[LARTC] U-turn route

[LARTC] U-turn route

[mingching.tiew]

 I have tested this, it seems to be working but I need double confirmation
:-

Given for example,

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

In other words, A,B,C are sitted in the same LAN, while
B and C each has an external network connected to it.

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 ?

Regards.


_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/

Re: [LARTC] U-turn route

[Martin A. Brown]

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

_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/