On Wednesday 26 April 2006 20:26, John Hemeon wrote:
> Hello, after moving to a 2.6 host I am noticing that a bridge created
> with brctl drops any packets exceeding the MTU of 1500 (1514 including
> eth header).
> Attached to the bridge are tun-taps . Searching around it appears that
> as of 2.6 the bridge uses the MTU of the attached interfaces (1500 in
> this case). As I need to send jumbo frames I need to get the MTU up to
> 9100 on the tun/tap interfaces.
> Is this possible?
> How can I configure tun/tap to behave as gigabit Ethernet interfaces as
> opposed to standard Ethernet interfaces?
Currently not (tested with ifconfig tap0 mtu 9100).
I think it's a simple patch to write, but you should ask to networking guys to
do it. (Some mailing list like netdev-something). Possibly the support must
also be added in the application using TUN/TAP (i.e. UML), but I think that
mustn't be that difficult anyway. A few thoughts are needed, though.
The code is in net/ethernet/eth.c:
static int eth_change_mtu(struct net_device *dev, int new_mtu)
{
if (new_mtu < 68 || new_mtu > ETH_DATA_LEN)
return -EINVAL;
dev->mtu = new_mtu;
return 0;
}
You could remove the part " || new_mtu > ETH_DATA_LEN", but
a) this applies to any ethernet interface
b) I'm unsure whether that's enough
I've tested on my system:
zion [~: 14:31 (0)] # tunctl
Set 'tap0' persistent and owned by uid 0
zion [~: 14:36 (0)] # ifconfig tap0
tap0 Link encap:Ethernet HWaddr 96:03:D2:70:7F:C6
BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
zion [~: 14:36 (0)] # ifconfig tap0 mtu 9000
SIOCSIFMTU: Invalid argument
zion [~: 14:36 (0)] # ifconfig tap0 mtu 150
zion [~: 14:36 (0)] # ifconfig tap0
tap0 Link encap:Ethernet HWaddr 96:03:D2:70:7F:C6
BROADCAST MULTICAST MTU:150 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
zion [~: 14:36 (0)] # tunctl -d tap0
Set 'tap0' nonpersistent
--
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Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade
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