Root Filesystem

This directory contains the root filesystem for the gateway.

Overview

The root filesystem is a minimal Linux system based on:

• BusyBox — Essential Unix utilities in a single binary
• Dropbear — Lightweight SSH server
• musl libc — Modern, lightweight C library

The filesystem is packaged as SquashFS (read-only, compressed) for reliability and space efficiency.

Key Design: Bootstrap + Symlinks

The rootfs contains only a minimal bootstrap in /etc:

File	Type	Purpose
/etc/inittab	Real file	Init configuration (required at boot)
/etc/init.d/rcS	Real file	Bootstrap script (mounts /userdata, runs init scripts)
/etc/passwd	Symlink	→ /userdata/etc/passwd
/etc/group	Symlink	→ /userdata/etc/group
/etc/hostname	Symlink	→ /userdata/etc/hostname
/etc/profile	Symlink	→ /userdata/etc/profile
/etc/dropbear/	Symlink	→ /userdata/etc/dropbear/
/etc/hosts	Symlink	→ /var/hosts (dynamic, ramfs)
/etc/resolv.conf	Symlink	→ /var/resolv.conf (dynamic, ramfs)

This architecture provides several benefits:

Benefit	Description
Updatable config	Configuration can be modified without rebuilding the read-only rootfs
Persistent settings	Changes survive rootfs updates
Easy customization	Users can tweak settings in the writable userdata partition
Safe updates	Rootfs can be reflashed without losing configuration

Boot Sequence

Kernel
  ↓
Mount SquashFS rootfs
  ↓
/sbin/init reads /etc/inittab
  ↓
/etc/init.d/rcS (bootstrap):
  1. Mount /proc, /sys, /var (ramfs), /dev/pts
  2. Create MTD device nodes if needed
  3. Mount /userdata (JFFS2 on mtdblock3)
  4. Execute /userdata/etc/init.d/S??* scripts
  ↓
System ready

The bootstrap script (/etc/init.d/rcS) is the only init script in the rootfs. All user services (network, SSH, applications) are started from /userdata/etc/init.d/.

Contents

Directory/File	Description
skeleton/	Base filesystem structure (etc, init scripts)
busybox/	BusyBox build directory
dropbear/	Dropbear SSH build directory
build_rootfs.sh	Script to assemble and package the rootfs

Building

# Build BusyBox
cd busybox && ./build_busybox.sh && cd ..

# Build Dropbear
cd dropbear && ./build_dropbear.sh && cd ..

# Assemble and package rootfs
./build_rootfs.sh

Output

• rootfs.bin — Flashable SquashFS image with Realtek header (~2 MB)

Customizing BusyBox

The BusyBox configuration can be easily customized using the interactive menuconfig interface:

# Default version + interactive config
./build_busybox.sh menuconfig

# Specific version + interactive config
./build_busybox.sh 1.36.1 menuconfig

# Via environment variable
BB_VER=1.36.0 ./build_busybox.sh menuconfig

After exiting menuconfig, you can choose where to save your configuration: 1. busybox.config — Generic base config (used as fallback) 2. busybox-X.Y.Z.config — Version-specific config (takes priority) 3. Both files 4. Don't save (temporary build only)

This allows you to enable/disable applets (utilities), adjust features, and optimize the binary size for your needs.

Customizing Dropbear

Dropbear is a lightweight SSH server designed for embedded systems. The build script supports version selection:

# Default version (2025.88)
./build_dropbear.sh

# Specific version
./build_dropbear.sh 2024.86

The build produces a multi-call binary (dropbearmulti) with symlinks for dropbear and dropbearkey.

To customize Dropbear features, edit the ./configure options in build_dropbear.sh. Current configuration disables zlib, utmp/wtmp logging, PAM, and shadow passwords to minimize binary size.

Resources: - Dropbear SSH — Official website - GitHub repository — Source code and releases - dropbear(8) — Server options and configuration - dropbearkey(1) — Key generation utility

Filesystem Structure

/
├── bin/                    # BusyBox symlinks
├── sbin/                   # System binaries
├── etc/                    # Configuration (minimal bootstrap + symlinks)
│   ├── inittab             # Init config (real file)
│   ├── init.d/
│   │   └── rcS             # Bootstrap script (real file)
│   ├── passwd              # -> /userdata/etc/passwd
│   ├── group               # -> /userdata/etc/group
│   ├── hostname            # -> /userdata/etc/hostname
│   ├── profile             # -> /userdata/etc/profile
│   ├── dropbear/           # -> /userdata/etc/dropbear/
│   ├── hosts               # -> /var/hosts (dynamic)
│   └── resolv.conf         # -> /var/resolv.conf (dynamic)
├── dev/                    # Device nodes
├── proc/                   # Proc filesystem (mounted at boot)
├── sys/                    # Sysfs (mounted at boot)
├── tmp/                    # -> /var/tmp
├── var/                    # Ramfs (mounted at boot)
├── userdata/               # Mount point for JFFS2 partition
└── lib/                    # Shared libraries (musl libc)

Default Access

• Root password: Set in /userdata/etc/passwd
• SSH: Enabled on port 22 (Dropbear)
• Serial console: 38400 8N1 on /dev/ttyS0