Unusual Router

[CDLM]

I've got a wierd question - keeping in mind im just getting into linux routers and such so if this just isnt possible let me know.


i have a box with say, three nics. two (eth1 and eth2) are connected to cable modems. the other (eth0) to the internal network. eth0 has two ips, 10.0.0.1 and 10.0.0.2.

some clients on the internal network have 10.0.0.1 as their router, some have 10.0.0.2 as their router.

now is it possible on the router box to have traffic on eth0 on IP 10.0.0.1 to be routed to eth1 and traffic on eth0 on IP 10.0.0.2 be routed to eth2?

and if i've confused the hell out of all you, let me know and ill draw up a diagram

- Dave -

[Arasi]

Fundamentally no you can not do this.

You have traffic coming in from a private subnet to either private ip but it is still all the same private subnet, you then want that traffic to route through, routing uses either known networks to route traffic or a routing protocol to learn networks and best path or they forward to a default gateway.

You can't have 2 default gateways and then have the router look at the source ip to determine the default gateway. Because your routing tables are not isolated for those externel nics.

If you had 2 physical boxes...or say went with user mode linux...another topic...then you would have 2 seperate routing tables on the machines and therefore could route traffic that way.

Question: Why are you looking to route like this? is there a practical application your trying to obtain that could be done another way?

Arasi

[Dracnor]

That shouldn't be a problem with IP tables. You can just set up a rule that says any traffic coming from 10.0.0.1 be routed to eth1 and 10.0.0.2 to the other one. You can just set up masquerading. You'd want to make sure that the clients use the external DNS servers of the appropriate nic.

[Arasi]

[Dracnor]

Well you can set up different subnets to route to different ethernet cards. So you could put one host on one subnet and another host on another subnet and when creating the subnet mask be able to span the subnets such that the hosts can communicate with each other. I can't remember if you can split a subnet (IIRC you can). But, if he's going to manually (statically) assign IP addresses he could easily create a rule each time. There should be multiple ways to do this.

[Arasi]

[CDLM]

alright, fundamentally what I'm trying to do is have a router box for multiple machines at a LAN party. I have four to five cable modems and I want it so that the network is basically divided so that certain IPs use certain cable modems:

ie first 30 ips go thru modem 1
next 30 ips go thru modem 2
etc

and optimally i'd like to only have 1 internal nic (5internal + 5external = 10nics which is gonna be hard to pull off on one box)

now what would be the easiest (or most correct) way to accomplish this?

alternatively if there's a way to "combine" the internet connections (spread the load across all the external connections) that would work fine for me too.

- Dave -

[Arasi]

Not sure what kind of costing you get, but for that many cable lines I could get a fibre line in to simplify the whole process.

Admittedly fibre is a matter of availability and costs are dictated by said availability so it may not be feasible in your area.

On the other hand if you want to run 5 cable modems and use it to the extent you've described I'd recommend getting a 3Com NetBuilder Super Stack II used off ebay for 40.00 US and configure it up to segment and direct the gateways as you have said. Then you could share a local ip subnet, and direct outbound traffic without having such a massive configuration to set up in software.

The application you are describing sounds more to me like a candidate for a hardware solution rather than a software one even though I'm sure with enough configuration you could manage something in software.

Arasi

[CDLM]

cost is not an issue, the local cable provider is sponsoring us by providing hookup/modems/service for the LAN

i've seen other lan groups do this with linux, ill ask around. if i find a solution ill post it but i'd still appreciate if someone could point me in the general direction of how to do this.

- Dave -

[jhmartin]

I believe some usage of 'policy routing' will allow what you are trying to do. Also, if your isp doesn't do egress filtering it should be possible to round-robin the outgoing packets to get what amounts to a really big outgoing pipe.

[Arasi]

[jhmartin]

Although it is true that they'll all be going through one machine, but what is the capacity of that machine vs the up/down capacity of the combined cable modems? If it is at all a decent-sized machine it should be able to handle the traffic.

[Arasi]

[CDLM]

looks like its going to be four cable modems like the last LAN - some windows app was used for the last one that round-robin-ized all four connections

the reason we need a lot of bandwidth is for a few reasons, all of which crop up at the start of the lan and in the wee hours of the morning/when no tourneys are going:

1. patch downloading - while I make a local ftp for most patches, there's always people wanting other mods/maps/etc
2. online gaming - for some reason people like to play online at a LAN... don't ask me why, i have no idea
3. massive amounts of downloading of "other stuff" just because the internet is there - we try to stop warez/movie/porn downloading but there's always someone sucking bandwidth


all of which are a detriment to normal internet surfing/IM-ing

the rest of the network is all switched, intel and cisco switches, so its not that the network is badly arranged.

i'd like to use a linux solution, but the pro-windows staff (basically everyone else) is pushing for the windows solution for ease of administration

- Dave -

[CDLM]

side-note - anyone know of some windows network monitoring software (monitor traffic by interface?)

[Mnemia]

While I agree that you might run into bandwidth/latency issues due to having the single egress point, I strongly disagree that what you want to do is difficult or anything with Linux.

All you have to do to round-robin the connections is simply set up an iptables rule (in the PREROUTING table) that uses the "nth" packet match module to mark packets sequentially with 4 different FWMARKS. This method of selecting packets neatly avoids the problem of static vs. dynamic IPs etc. However, you might want to check to make sure you have similar latency across the connections before you do this if you want that to be consistent within connections. If so a slightly more complicated packet selection method might be order.

Then you can use the iproute utility to set up some policy routing to send packets with each of the FWMARKS (1-4) to different cable modems. You can in fact have multiple different routing tables and default gateways in Linux; you just can't access them through the old "route" utility. Emerge iproute and you can use the "ip" command which allows access to the much more advanced routing capabilities as well as offering a nicer user interface for controlling regular routing.

[oegat]

Since we are talking NAT and not routing of "real" IP:s, I see no problems other than bandwidth with your solution. It would be perfectly doable with 2 outgoing nics as in your first example, I have friends doing exactly that in their apartment, each user there chooses gateway from which outgoing line they want.

4-5 modems might be heavy for a single pci machine to handle, but a linux solution can't be after all be doing worse that the windows solution you mentioned earlier. Can you afford to have 2 machines NAT:in 2-3 modems each? That would surely do.

[Arasi]

[hummus]

[CDLM]

alright, could someone point me at a good doc for actually understanding iptables? Thanks.

- Dave -

[Dracnor]

I used the tutorial on yolinux when I set up my gateway. This should give you a starting point into setting up the complex situation you have

[Crg]

[Crg]

[Crg]