Date: Sat, 4 Jan 2003 13:04:23 -0500 From: David Magda <dmagda+fstable@magda.ca> To: Evren Yurtesen <eyurtese@turkuamk.fi> Cc: Sten Daniel S?rsdal <sten.daniel.sorsdal@wan.no>, "Wright, Michaelx L" <michaelx.l.wright@intel.com>, fkittred@gwi.net, Michael Sierchio <kudzu@tenebras.com>, freebsd-stable@FreeBSD.ORG, wpaul@ctr.columbia.edu Subject: Re: wi0 and mtu setting [bad idea] Message-ID: <20030104180422.GA607@number6.magda.ca> In-Reply-To: <Pine.A41.4.10.10301041823050.19242-100000@bessel.tekniikka.turkuamk.fi> References: <20030104154328.GA266@number6.magda.ca> <Pine.A41.4.10.10301041823050.19242-100000@bessel.tekniikka.turkuamk.fi>
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On Sat, Jan 04, 2003 at 06:25:09PM +0200, Evren Yurtesen wrote:
> I want to change MTU because I will use PPPoE on wireless interfaces
> So I need to set the MTU to 1508 so that 1500 byte ethernet frames can be
> encapsulated in the access point and sent to the client. 1500byte ethernet
> plus 8byte pppoe overhead makes a total of 1508 bytes MTU which I need.
Why not hack /sys/dev/wi/if_wi? Simply change "ifp->if_mtu = ETHERMTU;"
to "ifp->if_mtu = 1508;" and see what happens. If things break you go
back, and if they work all the better.
> You showed what is the ifmtu default, but how does ifconfig decide what is
> the maximum MTU for this interface that a user can set? or it doesnt care
> about the interface and maximum that you can set is 1500?
This is all in the source:
Doing a 'grep mtu /usr/src/sbin/ifconfig/*', you find in ifconfig.c:
setifmtu(val, dummy, s, afp)
const char *val;
int dummy __unused;
int s;
const struct afswtch *afp;
{
strncpy(ifr.ifr_name, name, sizeof (ifr.ifr_name));
ifr.ifr_mtu = atoi(val);
if (ioctl(s, SIOCSIFMTU, (caddr_t)&ifr) < 0)
warn("ioctl (set mtu)");
}
So it tries an ioctl(2) and checks the return status. The device driver
enforces length limits internally and return()s appropriately.
--
David Magda <dmagda at ee.ryerson.ca>
Because the innovator has for enemies all those who have done well under
the old conditions, and lukewarm defenders in those who may do well
under the new. -- Niccolo Machiavelli, _The Prince_, Chapter VI
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