Re: Issues to set up a tap network backend

[Steve Litt]

Hi Daniele,

I can't completely understand either your needs or what you've done.
I'd suggest every time anybody uses a word like "bridge" or "tap" or
"DHCP" or names a file, they specify whether it's on the physical host
or the virtual guest. This can save a lot of misunderstanding and
confusion.

Meanwhile, just for fun, check out
http://www.troubleshooters.com/linux/qemu/nobs.htm . It's possible your
question might be answered from there. I know for sure that the moment
your guest instantiation command called for "net nic", you foreclosed
the possibility of your guest having an IP contactable from other
computers on the same subnet.

I don't normally top-post, but in this case I have no comments on
specifics of your message.

SteveT


Daniele Palmisano said on Sun, 11 Apr 2021 17:37:09 +0200

>Hi,
>I am writing as I am having some networking issues while I emulate a
>Raspberry PI image using QEMU on my Ubuntu 18.04 host machine.  I was
>wondering whether you could give me a hand to troubleshoot the issue.
>
>I would like to use a bridged adapter as a guest network type.
>Following the official Qemu documentation
>https://wiki.qemu.org/Documentation/Networking#Tap and an Archlinux
>section
>https://wiki.archlinux.org/index.php/QEMU#Bridged_networking_using_qemu-bridge-helper
>I ran the Rapberry PI using the following command:
>
>
>sudo qemu-system-arm \
>-kernel ./qemu-rpi-kernel/kernel-qemu-4.4.34-jessie \
>-append "root=/dev/sda2 panic=1 rootfstype=ext4 rw" \
>-hda 2021-01-11-raspios-buster-armhf-full.img \
>-cpu arm1176 -m 256 \
>-M versatilepb \
>-no-reboot \
>-serial stdio \
>-net nic,netdev=mynet0 -netdev bridge,br=br0,id=mynet0
>
>Before running the `qemu-system-arm` command, on the host, I created a
>bridge br0 with an IP address (192.168.0.14/24) removing the address
>previously assigned to the physical interface.
>My `ip address` output will look like:
>
>152: eth0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc
>fq_codel master br0 state UP group default qlen 1000
>    link/ether 4c:e1:73:4a:a7:80 brd ff:ff:ff:ff:ff:ff
>
>157: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
>state UP group default qlen 1000
>    link/ether 4c:e1:73:4a:a7:80 brd ff:ff:ff:ff:ff:ff
>    inet 192.168.0.14/24 brd 192.168.0.255 scope global dynamic br0
>       valid_lft 80984sec preferred_lft 80984sec
>    inet6 fe80::4ee1:73ff:fe4a:a780/64 scope link
>       valid_lft forever preferred_lft forever
>
>After running the `qemu-system-arm` command, a `tap0` interface will
>also be created and added into the bridge br0.
>
>XPS-13-7390 /etc/netplan » sudo brctl show
>bridge name bridge id STP enabled interfaces
>br0 8000.4ce1734aa780 no eth0
>tap0
>
>
>XPS-13-7390 /etc/netplan » ip ad
>1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc 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
>    inet6 ::1/128 scope host
>       valid_lft forever preferred_lft forever
>...
>152: eth0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc
>fq_codel master br0 state UP group default qlen 1000
>    link/ether 4c:e1:73:4a:a7:80 brd ff:ff:ff:ff:ff:ff
>157: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
>state UP group default qlen 1000
>    link/ether 4c:e1:73:4a:a7:80 brd ff:ff:ff:ff:ff:ff
>    inet 192.168.0.14/24 brd 192.168.0.255 scope global dynamic br0
>       valid_lft 80720sec preferred_lft 80720sec
>    inet6 fe80::4ee1:73ff:fe4a:a780/64 scope link
>       valid_lft forever preferred_lft forever
>158: tap0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel
>master br0 state UNKNOWN group default qlen 1000
>    link/ether fe:c1:a1:b7:45:bc brd ff:ff:ff:ff:ff:ff
>    inet6 fe80::fcc1:a1ff:feb7:45bc/64 scope link
>       valid_lft forever preferred_lft forever
>
>From the host I have got the Internet connectivity.
>From the guest OS, It is not able to get an IP address using the DHCP
>protocol, therefore I am forced to assign a static address to the guest
>interface (192.168.0.30/24). Nevertheless it shows that the interface
>is in UNKNOWN state.
>
>From here ( guest machine) I can ping the host but I cannot reach any
>other machines inside the host LAN. Not even the gateway. Hence, I
>don't have Internet connectivity either.
>
>In fact, using TCPDUMP tool I can see ICMP requests on the BR0
>interface but I can't on the host physical interface ETH0. On the
>other hand, if I try to ping the guest machine from any other machine
>inside the LAN, I can see ICMP request packets on the BR0 but not on
>the tap interface tap0.
>
>IP forwarding is also enabled:
>
> cat /proc/sys/net/ipv4/ip_forward
>1
>
>I am trying to troubleshoot this issue for one month now. I really
>hope I can hear back from you.
>
>As an additional note,  my host physical network interface is not an
>embedded one. I am using an USB adapter with the ethernet cable. If I
>try to emulate a linux machine with VirtualBox, instead, using the
>same host network configuration, I can reach the Internet from the
>Guest machine.
>
>I look forward to hearing from you.
>Many Thanks and regards,
>Daniel