Routing traffic on an ethernet alias to a default gateway

You would need to elaborate somewhat further what it is you are trying to do. A rough sketch with IP addresses of the hosts involved and a listing of your routing table(s) would help a lot understanding your problem.

my server doesn't seem to configure the tun0 device properly

It is possible for an ifconfig command to fail - maybe you should check the logs for that and post the relevant excerpts.

I want to set up the dual client box such that a computer whose gateway is set to eth0:0 gets all their traffic routed through one OpenVPN tunnel, and a computer whose gateway is set to eth0:1 gets all their traffic routed through a different OpenVPN tunnel

ip rule add from $IP_ETH00 table us_table

That's probably not the best way to achieve what you really want - which seems to be different routes for different clients. While it is possible to add iptables -t mangle rules to mark packets for different criteria, there would be no set of criteria being able to distinguish between eth0:0 and eth0:1 as the input interface (which is due to the way IP aliasing is implemented).

What you can do however is simply set up something like

ip rule add from <ip-of-your-client-for-the-us-table> table us_table

which would eliminate the need for IP aliases in your configuration entirely since the routing decision would be done based on source and destination IP addresses, no matter which interface the packet came in at.

copy_routing_table "us_table"

You've omitted the source of copy_routing_table - if it does what I suspect it does, you would end up with your entire main routing table in us_table. If your main routing table already contains routes potentially conflicting with what you're defining in the script, you might end up using them instead of your newly-added routes. This is especially a concern since you are adding a new default route in your up-script:

 ip route add default via $4 table us_table

As you already have a default route in your "main" table and add another one "via $4" (which is wrong BTW, as $4 would represent a local IP address of the router's own tun interface - you should use "dev $1" instead) without deleting the old route. You should prepend ip route del default table us_table here - and probably something similar for the other routes you add as well.

And this here:

From 192.168.1.133: icmp_seq=2 Redirect Host(New nexthop: 192.168.1.1)

is a message from 192.168.1.133 which is getting the packet for 98.137.149.56 (yahoo.com) and routing it out through 192.168.1.1. Since 192.168.1.133 knows (by evaluation of the interface netmask) that your host is in the same network as 192.168.1.1, you get notified to use 192.168.1.1 directly in the first place.

I can think of one way to do it which is as follows.

1. Define two IP addresses on your NIC like 10.0.0.1 & 10.0.0.2. This should be easy and straight-forward. This can be done using ifconfig.
2. Configure source based routing such that each source IP is routed to one of the default gateways. This link can help you.
3. Finally, you can tell your application to use bind one of the addresses 10.0.0.1 or 10.0.0.2. This way you can choose one of the gateways depending on the source IP selected. To test this, you can use telnet with -b source_ip option.

Here are the commands I used previously to enable source-based routing:

$ sudo ip rule add from 10.0.0.2 tab 1 priority 500
$ sudo ip route add default via 10.0.0.101 dev eth0 tab 1
$ sudo ip route flush cache

If you have the default gateway set to 10.0.0.100, then this should work for you. The packets sent from 10.0.0.1 should be sent to default gateway, and packets sent from 10.0.0.2 should be sent to 2nd gateway 10.0.0.101 as instructed above.