Cfrtjryk

Here is my interfaces file:

auto lo

iface lo inet loopback



auto eth0

iface eth0 inet dhcp



auto eth0:0

iface eth0:0 inet static

        address 10.20.8.231

        netmask 255.255.255.0

        gateway 10.20.8.1

        up /sbin/route add 1.2.3.4 gw 10.20.8.1

        down /sbin/route del 1.2.3.4 gw 10.20.8.1

From the multiple examples I have found online this should add routes when eth0:0 comes up but for some reason I can't get it to work.

When I try to bring up the interface with ifup eth0:0 I get:

RTNETLINK answers: File exists

Failed to bring up eth0:0.

Then, ifconfig shows eth0:0 is up but route -n does not show the new route. In fact it shows the exact same routes I had before. Nothing changed.

Kernel IP routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

0.0.0.0         10.20.8.1       0.0.0.0         UG    0      0        0 eth0

10.20.8.0       0.0.0.0         255.255.255.0   U     0      0        0 eth0

This config is to be used on remote systems that can only be accessed via SSH. The whole idea is to keep eth0 set to dhcp as a backup when the site's network is changed (happened twice already) rendering the static ip unaccessible. The problem with this is that SSH (used by built in processes) uses the default route via eth0 and that doesn't work as the firewall (not managed by us) has been set to allow ssh only for the static IP on eth0:0.

Note: In the example above 1.2.3.4 represents our ssh server and I am trying to route traffic via eth0:0 for that single host but after testing I may want to set the default route to use eth0:0 instead, but I also tried that and it failed the same way. I was using this line:

up route add -net 0.0.0.0 netmask 0.0.0.0 gw 10.20.8.1

Thank you for your assistance.

I do not find particularly a good idea to place secondary IP addresses first.

The problem is far greater than you using the default route of eth0, as it is the first interface, when routing to other networks, your source IP is the DHCP IP address de facto and not 10.20.8.231.

As such I would recommend placing your static address in eth0 in lieu of eth0:0 with the following configuration:

auto eth0

iface eth0 inet static

    address 10.20.8.231

    netmask 255.255.255.0

    gateway 10.20.8.1



auto eth0:0

iface eth0:0 inet dhcp

It seems ifupdown does not play well with lower-level tools like iproute2 or net-tools. It is mentioned as warning in the official Debian documentation:

Warning

Do not use low level configuration tools such as ifconfig(8) and ip(8)
commands to configure an interface in up state.

You may try to manually flush and disable your network interface before using ifupdown to fix your issue:

 # ip addr flush dev [interface_name]

 # ip link set down dev [interface_name]

 # ifdown [interface_name]

 # ifup [interface_name]