Re: Networking Problems with Debian Stretch + KVM + old Linux Guest

[Binarus <lists@binarus.de>]

On 20.01.2018 17:04, Hermann Himmelbauer wrote:
> Hi,
> I today upgraded my KVM host from Debian 8 to the latest Debian 9
> (Stretch). This worked perfectly, however, 2 old guest systems (SuSE
> 9.1, kernel 2.6.7 / 2.6.5) have no network access.
> 
> All other machines running on this host are Linux Debian machines and
> use the "virtio" networking drivere whereas those two old machines use
> RTL8139 (or e1000, makes no difference).
> 
> On the guest side, the networking interface (eth0 / rtl8139) is up, it
> states "Link Up / 100MBit" in the log file, everything looks fine, but I
> can't get out, no ping, empty arp table etc.
> 
> Basically, I use bridging for the virtual hosts, this looks like this:
> 
> br0             8000.0026186273f4       no              eth0
>                                                         vnet0
>                                                         vnet1
> 
> or like so:
> 
> port no mac addr                is local?       ageing timer
>   1     00:00:24:cc:c7:85       no                 0.42
>   1     00:19:66:b3:cb:34       no                 3.97
>   1     00:22:b0:cf:04:b2       no                 0.03
> 
> 
> What is interesting is that I cannot find the MAC Address of the 2
> machines in the above table, which is probably not good.

I had a similar problem some years ago. In my case, the network in the
guests generally worked, but was stuttering such extremely that it
actually could not be used.

After some research, it turned out that I had to assign a MAC address to
the guests' (virtual) NICs explicitly.

You haven't told us how you configure / start your VMs. I personally
don't use libvirt and colleagues, but I am starting the VMs from the
command line. Consequently, I don't know how to assign MAC addresses
using the high-level configuration tools. What I am doing on the command
line (as far as it concerns the network) is something like that (please
imagine the following code to be on one line):

/usr/bin/qemu-system-x86_64
[...]
-device virtio-net-pci,vlan=0,mac=02:01:01:01:02:01
-net
tap,vlan=0,name=dax,ifname=dax0,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown
[...]

Please note how the MAC address is explicitly assigned. script and
downscript are just two simple wrappers which add the virtual NIC to an
existing bridge which connects them with the host and the other VMs.

The disadvantage of this method is that you have to manage the MAC
addresses yourself (you ultimately must make sure that every VM gets its
own and that no other device in the world (in practice in most cases: in
your network) has the same as one of your virtual NICs). The MAC address
shown above is from the semi-official private MAC address space, so I am
hopefully out of trouble in this respect.

Please let us know if this solves your problem.

Regards,

Binarus