Why Qemu brings tap interface down after exit

Semant1ka · 07-07-2017, 07:06 AM

I want to set up tap device and then use this device instead of calling network setup script everytime qemu starts.

Code:

#!/bin/sh

sudo ip tuntap add tap0 mode tap
sudo ip link set dev tap0 up
sudo ip addr add 192.168.1.1/24 dev tap0

After tap0 created I start qemu like this:

Code:

$QEMU -m 768 -machine vexpress-a15 -serial stdio -net nic -net tap,ifname=tap0,script=no,downscript=no $user_flags -kernel $image_path/$image_name

However, after I am closing qemu tap0 interface goes down with its ip address removed from ip table, so instead of calling script above one time, I have to pass it as argument.

I wonder why so? How can I create persistent (at least till next reboot) tap interface for qemu? Qemu even doesn't have rights to change OS network settings, so it seems that OS itself switches off tap device.
My current OS is Debian Jessie.

dejank · 07-07-2017, 09:16 AM

Because, for persistent changes you must edit /etc/network/interfaces. And probably disable network-manager. Explained here: https://wiki.debian.org/QEMU.

Semant1ka · 07-07-2017, 10:02 AM

This device is not managed by network manager I guess

Code:

sudo nmcli dev status       
eth0    ethernet  connected     Wired connection 2 
eth1    ethernet  disconnected  --                 
wlan0   wifi      disconnected  --                 
lo      loopback  unmanaged     --                 
tap0    tap       unmanaged     --

I also added lines below to /etc/network/interfaces and this didn't help, maybe I forgot to add something

Code:

allow-hotplug tap0
iface tap0 inet manual
 pre-up ip tuntap add tap0 mode tap user root
 pre-up ip addr add 192.168.1.1/24 dev tap0
 up ip link set dev tap0 up

dejank · 07-07-2017, 10:23 AM

How about following that wiki article step by step? Did you install bridge-utils package? And then change your configuration to match that on the wiki.

Semant1ka · 07-07-2017, 11:04 AM

Maybe I am missing something from the wiki but I do not need and even do not want bridge or VDE, and can't see how repeating steps with bridge will help. And letting qemu to my company's network will be super bad and unwanted step, I need local network between qemu and host.

dejank · 07-07-2017, 01:09 PM

Without bridging, you can not do it like that, or at least I do not know how. Perhaps if you add some settings into the /etc/qemu-ifup, but not sure about it. You could do it with bridging, but without connecting your network device ( eth0 for example ) to the bridge. That should allow connection within host and guests connected to the bridge, but guests will not be able to see outside of that "localized" network.