Grant Taylor wrote:
On 06/21/07 11:00, Peter Rabbitson wrote:
Ah, here is part of the problem.
( eth1 ) --- (DSL Modem) / DSL Gateway
Server --- (DMZ) --- (Linux Router)
( eth2 ) --- (Cable Modem / Cable Gateway
Note: Globally routable DMZ is connected to eth0.
Traffic will be to / from servers in the DMZ and clients on the internet
at large.
My "Linux Router" (above) *IS* the system that would send the ICMP ...
unreachable message. So, there is not an upstream router to look for
traffic from.
I suppose that I could match traffic coming in eth1 or eth2, but I would
have to be careful about he source / destination. However the very
existence of inbound traffic means that the link is up for at least
inbound traffic. However I also need to know that I can send traffic
too.
You are misunderstanding how ICMP works. The modems themselves are hops,
and the thing they connect to is another hop. Just look at the first
several entries of a traceroute to any destination, and you will see
what I mean. If you still do not believe me - pull the ISP side cable
from the modem, while still having your router connected to it, and try
to do a ping to somewhere. Look at the source of the dest. unreachable
message - it will come from the modem, not from the linux box.
I've had situations where the traffic would come in but not go out
(Do NOT ask how why!).
This would be a problem with your router configuration. It is virtually
impossible to have an upstream problem that would cause this. It either
works both ways or does not at all.
I suppose such monitoring will work, but I still feel like there is a
better solution out there.
I thought so too, but it seems that the only thing that comes close (and
still does not cut it) are the DGD patches. And (this is my personal
opinion) the fact they have not been included in the kernel for such a
long time, indicates there is something fishy about them.
I myself am using a different approach as I am doing load balancing as
well. A script sends icmp ping packets with large payloads to several
destinations and computes the mean rtt. Then the ratio of both rtts is
used to assign link weights. When no pings come back one of the weights
will be 0, and effectively no routing will be performed through this link.
There is also the fact that I am wanting to use one route unless it is
down and then use the backup. If the primary route is up and traffic
comes in the backup, it is to go back out the primary.
Nothing above prevents you from doing this, although it is a bad idea.
Of course if you know what you are doing and still want to do it - it's
your system :)
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