[LARTC] ip routing problems in 2.4.20

[LARTC] ip routing problems in 2.4.20

[Ard van Breemen]

Hi,
Has anybody noticed that when:
ip addr add 192.168.0.1/32 dev eth0
ip route add 192.168.0.2/32 dev eth0

The system will not respond to arprequests from 192.168.0.2, even
when rp_filter=0, proxy_arp=1, etc...

I will investigate it further at another time, but this has
caused me a lot of headaches... Grrr...

-- 
mail          up  170+00:33,     7 users,  load 0.16, 0.22, 0.17
mistar1       up   28+18:37,    17 users,  load 0.22, 0.08, 0.04
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Re: [LARTC] ip routing problems in 2.4.20

[ulrich schwarz]

Ard van Breemen wrote:
> Hi,
> Has anybody noticed that when:
> ip addr add 192.168.0.1/32 dev eth0
> ip route add 192.168.0.2/32 dev eth0

/32? this would be a subnetmask with one host per subnet - so how could 
the host communicate with a router in it's own subnet? not at all.

it seems to me this doesn't make much sense. maybe i just haven't got 
the point.

greetings, ulric

-- 
ulrich schwarz, computing center, university of ulm, germany


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Re: [LARTC] ip routing problems in 2.4.20

[Ard van Breemen]

On Mon, May 05, 2003 at 01:21:52PM +0200, ulrich schwarz wrote:
> Ard van Breemen wrote:
> >Hi,
> >Has anybody noticed that when:
> >ip addr add 192.168.0.1/32 dev eth0
> >ip route add 192.168.0.2/32 dev eth0
> 
> /32? this would be a subnetmask with one host per subnet - so how could 
> the host communicate with a router in it's own subnet? not at all.

The use of whatever ip address to bind an interface to the ip
stack should not have any consequences to the arp behaviour if
rp_filter=0. If I said:
ip addr add 127.0.0.1/32 dev eth0
ip link set up eth0
echo 0 > /proc/sys/net/ipv4/conf/eth0/rp_filter

it should answer any arp request for any of it's ip addresses.
Adding a route to a device only makes sure that you can make
rp_filter=1.
At least that used to be the behaviour before 2.4.20. I've tested
a lot of this stuff, and it is used in a lot of places (not only
by me). It would hurt a lot of people if it does not work any
more.

> it seems to me this doesn't make much sense. maybe i just haven't got 
> the point.

Split a /24 in a lot of smaller subnets on a firewall with > 50
interfaces.

-- 
mail          up  170+01:42,     8 users,  load 0.08, 0.14, 0.20
mistar1       up   28+19:46,    17 users,  load 0.02, 0.01, 0.00
Let your government know you value your freedom: sign the petition:
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Re: [LARTC] ip routing problems in 2.4.20

[Julian Anastasov]

	Hello,

On Mon, 5 May 2003, Ard van Breemen wrote:

> Hi,
> Has anybody noticed that when:
> ip addr add 192.168.0.1/32 dev eth0
> ip route add 192.168.0.2/32 dev eth0

	It should respond after "ip route flush cache",
you are missing something, I assume eth0 is not down.

Regards

--
Julian Anastasov <ja@ssi.bg>

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Re: [LARTC] ip routing problems in 2.4.20

[Ard van Breemen]

On Tue, May 06, 2003 at 12:07:32AM +0300, Julian Anastasov wrote:
> On Mon, 5 May 2003, Ard van Breemen wrote:
> 
> > Hi,
> > Has anybody noticed that when:
> > ip addr add 192.168.0.1/32 dev eth0
> > ip route add 192.168.0.2/32 dev eth0
> 
> 	It should respond after "ip route flush cache",

Grrrr, thanks Julian.

> you are missing something, I assume eth0 is not down.

Correct. This was just the educational example...
Pfff, I thought I was going insane.... I had a 2.4.18 kernel
talking to a 2.4.20 kernel, except that the 2.4.20 kernel did not
reply on the arps... Now to find something soft to bounce my head
against (I already have the headache, so I don't want it to hurt
extra...).

Eh, just curious in how arp depends on routing... I've already
seen how neigh/gc_thresh[123] can influence the reachability of a
host, and that the routing cache can lock entries in the neigh
table while they are being flushed, filling up the neigh table
etc...
-- 
mail          up  171+00:12,     7 users,  load 0.08, 0.20, 0.16
mistar1       up   29+18:16,    17 users,  load 0.02, 0.04, 0.00
Let your government know you value your freedom: sign the petition:
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Re: [LARTC] ip routing problems in 2.4.20

[Julian Anastasov]

	Hello,

On Tue, 6 May 2003, Ard van Breemen wrote:

> Eh, just curious in how arp depends on routing... I've already
> seen how neigh/gc_thresh[123] can influence the reachability of a
> host, and that the routing cache can lock entries in the neigh
> table while they are being flushed, filling up the neigh table
> etc...

	When ARP packet is received the ARP code uses "input routing"
lookup (which can return cached entry) to determine what IP is
resolved. The result can be "local delivery", "forward", etc
but there are additional conditions that determine whether we
reply to this probe. In short, ARP follows the IPv4 routing
with additional checks.

	In your case I assume we already have cached
entry and without flushing it we can not provide actual/valid
lookup result. The rule is that if you care, after adding new
route you have to flush the cache. After adding IP addresses the
flush is automatically performed from the kernel but it is not
true for the routes.

Regards

--
Julian Anastasov <ja@ssi.bg>

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