Host Tools

This page discusses the tools/ directory containing miscellaneous scripts and host C programs that are important parts of the NuttX build system:

Tool documentation pages

mkpasswd — Build-time /etc/passwd Generation

tools/mkpasswd is a C host tool (compiled from tools/mkpasswd.c) that generates a single /etc/passwd entry at build time. It is invoked automatically by the ROMFS build step when CONFIG_BOARD_ETC_ROMFS_PASSWD_ENABLE=y is set.

Why build-time generation?

Shipping a hard-coded default password in firmware is a well-known security weakness (CWE-798). By generating the /etc/passwd entry from a user-supplied plaintext password at build time, each firmware image carries unique credentials. The build will fail if the password is left empty, preventing accidental deployments with no credentials.

For improved baseline security, the configured password must be at least 8 characters long.

How it works

  1. The host tool reads the plaintext password from CONFIG_BOARD_ETC_ROMFS_PASSWD_PASSWORD.

  2. The password is hashed using the Tiny Encryption Algorithm (TEA) — the same implementation used at runtime in libs/libc/misc/lib_tea_encrypt.c — with custom base64 encoding matching apps/fsutils/passwd/passwd_encrypt.c.

  3. The resulting hashed entry is written to etctmp/<mountpoint>/passwd and then embedded into the ROMFS image.

  4. The plaintext password is never stored in the firmware image.

Kconfig options

Enable the feature and configure credentials via make menuconfig:

CONFIG_BOARD_ETC_ROMFS_PASSWD_ENABLE=y
CONFIG_NSH_CONSOLE_LOGIN=y                     # required to enforce login prompt
CONFIG_BOARD_ETC_ROMFS_PASSWD_USER="root"          # default: root
CONFIG_BOARD_ETC_ROMFS_PASSWD_PASSWORD="<secret>"  # required, min length 8
CONFIG_BOARD_ETC_ROMFS_PASSWD_UID=0
CONFIG_BOARD_ETC_ROMFS_PASSWD_GID=0
CONFIG_BOARD_ETC_ROMFS_PASSWD_HOME="/"

The TEA encryption keys can be changed from their defaults via CONFIG_FSUTILS_PASSWD_KEY1..4.

/etc/passwd file format

user:x:uid:gid:home

Where:

  • user — user name

  • x — TEA-hashed, base64-encoded password

  • uid — numeric user ID

  • gid — numeric group ID

  • home — login directory

Verifying the generated entry

After enabling CONFIG_BOARD_ETC_ROMFS_PASSWD_ENABLE and setting a password, rebuild and verify:

  1. Configure and build:

    $ make menuconfig   # enable BOARD_ETC_ROMFS_PASSWD_ENABLE and set password
$ make

  2. Inspect the generated passwd line (written to the board build tree):

    $ cat boards/<arch>/<chip>/<board>/src/etctmp/etc/passwd
root:8Tv+Hbmr3pLVb5HHZgd26D:0:0:/

  3. Verify the plaintext is absent from firmware:

    $ grep <your-password> boards/<arch>/<chip>/<board>/src/etctmp.c
# must print nothing

Notes on savedefconfig

To avoid leaking credentials into board defconfigs, make savedefconfig does not save the following options in the generated defconfig:

  • CONFIG_BOARD_ETC_ROMFS_PASSWD_PASSWORD

  • CONFIG_FSUTILS_PASSWD_KEY1

  • CONFIG_FSUTILS_PASSWD_KEY2

  • CONFIG_FSUTILS_PASSWD_KEY3

  • CONFIG_FSUTILS_PASSWD_KEY4

If you need these values for local development, add them manually to your local defconfig after running make savedefconfig.