Re: Using Two ADSL Internet Connections Simultaneously

I have two telephone lines. I want to make both of these lines ADSL
> connections to the internet and use these connections to provide > internet access to several wireless laptop computers. A maximum of > about 20 laptops would be connected to the system at once. I would > like the laptops to be able to use both lines simultaneously so that I > don't have one line overloaded and one underloaded (ie: I want to > balance the traffic on the lines at any given time).

Are the lines connected to the same provider? Are they willing to bond the lines together on their end? (This may cost extra, or you may get the voice equivalent of a blank stare from the customer service representative in India.) You may be able to distribute outgoing traffic evenly between the two lines, but you have no control over how the incoming traffic is distributed unless you make arrangements with your ISP.

There are drivers in FreeBSD (and probably Linux as well) which permit distributing traffic over several lines which presumably have the same destination. In FreeBSD, this is the netgraph driver with the one2many module. The rest of the system pretends the two lines are one interface. This works best if the other end is also doing the same thing (and I suspect Cisco routers at the ISP can do it). You would use a FreeBSD or Linux machine as your router with multiple network cards (two for the two lines, one for internal net, perhaps one for wireless).

It is possible (especially if the lines are from different providers) that each DSL line will not accept outgoing packets except for those with "from" IP addresses assigned to the DSL lines. In other words, packets going out DSL line A have to be from netblock A, and the replies will probably be routed down DSL line A. Similarly for DSL line B. (ISPs do this to prevent untraceable spoofed flooding. If you flood, it's at least traceable to a specific box at the ISP. You might still be able to spoof your neighbor if he's on the same box.) Once you start a TCP connection, the IP address must be from either netblock A or B and traffic must go out the corresponding line for the duration of the connection.

Do you intend to accept incoming connections from the outside? (Mostly, this means servers, but some peer-to-peer and FTP issues are involved also). Then you need an arrangement with your ISP to fail over (and you want load balancing also) from line A to B and vice versa routing for the public IPs of your servers.

Anyone have any suggestions on how to set up something like this?

To do a good job, you need help from your ISP. To do a really good job, especially if the lines are from different ISPs, you need to talk BGP on both of them. For what you are doing, this is a bit like using nuclear weapons to solve a mosquito problem.

Gordon L. Burditt