Re: [Xen-users] slightly less random random macs

[Mike Wright <xktnniuymlla@xxxxxxxxxxxxxx>]

Tim Post wrote:
> Hello all, 
>
> There's been many posts lately about problems that stem from either
> duplicate macs, non static macs and other networking issues that would
> be averted if a mac was specified upon vm creation.
>
> I've been using this little function I wrote in my shell scripts that
> setup dom-u's from a csv file and its worked rather well, lets you
> control the prefix and characters used in making the mac. 
>
> As far as I know there is no "standard" mac prefix to denote a virtual
> nic. 
I seem to recall that the xen folks requested a mac prefix for virtuals 
and was awarded "00:16:3e:xx:xx:xx".

> Anyway here's the snippet / function which is very self explanatory and
> should work with bash / dash / etc. This can be accomplished with python
> using only a few lines of code, but bash seems more widely used when
> creating back end helpers. 
>
> Hope someone finds it of use.
>
> Best,
> -Tim
>
> # creates a random mac address and sets a global _MAC variable
> # to contain it.
>
> makemac()
> {
>         # edit prefix to suit, ideally matches the manufacturer
>         # prefix of the type of nic your using.
>         local prefix="00:00:6d"
>         local hextet[3]=""
>         local hextet[4]=""
>         local hextet[5]=""
>         local tmp=""
>         local i=3;
>         # these characters are used in making the mac
>         local digi='0a1b2c3d4e5f6789h'
>         local offset=$((${#digi} - 1))
>         _MAC="$prefix"
>
>         while [ "$i" -le 5 ]; do
>                 tmp[1]="${digi:$(($RANDOM%${offset})):1}"
>                 tmp[2]="${digi:$(($RANDOM%${offset})):1}"
>                 hextet[$i]="${tmp[1]}${tmp[2]}"
>                 _MAC="${_MAC}:${hextet[i]}"
>                 let "i += 1"
>         done
>
>         unset octet
>         unset tmp
>         unset prefix
> }
>
> # When done, the variable $_MAC will contain the generated mac address.
>
> Sample use :
>
> #!/bin/sh
>
> echo "I will make a mac address."
> makemac
> echo "The MAC address is ${_MAC}, Have a nice day."
> unset _MAC
>
> exit 0
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