[Bug 1759014] Re: Netplan has no way to control DHCP client

[Tom Matthews]

netplan.io 0.96
systemd 240

It seems that dhcp6-overrides don't work for ignoring DHCP provided DNS
servers :

# cat /etc/netplan/10-enp3s0-init.yaml
network:
    version: 2
    renderer: networkd
    ethernets:
        enp3s0:
            critical: true
            dhcp4: true
            dhcp4-overrides:
                use-dns: false
            dhcp6: true
            dhcp6-overrides:
                use-dns: false
            ipv6-privacy: true
            nameservers:
                search: [home]
                addresses: [1.0.0.1, 1.1.1.1]

# resolvectl status enp3s0
Link 2 (enp3s0)
      Current Scopes: DNS
DefaultRoute setting: yes
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: opportunistic
      DNSSEC setting: yes
    DNSSEC supported: yes
  Current DNS Server: 1.0.0.1
         DNS Servers: 1.0.0.1
                      1.1.1.1
                      fd50:a94:67b3:0:26a7:dcff:fe27:a60   <--- Why this DHCP6 server ?
          DNS Domain: home

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https://bugs.launchpad.net/bugs/1759014

Title:
  Netplan has no way to control DHCP client

Status in netplan:
  Fix Released
Status in netplan.io package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  Confirmed
Status in netplan.io source package in Bionic:
  Fix Committed
Status in systemd source package in Bionic:
  Confirmed
Status in netplan.io source package in Cosmic:
  Fix Committed
Status in systemd source package in Cosmic:
  Confirmed
Status in netplan.io source package in Disco:
  Fix Released
Status in systemd source package in Disco:
  Confirmed

Bug description:
  [Impact]
  DHCP configurations where custom settings (routes, nameservers, etc.) need to be applied.

  [Test case]
  1) Configure netplan for the particulars of the network by configuring an appropriate dhcp{4,6}-override stanza:

  network:
    version: 2
    ethernets:
      engreen:
        dhcp4: true
        dhcp4-overrides:
          use-dns: false
          use-routes: false
          route-metric: 3333

  Additionally, if so required, add a custom DNS / routes to the
  configuration. e.g.

        nameservers:
          search: [lab, kitchen]
          addresses: [8.8.8.8]

  (See https://netplan.io/reference#dhcp-overrides for the available
  options)

  2) Run 'netplan apply' or reboot to have the configuration applied.
  3) Validate that the routes / DNS are properly ignored and/or replaced by the defined values.

  [Regression potential]
  Minimal; this adds new values to the configuration generated for networkd or NetworkManager. Existing configurations will remain unchanged, but new configurations using the dhcp{4,6}-overrides fields will benefit from additional flexibility.

  
  ---

  
  Currently DHCP appears to be an all or nothing boolean, which is insufficient for many network configurations.

  Ideally all of the DHCP configuration options supported by systemd would also be supported in netplan:
  https://www.freedesktop.org/software/systemd/man/systemd.network.html#%5BDHCP%5D%20Section%20Options

  As an example, consider the following netplan configuration:

  network:
    version: 2
    renderer: networkd
    ethernets:
      enp0s3:
        dhcp4: yes
        nameservers: [8.8.8.8,8.8.4.4]

  After running netplan apply I check the nameservers with systemd-
  resolve --status and it shows:

  DNS Servers: 8.8.8.8
               8.8.4.4
               192.168.1.1

  Here, "192.168.1.1" was provided by my DHCP server.  On this
  particular node, I only want the manually configured DNS servers, but
  netplan has no way to indicate this.

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