[OLSR-users] ICMP Redirects

[Weber, Michael J. (US SSA)]

I just observed the same issue under Linux. A quick review of the code
indicates that the redirects are not being disabled under Linux, even
though the code to do so exists... it just isn't ever called (unless I
missed something!) Under Win32, there is a call made to disable
redirects. Apparently, linux will issue a redirect if it forwards a
packet out the same interface on which it was received (which is always
the case with a transit node in a MANET.)

I turn off the redirects manually under Linux; they add a lot of extra
traffic that otherwise hurts the network. In a MANET, you usually can't
make any assumptions about the reachability of a node from another
node's perspective, and so the redirect is hardly ever (never?) a good
idea. I have a simple script that kills the redirects and starts olsrd
in one shot.

echo 0 > /proc/sys/net/ipv4/conf/eth1/send_redirects

Best,
Mike


> -----Original Message-----
> From: (spam-protected) 
> [mailto:(spam-protected)] On Behalf Of Holger Mauermann
> Sent: Sunday, July 03, 2005 9:37 AM
> To: (spam-protected)
> Subject: [OLSR-users] ICMP Redirects
> 
> 
> Hi,
> 
> I just noticed that my nodes still send out ICMP redirects, 
> even if it is disabled by olsrd on startup.
> 
> cat /proc/sys/net/ipv4/conf/eth1/send_redirects shows 0 on 
> all nodes, but tcpdump captures lots of redirects:
> 
> 15:15:00.068250 IP ...6 > ...1: icmp 177: redirect ...4 to 
> host ...4 15:15:00.107527 IP ...6 > ...4: icmp 129: redirect 
> ...1 to host ...1 15:15:00.934280 IP ...2 > ...7: icmp 89: 
> redirect ...1 to host ...1 15:15:04.742682 IP ...2 > ...4: 
> icmp 113: redirect ...1 to host ...1
> 
> 4 can't see 1, so 4 has a route to 1 via 6 and 1 has a route 
> to 4 via 6. However, 6 sends back to 1 that it should try 4 
> directly and to 4 that it should try 1 directly... Can this 
> lead to problems? Should I ignore this or is it better to 
> block this ICMP type with the firewall?
> 
> 
> -- 
> \-- Holger Mauermann
>  \-- (spam-protected)
>   \-- PGP Key Id: 0x8EA8C301 
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