tap(4) SIOCSIFMTU patch

[Sean C. Farley]

On Fri, 13 Mar 2009, Bruce Simpson wrote:

> Sean C. Farley wrote:
>> 
>> Yes, this fixes my issue with bridging a tap device to an interface 
>> with an MTU higher than 1500.  I will probably commit this patch this 
>> weekend.
>
> I can't think of any reason why not, other than you might want to 
> ensure that tap's MTU is bounded within reasonable limits, 'cause yoi 
> don't want to exhaust the jumbo cluster pool if say mtu is more than 
> 9000.

I was letting ifhwioctl() perform the MTU limit check.  It insures: 
IF_MINMTU <= ifr->ifr_mtu <= IF_MAXMTU

I admitted to being new.  See!  :)

Exhausting the jumbo cluster pool refers to kern.ipc.nmbjumbo[p|9|16], 
yes?  em(4) has an upper limit of 16114 for MTU.  I could limit the MTU 
to TAPMRU (16384) which is the limit for a write to the driver anyway.

> I think ifconfig already performs such a check but you might want to double 
> check.

I noticed that ifconfig can report JUMBO_MTU, but few drivers actually 
flag it.  Should I set this for tap?

Sean
-- 
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