Hotplug usb0 without disconnecting wlan0

Turbocapitalist · 04-02-2018, 07:49 AM

I have a Raspbian 9.4 system set up as an access point but cannot hot-plug a network interface on usb0 without losing wlan0.

If I boot the machine I can connect from other machines to the network I've set up on wlan0 using dnsmasq either with or without usb0 in place:

A) If I have a network interface plugged into usb0 at boot time, the access point is available, including NAT (via iptables) as I expect.

B) If I have no network interface plugged into usb0 at boot time, then the access point is available, but only as a LAN as I expect.

The problem arises when I boot without a device on usb0 and then hot-plug it after the system is up and runnning. If I do that, then wlan0 disappears when usb0 comes up.

The contents of /etc/network/interfaces.d/wlan0 are:

Code:

iface wlan0 inet static
address 10.42.24.1
netmask 255.255.255.0

And the contents of /etc/network/interfaces.d/usb0 are:

Code:

allow-hotplug usb0
auto usb0
iface usb0 inet dhcp

What can I do to keep the access point on wlan0 available even after I hot-plug usb0?

jefro · 04-03-2018, 06:07 PM

This has been sitting here for a while so I'll make a guess.

I suspect that the metric of the usb nic is the reason. Maybe set metric for wifi over usb nic?

There other reason I guess could be that the dhcp of the usb affects the network or even static ip. In any case I don't really think it is a hotplug issue. Why do you suspect that?

Turbocapitalist · 04-16-2018, 01:33 AM

Thanks. I tried adding routing metrics to each interface with no identifiable results. The manual page interfaces(5) does not clarify what that option does. Which interface should have the higher metric integer? I'm not sure it matters because I've tried both ways, with one higher and the other lower and then vice versa.

What might be happening is that Raspbian 9.4 turns off the first interface when the second is added and then brings the first one up again. Other times the first interface just goes down and does not come back up. Either way this kills several daemons listening on the first interface which I need in order to reconnect and It seems that systemd does not seem to bring any of them back up automatically.

Turbocapitalist · 04-16-2018, 02:02 AM

As a followup, it seems that it is the interface that is down via the Network Manager. The Network Manager cannot find it after physically connecting the second networking device. If I bring the interface up manually via ifconfig it still remains unavailable to the outside.