Missing dns-nameservers in eth0 with config drive on openstack instance

[Sabari Murugesan]

Cloud Provider: OpenStack (Mitaka)
Distro: Ubuntu 16.04
Cloud-init version: 0.7.9 (cloud-init cfg is distro provider)

Other Info:
Nova Instance booted with Config Drive on a Neutron Network (without DHCP)

Problem:
when a nova instance is booted with config drive on a network with DHCP disabled, the global DNS servers passed in the network_data.json are not rendered to the eth0 interface in the /etc/network/interface.d/50-cloud-init.cfg file. It is only available on the 'lo' interface. As a result, /etc/resolv.conf is empty and all name resolutions fail.

/etc/network/interface.d/50-cloud-init.cfg looks like this
auto lo
iface lo inet loopback
    dns-nameservers 8.8.8.8

auto eth0
iface eth0 inet static
    address 192.168.111.183
    netmask 255.255.255.0
    post-up route add default gw 192.168.111.1 || true
    pre-down route del default gw 192.168.111.1 || true

instead, IMO, it should probably look like this:-

auto lo
iface lo inet loopback
    dns-nameservers 8.8.8.8

auto eth0
iface eth0 inet static
    address 192.168.111.183
    dns-nameservers 8.8.8.8 // Notice this additional dns info in eth0
    netmask 255.255.255.0
    post-up route add default gw 192.168.111.1 || true
    pre-down route del default gw 192.168.111.1 || true

I tried the fix in bug 1693265 i.e https://review.openstack.org/#/c/467699/ but it
still didn't add the dns-nameservers in the eth0 of the ENI file.

Then, I tried a custom fix (attached nova.diff) to pass the dns_namesevers
in the "networks" value of the network_data.json and that seems to fix the problem for me.

At this point, I am not clear if this is a bug in Nova or Cloud-Init to render the dns servers in ENI file correctly. Please let me know if you need more info.

[Dan Peschman (dpeschman)]

Affects me, too, on OpenStack Pike & cloud-init 17.2-35-gf576b2a2-0ubuntu1~16.04.2.

[Ryan Harper (raharper)]

Hi,

Thanks for filing the bug.

Can you confirm that /etc/resolv.conf does *not* contain your DNS server entry? In Ubuntu cloud-images, which include the 'resolvconf' package, do get the dns entries off of *all* interfaces included in /etc/network/interfaces.d/* and combine them in /etc/resolv.conf via resolvconf package.

root@x1:~# cat /etc/cloud/build.info
build_name: server
serial: 20190621
root@x1:~# cat /etc/issue
Ubuntu 16.04.6 LTS \n \l

root@x1:~# cat /etc/network/interfaces.d/50-cloud-init.cfg
# This file is generated from information provided by
# the datasource. Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
auto lo
iface lo inet loopback
    dns-nameservers 8.8.8.8

auto eth0
iface eth0 inet static
    address 192.168.111.183
    netmask 255.255.255.0
    post-up route add default gw 192.168.111.1 || true
    pre-down route del default gw 192.168.111.1 || true
root@x1:~# cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 8.8.8.8

[Dan Peschman (dpeschman)]

Hello. Yes I can confirm that, when this was written, the nameservers on lo would not be present in /etc/resolv.conf.

[Ryan Harper (raharper)]

Is this still reproducible on current Ubuntu cloud-images? I can't reproduce this behavior you're describing.

# cat /etc/network/interfaces.d/50-cloud-init.cfg
# This file is generated from information provided by
# the datasource. Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
auto lo
iface lo inet loopback
    dns-nameservers 8.8.8.8

auto eth0
iface eth0 inet static
    address 192.168.111.183
    netmask 255.255.255.0
    post-up route add default gw 192.168.111.1 || true
    pre-down route del default gw 192.168.111.1 || true

Whenever networking service starts up, ifup on the lo interface will trigger resolvconf via this ifup hook that the resolvconf package installs:

# cat /etc/network/if-up.d/000resolvconf

That builds up a list of nameservers and search paths from the IF_DNS_NAMESERVER and DNS_SEARCH values found under the interface in the eni file, and then pipes the values to resolvconf -a lo.inet which then updates /run/resolvconf/resolv.conf to which /etc/resolv.conf is a symlink that points to the file.

root@x1:~# ls -al /etc/resolv.conf
lrwxrwxrwx 1 root root 29 Aug 7 09:15 /etc/resolv.conf -> ../run/resolvconf/resolv.conf
root@x1:~# ifup --force lo
+ [ -x /sbin/resolvconf ]
+ :
+ R=
+ [ ]
+ [ ]
+ [ ]
+ STANDARD_IFS=

+ IFS=

+ IFS=

+ + echo/sbin/resolvconf -n -a
 lo.inet
+ [ -x /sbin/resolvconf ]
+ :
+ R=
+ [ ]
+ [ ]
+ [ ]
+ R=nameserver 8.8.8.8

+ STANDARD_IFS=

+ IFS=

+ IFS=

+ echo -n nameserver 8.8.8.8

+ /sbin/resolvconf -a lo.inet

[Launchpad Janitor (janitor)]

[Expired for cloud-init because there has been no activity for 60 days.]

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/2980