mjhjmtu

I have an Ubuntu 16.04 (Xenial) box with two wifi interfaces. One (wlxAABBCC) is managed by Network Manager, the other (wlan0) is manually managed via /etc/network/interfaces.d. I want the default route for the machine itself though to use wlxAABBCC, not wlan0.

The plan is to bridge an ethernet-only device to a wireless router via wlan0 and use wlxAABBCC for everything else. So wlan0 passes on DHCP information and I can set up a custom ip route table that only passes on traffic from eth0's address. wlan0 still needs an address on the router though, so I need DHCP to do... something.

I tried this in /etc/network/interfaces.d/wlan0:

auto wlan0 
iface wlan0 inet dhcp
 wpa-conf /etc/network/wlan0_wpa.conf

But then I get two default routes:

$ ip route show
default via 192.168.1.1 dev wlan0 
default via 192.168.1.1 dev wlxAABBCC proto static metric 600

I don't want that. All traffic is just going to go through wlan0 now.

I tried setting the metric of wlxAABBCC to 0 with nmcli connection modify id "SSID" ipv4.route-metric 0
, but it still gets relegated to 1 by Network Manager when wlan0 comes up.

Then I tried this in /etc/network/interfaces.d/wlan0:

auto wlan0 
iface wlan0 inet dhcp
 wpa-conf /etc/network/wlan0_wpa.conf
 up ip route del default dev wlan0

But then in the brief time between wlan0 coming up and wlxAABBCC coming up, a bunch of services seem to "choose" wlan0 as their preferred interface and don't recover when this default route is dropped (including SSH and Avahi, so I can't connect).

I found this Server Fault question which suggests using a manual stanza and a custom DHCP configuration. So I tried:

auto wlan0 
iface wlan0 inet manual
 up /sbin/dhclient -4 -v -pf /run/dhclient.$IFACE.pid 
 -lf /var/lib/dhcp/dhclient.$IFACE.leases -I 
 -df /var/lib/dhcp/dhclient6.$IFACE.leases 
 -cf /etc/network/dhclient_wlan0.conf $IFACE
 down /sbin/dhclient -4 -v -r -pf /run/dhclient.$IFACE.pid 
 -lf /var/lib/dhcp/dhclient.$IFACE.leases -I 
 -df /var/lib/dhcp/dhclient6.$IFACE.leases 
 -cf /etc/dhcp/dhclient_wlan0.conf $IFACE
 wpa-conf /etc/network/wlan0_wpa.conf

...where /etc/network/dhclient_wlan0.conf is based on the default DHCP conf, but sans routers and any ...static-routes options:

send host-name = gethostname();
request subnet-mask, broadcast-address, time-offset,
 domain-name, domain-name-servers, domain-search, host-name,
 dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers,
 netbios-name-servers, netbios-scope, interface-mtu,
 ntp-servers;

timeout 300;

It still sets two default routes.

I added this to the custom DHCP configuration file:

interface wlan0 
 supersede routers ""

IT STILL SETS TWO DEFAULT ROUTES.

Why does DHCP still add a default route with this custom config? What do I need to do so that wlan0 can get an address via DHCP but isn't the default interface?