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The Internet

Asynchronized Internet Connections? 12

Posted by Cliff from the optimizing-the-net-connections dept.
Zid asks: "I've got a cable modem, and the apartment I live in has a T1. The problem I have is that my cable goes out constantly, and the T1 is shared among the entire complex (200+ people) and it's behind a 3com NAT, so I'd like to be able to have my Linux box (which is a Masquerade box for the cable right now) to be able to automatically switch over to the T1 as a backup when cable goes out. My second question is whether it's possible to use the T1 as the uplink and cable as the downlink since the T1 can upload 4 times faster than the cable can."
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Asynchronized Internet Connections? More

Asynchronized Internet Connections?

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  • Something weird's going on.. this story seems to be only in "Ask Slashdot", but not on the main page..

    Of course, by the time I've posted this it might be fixed.

  • Perhaps the routing metric would do what you need (failover-wise).

    As for incoming/outgoing split, it *is* possible, since satellite internet feeds do it (incoming through satellite, outgoing through modem). Perhaps some ipchains rules would be able to rewrite the packets enough?
  • this is normal....taco and the crew do not always post "ask slashdot" features to the main site, though they do add it to the column.
  • In theory you can route traffic however you want. Here's the problem though:

    When you open a connection from one computer to another, it is done as a socket. When done using IP, you have a socket between IP.A:port1 and IP.B: port2. So, if you send a packet out your T1 with an address that belongs on that network, your return traffic will come back over the T1, because that's the correct route to get to the socket.

    To do what you want to do, you'd have to send the packets on the T1 network with your IP address from your Cable connection. That is called spoofing and /should/ be blocked by the ISP that services the T1.

    As for fail-over:
    1. Determine the IP address of your preferred gateway.
    2. Ping it once a minute.(using cron)
    3. When the ping fails, change your default route to the other gateway with a route command.

    This requires a bit of shell scripting, but should work. Note that all your connections will reset. Of course, this will happen whether you fail-over manually or automatically unless you are spoofing your cable IP and the T1 fails in which case you'd go back to using your cable IP the way it was intended.

    --

  • In theory this is possibal. However it won't work well.

    In theory your linux box just switches to the other connection, no problem. (Detection is a bit of a pain, but can be done) In parctice it doesn't help much. All your connections are to your cable IP address, which no longer reaches you. (it gets routed to your cable company who cannot reach you so they throw it away)

    If you have a friend with several stable IP addresses and a good connection, then you can probably tunnel to them (with some work). Otherwise your problem isn't sending data out the backup line, it is getting data to come back over that line without losing connections.

    If anyone has a solution please let me know - I'm on a dialup with poor line quality. I have a big problem with programs trying to hold a connction longer then the line will stay up at a time.

  • Gated speaks the OSPF routing protocol which can handle dynamic routing. This should allow you to switch to the T1 when the cable modem goes down.

    I don't think its possible to have all incoming packets come through the cable modem and all outgoing packets go through the T1. If your packets leave via the T1 they will have a different IP address than if they leave over the cable modem. To do this you would need an upstream router to readdress packets to the other ip address.

    You may want to look at Linux 2.4 Advanced Routing HOWTO [unc.edu] which lets you set up routing rules based on things other than destination address, including port number which may be of interest to you.
  • Yeah, you should be able to mangle incoming packets from your network out to the T1, and receive normally from cable... the only problem is you'd need to adjust the IP address so that the other system would send the responses to the cable modem... and the gateway on the other side of the T1 would still route you... hmmm... a little tricky.

    The fail-over could be more easily implemented. You'd have a couple of seconds of downtime, but that's better than a few minutes or an hour 8^)
  • I wish I had the problem of switching between cable and T1. I'm too far away from the CO for DSL and the only cable is one way. I.e. downstream over cable, but upstream over POTS modem (which is a tad slower than T1).
  • the only problem is you'd need to adjust the IP address so that the other system would send the responses to the cable modem... and the gateway on the other side of the T1 would still route you... hmmm... a little tricky. Perhaps Divert sockets and IPchains would do the job? Apparently, its a BSD thing, but there's a mini howto for getting them to work under Linux.
  • Satellite operators such as http://www.ihug.net/bandwidth.html routinely associate satellite downlink with terrestrial uplink to provide asymetrical service. I do not know how thy manage the routing, but this might be an interesting track to follow.

    http://varinfo.direcpc.com/what/work.html
    Direcpc uses an application level proxy using push methods :
    "The process in detail: When a customer requests a URL, the request gets delivered by modem to the ISP. However, before that request leaves the customer's PC, the DirecPC software attaches a tunneling code to the URL. That code instructs the ISP to forward the URL request to the DirecPC Network Operations Center (NOC). Once HNS receives the request, the tunneling code is stripped away and the request is forwarded to the appropriate site from which the desired content is retrieved. The NOC then uploads the information to the Hughes satellite, which beams it down to the customer's DirecPC dish and into his or her PC."

    http://hypercable.net/satellite/isp/body_isp.htm gives more precisions.

    If you do not want to proxy at application level, I would say that unless you have a host somewhere up in the Cloud, that you can tunnel both your connexion to (why not using specially load balanced multilink ppp over pptp) and let do the splitting, there is no way a connexion is going to originate somewhere and come back around another way.

  • As everyone is twisting their brains into knots to solve this problem, how about a much easier question. I have an external DSL modem/router with ONE real live static IP for it. How would I host a web server on my machine? Two static addresses would be really easy, but they refuse to do this for me without a "business" account (at more than double the monthly charge). I'm assuming I need some sort of forwarding rule setup on my router (a Cisco 675). This has to be a common thing to do, I just haven't been able to find any resources on it yet. Is it even possible?
  • Check out http://adsl.contamination.org/how_tos.ht ml [contamination.org]. This is for Zoomtown (cincinnati bell) but they use the same cisco, so i bet that the setup is similar.

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