Re: Request for TEXT: [LARTC] "onlink" option for ip route

[Rogerio Brito <rbrito@iname.com>]

On Mar 24 2001, bert hubert wrote:
> If someone manages to write a reasonable explanation for the
> 'onlink' option, I'll put it in the HOWTO ASAP.

	As far as I understand it, the situation is as follows:

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	Sometimes, you may want to send packets via a machine
	(i.e., a gateway) which is physically connected to your
	network, but to which you don't specifically have a route in
	your kernel routing table.

	In a situation like this, if you try to add a route through
	such a gateway, the kernel will perform consistency checks on
	the route you're trying to add and will detect that it doesn't
	know how to send packets to that gateway, rejecting the route.

	The "onlink" option of the "ip" command tells the kernel that
	the it does not have to check if the gateway is reachable
	directly by the current machine (i.e., the kernel does not
	need to check if the gateway is attached to the local
	network), so that you can insert your route in the kernel
	table without it being complained about.

	One possible use of this feature is when you're setting up a
	tunnel: you have to tell the kernel in some way that the other
	side of the tunnel is the gateway for your tunneled
	connection. But this other side is frequently very, very far
	away, only reachable via your default route (and, thus, the
	kernel doesn't have a direct route to it). So, to tell the
	kernel that the rest of your tunneled connection is reachable
	via this distant machine, you use the "onlink" option when
	you're setting this special kind of route.

	Of course, "onlink" should be used with caution.
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	Please let me know if this is acceptable. I could write with
	more details (giving a more concrete example), but I'm very
	busy right now. :-(

	I hope that this serves at least as a first approximation to
	the text that will be featured on the HOWTO.


	Hope this helps, Roger...

-- 
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-  Rogerio Brito - rbrito@iname.com - http://www.ime.usp.br/~rbrito/
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