SSH login to single board computer with fixed ip address via many OS's flashed to many micro sd cards

yorkearwaker · August 25, 2026, 3:21pm

Overview

The Scene

One single board computer with fixed ip address and multiplle micro sd cards with different operating systems from different vendors. Login via SSH to a different os instance when the micro sd card it is flashed to is booted in the single board computer.

The Cast

SSH client machine, Dell Intel + internal SSD with Ubuntu Desktop 24.04 LTS 64bit

SSH ‘server’ machine, Raspberry Pi Zero 2 W +

Micro SD Card 1 with Raspberry Pi OS Trixie Lite 64 bit
Micro SD Card 2 with Ubuntu Server 24.04 LTS 64 bit
Micro SD Card 3 with Raspberry Pi OS Trixie Lite 64 bit
Micro SD Card 4 with Raspberry Pi OS Bookworm 32 bit
Micro SD Card 5 with Ubuntu Core 24.04 LTS 64bit
Micro SD Card N with X OS
Micro SD Card N+1 with Y OS

Ubuntu One account WS, for use with Ubuntu Core instances

Plot Summary

~/.ssh/config

Use .ssh/config on the client machine and make one entry for each operating system. The .ssh directory is in the home directory of the client machine user.

The details as explained below in the play.

The Play

A question and answer session with Brave AI Leo.

Q. single ip address and multiple os’s for that ip address and ssh sign on conflicts with ip address registered in ssh file against an ssh fingerprint

Quick Answer
To resolve SSH conflicts when dual-booting or running multiple operating systems on the same IP address, the most secure solution is to synchronize the host keys across all OS installations.

Q. are there any down sides to synchronizing os instance host keys for ssh for example one os is raspberry pi os trixi llite and another os is ubuntu server 24 both os’s deployed on separate micro sd cards to the same raspberry pi zero 2 w

Finished
Yes, there are specific security and operational downsides to synchronizing SSH host keys between Raspberry Pi OS and Ubuntu Server on the same hardware, even though it solves the fingerprint conflict.

Security Risks

Cascading Compromise (Blast Radius) The primary risk is that the host private key is no longer unique to an OS instance. If an attacker compromises either the Trixie Lite or Ubuntu Server installation (e.g., via a software vulnerability or weak password), they can extract the shared host private key. With this key, they can perform a Man-in-the-Middle (MitM) attack against clients connecting to the other OS, impersonating it perfectly without triggering warnings.
Loss of Identity Integrity SSH host keys serve as a unique fingerprint for a specific software environment. By sharing them, you lose the ability for clients to cryptographically verify which OS they are connecting to. A client intending to reach Ubuntu might be silently redirected to the Raspberry Pi OS instance (due to network glitches or ARP spoofing), and the SSH client would accept the connection as valid because the key matches.
Key Rotation Complexity If the shared key is ever suspected of being leaked, you cannot simply regenerate it on one OS. You must simultaneously update and synchronize the keys on both SD cards and force all connecting clients to clear their known_hosts entries again. This couples the maintenance lifecycle of two independent systems.

Technical Compatibility

File Permissions and Ownership: Ubuntu Server 24 and Raspberry Pi OS may have slightly different default permissions or ownership requirements for /etc/ssh/. Copying keys directly can sometimes result in sshd refusing to start on the target OS if permissions are too open (e.g., private keys must be 600 and owned by root).
Key Type Support: Ensure both OS versions support the same key algorithms (e.g., Ed25519, RSA). While both modern distributions support standard types, an older OS might not support a newer algorithm generated by the other.

Recommendation for Your Setup

Since these are mutually exclusive boot options (you cannot run both OSs simultaneously on a single Pi Zero 2 W), the risk of one OS sniffing the network traffic of the other is negligible while running. However, the risk of storage compromise remains: if someone steals the SD card for the less-secure OS, they steal the identity of the secure one.

Better Alternative: Client-Side Configuration Instead of synchronizing keys, configure your client machine to handle the dual identities securely. This maintains the security boundary between the two OSs.

Edit your local ~/.ssh/config:

Host pi-trixie
HostName 192.168.1.XX
UserKnownHostsFile ~/.ssh/known_hosts_trixie
HostKeyAlias pi-trixie-key

Host pi-ubuntu
HostName 192.168.1.XX
UserKnownHostsFile ~/.ssh/known_hosts_ubuntu
HostKeyAlias pi-ubuntu-key

This approach allows you to connect to ssh pi-trixie or ssh pi-ubuntu without conflicts, keeping the host keys distinct and preserving the security integrity of each installation.

Q. so in a scenario where the client machine is a Dell Intel ubuntu desktop 24.04 lts and connects to the raspberry pi zero 2 w via ssh but with two more possible os’s installed on two or more separate micro sd cards with one os raspberry pi trixi lite and another os ubuntu server 24.04 lts the ssh/config file is the better option in this scenario is it only the ip address that connects the separate os’s on the client machine and what are the contents of ssh/known_hosts_trixie and ssh/known_hosts_ubuntu and what is the hostkeyalias that is what are the dependency chains

Finished
Yes, the ssh/config file is the superior option for your scenario. It isolates the identity of each OS without compromising the security of either installation.

The Role of the IP Address

The IP address is the only common network identifier because your Raspberry Pi Zero 2 W hardware presents the same network interface regardless of which SD card is booted.

The Conflict: SSH clients identify servers by IP (or hostname). When you swap SD cards, the IP remains 192.168.x.x, but the underlying Host Key (the server’s cryptographic identity) changes completely because Trixie Lite and Ubuntu Server generate different keys upon installation.
The Solution: The ssh/config file tells your Dell desktop to ignore the IP address when looking up saved fingerprints, using a custom alias instead.

Configuration Breakdown & Dependency Chains

Here is the precise configuration for your ~/.ssh/config file on the Dell Ubuntu desktop, followed by an explanation of the files and aliases.

Host pi-trixie
HostName 192.168.1.50
UserKnownHostsFile ~/.ssh/known_hosts_trixie
HostKeyAlias pi-trixie-key
User pi # Default user for Raspberry Pi OS

Host pi-ubuntu
HostName 192.168.1.50
UserKnownHostsFile ~/.ssh/known_hosts_ubuntu
HostKeyAlias pi-ubuntu-key
User ubuntu # Default user for Ubuntu Server

The HostKeyAlias (The Logical Link)
What it is: A arbitrary string you invent (e.g., pi-trixie-key).
Function: It decouples the verification identity from the network address.
Dependency: When you connect, SSH looks for a fingerprint labeled pi-trixie-key instead of 192.168.1.50.
Why it matters: Even if both OSs somehow had the same IP entry in a generic file, the alias ensures the client only accepts the specific key associated with that alias.
The UserKnownHostsFile (The Physical Storage)
What it is: A path to a specific text file on your Dell desktop.
Function: It creates separate “address books” for each OS.
Contents: These files are not manually written; they are auto-generated by SSH the first time you connect.
~/.ssh/known_hosts_trixie will contain exactly one line (after first connect):
pi-trixie-key ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAI… (Trixie’s Public Key)

~/.ssh/known_hosts_ubuntu will contain exactly one line:
pi-ubuntu-key ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAI… (Ubuntu’s Public Key)

Note: The first field is the Alias, not the IP address.

The Dependency Chain
The connection logic flows strictly in this order:

Command: You type ssh pi-trixie.
Config Lookup: SSH reads ~/.ssh/config, finds the Host pi-trixie block.
Resolution:
Resolves HostName to 192.168.1.50 (Network Target).
Resolves HostKeyAlias to pi-trixie-key (Identity Target).
Resolves UserKnownHostsFile to ~/.ssh/known_hosts_trixie (Storage Target).
Verification: SSH connects to 192.168.1.50, receives its public key, and checks if that key matches the entry for pi-trixie-key inside known_hosts_trixie.
Result:
If you booted Trixie: The keys match. Connection succeeds.
If you accidentally booted Ubuntu: The keys fail to match (because Ubuntu’s key is stored under pi-ubuntu-key in a different file). SSH warns you: “WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!” preventing a potential misconnection.

Summary of Benefits for Your Setup

Security: If the Trixie OS is compromised, the attacker cannot get the Ubuntu host key because they are stored separately on your Dell client and generated separately on the Pi.
Clarity: You explicitly know which OS you are talking to. If you try to connect to pi-ubuntu but the Pi is running Trixie, the connection will safely fail rather than silently connecting to the wrong environment.
No Key Syncing: You do not need to touch the SD cards or copy files between OSs.

SSH config HostKeyAlias multiple OS same IP

Q. when in the process does the known_hosts_trixie file or known_hosts-ubuntu file get generated that is does pre configuring ssh/config with these names before first ssh contact on the client machine mean the files are generated with these names that is what is the order of operations and if the known_hosts_* file is only generated after first ssh contact there appears to be a chicken and egg issue here and what determines the naming convention of the known_hosts_* file for example if there were multiple instances of ubuntu server 24.04 lts for different develop projects dito for raspberry pi os trixi lite

Finished
There is no chicken-and-egg issue. The known_hosts_trixie and known_hosts_ubuntu files do not exist until you successfully complete your first SSH connection. Pre-configuring the ssh/config file with these filenames is perfectly safe; SSH simply creates them on-demand.

Order of Operations

Pre-Configuration (Manual) You edit ~/.ssh/config on your Dell desktop. You define the Host, HostName, and the path to the UserKnownHostsFile (e.g., ~/.ssh/known_hosts_trixie).
Status: The config file exists. The known_hosts_* files do not exist yet.
First Connection Attempt (Trigger) You run ssh pi-trixie.
SSH reads the config, sees the path ~/.ssh/known_hosts_trixie.
SSH attempts to connect to the IP 192.168.1.50.
The Pi (running Trixie) presents its host key.
SSH checks ~/.ssh/known_hosts_trixie. It finds the file missing (or empty).
Verification Prompt Since the file is missing, SSH cannot verify the key. It displays the standard authenticity warning:
The authenticity of host ‘pi-trixie (192.168.1.50)’ can’t be established.
ED25519 key fingerprint is SHA256:…
Are you sure you want to continue connecting (yes/no/[fingerprint])?

File Generation (Automatic) You type yes.
SSH creates the file ~/.ssh/known_hosts_trixie immediately.
It writes the line: pi-trixie-key ssh-ed25519 AAAA… into that new file.
The connection proceeds.
Subsequent Connections SSH reads the now-existing ~/.ssh/known_hosts_trixie, finds the matching alias pi-trixie-key, verifies the fingerprint, and logs you in without prompts.

Naming Convention Strategy

The naming convention for UserKnownHostsFile is entirely up to you. SSH does not enforce a specific name; it only requires the path to be valid. The “alias” logic (HostKeyAlias) is what links the file content to the specific OS, not the filename itself.

For multiple projects (e.g., three Ubuntu servers and two Trixie instances), you create unique entries in config with descriptive filenames:

Example ~/.ssh/config for Multiple Projects:

Project Alpha - Ubuntu

Host pi-ubuntu-alpha
HostName 192.168.1.50
UserKnownHostsFile ~/.ssh/known_hosts_ubuntu_alpha
HostKeyAlias pi-ubuntu-alpha-key
User ubuntu

Project Beta - Ubuntu

Host pi-ubuntu-beta
HostName 192.168.1.50
UserKnownHostsFile ~/.ssh/known_hosts_ubuntu_beta
HostKeyAlias pi-ubuntu-beta-key
User ubuntu

Project Gamma - Trixie

Host pi-trixie-gamma
HostName 192.168.1.50
UserKnownHostsFile ~/.ssh/known_hosts_trixie_gamma
HostKeyAlias pi-trixie-gamma-key
User pi

Key Takeaways:

Filename Freedom: You can name the files known_hosts_project_alpha, my_pi_keys, or test_file_01. The content inside (specifically the HostKeyAlias column) is what matters for verification.
No Pre-creation Needed: Do not manually create these files. Let SSH generate them upon your first yes confirmation.
Isolation: Each project gets its own file, ensuring that swapping SD cards or re-imaging a specific project only requires clearing that specific file (e.g., rm ~/.ssh/known_hosts_ubuntu_alpha), leaving your other project keys untouched.

SSH UserKnownHostsFile dynamic creation behavior

Q. and how would an ubuntu core 24.04 lts work in this scenario if the core iso image has to be recreated several times to add or modify snaps during development and testing that is would it be okay just to delete the known_hosts_core_omega file every time a new core os image is burned to the micro sd disk so the ssh/config entry would remain the same but the known_hosts_core_omega file would be recreated on the client machine every time a new core os image was first ssh’d into

Finished
Yes, deleting the known_hosts_core_omega file every time you re-flash the Ubuntu Core 24 SD card is the correct and standard workflow for your development scenario.

Why This Works Perfectly

Ephemeral Identity: Unlike standard Ubuntu Server or Raspberry Pi OS, which often preserve host keys if you back up/restore partitions, flashing a new Ubuntu Core ISO image generates a completely new set of SSH host keys from scratch during the first boot (cloud-init or systemd-firstboot).
Clean Slate: Since the server’s identity (private/public key pair) changes with every flash, the old fingerprint stored on your Dell client becomes invalid.
The Workflow:
Flash: Burn new Ubuntu Core ISO to SD card.
Cleanup: Run rm ~/.ssh/known_hosts_core_omega on your Dell desktop.
Connect: Run ssh pi-core-omega.
Verify: SSH sees the missing file, prompts you to trust the new fingerprint, and regenerates known_hosts_core_omega with the fresh key.

Is It Safe?

Yes, provided you are physically swapping the SD card yourself.

Security Context: The “Man-in-the-Middle” warning SSH gives you is designed to alert you if a remote server’s key changed unexpectedly (implying an attack).
Your Context: Since you are the one physically re-imaging the device, you know the key change is legitimate. Deleting the file simply acknowledges this intentional reset.
Optimized Command Alias
To streamline this repetitive process, add an alias to your ~/.bashrc or ~/.zshrc on the Dell desktop:

alias reset-pi-core=‘rm -f ~/.ssh/known_hosts_core_omega && echo “Known hosts file cleared. Ready to connect.”’

Usage:

Flash SD card.
Run reset-pi-core.
Run ssh pi-core-omega.
Your ssh/config entry remains unchanged forever; only the known_hosts file and the server’s internal keys cycle.

Q. what is the outcome of using ubuntu one account in this scenario and is this scenario compatible with ubuntu one account that is what other things might need to be taken into consideration using the ~/.ssh/config method when using ubuntu one account

Finished
Using an Ubuntu One account with Ubuntu Core 24 is fully compatible with the ~/.ssh/config method, but it introduces a specific username dependency and a key management workflow you must account for.

Outcome and Compatibility

Mandatory Key-Based Auth: Ubuntu Core does not allow password authentication by default. It requires an SSH key linked to your Ubuntu One account to create the initial user.
Username Constraint: The SSH username on the Pi must match your Ubuntu One email username (the part before @).
Example: If your Ubuntu One email is developer@example.com, your SSH command must use developer as the user, not ubuntu or pi.
Config Compatibility: The ~/.ssh/config method works perfectly, provided you explicitly define the User field to match your Ubuntu One username.

Critical Considerations for Your Workflow

The “Re-Flash” Key Sync Issue
When you re-flash the SD card with a new Ubuntu Core image, the device pulls the SSH public key from your Ubuntu One account only during the very first boot setup.

The Risk: If you add a new SSH key to your Ubuntu One account after the Pi has already booted once, the new key will not automatically appear on the Pi. Ubuntu Core does not continuously sync keys from the cloud after initialization.
The Fix: If you change keys frequently, you must either:
Ensure the correct key is in Ubuntu One before the first boot of the new image.
Or, after the first login, manually append new public keys to /writable/user-data//.ssh/authorized_keys on the Pi itself.

Updated ssh/config Entry
You must update your configuration to include the correct User (your Ubuntu One username) and potentially the specific IdentityFile if you use multiple keys.

Project Omega - Ubuntu

Host pi-core-omega
HostName 192.168.1.50
User # CRITICAL: Must match SSO username
UserKnownHostsFile ~/.ssh/known_hosts_core_omega
HostKeyAlias pi-core-omega-key
IdentityFile ~/.ssh/id_ubuntucore # Optional: If not using default key name

The known_hosts Deletion Workflow Remains Valid
Your previous plan to delete known_hosts_core_omega before every re-flash remains the correct procedure.

Why: Re-flashing generates a new Host Key (server identity).
Ubuntu One Role: Ubuntu One manages the User Key (client authentication).
Independence: These are separate. Changing the SD card changes the Host Key (requiring known_hosts deletion), but the User Key (from Ubuntu One) remains constant, allowing you to log in immediately after accepting the new host fingerprint.
Summary of Dependency Chain
Ubuntu One Account: Stores your Public User Key.
First Boot: Pi downloads Public User Key → Creates user → Authorizes that key.
Client Config: ~/.ssh/config specifies User and path to Private User Key.
Re-Flash: Pi generates new Host Key → You delete known_hosts_core_omega → You reconnect → Trust new Host Key → Login succeeds using existing User Key.

Q. can’t find ~/.ssh/config on client machine the .ssh directory exists but the config file does not

It is completely normal and acceptable for the ~/.ssh/config file to not exist. OpenSSH clients are designed to function without a configuration file, relying on default system settings and command-line arguments if no user-specific config is found.

If you need to define custom connection parameters (such as aliases, specific keys, or port numbers), you can create the file manually. Ensure you set the correct permissions immediately after creation:

Create the file:
touch ~/.ssh/config

Set strict permissions (SSH will ignore the file if it is readable by others):
chmod 600 ~/.ssh/config

If you are experiencing issues where SSH is not reading a config file that does exist, check the following common causes:

Permissions: Ensure the file is 600 and the ~/.ssh directory is 700.
Host Matching: The Host alias in the config must match the text you type in the terminal (e.g., ssh myserver matches Host myserver), not necessarily the HostName (actual IP or domain).
Windows Environment: If using Git Bash or Cygwin on Windows, ensure you are not using a Cygwin-based ssh.exe which may look for config in Linux-style paths rather than %USERPROFILE%.ssh\config.

Q. should the ~/.ssh/config file have a file extension

No, the SSH config file should not have a file extension. It must be named exactly config without any suffix like .txt or .config.

File Name: The file is strictly named config in the .ssh directory (e.g., ~/.ssh/config on Linux/macOS or %USERPROFILE%.ssh\config on Windows).
Why: OpenSSH clients look for a file named config with no extension. Adding an extension (such as .txt) will cause the SSH client to ignore the file, resulting in configuration settings not being applied.
Common Mistake: When creating the file using a text editor on Windows, ensure the file type is set to All Files rather than Text Documents, to prevent the editor from automatically appending .txt.

Q. for ~/.ssh/config do the entries for Host and HostKeyAlias have to match with exception of -key added the the HostKeyAlias entry , for example Host pi-trixie-acdc HostKeyAlias pi-trixie-acdc-key that is can the values of Host and HostKeyAlias be entirely arbitrary and unconnected syntactically

Finished
No, the values for Host and HostKeyAlias do not need to match or share any syntactic connection. They can be entirely arbitrary and independent strings.

Independence of Host and HostKeyAlias

The Host directive defines the alias you type on the command line (e.g., ssh pi-trixie-acdc), while HostKeyAlias specifies the exact label used to look up or save the server’s public key in your ~/.ssh/known_hosts file.

Arbitrary Values: You can set Host to any convenient name (like pi-trixie-acdc) and HostKeyAlias to any other string (like pi-trixie-acdc-key, server-123, or even a completely unrelated word like banana), provided that the known_hosts file contains an entry matching that specific HostKeyAlias.

No Syntax Requirement: There is no requirement to append suffixes like -key or to maintain similar naming conventions. The SSH client treats them as independent identifiers: one for user convenience and one for key database lookup.

Practical Use Cases

This independence is particularly useful in scenarios where the connection method differs from the identity verification:

Dynamic IPs: If a server’s IP address changes frequently, you might use a stable HostKeyAlias (e.g., myserver-identity) while the HostName changes, ensuring the key lookup remains consistent regardless of the IP.
Multiple Servers on One Host: When tunneling to multiple virtual machines running on the same physical host (same IP/Hostname), each Host entry must have a unique HostKeyAlias to distinguish their keys in known_hosts, even though they share the same HostName.
Clean Separation: Users often choose descriptive but distinct names to clearly separate the “connection alias” from the “key identity,” but this is purely for human organization, not a technical requirement.

Q. for ~/.ssh/config when does the User entry have to be present when can it be omitted that is tying a single user to an entry might have security advantages in a production environment but might be burdensome in a development environment

Finished
The User entry in ~/.ssh/config is optional and its necessity depends entirely on whether your local username matches the remote username.

When User Can Be Omitted

You can omit the User directive if your local username is identical to the remote username you intend to log in as.

Default Behavior: If User is not specified, the SSH client automatically uses the username of the currently logged-in local user.
Development Use Case: In development environments where developers often create identical user accounts on local machines and VMs (e.g., dev on both), omitting this entry reduces configuration overhead and allows the config file to be more portable across different team members’ machines without editing.

When User Must Be Present

The User entry is required when the remote username differs from your local username.

Common Scenarios: Connecting to a server where the account is named ubuntu, ec2-user, root, or a specific service account while your local machine uses your personal name (e.g., alice).
Explicit Override: It is also used when you want to enforce a specific user for a host alias regardless of who is running the command, ensuring consistency in shared scripts or team configurations.

Security Implications: Production vs. Development

Production Environments (Strict) In production, explicitly defining the User entry is a security best practice.

Prevents Accidental Access: It ensures that even if a developer named admin tries to connect, they cannot accidentally log in as root or another privileged user unless explicitly configured to do so.
Audit Clarity: It enforces a specific identity for the connection, making logs predictable and easier to audit. You know exactly which user account is being targeted for a specific host alias.
Least Privilege: It forces the connection to use a dedicated service account (e.g., deploy_bot) rather than relying on the human operator’s current username.
Development Environments (Flexible) In development, omitting the User entry can be advantageous for agility.

Portability: A single config file can be shared among a team where everyone has their own personal user account on the dev server. If User were hardcoded, every developer would have to edit the file to change User=alice to User=bob.
Convenience: It reduces typing and configuration noise when the mapping is 1:1.

Things todo to iterate this entry

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<todo: consider, editing and rewording of some of the text entries, >