Re: Ensuring predictable MAC address for bond interface

[Ed Swierk <eswierk skyportsystems com>]

For these experiments I upgraded to NetworkManager 0.9.10.0.

I tried setting primary=eth0 in the bond options. This option has an effect in only one case: if I start with only eth1 plugged in (i.e. carrier up), and then plug in eth0, the bond driver will switch the active slave from eth1 to eth0 (whereas normally it leaves eth1 active, switching to eth0 only when link drops on eth1). But this has no effect on the MAC address of the bond device--either way, the bond device gets eth1's MAC address and keeps it, since eth1 was the first enslaved device.

I also experimented with setting ignore-carrier=eth0,eth1 in the main section of NetworkManager.conf, in the hope of tricking the nm_device_state_changed() logic into transitioning the slave devices to DISCONNECTED and enslaving them regardless of their carrier state. This works, sort of: now NetworkManager starts up, creates the bond device, and enslaves eth0 and eth1, whether or not they are plugged in. The bond device gets eth0's MAC address. It's still pretty easy to mess things up, for example by bringing down the bond device (via nmcli conn down) and then bringing up eth1.

I'm getting the sense that I'm fighting a losing battle against a fundamental aspect of NetworkManager, which is to react dynamically to link events, and only secondarily try to guarantee a certain configuration. I'm going to try systemd-networkd next, and see if it's a better fit for my application.

--Ed

On Tue, Oct 7, 2014 at 7:34 AM, Dan Williams <dcbw redhat com> wrote:

On Mon, 2014-10-06 at 12:51 -0700, Ed Swierk wrote:
> I'm using NetworkManager on a server with two wired Ethernet interfaces
> (eth0 and eth1) configured as slaves of a bond in active-backup mode. I'd
> like the bond interface to always be assigned eth0's MAC address.
>
> This is easy with old-school static network configuration like ifupdown:
> just make eth0 the first slave of the bond interface, and Linux will copy
> eth0's MAC address to the bond.
>
> When NetworkManager starts up and reads system-interface config files, it
> creates the bond interface right away, but doesn't add a slave until it
> notices that the slave's link is up (i.e. carrier is 1). And of course the
> link state depends on lots of things, like whether a cable is plugged in
> and the state of the switch or host at the other end. Thus whether eth0 or
> eth1 gets enslaved to the bond first is unpredictable, meaning the bond
> interface's MAC address is unpredictable. This is troublesome in some
> environments, such as ones where a DHCP server assigns IP addresses based
> on the MAC address of the client.

Does setting the "primary" bond option when in active-backup mode to
"eth0" make things better?  In nm-connection-editor it'll be in the page
for Bond options, in nmtui it'll be in about the same place, and in
nmcli you'd do:

nmcli con mod Bond1 +bond.options "primary=eth0"

and then re-activate the connection.  Let me know if that does/doesn't
help.  (note; this may only work with 0.9.10+)

Dan

> NetworkManager already treats a bond slave interface differently when its
> link goes down, leaving it in DISCONNECTED state rather than switching it
> to UNAVAILABLE. So there's precedent for having a bond interface with one
> or more link-down slave interfaces. I think the easiest way to achieve a
> stable MAC address is to extend that behavior to the startup case: as soon
> as NetworkManager sees an interface that's configured as a bond slave, it
> should move it from UNAVAILABLE to DISCONNECTED. That way the first
> configured slave interface, rather than the first one with link up, is
> enslaved to the bond.
>
> I couldn't figure out a way to configure this in NetworkManager 0.9.8.10,
> nor in mainline code. As a proof of concept, I hacked
> nm_device_state_changed() in src/nm-device.c: in the second switch(state),
> UNAVAILABLE case, I force the transition to DISCONNECTED for eth0 and eth1.
> This works as I'd hoped: both interfaces are enslaved right away, with eth0
> always first.
>
> I didn't see any easy way to implement this behavior cleanly, though. This
> new behavior should apply only to bond slave interfaces. At the point where
> nm_device_state_changed() is called, there's not yet a corresponding
> connection for eth0 or eth1, so I can't check whether the interface is
> configured as a bond slave. I thought I'd ask for advice before spending
> more time on this.
>
> Any help would be appreciated!
>
> --Ed
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