RRZE Anleitungen

How to add a second (third, …) network interface to a VM?

By default a VM has only one network interface and the DHCP assignment works out of the box. With two or more interfaces, there’s a problem. Every interface tries to set the default route, but there can be only one. So, the first interface to be set up wins, and it’s not given that it is the first one specified in the template. The solution consists in specifying the priority, or metric, of the interfaces. Each one sets the default gateway given by the DHCP but with a different priority.

If you plan to have more than one network interface in your VM, then give maximum priority (or lowest metric) to the public one, otherwise the VM won’t be reachable. As being said, if the VM has only only one network card, then there is no need to specify a metric.

Now, some details on how to configure the network card of a VM. We consider here some well-known Linux distributions.

Debian, Ubuntu and derivatives

Debian and Ubuntu have only one file, /etc/network/interfaces. A typical example is

auto lo
iface lo inet loopback
auto eth0
iface eht0 inet dhcp

It is obvious that the loopback interface and eth0 are brought up automatically at boot (auto) and then configured.

For each additional interface, the auto statement, the iface definition, and the entry

metric 200

should be added

auto lo
iface lo inet loopback

auto eth0
iface eht0 inet dhcp

auto eth1
iface eht1 inet dhcp
metric 200

Ubuntu 18 and later (Netplan)

version: 2
ethernets:
    ens3:
        dhcp4: true
        match:
            macaddress: fa:16:3e:30:36:0c  # MAC address of first network interface
        mtu: 1500
        set-name: ens3

    ens6:
        dhcp4: true
        dhcp4-overrides:
          route-metric: 200
        match:
            macaddress: fa:16:3e:30:36:0d  # MAC address of second network interface
        mtu: 1500
        set-name: ens6

Fedora, CentOS and derivatives

The approach is very similar to SUSE. For each interface there is a script located in /etc/sysconfig/network-scripts, named ifcfg-, such as /etc/sysconfig/network-scripts/ifcfg-eth0 and so on.

The content should be

DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes

For every other interface, the DEVICE and the file name should be adapted, while the parameter METRIC=200 should be added. METRIC increases for every new interface. A higher metric means a decreasing priority of the route associated to that interface. The public interface has to be the one without the METRIC parameter, that is the one whose default route is the preferred.

SUSE

for each interface there is a script named /etc/sysconfig/network/ifcfg-, such as ifcfg-eth0, ifcfg-eth1 and so on. The content should be

DEVICE=eth0 #this is an example, change to eth1 in ifcfg-eth1 ...
STARTMODE=auto
BOOTPROTO=dhcp4
DHCLIENT_PRIMARY_DEVICE=yes

The value of the DEVICE directive should be adapted accordingly (i.e., eth1) while DHCLIENT_PRIMARY_DEVICE should be changed to no, then save the new file in a consistent way, i.e., ifcfg-eth1.

Remember: the device attached to the public network needs DHCLIENT_PRIMARY_DEVICE=yes, for all the others it should be set to no.

Can I reach my VM via a domain name?

All Floating IPs have a generic DNS Record. For Example 10.76.31.111 has the DNS A Record „float-10-76-31-111.cc.rrze.de“