Ubuntu: Multiple interfaces, one lan only

Question

I have 2 interfaces:

wlan0: Connected to normal network with DHCP (has internet)
eth0: Connected to LAN only. Used for PXE boot and NFS

The network-manager service is not installed on this machine.

If I leave /etc/network/interfaces totally blank, both interfaces come up OK but each has a default gateway set. I run ip route:

default via 192.168.10.1 dev eth0 src 192.168.10.100 metric 202
default via 192.168.0.1 dev wlan0 src 192.168.0.20 metric 303
192.168.0.0/24 dev wlan0 proto kernel scope link src 192.168.0.20 metric 303
192.168.10.0/24 dev eth0 proto kernel scope link src 192.168.10.100 metric 202

The default route on 192.168.10.1 is in the way of internet connectivity. If I run sudo route del default and the ip route again:

default via 192.168.0.1 dev wlan0 src 192.168.0.20 metric 303
192.168.0.0/24 dev wlan0 proto kernel scope link src 192.168.0.20 metric 303
192.168.10.0/24 dev eth0 proto kernel scope link src 192.168.10.100 metric 202

Everything is good:

adam@sheep:~$ ping google.com
PING google.com (172.217.5.238) 56(84) bytes of data.
64 bytes from iad30s07-in-f238.1e100.net (172.217.5.238): icmp_seq=1 ttl=54 time=38.3 ms
64 bytes from iad30s07-in-f238.1e100.net (172.217.5.238): icmp_seq=2 ttl=54 time=38.7 ms

If I put anything at all in /etc/network/interfaces, wlan0 will not come up and the machine hangs at this boot step for about a minute: a start job is running for Raise network interfaces. Even this fails:

auto wlan0
iface wlan0 inet dhcp


auto eth0
iface eth0 inet dhcp

Am I missing something? Have I configured something incorrectly?
How do I get that default route on eth0 to never be set in the first place?

I get the feeling the end solution looks something like this:

auto wlan0
iface wlan0 inet dhcp


auto eth0
iface eth0 inet dhcp
up route del default

but I'm currently stuck because even a more basic config fails.

Could you provide more detail? I assume you're talking about the PXE server (which is serving DHCP that eth0 is connected to, but it's not clear from your comment. — Adam Brakhane, Apr 07 '19 at 05:14
Your wired network has some DHCP server which is assigning a gateway address, and should not be. — Michael Hampton, Apr 07 '19 at 14:03
That's it! I had to convince my dnsmasq dhcp server to stop sending out gateway and dns addresses. — Adam Brakhane, Apr 07 '19 at 18:22

score 0 · Answer 1 · answered Apr 07 '19 at 18:26

My problem was with the dnsmasq configuration on the PXE server. It was sending out a gateway address so even when that address was removed from a client, it would eventually be resent as the client got a new DHCP lease. The permanent fix suggested by Michael Hampton is to stop the DHCP server from suggesting a gateway.

I had

# DHCP server for PXE
interface=eno1
dhcp-range=192.168.10.50,192.168.10.150,12h
dhcp-option=66,192.168.10.1 #option 66 sets the address of the TFTP server

but needed to explicitly disable the gateway address from being sent:

interface=eno1
dhcp-range=192.168.10.50,192.168.10.150,12h
dhcp-option=66,192.168.10.1 # option 66 sets the address of the TFTP server
dhcp-option=3
dhcp-option=6

This sets the gateway (option 3) and dns (option 6) to nil

Ubuntu: Multiple interfaces, one lan only

1 Answers1