HOWTO Combine Multiple ADSL Connections

I have installed a Redhat 7.0 server and my machine is a Celeron 666 MHz with 512 MB of RAM.  I currently have one ADSL connection that is handled by the Linux server.  But my question is how would a person set up their Linux server to handle more than one ADSL connection, effectively doubling the connection speed.  I have installed 2 NICs.  One handles the ADSL line and the other communicates on the LAN.  Thanks for your assistance.
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jlevieConnect With a Mentor Commented:
Well, I read their product blurb also. It wasn't completly clear to me whether some of their statements applied only to the VPN traffic or whether they applied to generic Internet access.

In the general case, it doesn't matter what you do at the local system with respect to any given Internet server. The remote server is going to get a request from a specific IP and it will respond to that IP. So even if you have multiple ADSL lines on a system, the max rate for any given site will be that of a single ADSL line. As I said above, clever use of routing statments can allow one ADSL line to be used for one site while the second is used for another. And that may be what they are doing, just dynamically. But if you're trying to get 2x the download or upload speed to a single site it isn't going to happen because that would require the cooperation of the remote server.
Adding a second ADSL connection is possible, but you won't be able to use the combination of the two ADSL lines as "one big pipe to the Internet". You could, with adroit use of routing commands, arrange things so that traffic to specific sites uses a specific ADSL connection. That would allow you to use both lines at the same time if you were accessing a site that is routed via one ADSL line and accessing a site that used the default route via the other ADSL line.

Trying to use both lines for inbound traffic is more of a problem. The system, unless told otherwise via a route statement, is going to send replies back out the default route, regardless of which ADSL line the request arrives on.

As far as I'm aware, with xDSL, you can't do 'channel bundling' in the way you can with ISDN to create a large logical pipe.

If you are just providing internet access to LAN users then you could easily steer a subset of the traffic to one or other interface. (You'll have three with two DSL lins).

For example you could point clients on one subnet to eth1 and others to eth2 :

/sbin/iptables -t nat -A POSTROUTING -i eth0 -s -o eth1 -j MASQUERADE

/sbin/iptables -t nat -A POSTROUTING -i eth0 -s -o eth2 -j MASQUERADE

Or you could filter all http traffic down one line and everything else down the other :

/sbin/iptables -t nat -A POSTROUTING -i eth0 -p tcp --dport 80 -o eth1 -j MASQUERADE

/sbin/iptables -t nat -A POSTROUTING -i eth0 -p tcp --dport ! 80 -o eth2 -j MASQUERADE

/sbin/iptables -t nat -A POSTROUTING -i eth0 -p ! tcp -o eth2 -j MASQUERADE

I presume you would already be using IP masquerading (nat) with ipchains (2.2.x kernels) or iptables (2.4.x kernels). The above examples use iptables which is somewhat easier than its predecessor.

Of course, you need the following command in effect to enable packet forwarding to happen...

echo 1 > /proc/sys/net/ipv4/ip_forward

For more info see :

Rgds, Iain

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emery_kAuthor Commented:
Stallion Technologies ( has a product that `bonds" multiple ADSL lines. It is built on a Linux Kernel. Their primary focus is setting up VPN between various sites and their focus is with multiple DSL providers if one is done you are still in business. I was Wondering if this is being done commonly rather than their $1000/site for the license which apparently buys you the software they have set up
I can think of a couple of things one could do if the goal was to bond multiple ADSL lines for VPN links. In that case you control both ends of the link and the routing issues don't apply. I assumed we were talking about the general case of Internet connectivity, not VPN links.
emery_kAuthor Commented:
Their solution also increases available bandwidth for internet connectivity and VPN. Their limit is 8 NICs which seems to mean up to 7 ADSL Connections can be combined and it is supposedly transparent to the ISP(s).
emery_kAuthor Commented:
It's interesting how it works out. You are correct in everything you said. They will get the aggregate rate between sites running their software (limited by the slowest aggregate). At the same time if you have one site who acts as an ISP you could get the aggregate rate to that site and then only be limited by their connection to the internet. Which means that with the right infrastructure you could get multiple DSL speed. It's not a solution for most people (unless someone loaded their software, or equivalent, to share a t1 or t3 connection and acted as an ISP) but it could be a solution for some companies. Your feedback helped clarify things. I'm not sure how to close out the question.
emery_kAuthor Commented:
Thanks for the insight
I am using ML-IP software to bond ADSL now. Would like a new way to do this. I have a server colocated at the ISP so the ML-IP tunnal bonding works great but it's price is to much. Cant anyone give me any ideas on how to do this. We have 2 C blacks to work with. would BGP work for us.
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