Re: Web mirroring and ifconfig

sfahey@niu.edu
Mon, 09 Feb 98 16:06:30 -0600

>The problem is that packets get to your machine by the base network
>address (probably an ethernet address). When something is sent to your
>IP address, the sending host or router must map that to an ethernet
>address using its arp table. Thus it will not send to the new machine
>until the entry in its arp table has timed out. But, if that entry is
>getting constant traffic, it may not time out. You might be able to
>force an arp table update by sending the router a packet from the aliased
>address. But I suspect that your aliased address is receive-only, and
>you cannot send packets with it.

The alias address can send packets. Can I force the arp update with
the arp command somehow? How about arp -f to delete the arp table
then two ifconfig commands, one for each address?

>A better plan would be to use a phony hostname for your web server. Let
>me illustrate with our cs department server. We currently are using the
>hostname 'WWW.CS.NIU.EDU' for our web server. At present, that address
>is defined in DNS to be an alias for MP.CS.NIU.EDU. As an alternative, I
>could give WWW.CS.NIU.EDU its own IP address. In fact, I could give it
>two addresses. Suppose that I gave it the two address 131.156.1.2
>(really the address of MP.CS.NIU.EDU) and 131.156.68.2 (really the
>address of UX.CS.NIU.EDU). Note that it is allowable to give the same
>address to two hostnames. The result would be that anyone wanting to
>make a connection to WWW.CS.NIU.EDU would receive two addresses from the
>nameservers, and would try to connect to one, then if that failed it
>would try to connect to the other. When both machines are running,
>MP.CS.NIU.EDU would pick up about half the web traffic, and UX.CS.NIU.EDU
>would pick up the other half. When one machine is down, the other would
>get all of the traffic, although half the connection attempts would first
>time out on the machine that was down.

This presents several problems. The first is that I will have to keep both
machines mirrored almost instantaneously. The second concerns log analysis.
Too much hassle to merge and sort the logs. In addition, I don't like the
idea of waiting for timeouts when MIRROR is down.

Thanks for the hints,

Sean