broadcast oddity

[Daniel Braniss]

> Tue, Jul 19, 2011 at 10:40:11AM +0300, Daniel Braniss wrote:
> > > And that non-broadcast ethernet address is the MAC of your
> > > default router?
> > yes.
with dest_addr = INADDR_BROADCAST on the non diskless:

09:44:29.850576 00:0d:b9:00:72:a8 (oui Unknown) > 00:04:38:a0:c6:07 (oui 
Unknown), ethertype IPv4 (0x0800), length 61: wrap-1.cs.huji.ac.il.53016 > 
255.255.255.255.12345: UDP, length 19
and 00:04:38:a0:c6:07 belongs to the router:
wrap-1# arp -a | grep 00:04:38:a0:c6:07
router-80.cs.huji.ac.il (132.65.80.1) at 00:04:38:a0:c6:07 on sis0 expires in 
1200 seconds [ethernet]

> 
> Fine, that is more-or-less expected, since the network subsystem
> just routes 255.255.255.255 to the default gateway.  The issue
> you're seeing were already seen before,
>   http://lists.freebsd.org/pipermail/freebsd-net/2005-October/008626.html
>   http://www.freebsd.org/cgi/query-pr.cgi?pr=3D72468
>   http://lists.freebsd.org/pipermail/freebsd-net/2007-January/012874.html [=
> 1]

the more I read the more confused i get :-)

> and bms@ told me that in this case the default gateway routing is the
> correct historical behaviour of FreeBSD.
> 
> [1] I finally remembered that I had seen this issue too ;))
> 
> > > What's your routing table (netstat -rn) for the PXE-booted host?
> >
> > it's ok, same in both cases. it's picked up via DHCP, in the diskless
> > case by the boot/loader in the second via dhcpclient.
> 
> Still, can you show both of them?
from the diskless:
els-01# ifconfig
vr0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=8280b<RXCSUM,TXCSUM,VLAN_MTU,WOL_UCAST,WOL_MAGIC,LINKSTATE>
        ether 00:0d:b9:22:57:18
        inet 132.65.91.1 netmask 0xfffff000 broadcast 132.65.95.255
        media: Ethernet autoselect (100baseTX <full-duplex>)
        status: active
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
        options=3<RXCSUM,TXCSUM>
        inet 127.0.0.1 netmask 0xff000000 
els-01# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
default            132.65.80.1        UG          0    16606    vr0
127.0.0.1          link#4             UH          0       36    lo0
132.65.80.0/20     link#1             U           0    86612    vr0
132.65.91.1        link#1             UHS         0       12    lo0

from the non-diskless:
wrap-1# ifconfig
sis0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 
1500
        options=83808<VLAN_MTU,WOL_UCAST,WOL_MCAST,WOL_MAGIC,LINKSTATE>
        ether 00:0d:b9:00:72:a8
        inet 132.65.80.181 netmask 0xfffff000 broadcast 132.65.95.255
        media: Ethernet autoselect (100baseTX <full-duplex>)
        status: active
sis1: flags=8802<BROADCAST,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=83808<VLAN_MTU,WOL_UCAST,WOL_MCAST,WOL_MAGIC,LINKSTATE>
        ether 00:0d:b9:00:72:a9
        media: Ethernet autoselect (none)
        status: no carrier
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
        options=3<RXCSUM,TXCSUM>
        inet 127.0.0.1 netmask 0xff000000 

wrap-1# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
default            132.65.80.1        UGS         0    16936   sis0
127.0.0.1          link#4             UH          0       76    lo0
132.65.80.0/20     link#1             U           0    67433   sis0
132.65.80.181      link#1             UHS         0        0    lo0

> 
> > > You nailed it: you should send packets to the network's broadcast,
> > > not to the 0xffffffff.
> > >=20
> > but I'm at the user/ip level!, have no way to set mac/ethernet address.
> 
> I meant the IP's network broadcast and by 0xffffffff I meant
> 255.255.255.255.  Please, look at the posted code.
> 
> > still, the question is why it works in one case, and failes in the other.
> 
> Yes, it is.  But ip_output.c has the following code,
> {{{
>                 if (rte->rt_flags & RTF_GATEWAY)
>                         dst =3D (struct sockaddr_in *)rte->rt_gateway;
>                 if (rte->rt_flags & RTF_HOST)
>                         isbroadcast =3D (rte->rt_flags & RTF_BROADCAST);
>                 else
>                         isbroadcast =3D in_broadcast(dst->sin_addr, ifp);
> }}}
> 
> So, if the route that is selected is the gateway, then there will be
> no broadcast on the L2.  At least in my understanding of the code.
> Thus, I am interested in the routing tables and route flags.

so it boils down to a problem in selecting the route?
cheers,
	danny