Home

Awesome

(c) 2014-2018 Paul Sokolovsky. Portions of content (c) by their respective authors.

Linux

How to find out version of kernel running

If that kernel is booted and you have access to console:

uname -a

Alternatively:

cat /proc/version

If you have only kernel image file ... (tba).

How to see what devices/partitions are mounted and their filesystem, types

mount

Alternatively:

cat /proc/mounts

How to read kernel message log (including boot log)

dmesg

Kernel log contains all messages from the beginning of kernel boot, but it is implemented as a circular buffer (of configurable size), so newer messages may replace older.

Is it possible to read kernel log if dmesg is not available?

cat /proc/kmsg will block and dump ''new'' kernel messages. This will work reliably only if there are no other consumer(s) of /proc/kmsg (note: on typical fully booted Linux system there's - syslogd, etc.), otherwise the output may be garbled.

How to read system log

On a full-fledged Linux system, system log is usually accessible as /var/log/messages. Recently, some particular distributions started to play original, for example, in Ubuntu 12.04 /var/log/messages is empty and instead /var/log/syslog may contain some, or all, of system log messages.

On an embedded system, a busybox syslogs may be used, providing circular buffer without file backing storage. It can be accessed using logread command.

How to write to system log

logger command does that.

How to find out config of a kernel?

With a running kernel, if it was built with CONFIG_IKCONFIG_PROC, the config can be read as /proc/config.gz. For a zImage file, scripts/extract-ikconfig can be used to extract it (just CONFIG_IKCONFIG is required for this).

How to list loaded kernel modules?

lsmod

Alternatively:

cat /proc/modules

How to load kernel modules?

Low-level command to load a specific filename is:

insmod /path/<module>.ko

Higher level command is:

modprobe <module>

This will look for modules located in /lib/modules/<kernel_version> and requires support files generated by depmod.

How to force loading of kernel module?

Module loading is subject to many checks (see below), so and kernel may refuse to load improper one. If CONFIG_MODULE_FORCE_LOAD is defined, it is possible to force-load module nonetheless. Unfortunately, it's not possible to force load with (recent) insmod. It can be done with modprobe -f (modprobe ---help for other force options).

How to unload kernel modules?

rmmod <module>

Module unloading usually available, but still depends on CONFIG_MODULE_UNLOAD. Module may refuse to unload if other modules depend on it, or if it's in particular state (which also includes states like "hanged" and "gone awry"), in which case it's possible to force unload (rmmod -f), if CONFIG_MODULE_FORCE_UNLOAD was defined.

Can kernel modules built for one version be used with another kernel version?

In general, this is not possible. With Linux kernel's "stable API is nonsense" attitude, anything in the kernel can be changed at any time

That said, for close kernel versions and small differences in configuration, chances of fatal incompatibilities of modules are low. However, kernel itself has "helpful guards" to preclude usage of modules with different kernel version. By default (CONFIG_MODVERSIONS is not defined), kernel's and module's vermagic strings are compared, which are of the form: "3.2.0-32-generic SMP mod_unload modversions 686", i.e. include kernel version, extraversion, then tags for most important configuration params. If vermagics don't match, module loading fails.

Even more annoying "protection" is employed if CONFIG_MODVERSIONS is defined. In this case, each kernel exported symbol gets hash (CRC) signature based on the parameter signature on the underlying C function, stored both in kernel and module. These signatures are compared, and on any mismatch, loading fails. The only easing with this is that when vermagics comparison is done, kernel versions are ignored, i.e. if different kernel versions don't have param signature changes and have same core config params, a module will work across such versions.

There's also support for forced module load, but that will work only if enabled in kernel config (CONFIG_MODULE_FORCE_LOAD).

How to get detailed information about a kernel module

modinfo <module>

This will dump main module properties and vermagic string. As a parameter, both a filename and system module name (as available in /lib/modules) can be given.

To dump modversions hashes:

modprobe --dump-modversions <module>.ko

How to build out of tree kernel module

From official kernel docs: Documentation/kbuild/modules.txt, for a module source in current dir:

make -C <path_to_kernel_src> M=$PWD

Build against currently running kernel (requires linux-headers package installed):

make -C /lib/modules/$(uname -r)/build M=$PWD

How to install built kernel modules to a specific location

Useful when cross-compiling, etc.

make modules_install INSTALL_MOD_PATH=/path/where

More details: Documentation/kbuild/modules.txt.

How to setup NAT (masquerading)

If you have another host/embedded device/QEMU connected to a local host interface, and you would like to let that device connect to the Internet, masquerading should be set up for device's IP address:

iptables -t nat -A POSTROUTING -j MASQUERADE -s 192.168.2.2

(This assumes that the host has Internet connection and has IP forwarding enabled).

How to connect to WiFi network from bare command-line (using wpa_supplicant & wpa_cli)

If you're not on bare command-line, make sure you killed already running wpa_supplicant as well as other daemons which may respawn it (e.g. NetworkManager on Ubuntu).

Make sure that WiFi interface is up. It otherwise doesn't need to have any special configuration:

ifconfig wlan0 up

Run:

wpa_supplicant -i wlan0 -g /tmp/wpa -C /tmp

-g specifies "global control socket". The caveat about is that many commands are not supported via it, issuing them via it leads to obfuscated "UNKNOWN COMMAND" reply. So, it's important to specify -C switch, which takes directory path under which individual control sockets for each WiFi interface will be stored.

In another window, run:

wpa_cli -p /tmp

-p option gives path to per-WiFi-interface control socket dir (-C of wpa_supplicant). Issue following commands in wpa_cli interactive session (ignore # lines):

# Scan for available APs
> scan
OK
# Wait a bit, then view results
<3>CTRL-EVENT-SCAN-STARTED 
<3>CTRL-EVENT-SCAN-RESULTS 
> scan_results 
...
# Create new AP entry, returned is ID of new entry
> add_network
0
# Start setting AP parameters
> set_network 0 ssid "MY_AP"
OK
> set_network 0 psk "password"
OK
# These should be all params requires for WPA
# Activate network
> select_network 0
OK
# Trigger connection
> reconnect
OK
<3>SME: Trying to authenticate with xx:xx:xx:xx:xx:xx (SSID='MY_AP' freq=2462 MHz)
<3>Trying to associate with xx:xx:xx:xx:xx:xx (SSID='MY_AP' freq=2462 MHz)
<3>Associated with xx:xx:xx:xx:xx:xx

This will have wlan0 connected to AP. Then get IP address via DHCP:

dhclient wlan0

And configure /etc/resolv.conf to suitable DNS address (e.g. 8.8.8.8 to be monitored by Google):

nameserver 8.8.8.8

The instructions above show how to run wpa_supplicant without any stored configuration, and instead configure it dynamically (e.g., programmatically). Alternatively, using config:

# Create simple config file
wpa_passphrase MY_AP >wpa.conf
# Enter password
...
wpa_supplicant -i wlan0 -c wpa.conf

How to test network multicasting

Multicasting is basis of many usability protocols and services (e.g. mDNS, UPNP, DLNA, etc.), and yet means to query/test/diagnose it are not widely known, if not to say obfuscated. At the same time, multicasting has known andregular issues, when used on network interfaces differing from Ethernet, for which it was initially conceived (these other networking types include WiFi/other wireless, loopback, etc.)

Multicast is similar to broadcast, except individual hosts don't receive datagrams unconditionally, but need to join "multicast group" (other way to think about it is a PubSub pattern). Multicast groups are represented by special IP addresses, e.g. for IPv4 it's 224.0.0.0 to 239.255.255.255. Some multicast IPs have predefined (STD/RFC) meaning, other are supposedly can be (dynamically) allocated per purpose.

224.0.0.1 is a "local network" predefined address. All hosts on current subnetwork are supposed to (auto)join it. In this regard, that address is similar to local network broadcast, 255.255.255.255. The basic means to test multicasting is ping this address:

ping 224.0.0.1

The expected outcome is that each host which is member of multicast group will respond (ping thus will receive duplicate responses and will report so).

However:

echo "0" >/proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

(needs to be done on each machine from which you want to receive responses of course).

If you have a normal Ethernet/WiFi interface, following first 2 suggestions should lead to ping 224.0.0.1 to get responses at least from the host you run it on.

For other multicast groups, they should be pingable as long as a socket bound to that group is active on a host.

To further diagnose multicast settings, refer to following /proc files:

Few other points:

Android

How to enable ADB connection on device

This is called "USB debugging" and is available in Settings -> Developer options. Developer options themselves are concealed by default in Android 4.2 and higher. From https://developer.android.com/studio/debug/dev-options:

On Android 4.1 and lower, the Developer options screen is available by default. On Android 4.2 and higher, you must enable this screen. To enable developer options, tap the Build Number option 7 times. You can find this option in one of the following locations, depending on your Android version:

How to configure udev rules for using adb over USB

Create /etc/udev/rules.d/99-android.rules with content:

SUBSYSTEM=="usb", ATTR{idVendor}=="0bb4", MODE="0660", GROUP="plugdev"

Here, 0bb4 is a USB vendor ID, you may need to adjust it for your device; plugdev - is typical pluggable device access group on most modern Linux distros.

Based on: http://developer.android.com/tools/device.html

How to connect adb over WiFi (or TCP/IP in general)

On Android command line:

su
stop adbd
setprop service.adb.tcp.port 6565
start adbd

Use netcfg to check device IP. (In newer Android versions, standard ifconfig or ip addr may work.)

Per above (su), this requires a rooted device. An alternative is to connect device using USB first, and run adb tcpip 6565, after which USB cable can be disconnected.

After that, on host:

adb connect <IP>:6565
adb shell

How secure is ADB connection?

ADB protocol does not seem to support encryption (protocol description: https://android.googlesource.com/platform/system/core/+/master/adb/protocol.txt). This means that using it over broadcasting non-encrypted physical medium (like Ethernet or public WiFi) is insecure.

Until Android 4.2.2, protocol did not support authentication, which made it highly insecure. There are known exploits of trojan "charging stations" used to break into devices.

In Android 4.2.1_r1, public key authentication was added to adb protocol: https://github.com/android/platform_system_core/commit/d5fcafaf41f8ec90986c813f75ec78402096af2d This requires ADB version 1.0.31 (available in SDK Platform-tools r16.0.1 or higher). User is required to confirm connection to a new ADB host on a device itself, for one time connection or to remember the approval. In the latter case, host key(s) are saved in /data/misc/adb/adb_keys. There can be pre-installed keys in /adb_keys.

How to verify ADB host fingerprint

When connecting to a new ADB host, an Android device shows an RSA fingerprint, however, it takes some effort to check that this fingerprint actually belongs to a host.

On a host, ADB public key is in $HOME/.android/adb_key.pub. To get its fingerprint:

awk '{print $1}' < adbkey.pub|openssl base64 -A -d -a | openssl md5 -c

Based on: http://nelenkov.blogspot.de/2013/02/secure-usb-debugging-in-android-422.html

How to read Android logs

On Android command line:

logcat

See logcat --help

From host:

adb logcat

How to list installed Android packages?

On device:

pm list packages

How to install Android package (.apk)?

On device:

pm install app.apk

How to install .apk using adb (from host command line)

From host:

adb install app.apk

To force install even if already exists:

adb install -r app.apk

adb --help for more.

How to remove installed Android package?

On device:

pm uninstall <package.name>

This won't work for system packages located in /system/app, see question below on how to remove them.

How to get around lack of cp in default Android toolbox

Try:

cat /path/from >/path/to

Is there a writable location on the main filesystems on non-rooted device?

First of all, there's /mnt/sdcard, which is accessible to default adb user. However, that filesystem is mounted noexec, so it's not possible to run any executables from there. However, there's /data/local which is both writable and executable. On newer Android versions (checked with Android 7), only /data/local/tmp is such.

How to get temporary root access on ro.secure=1 devices

An exploit executable is needed. For Android 2.x, zergRush is a well-known app. On host:

unzip zergRush.zip
adb push zergRush /data/local
adb shell
cd /data/local
chmod 770 zergRush
./zergRush

If you get error like [-] Cannot copy boomsh.: Permission denied, it's likely zergRush was already run previously and left some cruft, remove it and retry (on device):

rm tmp/sh
rm tmp/boomsh
./zergRush

How to install Android su with frontend (permanent root)

Get root shell as described above. Then (you can get newer versions than 3.0.7, but they're kinda bloat):

http://downloads.noshufou.netdna-cdn.com/superuser/Superuser-3.0.7-efghi-signed.zip
unzip Superuser-3.0.7-efghi-signed.zip
adb push system/bin/su /system/bin/
adb push system/app/Superuser.apk /system/app/
adb shell chmod 06755 /system/bin/su

Where vendor-provided software is located in filesystem?

Preinstalled application are in /system/app, unlike user-installed applications, which are in /data/app .

How to remove bloatware from /system/app

This requires root.

On host:

adb remount
adb shell

Then on device:

cd /system/app
rm bloatware.apk
sync

You may want to reboot to clear app cache, though apps should be gone immediately.

What are user and group ids used by Android?

These are defined in system/core/include/private/android_filesystem_config.h

Is it possible to use stdout/stderr/printf/System.out.println/etc. in Android?

Reference: http://developer.android.com/tools/debugging/debugging-log.html#viewingStd

By default, Android redirects stdout, etc. to /dev/null. However, log.redirect-stdio system property can be set to true to redirect to system log (logcat) instead. This takes effect after system restart (and has effect during current system session), so the complete command sequence to enable stdout logging is:

stop
setprop log.redirect-stdio true
start

How to list Android services?

service list

How to call Android services from command line?

Based on http://www.slideshare.net/jserv/android-ipc-mechanism

Getting Java class implementing service:

service call activity 1598968902

1598968902 (0x5f4e5446) is INTERFACE_TRANSACTION.

Prepare to dial number "123":

service call phone 1 s16 123

"1" is a method #1 per ITelephony.aidl, i.e. dial().

How to start/stop Android daemons

start & stop commands do that. E.g.:

stop vold

Alternatively, this can be done by setting system properties:

setprop ctl.stop vold
setprop ctl.start vold

How to start/stop complete Android environment

For this, zygote daemon should be stopped:

stop zygote

zygote is also a default argument for start/stop, so following will work too:

stop

How to dump detailed service state?

dumpsys <service> dumps loadsome of information about internal service state. Omit service name to dump state of all services.

How to Allow Apps To Write Files to USB Mass Storage Devices in Android

Based on:

Edit /system/etc/permissions/platform.xml to add <group gid="media_rw" /> to WRITE_EXTERNALS_STORAGE permission:

<permission name="android.permission.WRITE_EXTERNAL_STORAGE">
<group gid="sdcard_rw" />
<group gid="media_rw" />
</permission>

Reboot required. Tested with Android 4.

How to list all modules available in Android platform source tree?

make modules

Is it possible to write console Dalvik apps?

Yes, actually "am", "pm" and few other standard Android tools are actually such, and are run via shell wrapper: https://android.googlesource.com/platform/frameworks/base/+/gingerbread/cmds/am/

hello-world.html from platform/dalvik/docs/ explains how:

# Create file Foo.java
class Foo {
    public static void main(String[] args) {
        System.out.println("Hello, world");
    }
}

javac Foo.java
dx --dex --output=foo.jar Foo.class
adb push foo.jar /sdcard
adb shell dalvikvm -cp /sdcard/foo.jar Foo

I tried to flash a custom image to recovery partition (Android 4.x), however it doesn't boot and instead "red triangle of doom" is shown

Assuming the image is correct for your type of device and flashed into right place for it, known "feature" which causes this behavior is Android 4.x "recovery from boot". The idea is following: Recovery allows you to restore main Android install if something goes wrong. But what if recovery itself gets corrupted? Then if main image gets corrupted too, the device is bricked. So, main image and recovery were made to "watch after another". In particular, during boot of main image, recovery partition is checked, and if it doesn't contain "know good" image, it gets silently reflashed with copy of such image kept in main partition.

The backup of recovery image is commonly held in /system/recovery-from-boot.p . Actual boot-time checking and reflashing is handled by /system/etc/install-recovery.sh . Thus, to disable this behavior which gets into way of advanced Android usage, do following:

mv /system/recovery-from-boot.p /system/recovery-from-boot.p.old

or:

chmod 644 /system/etc/install-recovery.sh

Also, "red triangle of doom" is actually how a stock recovery of Android 4.x looks - while previous versions offered interactive means to investigate and fix your device, that's what to it was reduced in Android 4.x.

Based on: http://android.stackexchange.com/questions/18932/why-isnt-clockworkmod-recovery-sticking , http://androtab.info/clockworkmod/rockchip/install/