Friday, 8 August 2008

Mobile Internet over Bluetooth Part 2

Following on from configuring your mobile phone for Internet over bluetooth...

Once you have your device configured, the next thing you need is to connect over bluetooth to some machine to route your traffic for you. I have a laptop with built-in bluetooth and I'm running Redhat Enterprise Linux version 5. You need to configure and setup bluetooth, a ppp bluetooth daemon, and some simple routing to get your phone on the Internet. First off you need to pair your laptop with your phone, there's plenty of info on how to do this around the web and it's simple enough I'll assume you now have a GnuBox configured phone paired with your laptop.

First off you need to set up Internet Connection Sharing (in windows speak) or IP Masquerading (to anybody who know's what they're talking about), on Linux this is done with iptables. Insert the following rules:
  • iptables -A FORWARD -i ppp0 -j ACCEPT
  • iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
  • iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
The last rule for the postrouting chain should point to the interface on your laptop where you have an outgoing Internet connection enabled. In my case this can be either eth0 (the first Ethernet device) or wlan0 (my wireless device, could also be ath0, eth1, etc), so I insert two rules to cover both devices, even though I only show one of these above.

Enable IP forwarding:
  • echo 1 > /proc/sys/net/ipv4/ip_forward
With your bluetooth device turned on, and previously paired with your phone, now all you need to do is set up a ppp daemon listening for incoming bluetooth "serial" type connections. This can be done in a couple of easy commands, first to add the serial protocol to your bluetooth SDP server, then to set up the listening daemon:
  • sdptool add SP
  • dund -n --listen --msdun noauth crtscts 115200 ms-dns lock
The second command will start a dun daemon on the command line that doesn't background itself so you can debug your connection if necssary. It will set up a point to point connection between your laptop and the bluetooth connecting device with IP addresses and, you can replace these with any addresses you like. It will also ensure the connection is operating at a sensible speed and you should provide a DNS server that will be passed to the device to resolve Internet server names - you can see this from the contents of your current /etc/resolv.conf file.

Start your web browser on your phone and tell it to connect using the Bt access point you have already configured. On the command line on your laptop you should see something similar to:
    Using interface ppp0
Connect: ppp0 <--> /dev/rfcomm1
local IP address
remote IP address
This shows your phone and your laptop are now connected through a ppp serial type connection over bluetooth using the /dev/rfcomm1 device, your phone will have the IP address and your laptop ppp0 device will have the ipaddress Your phone should have set as the gateway automatically by GnuBox and the DNS address will be passed when dund sets up the ppp connection as per your command line options. Once connected, you should be able to browse the web on your phone for free through your laptop!

As I said, I run Red Hat, which provides for several handy interfaces to help with the setup of Internet over bluetooth from my phone. Here are some further instructions specific, but not limited to, how I've done things on my machine so I everything happens neatly during boot and I don't need to run any commands to set up the connection...
  • You'll want your iptables rules in place when you boot, you can append rules to /etc/iptables.d/ to do this:
    • Add the following lines to a file in /etc/iptables.d/filter/FORWARD/
      • -A FORWARD -i ppp0 -j ACCEPT
      • -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
    • Add the following lines to a file in /etc/iptables.d/nat/POSTROUTING/
      • -A POSTROUTING -o eth0 -j MASQUERADE
      • -A POSTROUTING -o ath0 -j MASQUERADE
  • Use sysctl to configure IP forwarding at boot time
    • Add (or change) the following definition in /etc/sysctl.conf
      • net.ipv4.ip_forward = 1
  • Register the serial protocol at boot time for your SDP server
    • Add the following line to the start definition after SDP is started in /etc/init.d/bluetooth
      • sdptool add SP
  • Start the dund daemon at boot time with the correct configuration options
    • chkconfig dund on (to start the service at boot time)
    • Create a file called /etc/ppp/peers/dun with the following contents
      • noauth
      • crtscts 115200
      • ms-dns
      • lock
    • Edit the file /etc/sysconfig/dund so you have the following configuration
      • DUNDARGS='--listen --msdun call dun'
These settings should be enough that you can connect your phone to your laptop without having to modify any settings or start any services every time you boot. Once all the services are configured and started, and your phone is paired with the laptop, you will be able to connect to the phone's Bt access point to access the Internet any time you're in range of your laptop.

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