Android on the Beagle Board Gains Internet Access by Connecting with the Host Machine Acting as NAT through Ethernet over USB

Brief introduction

There are already solutions to share the mobile network connections of Android phone by the host PC via USB cable, so that browser in the host PC can surf Internet by using Android’s mobile network subscription. This is called tethering. However, today I’m going to present a complete solution to connect Android with the host PC by using ethernet via USB and enable Andriod to access the Internet through the Internet connection of the host PC.

I’m using the Beagle Board (http://www.beagleboard.org) as the platform to run Android (http://labs.embinux.org/index.php/Android_Porting_Guide_to_Beagle_Board).

Enable ethernet over USB in the linux kernel

- go to the “kernel” directory in the android source, use menuconfig (CC_PATH is mentioned in the link above)

make CROSS_COMPILE=$CC_PATH menuconfig

- enable usb-eth by choosing “Ethernet Gadget (with CDC Ethernet support)” for “Device Drivers -> USB support -> USB Gadget Support -> USB Gadget Drivers”, exit and save
- re-build the uImage

make CROSS_COMPILE=$CC_PATH uImage

the previous built uImage might need to be removed first

rm arch/arm/boot/uImage

- copy the uImage to the first partition of the sdcard
- make sure the kernel on the host machine is built with usbnet (“Device Drivers -> Network device support -> USB Network Adapters -> Multi-purpose USB Networking Framework” in the menuconfig). Most GNU Linux distributions already come with usbnet built-in, including Ubuntu in my case

Config ethernet over USB

- refer to http://benno.id.au/blog/2007/11/14/android-busybox to download and install the static-linked busybox built for ARM. It’ll be assumed busybox is installed at “/data/busybox” below
- in minicom connected with Beagle Board, wait until Android is fully booted up
- use ifconfig from busybox mentioned above

/data/busybox/ifconfig usb0 192.168.1.2 up

- config the gateway as well

route add default gw 192.168.1.1 dev usb0

- similarly, type the following command in the shell of the host machine

sudo ifconfig usb0 192.168.1.1 up

Use ADB

- the ADB daemon may require to be terminated first if already running:

ADBHOST=192.168.1.2 adb kill-server

- ADBHOST requires to be set if ADB works over ethernet

ADBHOST=192.168.1.2 adb shell

Config the host machine to act as NAT

- load necessary kernel modules

sudo modprobe ip_tables
sudo modprobe iptable_nat
sudo modprobe ip_nat_ftp
sudo modprobe ip_nat_irc

- enable IP forwarding

echo "1" | sudo dd of=/proc/sys/net/ipv4/ip_forward

- in case the IP address is assigned dynamically

echo "1" | sudo dd of=/proc/sys/net/ipv4/ip_dynaddr

- enable SNAT functionality on the corresponding network interface

sudo iptables -t nat -A POSTROUTING -o [NETWORK_INTERFACE] -j MASQUERADE

Note: [NETWORK_INTERFACE] is the corresponding network interface your host is using to access the Internet.

Proxy

It’s not uncommon that computer access Internet via network proxy. But WIFI and other network connections in Android require direct network connection and network proxy is not supported.

The code handling network proxy in Android is mainly in “frameworks/base/core/java/android/net/http/RequestQueue.java”. In the function setProxyConfig(), getHost() from “frameworks/base/core/java/android/net/Proxy.java is invoked to get the proxy host. However, as illustrated in “frameworks/base/core/java/android/net/ConnectivityManager.java”, Android seems to only support WIFI and mobile network as network connection type. WIFI happens to be the default connectivity type, the reason of which is shown below taken from the file “[ANDROID_SRC_DIR]/frameworks/base/core/java/android/net/http/RequestQueue.java”:

                /*
                 * Initializing the network type is really unnecessary,
                 * since as soon as we register with the NCL, we'll
                 * get a CONNECTED event for the active network, and
                 * we'll configure the HTTP proxy accordingly. However,
                 * as a fallback in case that doesn't happen for some
                 * reason, initializing to type WIFI would mean that
                 * we'd start out without a proxy. This seems better
                 * than thinking we have a proxy (which is probably
                 * private to the carrier network and therefore
                 * unreachable outside of that network) when we really
                 * shouldn't.
                 */

Then no proxy is set in case of WIFI network, which means WIFI requires direct Internet connection as shown below also taken from the file “frameworks/base/core/java/android/net/http/RequestQueue.java”. The way to solve it is to comment out the “if (mNetworkStateTracker.getCurrentNetworkType() == ConnectivityManager.TYPE_WIFI)” statement:

    private synchronized void setProxyConfig() {
        /* get rid of this branch */
        if (mNetworkStateTracker.getCurrentNetworkType() == ConnectivityManager.TYPE_WIFI) {
            mProxyHost = null;
	    } else {
            String host = Proxy.getHost(mContext);
            if (HttpLog.LOGV) HttpLog.v("RequestQueue.setProxyConfig " + host);
            if (host == null) {
                mProxyHost = null;
            } else {
                mActivePool.disablePersistence();
                mProxyHost = new HttpHost(host, Proxy.getPort(mContext), "http");
	        }
	     }
     }

Still in “frameworks/base/core/java/android/net/http/RequestQueue.java”, setProxyConfig() is only invoked when the NetworkStateTracker sends out message that the network state is changed. And setProxyConfig() is not called during initialization because as soon as Android registers with the NCL, a CONNECTED event is received for the active network. However, this is only true to mobile network, not for WIFI or wired networks. The way to fix this is to invoke setProxyConfig() at the end of enablePlatformNotifications(), so that proxy is properly configured at initialization regardless of what network type it is. The corresonding code is illustrated below:

    public synchronized void enablePlatformNotifications() {
        if (HttpLog.LOGV) HttpLog.v("RequestQueue.enablePlatformNotifications() network");

        if (mProxyChangeReceiver == null) {
            mProxyChangeReceiver =
                    new BroadcastReceiver() {
                        @Override
                        public void onReceive(Context ctx, Intent intent) {
                            setProxyConfig();
                        }
                    };
            mContext.registerReceiver(mProxyChangeReceiver,
                                      new IntentFilter(Proxy.PROXY_CHANGE_ACTION));
        }

        /* Network state notification is broken on the simulator
           don't register for notifications on SIM */
        String device = SystemProperties.get("ro.product.device");
        boolean simulation = TextUtils.isEmpty(device);

        if (!simulation) {
            if (mNetworkStateTracker == null) {
                mNetworkStateTracker = new NetworkStateTracker(mContext);
            }
            mNetworkStateTracker.enable();
        }

        /* call setProxyConfig() as part of the initialization.
          Otherwise, setProxyConfig() will only be invoked in case of
          network state change, and only mobile network get CONNECTED
          event after registration with NCL */
        setProxyConfig();
    }

Then in Proxy.java, it uses “frameworks/base/core/java/android/provider/Settings.java”, which is actually an utility wrapper of sqlite database to get the proxy and port settings. The corresponding table is “secure” from “/data/data/com.android.providers.settings/databases/settings.db”. The “name”, “value” pair should be “http_proxy”, “[HTTP_PROXY_HOST]:[HTTP_PROXY_PORT]“. The way to append the record is:

adb shell sqlite3 /data/data/com.android.providers.settings/databases/settings.db "INSERT INTO secure VALUES(99,'http_proxy','[HTTP_PROXY_HOST]:[HTTP_PROXY_PORT]');"

If nothing is got from the above database, then the system property “net.gprs.http-proxy” is used instead. The way to put it is to add the following command in “on boot” trigger in “init.rc”:

setprop net.gprs.http-proxy [HTTP_PROXY_HOST]:[HTTP_PROXY_PORT]

One more thing to mention is DNS has to be configured to resolve domain names. The way to do that is to add the following commands in “on boot” trigger in “init.rc” as well:

setprop net.dns1 [DNS_SERVER_1_IP]
setprop net.dns2 [DNS_SERVER_2_IP]

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