cloud-init and UseDomains=yes

[victor_virtual]

1. Setup

• I use the PROXMOX 6 (proxmox-ve: 6.1-2)
• I set up a DNS server with dnsmasq
• dnsmasq is configured to send search-domains
• The VM is running Arch Linux
• Within the VM systemd-networkd and systemd-resolved are used
• I use "cloud-init" 19.3 with netplan 0.98-1

2. Problem

There are no search domains configured in "/etc/resolv.conf" although "dnsmasq" sends them to the VM.

# [...]
nameserver 10.0.0.2

3. Logs

Extract from dnsmasq logs: This shows the options transferred from dnsmasq to the dhcp client (VM). I replaced all "internal" IP-addresses.

option: 53 message-type  5
option: 54 server-identifier  10.0.0.3
option: 51 lease-time  12h
option: 58 T1  6h
option: 59 T2  10h30m
option:  1 netmask  255.255.255.0
option: 28 broadcast  10.0.0.255
option: 12 hostname  server1
option: 42 ntp-server  213.239.239.166
option: 15 domain-name  in.exmple.com
option:119 domain-search  xxxxx
option:  6 dns-server  10.0.0.2
option:  3 router  10.0.0.1

4. Configs

The configuration of resolved looks like this:

resolvectl status

Link 2 (eth0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
DefaultRoute setting: yes
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: allow-downgrade
    DNSSEC supported: yes
  Current DNS Server: 10.0.0.2
         DNS Servers: 10.0.0.2

The network cloud-init configuration for the vm contains this:

$ sudo qm cloudinit dump 1 network

version: 1
config:
    - type: physical
      name: eth0
      mac_address: 'xxxxx'
      subnets:
      - type: dhcp4
      - type: dhcp6
    - type: nameserver
      address:
      - '10.0.0.2'
      search:
      - 'in.example.com'

This is the data netplan got via DHCP:

% sudo netplan ip leases eth0

# This is private data. Do not parse.
ADDRESS=10.0.0.3
NETMASK=255.255.255.0
ROUTER=10.0.0.1
SERVER_ADDRESS=10.0.0.2
NEXT_SERVER=10.0.0.2
BROADCAST=10.0.0.255
T1=21600
T2=37800
LIFETIME=43200
DNS=10.0.0.2
NTP=213.239.239.166
DOMAINNAME=in.example.com
DOMAIN_SEARCH_LIST=in.example.com
HOSTNAME=server1
CLIENTID=xxxxxx

The generated .network-file for networkd looks like this:

% cat /run/systemd/network/10-netplan-*.network

[Match]
MACAddress=xxxxxxx
Name=eth0

[Network]
DHCP=yes
LinkLocalAddressing=ipv6

[DHCP]
RouteMetric=100
UseMTU=true

5. Findings

If I modify the .network file manually and restart networkd + resolved, the /etc/resolv.conf contains the configured domains.

% cat /run/systemd/network/10-netplan-*.network

[Match]
MACAddress=xxxxxxx
Name=eth0

[Network]
##### MODIFICATION: BEGIN #########
UseDomains=yes
##### MODIFICATION: END #########
DHCP=yes
LinkLocalAddressing=ipv6

[DHCP]
RouteMetric=100
UseMTU=true

6. Question

What do I need to do to make proxmox/cloud-init/netplan generate the "correct" .network file?

 

[Alwin]

option: 15 domain-name in.exmple.com

I assume this is a typo.

The network cloud-init configuration for the vm contains this:

There is no need to configure a cloud-init network, if a DHCP server pushes those to the VM anyway.

UseDomains=yes

Sadly this is the only thing that needs to be set beforehand. This doesn't seem to be done by cloud-init. Maybe you can use the runcmd [0] for this. You will need to try with a custom could-init config [1].

Another thing, might be that LLMNR [2] gets in the way.

[0] https://cloudinit.readthedocs.io/en/latest/topics/modules.html#runcmd
[1] https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_deploying_cloud_init_templates
[2] https://wiki.archlinux.org/index.php/Systemd-networkd#systemd-resolve_not_searching_the_local_domain

 

[Hatifnatt]

For anybody who found this during search

UseDomains inside [Network] section was added in systemd version 256.
https://www.freedesktop.org/software/systemd/man/latest/systemd.network.html#UseDomains=

If you running earlier versions of systemd you probably can use
UseDomains inside [DHCP] section, this option was added in systemd version 216.
https://www.freedesktop.org/software/systemd/man/latest/systemd.network.html#UseDomains=1

Also since systemd version 256 you can set UseDomains inside [Network], [DHCPv4], [DHCPv6] sections of systemd-networkd global configuration
i.e. in /etc/systemd/networkd.conf or better in drop-in config like /etc/systemd/networkd.conf.d/dhcp-use-domains.conf
https://www.freedesktop.org/software/systemd/man/latest/networkd.conf.html#UseDomains=
https://www.freedesktop.org/software/systemd/man/latest/networkd.conf.html#UseDomains=2
https://www.freedesktop.org/software/systemd/man/latest/networkd.conf.html#UseDomains=3

As of 2025.04.16 there is still no easy way to add

dhcp4-overrides:
  use-domains: true

to each interface of the VM via cloud-init, at least not any way I'm aware of.

In commits referencing this https://github.com/canonical/cloud-init/issues/4764 issue, you can find workaround using python scrip to enrich each of ethernets in Netplan configuration created by cloud-init with

dhcp4-overrides:
  use-domains: true

configuration.