Re: Automatically setiting mtu
glenn <vmsadsl@tpg.com.au> writes:
> Firstly just because I used the tla MTU doesn't mean I undertand it,
"Maximum transmission unit", it's the largest packet size that can be
sent out over some particular interface. For Ethernet the standard
MTU is 1500 bytes; you can apparently get better interactive
performance over very slow connections (modem) by setting a lower MTU,
and some odd types of network connections may also require a lower MTU
(e.g., IP-over-IP tunnels consume 40 bytes of the packet, so the MTU
on a tunnelled connection is generally 1460 bytes). In theory IP can
deal just fine with fragmenting packets larger than a given
connection's MTU, but in practice some broken sites (slashdot.org is a
good one) set flags that cause fragmented packets to be dropped.
> I look after a box where I need to set it, and would like it done
> automatically.
You can set the MTU on an interface from /etc/network/interfaces:
iface eth0 inet static
address 10.20.30.53
netmask 255.255.255.240
broadcast 10.20.30.63
gateway 10.20.30.49
mtu 1460
> After a bit of googling the only way, out of many, that seems to work on
> this box is to delete the default route and use the route command to
> reinstate it with the mss parameter.
You could probably do that too; is this a particularly complicated
router machine?
> I've experimented with the option interface-mtu parameter in
> dhclient.conf
In my experience that option is completely ignored; it *might* work if
the DHCP server is configured to send it and the client is configured
to receive it, but there's no guarantees.
> Anyone ever had to play with this to get through the cursed ms
> internet sharing reigiem?
(!) My (uninformed) impression is that that was just a NAT, which
shouldn't require resetting your MTU. You might run Debian on your
gateway machine instead; it's much more configurable, and much easier
to find out what exactly is going on. 'apt-get install ipmasq' should
get you going for straightforward things.
What exactly *is* your configuration here?
--
David Maze dmaze@debian.org http://people.debian.org/~dmaze/
"Theoretical politics is interesting. Politicking should be illegal."
-- Abra Mitchell
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