Running `sshd` in Rootless Mode

This article explains how to run sshd as a normal user without root privileges. The setup uses a custom configuration directory under the user’s home directory and listens on a non-privileged port.

Overview

Normally, sshd is started by the system as root and listens on port 22. In some environments, however, it is useful to run an SSH server without root access. A rootless setup can be used for testing, development, or temporary remote access in user space.

The example here creates a private SSH server environment in ~/.sshd, generates a host key if needed, writes a minimal sshd_config, validates the configuration, and then starts the daemon in the foreground.

Directory and Variable Setup

The setup.sh script begins by defining a few variables:

BASE="$HOME/.sshd"
PORT=2222
SSHD="$(command -v sshd)"

BASE is the working directory for the SSH server files. PORT is set to 2222, which is a non-privileged port and can be used without root. SSHD stores the path to the sshd executable.

The script then creates the required directories:

install -d -m 700 "$BASE" "$HOME/.ssh"

This ensures that both ~/.sshd and ~/.ssh exist with secure permissions.

Generating the Host Key

An SSH server needs its own host key. The script checks whether an Ed25519 host key already exists:

if [ ! -f "$BASE/ssh_host_ed25519_key" ]; then
  ssh-keygen -q -t ed25519 -N '' -f "$BASE/ssh_host_ed25519_key"
fi
chmod 600 "$BASE/ssh_host_ed25519_key"

If the key is missing, it generates one. The permission is then restricted to the owner only. This is important because SSH rejects private keys that are too broadly accessible.

Creating the sshd Configuration

The script writes a custom sshd_config file into the user-owned directory.

cat > "$BASE/sshd_config" <<EOF
Port $PORT
ListenAddress 0.0.0.0

UsePAM no
PasswordAuthentication no
KbdInteractiveAuthentication no
PubkeyAuthentication yes

HostKey $BASE/ssh_host_ed25519_key
PidFile none

PermitRootLogin no
PrintMotd no
PrintLastLog no
X11Forwarding no
AllowUsers $USER

Subsystem sftp internal-sftp
EOF

Key Settings

Port and Listen Address

• Port $PORT tells the server to listen on port 2222.
• ListenAddress 0.0.0.0 allows connections on all network interfaces.

Authentication

• UsePAM no disables PAM because PAM generally requires a system-level setup.
• PasswordAuthentication no disables password login.
• KbdInteractiveAuthentication no disables keyboard-interactive authentication.
• PubkeyAuthentication yes enables public key login.

This makes the server simpler and safer for rootless use by relying only on SSH keys.

Host Key and PID File

• HostKey $BASE/ssh_host_ed25519_key points to the host key generated earlier.
• PidFile none avoids writing a PID file, which is useful in lightweight user-space execution.

Restrictions

• PermitRootLogin no disallows root login.
• PrintMotd no and PrintLastLog no suppress extra login messages.
• X11Forwarding no disables X11 forwarding.
• AllowUsers $USER limits access to the current user only.

SFTP Support

• Subsystem sftp internal-sftp enables SFTP using the built-in internal subsystem.

Validating the Configuration

At the end of setup.sh, the script verifies the configuration:

"$SSHD" -t -f "$BASE/sshd_config"

This is a useful step because it checks the syntax before starting the server.

Starting the SSH Server

The start.sh script is very small:

#!/bin/bash
BASE="$HOME/.sshd"
SSHD="$(command -v sshd)"
exec "$SSHD" -D -e -f "$BASE/sshd_config"

What This Does

• -D keeps sshd in the foreground.
• -e sends log output to standard error.
• -f "$BASE/sshd_config" tells sshd to use the custom configuration file.

Using exec replaces the shell process with sshd, which is a clean way to launch the server.

How to Use It

A typical flow is:

1. Run setup.sh once to create the configuration and host key.
2. Make sure your public key is present in ~/.ssh/authorized_keys.
3. Run start.sh to launch the SSH server.
4. Connect to it on port 2222.

For example:

ssh -p 2222 user@host

Benefits of This Approach

This rootless SSH setup has several advantages:

• It does not require system-wide configuration.
• It avoids privileged ports.
• It works well for personal environments and temporary setups.
• It keeps all SSH server files under the user’s home directory.

Conclusion

This example provides a compact way to run sshd without root privileges. By using a private configuration directory, a non-privileged port, and public key authentication only, it creates a simple and practical SSH server for user-space operation.

Scripts

setup.sh

#!/bin/bash 
BASE="$HOME/.sshd" 
PORT=2222 
SSHD="$(command -v sshd)" 
 
install -d -m 700 "$BASE" "$HOME/.ssh" 
 
# server host key 
if [ ! -f "$BASE/ssh_host_ed25519_key" ]; then 
  ssh-keygen -q -t ed25519 -N '' -f "$BASE/ssh_host_ed25519_key" 
fi 
chmod 600 "$BASE/ssh_host_ed25519_key" 
 
cat > "$BASE/sshd_config" <<EOF 
Port $PORT 
ListenAddress 0.0.0.0 
 
UsePAM no 
PasswordAuthentication no 
KbdInteractiveAuthentication no 
PubkeyAuthentication yes 
 
HostKey $BASE/ssh_host_ed25519_key 
PidFile none 
 
PermitRootLogin no 
PrintMotd no 
PrintLastLog no 
X11Forwarding no 
AllowUsers $USER 
 
Subsystem sftp internal-sftp 
EOF 
 
"$SSHD" -t -f "$BASE/sshd_config" 

start.sh

#!/bin/bash 
BASE="$HOME/.sshd" 
SSHD="$(command -v sshd)" 
exec "$SSHD" -D -e -f "$BASE/sshd_config"