---
title: "SSH Customization"
slug: "fmadio-os-custom-ssh-customization"
updated: 2025-04-20T10:18:32Z
published: 2025-04-20T10:18:32Z
canonical: "docs.fmad.io/fmadio-os-custom-ssh-customization"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fmad.io/llms.txt
> Use this file to discover all available pages before exploring further.

# SSH Customization

FMADIO devices run exclusively from pseudo-ROM where any changes on the file system between reboots is lost. This ROM approach provides consistency and system predictability making maintenance simpler.

## Shell Environment

One problem with this approach is shell customization becomes quite difficult. To allow small modifications in the shell environment when a user logs into the system it can run the shell script for each SSH session. Configuration file is:

```plaintext
/opt/fmadio/etc/fmadio.rc
```

Please do not use this excessively, typically its used for setting ENV variables.

Example below

```plaintext
$ cat /opt/fmadio/etc/fmadio.rc
# local shell prompt configuration (ash) ran on at boot time

export TEST="random test variable"
```

## Persistent authorized_keys

This file is usually located in ~/.ssh/ directory. As that is part of the volatile file system, the persistent version of this is placed into

```plaintext
/opt/fmadio/etc/authorized_keys
```

This allows SSH keys to be used in a persistent way across reboots and power cycles. Note the file in /opt/fmadio/etc/authorized_keys is only copied during bootup. Updates made after reboot are not copied to the user .ssh directory.

## Custom SSHD config

A customized sshd configuration file can be used by placing the customized configuration into

```plaintext
/opt/fmadio/etc/sshd_config
```

This is helpful for example to force exclusive RSA based login / disable password login. Which is a good practice if the device is on a public network.

## Custom SSH RSA ID

In many cases using the default fmadio SSH RSA ID is not a good security practice. As such custom SSH RSA keys both public and private can be copied into

```plaintext
/opt/fmadio/etc/fmadio_id_rsa
/opt/fmadio/etc/fmadio_id_rsa.pub
```

These will copied into ~/.ssh/idrsa and idrsa_pub on boot

## Custom sudoers File

A customized persistant /etc/sudoers file can be created at the following location

```plaintext
/opt/fmadio/etc/sudoers
```

On boot the system will override the default /etc/sudoers file with the file created per above

```plaintext
/opt/fmadio/etc/sudoers
```