kern/181388: [route] Routes not updated on mtu change

[Joe Holden]

On 20/08/2013 02:12, John-Mark Gurney wrote:
> Joe Holden wrote this message on Mon, Aug 19, 2013 at 09:32 +0100:
>> Hm, I hadn't considered that... how do other OSes and vendors handle
>> this?  (eg Linux?)
>>
>> Just changing the connected route would probably suffice, or maybe if
>> any routes not added with default interface mtu could/are be flagged,
>> then those could be changed or not depending on what added them?
>
> How do you know which routes are which?  I believe that FreeBSD will
> automaticly reduce the MTU if you decrease it, but it won't increase
> it..  How do you know the difference between someone increasing the MTU
> on the interface to allow a specific host to talk at the larger MTU and
> wanting the rest of the hosts to talk at the larger MTU...
>
connected route is the one that should be changed, not others as you are 
right in that regard, there may be routes that shouldn't be changed like 
the default gateway or other hosts. Make it a tunable exposed via sysctl 
or something, job done.

> At a previous work place, we used this feature so that we could use
> MTU 9k to other FreeBSD boxes to get better NFS performance, and but
> keep the other windows boxes which didn't have MTU 9k compatible
> interfaces talking on the same LAN...
>
vlan interfaces achieve the same thing without having to mess about with 
mtus on routes and also give you an interface to work with, a much nicer 
method comparatively.

>> Perhaps need someone with more experience of the network stack to wade
>> in here...
How difficult would it be to have ifconfig do it? As in, when MTU is 
changed on an interface, if there is a prefix configured, update the MTU 
as an rtsock message?
>>
>> Cheers,
>> Joe
>>
>> On 19/08/2013 08:00, Julian Elischer wrote:
>>> The following reply was made to PR kern/181388; it has been noted by GNATS.
>>>
>>> From: Julian Elischer <julian at elischer.org>
>>> To: bug-followup at FreeBSD.org, joe at rewt.org.uk
>>> Cc:
>>> Subject: Re: kern/181388: [route] Routes not updated on mtu change
>>> Date: Mon, 19 Aug 2013 14:57:22 +0800
>>>
>>>   The problem is that this is not as simple as it seems.
>>>   The route MTU MIGHT have been set by something other than the
>>>   interface MTU
>>>   in the first place.
>>>   The interface MTU is a default for the route MTU but is not the only
>>>   source.
>>>   This actuall bit me a couple of days ago when I was wonderign why my
>>>   interface was not sending 9K packets..  turns out you need to do
>>>   'ifconfig_xn0="DHCP mtu 9000"' in order to have your dncp
>>>   configured interface routes  have the right size.
>>>
>>>   so, I'm agreeing with you , but noticing that there are complications.
>