Complete Guide to Modifying hosts File on Android: From Root Access to Filesystem Mounting

In Android development and system customization, modifying the hosts file is a common yet sometimes challenging task. Users typically expect to freely modify system files after obtaining root access, but the reality is often more complex. Based on real cases from technical Q&A communities, this article systematically analyzes the core technical points of this process.

Problem Background and Common Misconceptions

Many Android users encounter insufficient permission errors when attempting to modify the /system/etc/hosts file even after successfully rooting their devices. This typically manifests in two scenarios: receiving a "Read-only file system" prompt when pushing files via ADB, or being unable to modify files in the device shell even with the su command. At first glance, this seems counterintuitive—why can't certain system files be modified with superuser privileges?

Root Cause: Filesystem Mount Status

The core issue lies in Android's security mechanism design. Even with root access, the /system partition is mounted as read-only by default after normal boot. This design is not a flaw in permission restrictions but an important safeguard for Android system integrity and security. When the system partition operates in read-only mode, no user (including superusers) can directly modify its contents, effectively preventing malware or accidental operations from damaging core system files.

Solution: Remounting the System Partition

To modify the hosts file, the /system partition must first be remounted as read-write. According to the best answer in the Q&A data (score 10.0), the most reliable method is using the following ADB command sequence:

$ adb shell
$ su
$ mount -o rw,remount -t yaffs2 /dev/block/mtdblock3 /system

The execution logic of this command sequence warrants in-depth analysis: first, enter the device command-line environment via adb shell, then switch to superuser with su, and finally execute the critical mount command. The -o rw,remount parameter of the mount command specifies remounting in read-write mode, -t yaffs2 declares the filesystem type (particularly important on some older Android devices), and /dev/block/mtdblock3 is the block device path for the /system partition on specific devices.

Simplified Methods and Compatibility Considerations

The second answer in the Q&A data (score 6.8) offers a more simplified approach:

adb shell
su
mount -o remount,rw /system

This method omits specifying the filesystem type and specific device path, relying on system auto-detection. On most modern Android devices, this simplified command indeed works correctly because it utilizes existing system mount information. However, on some custom ROMs or older devices, explicitly specifying parameters may be more reliable. Developers need to choose the appropriate method based on the specific device environment.

Special Handling for Android Emulators

The third answer (score 2.1) mentions special circumstances in Android emulator environments. When modifying the hosts file in an emulator, it may be necessary to first adjust the partition size:

./emulator -avd myEmulatorName -partition-size 280

Then use simplified ADB commands:

./adb remount
./adb push /tmp/hosts /system/etc

Here, the adb remount command is a special shortcut in emulator environments that automatically handles remounting the /system partition. This method is generally unavailable on real devices but offers convenience in development and testing environments.

Practical Steps and Verification

In practice, it is recommended to follow these systematic steps:

1. Ensure the device is properly rooted and ADB debugging is enabled.
2. Connect the device and open a command-line tool, entering adb shell to access the device environment.
3. Execute the su command to obtain superuser privileges (some devices may require authorization confirmation).
4. Attempt to remount the system partition using the simplified command mount -o remount,rw /system.
5. If the simplified command fails, try the complete command specifying the filesystem type and device path.
6. After successful mounting, verify the mount status with cat /proc/mounts | grep system.
7. At this point, the hosts file can be modified via vi, echo, or ADB push.
8. After modification, it is advisable to execute mount -o remount,ro /system to restore the partition to read-only mode for system security.

Security Considerations

While modifying the hosts file is useful in development and testing, the following security risks must be noted:

• Mounting the /system partition as read-write temporarily reduces system security and should only be done when necessary.
• Modifying the hosts file may affect network connectivity, especially if critical domains are incorrectly blocked.
• Different Android versions and device manufacturers may have varying protection mechanisms for system partitions; consult device-specific documentation before operation.
• It is recommended to back up the original hosts file before modification for quick recovery in case of issues.

In-Depth Technical Principles

From an operating system perspective, Android is based on the Linux kernel, and its filesystem mounting mechanism follows Linux standards. The remount option of the mount command allows changing mount parameters without unmounting the filesystem. When the rw parameter is specified, the kernel updates the filesystem's superblock flags to permit write operations. However, some Android devices incorporate additional verification mechanisms, such as SELinux policies or dm-verity, which may require extra handling.

For developers, understanding these underlying mechanisms not only helps resolve hosts file modification issues but also lays the foundation for deeper system customization and debugging. By mastering filesystem mounting, permission management, and the use of ADB toolchains, developers can conduct Android system-level development and testing more effectively.