Re: [Xen-users] MAC address management (and a guy with poor math skils talks about the birthday paradox!)

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

Luke S. Crawford wrote:
> On Fri, 30 Mar 2007, Nico Kadel-Garcia wrote:
> > Luke S. Crawford wrote:
> > Assign each Xen *server* a MAC address range, and incorporate it into 
> > their MAC addresses. Server 1 gets the last four digits of 01:00 
> > thorugh 01:FF, server two gets 02:00 through 02:FF, etc. That allows 
> > you plenty of addresses for each Xen environment, and a meaningful 
> > way to look up hostnames of Xen servers based on MAC address as well.
> > Then hardcode both the MAC addresses and the vifnames accordingly, 
> > for any SNMP resource tracking use.
> Hm. those are good ideas.  (oh, and vifname= looks like it is going to 
> solve a bunch of my problems at my other venture where we do properly 
> hard-code the mac and IP.  Thanks!)    But like I said, this shop 
> tracks IP addresses via ping.... when I brought this issue up, one of 
> the  guys said "Can't you just make it check to see if the MAC address 
> is taken before it brings up the interface?"  so something that 
> requires less accounting is probably in order.
> I'm considering changing vif-common to send out an 'arp who-has'  
> before grabbing an arp, and then having it back off (and trying 
> another, if it's randomly generated)  if it's taken, much like the 
> RHEL network scripts. are there any obvious problems with this 
> approach.   It seems like it would work reasonably well if that change 
> is accepted to the mainline xen kernel, and assuming that the mainline 
> xen kernel uses a unique prefix.
That's why you assign each server a generous allocation, such as the 
last 2 bytes. With 256 addresses, you're unlikely to run out. And by 
simply keeping your old Xen config files around for reference, you can 
grep the vifname entries for used addresses.
Mind you, I set up a subdirectory called /etc/xen/[servername], and 
symlink my entries from that to /etc/xen. That helps keep them separated 
out and easier to parse. To disable one, I simply break the symlink.
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