[Howto] Automated DNS resolution for KVM/libvirt guests with a local domain

libvirt_logo-svg

I often run demos on my laptop with the help of libvirt. Managing 20+ machines that way is annoying when you have no DNS resolution for those. Luckily, with libvirt and NetworkManager, that can be easily solved.

The problem

Imagine you want to test something in a demo setup with 5 machines. You create the necessary VMs in your local KVM/libvirt environment – but you cannot address them properly by name. With 5 machines you also need to write down the appropriate IP addresses – that’s hardly practical.

It is possible to create static entries in the libvirt network configuration – however, that is still very inflexible, difficult to automate and only works for name resolution inside the libvirt environment. When you want to ssh into a running VM from the host, you again have to look up the IP.

Name resolution in  the host network would be possible by adding each entry to /etc/hosts additionally. But that would require the management of two lists at the same time. Not automated, far from dynamic, and very ponderous.

The solution

Luckily, there is an elegant solution: libvirt comes with its own in-build DNS server, dnsmasq. Configured properly, that can be used to serve DHCP and DNS to servers respecting a previous defined domain. Additionally, NetworkManager can be configured to use its own dnsmasq instance to resolve DNS entries – forwarding requests to the libvirt instance if needed.

That way, the only thing which has to be done is setting a proper host name inside the VMs. Everything else just works out of the box (with a recently Linux, see below).

The solution presented here is based on great post from Dominic Cleal.

Configuring libvirt

First of all, libvirt needs to be configured. Given that the network “default” is assigned to the relevant VMs, the configuration should look like this:

$ sudo virsh net-dumpxml default
<network connections='1'>
  <name>default</name>
  <uuid>158880c3-9adb-4a44-ab51-d0bc1c18cddc</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0'/>
  <mac address='52:54:00:fa:cb:e5'/>
  <domain name='qxyz.de' localOnly='yes'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.128' end='192.168.122.254'/>
    </dhcp>
  </ip>
</network>

The interesting part is below the mac address: a local domain is defined and marked as localOnly. That domain will be the authoritative domain for the relevant VMs, and libvirt will configure dnsmasq to act as a resolver for that domain. The attribute makes sure that DNS requests regarding that domain will never be forwarded upstream. This is important to avoid loop holes.

Configuring the VM guests

When the domain is set, the guests inside the VMs need to be defined. With recent Linux releases this is as simple as setting the host name:

$ sudo hostnamectl set-hostname neon.qxyz.de

There is no need to enter the host name anywhere else: the command above takes care of that. And the default configuration of DHCP clients of recent Linux releases sends this hostname together with the DHCP request – dnsmasq picks the host name automatically  up if the domain matches.

If you are on a Linux where the hostnamectl command does not work, or where the DHCP client does send the host name with the request – switch to a recent version of Fedora or RHEL 😉

In such cases the host name must be set manually, according to the documentation of the OS. Just ensure that the resolution of the name works locally. Also, the DHCP configuration must be altered to send along the host name. In older RHEL and Fedora versions for example the option

DHCP_HOSTNAME=neon.qxyz.de

had to be added to /etc/sysconfig/network-scripts/ifcfg-eth0.

At this point automatic name resolution between VMs should already work after a restart of libvirt.

Configuring NetworkManager

The last missing piece is the configuration of the actual KVM/libvirt host, so that the local domain, here qxyz.de, is properly resolved. Adding another name server to /etc/resolv.conf might work for a workstation with a fixed network connection, but certainly does not work for laptops which have changing network connections and DNS servers all the time. In such cases, the NetworkManager is often used anyway so we take advantage of its capabilities.

First of all, NetworkManager needs to start its own version of dnsmasq. That can be achieved with a simple configuration option:

$ cat /etc/NetworkManager/conf.d/localdns.conf 
[main]
dns=dnsmasq

This second dnsmasq instance just works out of the box. All DNS requests will automatically be forwarded to DNS servers acquired by NetworkManager via DHCP, for example. The only notable difference is that the entry in /etc/resolv.conf is different:

# Generated by NetworkManager
search whatever
nameserver 127.0.0.1

Now as a second step the second dnsmasq instance needs to know that for all requests regarding qxyz.de the libvirt dnsmasq instance has to be queried. This can be achieved with another rather simple configuration option, given the domain and the IP from the libvirt network configuration at the top of this blog post:

$ cat /etc/NetworkManager/dnsmasq.d/libvirt_dnsmasq.conf 
server=/qxyz.de/192.168.122.1

And that’s it, already. Restart NetworkManager and everything should be working fine.

As a side node: if the attribute localOnly would not have been set in the libvirt network configuration, queries for unknown qxyz.de entries would be forwarded from the libvirt dnsmasq to the NetworkManager dnsmasq – which would again forward them to the libvirt dnsmasq, and so on. That would quickly overload your dnsmasq servers, resulting in error messages:

dnsmasq[15426]: Maximum number of concurrent DNS queries reached (max: 150)

Summary

With these rather few and simple changes a local domain is established for both guest and host, making it easy to resolve their names everywhere. There is no need to maintain one or even two lists of static IP entries, everything is done automatically.

For me this is a huge relief, making it much easier in the future to set up demo and test environments. Also, it looks much nicer during a demo if you have FQDNs and not IP addresses. I can only recommend this setup to everyone who often uses libvirt/KVM on a local machine for test/demo environments.

[Short Tip] dnsmasq and /etc/hosts

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In case you do simple network tests with KVM virtual machines on your host, you might want to add some host names and IPs to /etc/hosts. However, that may not work: KVM, or better said libvirt ignores the entries in /etc/hosts. That is due to the fact that dnsmasq reads the entries of the file once: at startup. So you need to restart dnsmasq, or simply send it the SIGHUP signal:

killall -HUP dnsmasq

[Howto] Share Ethernet via Wifi with NetworkManager in KDE

KDE logoI recently was taking part in a training at a place which had poor cellular reception, no wifi and only one single ethernet connection. Thus we had to the ethernet via wifi. I tried to do just that with my laptop via NetworkManager – and it worked out of the box.

The basic situation is rather common: you have one single network connection, and want to share it to other people or devices via wifi. If you want to do that manually you have to set up the wifi network on your own, including encryption, need to bring up a dhcp server, configure the routing and NATing, and so on. That can take quite some time, and is nothing you want to do during some precious training hours.

Thus I simply tried to bring up a shared wifi via the NetworkManager applet in KDE:
Share-Wireless
After providing a SSID name and configuring some security credentials the process was already done, and I was notified that the network was set up and ready. It was also shown in the plasma applet besides the ethernet connection:
Plasma-Connection-Established
Plasma-Applet-Conncetions

And that was it already: everyone was able to connect to the network without any problems – and it didn’t even took me a minute to bring it all up. Since I know how much trouble it can be to bring such a connection up manually I was really impressed!

In case you want to give it a try, make sure that your wifi hardware and thus the appropriate driver; do support Access Point (AP) mode which is needed to bring up the wifi. If it says “for some devices only” you have no choice but give it a try.

By the way, in case you wonder about DNS and DHCP: both is done via dnsmasq as a local server, offering both. The DNS queries are forwarded to the DNS servers you got via DHCP from the ethernet connection (or, presumably the one you configured in NetworkManager).
However, I was not able to find any temporary configuration in /run or /var/ which showed the actual DNS servers – I had to call nm-tool to figure that one out:

$ nm-tool
- Device: eth0  [Standard-Ethernet] --------------------------------------------
[...]

  IPv4 Settings:
    Address:         192.168.3.27
    Prefix:          24 (255.255.255.0)
    Gateway:         192.168.3.1

    DNS:             192.168.2.4
    DNS:             192.168.2.3

If you know of any other way to find out these information, or even better simply the entire configuration of dnsmasq for that purpose please let me know =)

Besides, while the Plasma applet gave me the option to shut down the shared wifi network, I wasn’t able to bring it up again. There simply is no option in the network overview to fire up again such a network, thus I filled a bug report.

But, besides these two smaller issues, the plasma-nm applet and thus NetworkManager did a great job making sharing networks very easy.