Date: Wed, 08 Jan 2003 09:52:14 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: gallatin@cs.duke.edu Cc: freebsd-stable@FreeBSD.ORG Subject: Re: wi0 and mtu setting [bad idea] Message-ID: <20030108.095214.36936663.imp@bsdimp.com> In-Reply-To: <15897.56408.793022.131019@grasshopper.cs.duke.edu> References: <Pine.A41.4.10.10301061918340.116252-100000@bessel.tekniikka.turkuamk.fi> <20030106.102538.57922346.imp@bsdimp.com> <15897.56408.793022.131019@grasshopper.cs.duke.edu>
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In message: <15897.56408.793022.131019@grasshopper.cs.duke.edu>
Andrew Gallatin <gallatin@cs.duke.edu> writes:
:
: M. Warner Losh writes:
:
: >
: > Well, this converstation needs to take place on the net@ list. In
: > -current, at least, if_ethersubr.c prevents 'ethernet' drivers from
: > setting the mtu higher. There's some experimental patches that make
:
: It doesn't prevent anything. It just sets the mtu to a default of
: ETHER_MTU, which is what 98% of ethernet drivers want. If you want
: something higher (or lower) as your default, you just need to reset
: the mtu after calling ether_ifattach().
:
: For example, the following code at the bottom of my attach function
: has worked fine for years..: (GM_IP_MTU is 9K on some firmware, 4K on
: older firmware)
:
: #if (__FreeBSD_version >= 500000)
: ether_ifattach (ifp, sc->is_addr);
: #elif (__FreeBSD_version >= 410000)
: ether_ifattach (ifp, ETHER_BPF_SUPPORTED);
: #else
: if_attach (ifp);
: ether_ifattach (ifp);
: #endif
: /*ether_ifattach resets mtu */
: ifp->if_mtu = GM_IP_MTU;
: return 0;
Interesting. I see the following code:
case SIOCSIFMTU:
/*
* Set the interface MTU.
*/
if (ifr->ifr_mtu > ETHERMTU) {
error = EINVAL;
} else {
ifp->if_mtu = ifr->ifr_mtu;
}
break;
But as others have pointed out, this can easily be defeated...
Warner
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