TELECOM Digest OnLine - Sorted: Re: Using Two ADSL Internet Connections Simultaneously

Re: Using Two ADSL Internet Connections Simultaneously

Gordon Burditt (
Wed, 07 Dec 2005 19:56:47 -0000

> 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

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