Two gateways, port-based load balancing

Question

Hello,
I have a simple question on Mac OS X Leopard/SL Server regarding the use of 2 distinct internet connections on a single LAN.
Gateway #1 : 10.0.1.1 (delivering IPs) - 18 mbps
Gateway #2 : 10.0.1.254 - 4 mbps
Any computer accessing the network is delivered an IP by the DHCP server (10.0.1.1), thus uses #1 as of main gateway.
The main server (10.0.1.16) is running DNS services and a Squid proxy-cache.
Now, is it possible to set all the computers that connect to the network up so that they use the main server as of main gateway and see their requests redirected to #1 or #2 according to the port in use ?
For example:
mail,http,https,jabber -> #1
skype,rtsp,... -> #2
Thank you very much for your help
Tha
Message was edited by: Kwintin

Answer 1

is it possible to set all the computers that connect to the network up so that they use the main server as of main gateway and see their requests redirected to #1 or #2 according to the port in use ?
No. routing is based on destination IP address, not port.
Therefore each client will send all traffic for a specific address to a specific router address. It doesn't matter whether it's talking HTTP, SMTP, IMAP, POP, AIM, or any other protocol - any traffic for that IP will go to the same router.
You have three ways of getting around this.
One is to install a router that supports dual WAN connections. Point all internal clients to the LAN address of the router and let it do the work of routing the traffic as needed, based on its routing policies (routers may be able to route based on port).
Option two is to setup a proxy server for specific services - for example you could setup a HTTP/HTTPS proxy server on a machine that has router #1 as its default gateway and configure the clients to talk to router #2. All traffic on the clients will go over router #2 except the proxied traffic which will go to the proxy and then out via router #1.
This is relatively simple to setup, but is limited to traffic that can be easily proxied (e.g. that probably excludes email).
The third option is static routing. Look at the servers each machine is contacting and setup static routes for the smaller set of addresses. For example, if you're only splitting off traffic to Skype's servers then set each client with a default route of router #1, and static routes to Skype's server to router #2. Now all traffic except that to Skype will use router #1.
This is really only viable if you have a relatively small number of destination addresses you're trying to divert. That's why it works well for Skype (single server address), but wouldn't work well for something more generic such as 'web traffic' since you cannot predict which web servers (and therefore which IP addresses) need static routes.
Of the three options, only option #1 will cover all protocols for all clients, but it's also the only option that costs $$s if your current router doesn't support multiple WAN interfaces.