aforementioned

Best regards,
Stefan Lendl

Stefan Hanreich <s.hanreich at proxmox.com> writes:

> This is a WIP patch series, since I will be gone for 3 weeks and
> wanted to
> share my current progress with the DHCP support for SDN.
> This patch series adds support for automatically deploying dnsmasq as
> a DHCP
> server to a simple SDN Zone.
> While certainly not 100% polished on some ends (looking at restarting
> systemd
> services in particular), the general idea behind the mechanism shows.
> I wanted
> to gather some feedback on how I approached designing the plugins and
> the
> config regeneration process before comitting to this design by
> creating an API
> and UI around it.
> You need to install dnsmasq (and disable it afterwards):
>   apt install dnsmasq && systemctl disable --now dnsmasq
> You can use the following example configuration for deploying a DHCP
> server in
> a SDN subnet:
> /etc/pve/sdn/dhcp.cfg:
>   dnsmasq: nat
> /etc/pve/sdn/zones.cfg:
>   simple: DHCPNAT
>           ipam pve
> /etc/pve/sdn/vnets.cfg:
>   vnet: dhcpnat
>           zone DHCPNAT
> /etc/pve/sdn/subnets.cfg:
>   subnet: DHCPNAT-
>           vnet dhcpnat
>           dhcp-dns-server
>           dhcp-range server=nat,start-address=,end-
> address=
>           gateway
>           snat 1
> Then apply the SDN configuration:
>   pvesh set /cluster/sdn
> You need to apply the SDN configuration once after adding the dhcp-
> range lines
> to the configuration, since the running configuration is used for
> managing
> DHCP. It will not work otherwise!
> For testing it can be helpful to monitor the following files (e.g.
> with watch)
> to find out what is happening
>   * /etc/dnsmasq.d/<dhcp_id>/ethers (on each node)
>   * /etc/pve/priv/ipam.db
> Changes from v1 -> v2:
>   * added hooks for handling DHCP when starting / stopping / .. VMs
> and CTs
>   * Get an IP from IPAM and register that IP in the DHCP server
>     (pve only for now)
>   * remove lease-time, since it is now infinite and managed by the VM
> lifecycle
>   * add hooks for setting & deleting DHCP mappings to DHCP plugins
>   * modified interface of the abstract class to reflect new
> requirements
>   * added helpers in existing SDN classes
>   * simplified DHCP configuration settings
> pve-cluster:
> Stefan Hanreich (1):
>   cluster files: add dhcp.cfg
>  src/PVE/Cluster.pm  | 1 +
>  src/pmxcfs/status.c | 1 +
>  2 files changed, 2 insertions(+)
> pve-network:
> Stefan Hanreich (6):
>   subnets: vnets: preparations for DHCP plugins
>   dhcp: add abstract class for DHCP plugins
>   dhcp: subnet: add DHCP options to subnet configuration
>   dhcp: add DHCP plugin for dnsmasq
>   ipam: Add helper methods for DHCP to PVE IPAM
>   dhcp: regenerate config for DHCP servers on reload
>  debian/control                         |   1 +
>  src/PVE/Network/SDN.pm                 |  11 +-
>  src/PVE/Network/SDN/Dhcp.pm            | 192
> +++++++++++++++++++++++++
>  src/PVE/Network/SDN/Dhcp/Dnsmasq.pm    | 186
> ++++++++++++++++++++++++
>  src/PVE/Network/SDN/Dhcp/Makefile      |   8 ++
>  src/PVE/Network/SDN/Dhcp/Plugin.pm     |  83 +++++++++++
>  src/PVE/Network/SDN/Ipams/PVEPlugin.pm |  64 +++++++++
>  src/PVE/Network/SDN/Makefile           |   3 +-
>  src/PVE/Network/SDN/SubnetPlugin.pm    |  32 +++++
>  src/PVE/Network/SDN/Subnets.pm         |  43 ++++--
>  src/PVE/Network/SDN/Vnets.pm           |  27 ++--
>  11 files changed, 622 insertions(+), 28 deletions(-)
>  create mode 100644 src/PVE/Network/SDN/Dhcp.pm
>  create mode 100644 src/PVE/Network/SDN/Dhcp/Dnsmasq.pm
>  create mode 100644 src/PVE/Network/SDN/Dhcp/Makefile
>  create mode 100644 src/PVE/Network/SDN/Dhcp/Plugin.pm
> pve-manager:
> Stefan Hanreich (1):
>   sdn: regenerate DHCP config on reload
>  PVE/API2/Network.pm | 1 +
>  1 file changed, 1 insertion(+)
> qemu-server:
> Stefan Hanreich (1):
>   sdn: dhcp: add DHCP setup to vm-network-scripts
>  PVE/QemuServer.pm                 | 14 ++++++++++++++
>  vm-network-scripts/pve-bridge     |  3 +++
>  vm-network-scripts/pve-bridgedown | 19 +++++++++++++++++++
>  3 files changed, 36 insertions(+)
> pve-container:
> Stefan Hanreich (1):
>   sdn: dhcp: setup DHCP mappings in LXC hooks
>  src/PVE/LXC.pm            | 10 ++++++++++
>  src/lxc-pve-poststop-hook |  1 +
>  src/lxc-pve-prestart-hook |  9 +++++++++
>  3 files changed, 20 insertions(+)
> Summary over all repositories:
>   20 files changed, 681 insertions(+), 28 deletions(-)
> --
> murpp v0.4.0
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