[Bug 1888391] Re: /etc/dhcp/dhclient-enter-hooks.d/resolved should not run in non-root network namespaces

[Steffan Karger]

You' re right. It's provided by isc-dhcp-client. Can' t recall how I
ended up with reporting this against systemd 4 years ago.

** Package changed: systemd (Ubuntu) => isc-dhcp (Ubuntu)

** Changed in: isc-dhcp (Ubuntu)
       Status: Invalid => New

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https://bugs.launchpad.net/bugs/1888391

Title:
  /etc/dhcp/dhclient-enter-hooks.d/resolved should not run in non-root
  network namespaces

Status in isc-dhcp package in Ubuntu:
  New

Bug description:
  Running dhclient in a (non-root) network namespace on a system with
  systemd enabled fails because the hook installed by the systemd
  package updates the DNS configuration of the core/root namespace,
  instead of simply the /etc/resolv.conf file of that particular network
  namespace.

  ("ip netns add <name>" nicely links /etc/resolv.conf in that namespace
  to /etc/netns/<name>/resolv.conf if it exists.)

  The following patch ensures that this script no longer misbehaves for
  this scenario:

  --- /etc/dhcp/dhclient-enter-hooks.d/resolved.bak	2020-07-21 14:05:23.390989468 +0200
  +++ /etc/dhcp/dhclient-enter-hooks.d/resolved	2020-07-21 14:03:18.566046842 +0200
  @@ -14,7 +14,7 @@
   #   (D) = master script downs interface
   #   (-) = master script does nothing with this

  -if [ -x /lib/systemd/systemd-resolved ] ; then
  +if [ -x /lib/systemd/systemd-resolved ] && [ -z "$(ip netns identify)" ] ; then
           # For safety, first undefine the nasty default make_resolv_conf()
           make_resolv_conf() { : ; }
           case "$reason" in

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