combining 2 ADSL Lines

Aaron Nichols adnichols at
Fri Dec 17 15:32:09 PST 2004

On Fri, 17 Dec 2004 15:32:50 +0100, Hexren <me at> wrote:
> I have the following problem, I have 2 ADSl Lines and would like to
> combine those 2 into 1 big. I have full control of both the gateway
> from my LAN and another Server in the internet which I coud use as a
> second gateway.
> What I am thinking of is something like virtualizing so that in the
> end I have VirtualInterface1 which is DSL1 and DSL2.

Assuming these are just normal DSL lines with no special options which
would allow them to be combined, your options are limited I think.
This message assumes these are your typical Residental/business type
DSL lines which provide a few IP's to the end Customer.

In general, when using two different connections (DSL, T1 or
otherwise) between two different providers you have to pick one
connection or the other to send traffic over when communicating with a
given host on the Internet. There are many ways to "share" the two
connections in an automated way, so that some traffic goes over one
connection and some traffic goes over the other, but this doesn't
allow you to bond the two connections. "Bonding" being defined as the
ability to combine the bandwidth of both connections to provide higher
peak transfer rates.

The basic problem is that each ISP is only going to route traffic to
the IP address they provide you. Since it's highly unlikley that both
providers are providing the same IP you have to use NAT. When you send
traffic out of your network, it has to originate from one or ther
other IP address and response traffic is only going to come back to
that IP address, thus only using that DSL line. You can certainly send
the next connection out the other DSL line, but again, it will only
use that one DSL line.

There have been threads in the past discussion the use of policy
routing to send traffic from different parts of your network over
different connections - thus sharing the overall load between the two
connections. This gives you some additional capacity, but does not
provide higher throughput that a bonded connection would provide.

Setting up each connection individually on the FreeBSD gateway and
then investigating the use of pf or ipfw to provide policy routing is
probably the closest you are going to get to using both connections at
the same time. Perhaps someone else has some suggestions for options
I'm not aware of.


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