[User-mode-linux-devel] Re: slip vs ethertap

[William S. <wst...@po...>]

Good afternoon, Jeff,

On Mon, 13 Mar 2000, Jeff Dike wrote:

> My current umn network driver uses slip as the kernel-to-user communications 
> mechanism.  I'm unhappy with this for two reasons:
> 	- the slip device needs to be ifconfig-ed, which means that either the kernel 
> needs to be run as root, or there needs to be a setuid ifconfig available 
> (i.e. um_ifconfig).

	Agreed.  Are Linux capabilities functional enough that one could
add CAP_NET_ADMIN to the uml-kernel file?  I suspect administrators would
be much happier typing "set_pcap +netadmin ./linux" than "chmod 4755
./linux".

> 	- there is no obvious way to configure both ends (especially the host end) of 
> the channel from the boot scripts.
> 
> I don't see that using ethertap helps either of these.  /dev/tap* still needs 
> to be ifconfig-ed, which still needs root privs somehow.  And using a 
> different communications mechanism to the hosting kernel doesn't seem to 
> change the problem of getting the host end ip address into the driver from the 
> networking layer above it.
> 
> Is this right?  Are there any advantages to using ethertap that I've 
> overlooked?

	The ethertap could be brought up once during host system boot and
left until uml-linux is started.  Just for example:  Boot scripts bring up
tap0 w/ 192.168.0.254, ptp -> 192.168.0.253, and add a route to .253.  
/dev/tap0 is given mode 660, owned by root/uml-group.
	tap0 just sits there until someone in uml-group start up uml with:
./linux tap=192.168.0.253,/dev/tap0
	at which point uml-linux assigns .253 to its internal tap0, opens
/dev/tap0 on the host filesystem as a normal file for read-write (perhaps 
locking it to avoid accidental opens by two uml's), and shuffles bytes
back and forth from its own /dev/tap0 to the host /dev/tap0.
	In fact, if I replace
route add -host 192.168.0.254 dev umn
route add default gw 192.168.0.254
	in the newly submitted 0.0.6 howto with
route add default dev umn
	, uml doesn't even have to know what IP is on the other end of the
tap-tap link.
	The right to access host networking is now down to whether ./linux
is able to read and write to a preconfigured /dev/tap0.  Is that a more
elegant approach?
	Cheers,
	- Bill

---------------------------------------------------------------------------
	Q: Will the tcp/ethernet SMP scaling changes be back-ported to 2.2.x?
	Mingo: yes, all SMP changes in 2.3 will be backported to 2.2 in the
next few months, but to not confuse it with 2.2 it will be named '2.4' ;)
	-- Ingo Molnar <mi...@ch...>
--------------------------------------------------------------------------
William Stearns (wst...@po...).  Mason, Buildkernel, named2hosts, 
and ipfwadm2ipchains are at:                http://www.pobox.com/~wstearns
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