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I have several VMs running on Ubuntu 9.10 via KVM+libvirt. I want to be able to find out the IP address that has been assigned to each host without physically opening a physical "console" to each machine and invoking ifconfig.

Consider:

rascher@localhost:~$ virsh -c qemu:///system list --all
Connecting to uri: qemu:///system
 Id Name                 State
----------------------------------
  1 machine1          running
  2 machine2          running
  - machine3          shut off

My network configuration looks like:

<network>
  <name>default</name>
  <uuid>1be...</uuid>
  <forward mode='route' dev="eth0"/>
  <bridge name='virbr0' stp='on' forwardDelay='0' />
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.254' />
    </dhcp>
  </ip>
</network>

So how can I get a listing which says:

machine1 IP address = 192.168.122.16
machine2 IP address = 192.168.122.238
...

I played with arp:

rascher@localhost:~$ arp
Address                  HWtype  HWaddress           Flags Mask            Iface
192.168.122.238          ether   00:16:36:00:61:b0   C                     virbr0
192.168.122.16           ether   00:16:36:52:e8:9c   C                     virbr0
...

But this doesn't map to a virtual machine's ID.

Is there some tool (via the command line, virsh or virt-*) I can ascertain this information? Or do I need to have some fancy script which runs on each individual VM, checks its own IP, and reports it back to the host OS?

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6 Answers 6

libvirt uses dnsmasq to provide DHCP to the guests, so you could trawl /var/log/daemon.log or dig through the leases file in /var/lib/libvirt to get an IP to hostname mapping.

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up vote 2 down vote accepted

So, when investigating this, I found that libvirt uses dnsmasq in order to do DHCP and DNS for guest OSes.

And dnsmasq will set the hostname in the hosts's DNS table based on whatever hostname it receives from the guest.

So in accordance with these instructions and a lot of googling, I simply needed to create and add this to /etc/dhclient.conf:

send host-name "machine1"

Now, from my host OS, I can ping machine1.

Does anyone know why I need to add the trailing "." in order for the DNS entry to resolve? How can I change this?

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1  
Without the trailing dot, your DNS resolver will append it's list of search domains to the hostname when it does a lookup. You could send a FQDN instead, e.g. machine1.example.com and then add example.com to your DNS search order. –  James Jun 23 '10 at 22:31
    
thanks for this. however the linked notes say to go on the main host machine and edit /etc/resolv.conf and add 192.168.122.1 as the First nameserver (i.e. add the libvirt dnsmasq as an NS), which of course doesnt work on most modern linuxes as there are multiple abstractions nowdays of the networking that rewrite /etc/resolv.conf. havent got mine quite figured out yet. –  don bright Jul 8 at 10:15

I had the same problem so I created the following script:

#!/bin/bash



function showMAC(){
    virsh dumpxml ${1}|grep "mac address"|sed "s/.*'\(.*\)'.*/\1/g"
}

function showIP(){
    for mac in $($0 -m $1); do
        grep $mac /var/log/daemon.log | tail -n 1 | awk '{print $7}'
    done
}

if test -z "${1}"; then
    echo "Usage: ${0} [-i | -m] <domain>"
    echo "  -i   Show IP address (the default)."
    echo "  -m   Show MAC address."
    exit
fi

addr_type="-i"

if test ${1} = "-i" || test ${1} = "-m"; then
    addr_type=${1}
    shift
fi

domain=${1}

test $addr_type = "-i" && showIP $domain || showMAC $domain
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On Ubuntu dnsmasq is used to provide DNS and DHCP services to the VMs. The dnsmasq processes on the host store their leases in this file:

/var/lib/misc/dnsmasq.leases

This is a normal text file and the lines in it might look similar to this here:

1362729847 52:54:de:ad:be:ef 192.168.122.254 vm-win7 01:52:54:de:ad:be:ef

The fields of interest to you are the third and the fourth column: the third field contains the IPv4 address of the VM and the fourth field either contains an asterisk or the hostname of the VM. This depends on the DHCP reply send by the guest to the dnsmasq service process.

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thanks for this. on my ubuntu machine the file is /var/lib/libvirt/dnsmasq/default.leases with the dhclient.conf file set to do send host-name "myvirtmachine"; as listed above –  don bright Apr 22 at 2:53

Lars Kellogg-Stedman has created a set of scripts to automate some of this process. He calls it 'virt-utils'.

He describes it in his blog post here: http://blog.oddbit.com/2013/10/04/automatic-dns-entrie/

He also has a github with some of the scripts he wrote, here:

https://github.com/larsks/virt-utils

You can basically just run this:

git clone https://github.com/larsks/virt-utils 
cd virt-utils 
sudo make install 
virt-hosts

and you will get a listing of each virtual machine by it's "domain name" inside libvirt's virtual-machine-manager. For example, on my machine I have 3 vms running.

don@serebryanya:~/src/virt-utils$ virt-hosts
192.168.122.23  mageia4.x64-net0.default.virt mageia4.x64.default.virt
192.168.122.197 debian7amd64-net0.default.virt debian7amd64.default.virt
192.168.122.15  freebsd10_amd64-net0.default.virt freebsd10_amd64.default.virt

Note, this is not the 'hostname' the VM itself is using, but for a large number of use-cases, it will be 'good enough' and solves the problem of having to 'ifconfig' from within each VM in dhcp land.

Lars' blog posting also shows a way for this to 'auto update' your own /etc/hosts file as libvirt starts and/or stops new VMs. This enables you to do things like ssh myname@fedora20vm or ssh myname@debian6vm without having to find the 192.168.122.x addresses by hand.

I have added a few very minor enhancements, like a script to spit out some ~/.ssh/config options (very very handy for using github on VMs, via Agent Forwarding), here:

https://github.com/donbright/virt-utils

I'd also like to note that the method of editing dhclient.conf to 'send host-name xxxxx' only works on systems that actually use dhclient.conf in a standard manner. Mageia, for example, has an unusual setup of how it's dhclient works, so the simple instructions wont necessarily work. However, with Lars' method, it works regarldess of the guest OS'es dhcp setup, because he is not relying on the VM to send it's hostname - he is using the 'domain names' within libvirt's machine manager.

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So how can I get a listing which says:

machine1 IP address = 192.168.122.16

machine2 IP address = 192.168.122.238

at least on fedora you can get that information this way:

cat /var/lib/libvirt/dnsmasq/default.leases

has an output similar to:

1412006226 52:54:00:fe:b3:c0 192.168.122.117 coreos0 01:52:54:00:fe:b3:c0

although that is a bit more than you asked for

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