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From: "Martin A. Brown" <mabrown-lartc@securepipe.com>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] U-turn route
Date: Fri, 07 Mar 2003 07:15:31 +0000	[thread overview]
Message-ID: <marc-lartc-104702139723539@msgid-missing> (raw)
In-Reply-To: <marc-lartc-104700808115166@msgid-missing>

Hello there Mingching,

 : hostA: 192.168.0.1/24
 :
 : routerB: 192.168.0.2/24
 :               192.228.118.2/24
 :
 : routeC: 192.168.0.3/24
 :             192.228.110.3/24
 :
 : Given the hostA is rather dumb and that we can only configure one
 : default route, and that we have defaulted the route to B. In order
 : for A to be able to access C's external network, we configure
 : a U-turn route at B, ie the packet hop onto B and then re-forwarded
 : on the same interface to C.
 :
 : Is this something commonly done ? Any issue with it ?

There is no problem with this.  This is not uncommon--typically, router B
will generate an ICMP redirect bound for host A, causing host A (if it
accepts redirects) to create a route (cache) entry for the destination.

If you wish traffic to move through routerB at all times, you can suppress
and/or enable the generation of redirects with the sysctl
net/ipv4/conf/$DEV/send_redirects [1] toggle.  If host A is a linux box,
you can also see if it will net/ipv4/conf/$DEV/accept_redirects

I have occasionally seen peculiar TCP resets as a result of ICMP redirects
not handled correctly be machines in the position of host A, but it causes
no problem for routers and should pose no problem for end hosts.

-Martin

 [1]  http://ipsysctl-tutorial.frozentux.net/ipsysctl-tutorial.html#AEN630
 [2]  http://ipsysctl-tutorial.frozentux.net/ipsysctl-tutorial.html#AEN574

-- 
Martin A. Brown --- SecurePipe, Inc. --- mabrown@securepipe.com

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      reply	other threads:[~2003-03-07  7:15 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2003-03-07  3:29 [LARTC] U-turn route mingching.tiew
2003-03-07  7:15 ` Martin A. Brown [this message]

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