Re: [Qemu-devel] [PATCH v2] net/net: Change the default mac address of nic

[Stefan Hajnoczi]

On Fri, Oct 18, 2013 at 11:44 AM, mike <address@hidden> wrote:
> On 10/18/2013 05:00 PM, Stefan Hajnoczi wrote:
>
> On Fri, Oct 18, 2013 at 10:54:13AM +0800, mike wrote:
>
> NACK
>
> I'm not going to merge this patch:
>
> If you terminate QEMU and launch it again the NIC gets a different MAC
> address.  Some guest operating systems are sensitive to this - under
>
> For these users must use -device <nic-model>,mac=XX:XX:XX:XX:XX:XX.
> I think no body will boot up the guest, which sensitive to this,
> without mac address.
>
> Actually, people use the command line without mac address, mean they mainly
> don't care about mac address, so give them random mac address is reasonable
> I think.
>
> In my opinion, if we fix this, for qemu side no any issue, we both support
> mac address set or unset correctly.
>
> What am I confuse is, *qemu supports mac address unset, why we force
> users must set the address when more than one guests*?
> This is unreasonable.
>
> many Linux distros the network interfaces names change due to the MAC
> address change.  As a result firewall configuration will break and other
> services may fail to start because they cannot find the interface.
>
> Agree, so this mac address should set in qemu command line as
> libvirt does :)
>
> If you have multiple guests or want control over the MAC address, set it
> explicitly using -device <nic-model>,mac=XX:XX:XX:XX:XX:XX.
>
> Currently, especially for developers, people mainly use qemu
> command line directly, and as qemu supports mac address
> unset, they may try the simplest command line to boot up
> lots of guests, they will confuse about why all this guest use
> the same mac address.
>
> Your argument is weak: *you* want to avoid specifying the MAC address so
> in exchange you want to *break* existing configurations and force other
> people to start specifying a MAC address.
>
> OK, I do not want to break the existing configurations.
>
> I'm fine if you do not want to merge this patch.
>
> But I think this should be an issue of qemu, and need to
> do something on it, so I make this patch.
>
> Can we try other solutions to solve this issue?
> (if you agree this should be an issue)

Options:
1. Set mac= on the command-line
2. Save the QEMU command-line in a script so the guest can be launched
without remembering the details
3. Use -writeconfig/-readconfig to save the command-line to file and restore it

Option 2 or 3 are more convenient than Option 1.  Does that help?

Finally, if random MACs are desirable for your particular use case,
create a wrapper script that does something like:
#!/bin/bash
exec qemu-system-x86_64 "$@" -set device.mynic.mac=04:ab:cd:$((RANDOM
% 256)):$((RANDOM % 256)):$((RANDOM % 256))

> Also this potential issue can happens if the user set the same mac address
> with more than one guest on one host.
>
> Can we avoid this ?

The same situation is possible on a physical LAN.  MAC or IP
collisions are configuration issues.  I don't see collisions as a QEMU
issue.

Stefan