[1808] in linux-net channel archive
Re: Static vs dynamic routing from ISP
daemon@ATHENA.MIT.EDU (wantling@wgc.rx.xerox.com)
Sat Feb 3 00:52:26 1996
Date: Fri, 2 Feb 1996 12:32:22 PST
From: wantling@wgc.rx.xerox.com
To: John Burton <john@piper.gats.hampton.va.us>, linux-net@vger.rutgers.edu
On Thu, 1 Feb 1996 05:40:48 PST John Burton wrote:
>
>Could you please tell *me* what "ICMP redirect" frames do ? I regularly
>get the following message in my /var/log/messages file and I am not sure
>what is causing them.
>
> Jan 27 11:41:01 ns kernel: ICMP redirect from xxx.yy.zz.1
> Jan 27 11:41:59 ns kernel: ICMP redirect from xxx.yy.zz.1
> Jan 27 11:50:01 ns kernel: ICMP redirect from xxx.yy.zz.1
>
>"ns" is our primary DNS name server and so gets alot of traffic from
>inside and outside our LAN. We currently have 3 Class C networks running
>on the same ethernet, with routing between them handled by the default
>router (xxx.yy.zz.1). "ns" is handling name service for all three Class C
>networks. I occasionally get the same message on other machines, but with
>*much* less frequency...
>
>Any suggestions ?
>
>John
>
>
ICMP stands for Internet Control Message Protocol and relates to several
networking features like PING and TRACEROUTE. One of the features it
attempts to provide is correct any mistaken ideas any host may have about how
to get to other hosts, this is ICMP redirect.
These ICMP redirect messages are sent by either a dedicated router or a
host having routing software. The purpose is to indicate that you are trying
to route messages by going to a gateway which is not the optimal way to get
to the intended destination. As a result of receiving these messages, routing
software should automatically take note and modify routing tables accordingly.
If you look at routing tables using "netstat -nr" you will see thatthere are
a bunch of letters against each route which indicates the type of route (i.e
route to a net or route to a host, whether it is static or dynamic etc.) What
you would expect to find is a route marked with a "D", indicating that it has
been set as a result of the ICMP redirect.
To avoid the messages, you need to make sure that any routes you have configured
are sensible, and change them if not. In some cases, particularly where kludges
are used, you might just have to put up with it.There is a penalty to be paid if
you rely on redirects. That is that certain hosts may either not handle the correctly,
or may totally ignore them. Specifically, Microsoft software seems not to act on the
redirects. Linux does.
Hope this helps,
Colin
