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

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

[bert hubert]

On Sat, Mar 24, 2001 at 12:43:07PM +0100, RoMaN SoFt / LLFB!! wrote:

>  Please, couldn't you summarize the meaning of "onlink" option? Is it
> necessary or could I remove it in my multipath setting? See my former
> posts; basically it's a default route with two nexthops for traffic
> balancing between them:

If someone manages to write a reasonable explanation for the 'onlink'
option, I'll put it in the HOWTO ASAP.

Regards,

bert

-- 
http://www.PowerDNS.com      Versatile DNS Services  
Trilab                       The Technology People   
'SYN! .. SYN|ACK! .. ACK!' - the mating call of the internet

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

[Rogerio Brito]

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:

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
	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.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

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

[bert hubert]

On Sun, Mar 25, 2001 at 12:43:07AM -0300, Rogerio Brito wrote:
> 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:
> 
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> 	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.

I'm more confused then ever!

Alexey says this: 
 A: Yes, in 2.2 the rule that {\em normal} gateway must reside on directly
 connected network has not any exceptions. You may tell kernel, that
 this particular route is {\em abnormal}:
 \begin{verbatim} 
   ifconfig tunl0 10.0.0.1 netmask 255.255.255.255
   ip route add 10.0.0.0/8 via 193.233.7.65 dev tunl0 onlink
 \end{verbatim}
 Note keyword \verb|onlink|, it is the magic key that orders kernel
 not to check for consistency of gateway address.
 Probably, after this explanation you have already guessed another method
 to cheat kernel:
 \begin{verbatim} 
   ifconfig tunl0 10.0.0.1 netmask 255.255.255.255
   route add -host 193.233.7.65 dev tunl0
   route add -net 10.0.0.0 netmask 255.0.0.0 gw 193.233.7.65
   route del -host 193.233.7.65 dev tunl0
 \end{verbatim}
 Well, if you like such tricks, nobody may prohibit you to use them.
 Only do not forget
 that between \verb|route add| and \verb|route del| host 193.233.7.65 is
 unreachable.

However, our HOWTO says this:

 ip tunnel add netb mode gre remote 172.19.20.21 local 172.16.17.18 ttl 255
 ip link set netb up
 ip addr add 10.0.1.1 dev netb
 ip route add 10.0.2.0/24 dev netb

We don't use onlink - might it be that 'onlink' is only useful for ipip
tunnels?

> 	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. :-(

Once this is cleared up, your text will do fine. Thanks!

Regards,

bert

-- 
http://www.PowerDNS.com      Versatile DNS Services  
Trilab                       The Technology People   
'SYN! .. SYN|ACK! .. ACK!' - the mating call of the internet

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

[Guy Van Den Bergh]

I'll try to give an example to see what the onlink option really does:

You have configured eth0 (on your machine) with an ip address/mask: 
192.168.0.1/24

For some reason, you want to use a gateway on the physical LAN of eth0
which has an inconsistent configuration. Assume it has an address 
10.0.10.1/24, which does not match the 192.168.0.1/24 configuration you 
gave your own machine.

If you say:

ip route add default dev eth0 via 10.0.10.1

then you will get an error response (network unreachable or something).
Why? Because (strictly speaking) it does not make sense to have
a machine 10.0.10.1 on the eth0 lan, because you told your computer this 
lan's network address is 192.168.0.0/24.

But here it comes: if you add "onlink" to the command, like this:

ip route add default dev eth0 via 10.0.10.1 onlink

then the route is accepted.

But now we come to another question: What does the kernel do if a packet 
has to routed to this "onlink" gateway? I checked (by experiment), and 
this is the result:
The kernel sends an arp request on the eth0-lan asking "who-has 
10.0.10.1", which is the "onlink" gateway you configured. When it 
receives a response, he has a MAC address where he can send his packets 
to and routing will be performed.

This means: when the gateway's address doesn't match the lan 
configuration on your computer (eth0), you can force the kernel to use
this gateway by adding "onlink" to the statement.

I guess there very rare situations where you have to use this feature.
Situations where you need the onlink option will mostly be a case of 
poor network design... I don't think the average tunnel setup needs 
"onlink" either.

Regards,
Guy

bert hubert wrote:

> On Sun, Mar 25, 2001 at 12:43:07AM -0300, Rogerio Brito wrote:
> 
>> 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:
>> 
>> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>> 	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.
> 
> 
> I'm more confused then ever!
> 
> Alexey says this: 
>  A: Yes, in 2.2 the rule that {\em normal} gateway must reside on directly
>  connected network has not any exceptions. You may tell kernel, that
>  this particular route is {\em abnormal}:
>  \begin{verbatim} 
>    ifconfig tunl0 10.0.0.1 netmask 255.255.255.255
>    ip route add 10.0.0.0/8 via 193.233.7.65 dev tunl0 onlink
>  \end{verbatim}
>  Note keyword \verb|onlink|, it is the magic key that orders kernel
>  not to check for consistency of gateway address.
>  Probably, after this explanation you have already guessed another method
>  to cheat kernel:
>  \begin{verbatim} 
>    ifconfig tunl0 10.0.0.1 netmask 255.255.255.255
>    route add -host 193.233.7.65 dev tunl0
>    route add -net 10.0.0.0 netmask 255.0.0.0 gw 193.233.7.65
>    route del -host 193.233.7.65 dev tunl0
>  \end{verbatim}
>  Well, if you like such tricks, nobody may prohibit you to use them.
>  Only do not forget
>  that between \verb|route add| and \verb|route del| host 193.233.7.65 is
>  unreachable.
> 
> However, our HOWTO says this:
> 
>  ip tunnel add netb mode gre remote 172.19.20.21 local 172.16.17.18 ttl 255
>  ip link set netb up
>  ip addr add 10.0.1.1 dev netb
>  ip route add 10.0.2.0/24 dev netb
> 
> We don't use onlink - might it be that 'onlink' is only useful for ipip
> tunnels?
> 
> 
>> 	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. :-(
> 
> 
> Once this is cleared up, your text will do fine. Thanks!
> 
> Regards,
> 
> bert



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