Should routers send redirects by default?

Eric J. Katanich ekat at
Sat Aug 21 01:08:17 UTC 2010

On Fri, 20 Aug 2010 19:49:43 -0400
"Ricky Beam" <jfbeam at> wrote:

> On Fri, 20 Aug 2010 13:20:58 -0400, Christopher Morrow  
> <christopher.morrow at> wrote:
> > Polling a little bit here, there's an active discussion going on
> > 6man at ietf about whether or not v6 routers should:
> >   o be required to implement ip redirect functions (icmpv6 redirect)
> >   o be sending these by default
> ...
> > In ipv4 there's a relatively widely used practice of disabling ip
> > redirects.
> I think it's almost universally disabled (by default) everywhere in IPv4  
> purely for security (traffic interception.)  In a perfectly run network,  
> redirects should never be necessary, so I'd think IPv6 should avoid going  
> down that road again. (support OPTIONAL, never enabled by default.) [It's  
> another insecure mistake IPv6 doesn't need to repeat.]

You're assuming the cost of always hair pinning traffic on an interface
is cheaper than issuing a redirect. Sometimes it won't be. 1 ICMP
redirect could result in potentially congestion inducing load being
shifted off of a single router's interface.

It seems that there might be a common and unstated assumption here that 
ever router uses hardware forwarding and has high speed 1Gbps+
interfaces that have <50% utilisation. The majority of routers - CPE -
don't meet that assumption.

> As I recall from long long ago, Cisco IOS would deal with traffic  
> differently depending on redirects... with redirects enabled, a redirect  
> was sent and the packet dropped; with redirects disabled, the router  
> hairpined the packets.  I honestly don't know what today's versions do  
> because I've never checked -- A can ping B, I move on.  I turn redirects  
> off on *outside* interfaces.  Inside (trustable) interfaces vary -- I  
> don't go out of my way to disable them.
> --Ricky

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