Date: Wed, 8 Jan 2003 12:08:06 -0500 (EST) From: Andrew Gallatin <gallatin@cs.duke.edu> To: "M. Warner Losh" <imp@bsdimp.com> Cc: freebsd-stable@FreeBSD.ORG Subject: Re: wi0 and mtu setting [bad idea] Message-ID: <15900.23286.323707.772202@grasshopper.cs.duke.edu> In-Reply-To: <20030108.095214.36936663.imp@bsdimp.com> 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> <20030108.095214.36936663.imp@bsdimp.com>
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M. Warner Losh writes:
> 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...
Yes, you just override SIOCSIFMTU in your driver's ioctl. See
sys/dev/bge/if_bge.c for an in-tree example.
Drew
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