networking question...

Matthew Dillon dillon at
Thu May 26 18:00:05 PDT 2005

:say there exist two servers, server0 and server1, both running
:each server has two ethernet ports, em0 and em1...
:em0 on each server is connected to the net at 10mbps, and say their
:public ip's are and respectively...
:the servers are connected together on their em1's at 1000mbps, and say
:their ip's are and respectively...
:crappy graph:
:internet                                               internet
:|                                                      |
:em0(                                        em0 (
:|                                                      |
:say server0 is very busy and its 10mbps connection to the internet is
:congested, while server1 doesn't use much bandwidth to the internet...
:is it possible to somehow route some of server1's public ip's through
:server1 to em1 on server0, so that server0 can make use of server1's
:bandwidth? if so, how would this be done? if not, any other
:suggestions? i hope it is clear what i am trying to say, networking is
:not my area of expertise (i'm sure that is clear) :)

    You can route outgoing bandwidth, but not incoming bandwidth.

    There are two ways to do this.

    First, when operating on connections initiated from the outside into
    one of the two servers, you can try routing the originating IP to 
    your other server and see if the ISP on the other server accepts the
    packet or filters it out.  The ISP may filter it out for not having
    the correct source IP address (and in fact, this is likely).

    Second, when initiating connections from within your network, since
    your internal addresses are NAT'd, you can route said connections based
    on the target address to your other server.  Assuming that both servers
    default out to their respective internet connections and both servers
    are running NAT.  This should work regardless.

    Doing actual load balancing is another problem entirely.

					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>

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