CODEWITHSUNDEEP
androidandroid-wifi
AllThatICode
AllThatICode

Reputation: 1300

Android: Error Resolving hostname to IP

I have been doing some low-level hacking on Android.

I'm trying to enable wifi i.e. establish Internet Connectivity through terminal (adb shell). I have written a C program to achieve this by hacking into android's bionic library and libnetutils library to get things working.

Everything works fine. I'm able to acquire an IP address via dhcp request. The problem is that whenever I try to open any site for ex. google.com via browser, it does not opens. But when I enter the IP address of the site "74.125.228.66" (google.com), the page gets loaded.

I have tried several options, like modifying the dns entries in file "resolv.conf" (present in /system/etc) and in file "20-dns.conf" (in /system/etc/dhcpcd/dhcpcd-hooks). I also tried using "setprop" call to set dns values manually for "dhcp.eth0.dns*" and "dhcp.wlan0.dns*".

But nothing seems to work. There's also an interesting behavior I noticed. If I turn on wifi manually from "settings" menu and then turn it off and then run my program, I don't face this issue anymore. Looks like it uses some settings that I could not figure out.

My guess is that this is DNS issue, but it may be something else. Let me know if anyone has faced this issue before.

Here's is what I do to enable wifi:

  1. Enable wpa_supplicant daemon using set_prop().

  2. Send a dhcp request to acquire IP (code from dhcpclient.c in libnetutils).

  3. Enable dhcpcd daemon using set_prop(). (Even without this, everything works. I ran this so that IP lease gets renewed automatically. ( Although I'm not sure about this, if dhcpcd daemon would take care of lease renewal or not) ).

Upvotes: 2

Views: 1622

Answers (1)

AllThatICode
AllThatICode

Reputation: 1300

In order to enable WIFI on Android via command line through a C program, you'll need to do the following:

1) Enable wpa_supplicant daemon. (Make sure you have the wpa_supplicant.conf file at /data/misc/wifi with AP (access point) information in it.). wpa_supplicant internally takes care of loading the driver and then establishes connectivity to the specified networks in its configuration file based on availability and strength.

2) Issue a dhcp request. ( To fetch IP, lease, dns1, dns2 etc.)

3) Start dhcpcd daemon. (For Lease renewal)

4) Set net.dns* properties. (Without this, DNS service would not work for any application)

In order to enable daemons (wpa_supplicant and dhcpcd) and set net.dns* properties , you'll need to use Android property system (property_set() and property_get() functions). To learn more about Android Property System, follow this link: http://rxwen.blogspot.com/2010/01/android-property-system.html

In order to use above 2 functions, you'll need to hack into bionic and core libcutils library. Location: /bionic/libc/bionic/system_properties.c /system/core/libcutils/properties.c

For issuing dhcp request, you need to hack some of the implementation of libnetutils Location: /system/core/libnetutils/*

Disabling Wifi:

1) Unload the driver manually. 2) Stop wpa_supplicant daemon. 3) Stop dhcpcd daemon. 4) Unset net.dns* properties.

In this case, driver needs to be unloaded manually unlike loading. If this step is not done, then any of the existing connections will not be torn apart even after 2, 3, and 4. To manually unload the driver, you need to issue "DRIVER STOP" request to wpa_supplicant which will take care of interacting with kernel to unload the driver. In order to communicate with wpa_supplicant, you'll need to hack into wpa_cli implementation to see how it works. It basically uses UNIX Domain Sockets to interact with the supplicant. You basically need to have a look at wpa_ctrl.c and wpa_cli.c

To learn more about wpa_supplicant, wpa_cli, follow below link: http://hostap.epitest.fi/wpa_supplicant/devel/

All above that I explained to you is done in Android at HAL layer. Location: /hardware/libhardware_legacy/wifi/wifi.c

So, basically whenever you toggle wifi switch from settings menu, control passes from wifi_app code to wifi_frameworks layer (WifiManager and WifiServices) which passes control to wifi.c (HAL layer) through JNI implementation (WifiNative).

A good starting point would be to look at wifi.c if you want to know things that are done at low level to enable wifi.

PS - All above is what I learned after several attempts trying to figure out stuff on my own. There are no documents or blogs (atleast I didn't find any!) that would specify what needs to be done and order of events that should be followed to enable/disable WIFI. So, it may be possible you might find a better way of doing things. This worked for me, something else might work for you!

Upvotes: 1

Related Questions