Routing Table Metric With Two Interfaces

Question

I've tried for many hours now to find a solution to my issue, but I can't seem to locate a permanent explanation - I'm also an ultra noob, so forgive me if I get the basics incorrect.

I have a server running on LTS Ubuntu 22.04 with two interfaces, eno1 and eno2 - one interface has an internet connection (eno1) which I would like to have priority over the interface without an internet connection (eno2). I have this setup because I'd like to transfer files between LAN over eno2 while utilising the eno1 network.

The issue I'm having is that when booting up the server or establishing a connection to eno2, a route entry is automatically generated giving the eno2 interface a metric of 0; which from what I've researched means that my network traffic will utilise that interface over others - which in turn, disables me from using the internet.

This is a server installation of Ubuntu, I'm looking for both interfaces to be on separate subnets as the PC connected to the server through LAN has a separate interface for an internet connection.

I don't need the hosts connected to eno2 to be able to have internet access, just for eno1 to be the default connection for the server.

What I've tried -

• Doing the command: sudo ip route del 192.168.0.0/24 dev eno 2 proto kernel scope link src 192.168.0.88 solves the issue, however, this is not permanent as when the system reboots or a connection is established through the interface it just recreates the route with a metric of 0.
• I've tried adding an identical route entry into /etc/netplan/00-installer-config.yaml but that just creates a new entry with a different metric.
• I've tried creating a script that will automatically delete the route when the system starts - but, this route is added back when a connection is made to the server's eno2 interface.

ip route:

default        via 192.168.0.1 dev   eno1   proto dhcp src 192.168.0.2  metric 100
192.168.0.0/24 dev eno2        proto kernel scope link src 192.168.0.88
192.168.0.0/24 dev eno1        proto kernel scope link src 192.168.0.2  metric 100
192.168.0.1    dev eno1        proto dhcp   scope link src 192.168.0.2  metric 100
194.168.4.100  via 192.168.0.1 dev   eno1   proto dhcp src 192.168.0.2  metric 100
194.168.8.100  via 192.168.0.1 dev   eno1   proto dhcp src 192.168.0.2  metric 100

route -n:

Destination     Gateway       Genmask           Flags Metric Ref Use Iface
0.0.0.0         192.168.0.1   0.0.0.0           UG    100    0   0   eno1
192.168.0.0     0.0.0.0       255.255.255.0     U     0      0   0   eno2
192.168.0.0     0.0.0.0       255.255.255.0     U     100    0   0   eno1
192.168.0.1     0.0.0.0       255.255.255.255   UH    100    0   0   eno1
192.168.4.100   192.168.0.1   255.255.255.255   UGH   100    0   0   eno1
192.168.8.100   192.168.0.1   255.255.255.255   UGH   100    0   0   eno1

/etc/netplan/00-installer-config.yaml:

network:
  ethernets:
    eno1:
      dhcp4: true
    eno2:
      dhcp4: false
      addresses: [192.168.0.88/24]
  version: 2

ip a

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdic noqueue state UNKNOWN group default qlen 1000
   link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
   inet 127.0.0.1/8 scope host lo
        valid_lft forever preferred_lft forever
   inet ::1/128 scope host
        valid_lft forever preferred_lft forever
2: eno1: <BROADCAST.MULTICAST.UP.LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
   link/ether (MAC) brd ff:ff:ff:ff:ff:ff
   altname enp3s0f0
   inet 192.168.0.2/24 metric 100 brd 192.168.0.255 scope global dynamic eno1
        valid_lft 86395sec preferred_lft 86395sec
   inet6 fe80::2fd:35ff:fefc:5648/64 scope link
        valid_lft forever preferred_lft forever
3: eno2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
   link/ether (MAC) brd ff:ff:ff:ff:ff:ff
   altname emp3s0f1
   inet 192.168.0.88/24 brd 192.168.0.255 scope global eno2
        valid_lft forever preferred_lft forever
   inet6 fe80::2fd:45ff:fefc:5649/64 scope link
        valid_lft forever preferred_lft forever

If there's anything you can help with, I'd be much appreciative. Also, if there's further information you need, I'll gladly oblige.

Thank you.

Answer 1

After some soul searching and furious trial and error I found the solution to my problem - which @mpboden may have contributed towards.

When setting the default route towards the router in netplan - I needed to add the nameservers for the internet to function.

The netplan that worked:

network:
  ethernets:
    eno1:
      dhcp4:false
      addresses: [192.168.0.2/24]
      routes:
        - to: default
          via: 192.168.0.1
          metric: 0
      nameservers:
        addresses: [8.8.8.8,8.8.4.4]
    eno2:
      dhcp4: false
      addresses: [192.168.1.88/24]
  version: 2

after saving the file, do netplan apply to apply the netplans changes.

Answer 2

You’ve stated that you want both eno1 and eno2 on separate subnets, but they’re actually on the same subnet: 192.168.0.0/24. So this appears to be your problem.

My suggestion then is to update your Netplan YAML file so that eno2 is on a different subnet. Of course, this would mean all hosts behind eno2 would need to be configured to be in this subnet as well.

If you want to be able to allow traffic to be forward between eno1 and eno2, then you need to enable IP forwarding.

To check if IP forwarding is enabled or not, use sysctl:

$ sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 0

In this example, net.ipv4.ip_forward is set to 0. That means it’s off. If it were set to 1, that would mean it’s enabled.

To enable, use the following command:

sysctl -w net.ipv4.ip_forward=1

Or…

echo 1 > /proc/sys/net/ipv4/ip_forward

To make the change persistent, edit /etc/sysctl.conf and add the following:

net.ipv4.ip_forward = 1

Then either reboot the system or run sysctl -p to load the new settings from /etc/sysctl.conf.

Finally, if you have a firewall configured on the system, then you may need to update that to allow traffic as well.