A SSHD (or Secure Shell Daemon) is a software program that runs as a background process on servers and other devices to handle incoming SSH connections. SSH stands for “Secure Shell” and is a network protocol that allows secure remote login and other network services over an unsecured network.
When a client wants to connect to a server via SSH, the SSHD on the server is responsible for authenticating the client, encrypting data sent over the connection, and spawning the requested shell or service. Some key responsibilities of the SSHD include:
- Listening for incoming SSH connections on port 22 (or other configured port)
- Handling the initial “handshake” when a client connects
- Authenticating clients using passwords, public keys, or other methods
- Negotiating encryption algorithms to establish an encrypted tunnel
- Providing access to a shell or forwarding connections to other services
- Logging connection attempts, sessions, and errors
So in summary, the main role of the SSHD is to securely handle remote access connections for clients according to the SSH protocol. The rest of this article will explore the SSHD in more detail.
Common Uses of SSHD
Some of the most common uses and capabilities provided by SSHD include:
Remote Command Line Access
One of the primary purposes of SSH is to allow administrators and users to securely access the command line shell on remote servers. This enables tasks like managing configurations, running commands, transferring files, and troubleshooting issues. The SSHD handles establishing the encrypted SSH session and providing shell access.
Automated Tasks and Configuration
Tools like Ansible, Chef, Puppet, and others rely on SSH connections to automate tasks and deploy configurations on remote servers. The SSHD allows them to run commands, copy files, and make changes through scripted SSH sessions.
Forwarding and Tunneling
SSH has the ability to forward ports and tunnel other protocols like HTTP(S) or RDP through an SSH connection. This allows administrators to access services running on private networks via the encrypted SSH tunnel. The SSHD facilitates setting up forwarded connections.
SFTP File Transfers
SSH includes support for file transfer capabilities via SFTP (Secure File Transfer Protocol). This provides an encrypted method of transferring files that is handled by the SSHD on the server side.
Git Version Control
Git can utilize SSH for secure connections to remote repositories instead of HTTPS. An SSHD must be running on the Git server to accept and handle these connections.
Remote Desktop
Some operating systems integrate their remote desktop protocols with SSH. For example, Linux distributions offer X11 forwarding over SSH connections. The SSHD sets up these secure remote desktop sessions.
VPN Concentrators
Hardware-based VPN devices often support site-to-site VPN connections over SSH. The SSHD runs on these devices to handle the secure VPN tunnel using the SSH protocol.
How Does SSHD Work?
When a client establishes a new SSH connection, the SSHD goes through several steps to handle the secure session:
Listens for Connections
The SSHD process runs in the background and binds to port 22 (or another configured port) to listen for incoming SSH connections. It has to be actively running for any SSH connections to be accepted.
Handles Handshake
When a client connects, the first thing sent is an SSH handshake message. The SSHD processes this handshake, verifies the protocol version, and responds to continue opening the connection.
Authenticates the Client
After the handshake, the SSHD then works to authenticate the client using a configured method like password auth, public keys, or Kerberos tickets. The client must provide valid credentials that are authorized for SSH access.
Negotiates Encryption
The server and client negotiate which encryption algorithms and ciphers to use to secure the SSH session traffic. This ensures all data sent over the connection will be encrypted.
Opens Requested Session
Once authenticated and encrypted, the SSHD grants access to the requested shell session, forwarded port, or other service depending on the client request. This may be a command shell, SFTP, tunnel, etc.
Handles Data Transfer
With the encrypted session now established, the SSHD securely transfers data between the client and server, encrypting outgoing data and decrypting incoming data. This continues until the client closes the connection.
Closes Session
When the client terminates the SSH session, the SSHD closes the connection, cleans up any temporary resources, and logs the session info like duration and bytes transferred. The process listens for additional connections.
So in summary, the SSHD handles the underlying tasks needed to open, encrypt, authorize, maintain, and close an SSH session on behalf of connecting clients.
SSH Daemon Software
There are a few common open source and commercial SSH daemon programs used on Linux, UNIX, and other operating systems:
OpenSSH
OpenSSH is the most widely used open source SSH implementation, included in Linux distributions like Ubuntu, RHEL, Debian, etc. The daemon is called sshd and works with the OpenSSH client programs ssh, scp, and sftp.
Dropbear
A smaller SSH server/client designed for embedded systems. The Dropbear SSHD is configured through the dropbear daemon and also integrates scp and sftp clients.
Tectia SSH
A commercial SSH suite sold by SSH Communications Security that includes an SSHD server, client tools, and administrative GUI. Often used by enterprises.
Cisco IOS SSH
Network devices like routers and switches from Cisco run IOS and can have an SSHD enabled. This allows device administration over SSH.
FTP-SSH
Implementation of SSHD focused on SFTP file transfer capabilities more than terminal shell access. Included with some FTP servers.
Bitvise SSH Server
Commercial Windows SSHD focused on features like preauthentication, public key management, and auditing. Provides sshd.exe process.
So in summary, OpenSSH sshd is the most common daemon used, but alternatives exist for specific use cases or operating systems. The core functionality remains similar between implementations.
SSH Daemon Configuration
To control the behavior of the SSHD, it must be configured by editing the main server configuration file. On Linux, the OpenSSH sshd_config file is typically located at /etc/ssh/sshd_config. Here are some key configuration options:
Listening Port
Specifies which TCP port number the SSHD should bind to for incoming connections. The default is 22 but it can be changed if needed.
Allowed Users
Access can be limited to only allow specific user accounts or groups to establish SSH sessions and log in.
Authentication Methods
The SSHD can be configured to enable password authentication, public key authentication, Kerberos tickets, and other login methods.
Banner Message
A banner containing legal, warning, or informational text can be configured and displayed to users upon connection.
Algorithms
The permitted ciphers, key exchange methods, MACs, and compression allowed during encryption negotiation can be refined.
Logging
Detailed logs containing usage statistics, connections, and user activity can be generated. Logs are essential for monitoring and troubleshooting.
Access Controls
Options like allowing TCP forwarding, port forwarding, X11 forwarding, and other capabilities can be controlled on a granular level.
So in summary, fine-tuning the SSHD configuration allows control over authentication, security, logging, and access options. Proper configuration is important for both security and usability.
Securing SSH Daemons
Since the SSHD directly exposes services over the network, hardening its security is crucial:
Use Public Key Authentication
Disable password authentication and require public/private key pairs, which are much harder to crack than passwords.
Limit Access
Only allow the minimum number of user accounts to authenticate via SSH and consider using access controls like firewall rules.
Disable Root Login
Direct root login over SSH should always be disabled. Require users to login then su or sudo to root.
Use Ports >1024
Move the SSHD to a non-default port above 1024 to avoid automated scans for port 22.
Update Regularly
Apply security patches promptly to ensure vulnerabilities are not exploitable. Use auto-updates if possible.
Use Fail2Ban
Fail2Ban can monitor logs and block IPs that show malicious activity like repeated failed passcodes.
Hardening SSHD is important for preventing unauthorized remote access and cyber attacks. A compromised SSHD can lead to data breaches, malware infections, and outages.
SSH Daemon Logging
Comprehensive logging is essential for monitoring SSHD activity and troubleshooting issues. The OpenSSH daemon can log to syslog or individual log files. Key events to capture include:
Successful Connections
Log entries showing user, IP, timestamp, and bytes transferred for all successful connections.
Failed Connections
Failed connection attempts indicating username supplied, source IP, and authentication error should be logged.
Session Activity
Commands entered, programs run, files accessed, and other events during an SSH session can be optionally logged.
Errors and Warnings
Operational errors like failed decryption or inability to resolve hostkeys should always be logged.
Admin Actions
Changes to configuration, keys, or user access controls made by administrators should log details.
Thorough SSHD logging provides an audit trail and assists diagnosing problems. They also supply audit data and alerts for security monitoring purposes. Log data should be archived and rotated to avoid filling disk space.
Troubleshooting SSH Daemon Issues
Some common issues with SSHD services and how to troubleshoot them include:
| Issue | Troubleshooting Steps |
|---|---|
| Can’t connect to SSHD | Verify SSHD process is running and listening on port 22. Check firewall rules blocking port 22. Test connection from server itself using “ssh localhost”. |
| Connection timeout | TCP connection blocked by firewall. Verify open path on port 22 between client and server. Check SSHD config has correct IP address. |
| Authentication failure | Enable debugging logs to see auth error. Verify accountUnlock account. Check public key or password. Test credentials locally. |
| Disconnected during session | Inspect network path for latency or drops. Add keepalives to SSHD config. Consider intermittent issue like firewall or proxy dropping idle connections. |
| File transfers fail | Verify permissions allow SFTP access if enabled. Logs will show file errors. Test SFTP on local server. |
Diagnosing SSHD problems involves verifying configurations, inspecting logs, testing connectivity, and enabling debug mode for verbose troubleshooting data.
Conclusion
The SSH daemon is the underlying service that enables secure remote shell access and other SSH-based connections for clients. It performs authentication, encryption, session management, logging, and access controls to handle SSH sessions.
Proper configuration ensures SSHD only allows authorized users to connect securely while enabling necessary capabilities. Hardening SSHD is also vital for blocking attacks and vulnerabilities. Detailed logging provides audit trails and data to assist troubleshooting issues.
Understanding how the SSHD works and its role in handling remote connections over SSH allows administrators to utilize it for secure, encrypted remote access while troubleshooting problems and improving security.