Terminal Access Protocols
These protocols play key roles in network communication by establishing sessions and enabling secure or remote access to devices and systems.
Protocol |
Description |
Use Case |
|---|---|---|
Telnet |
A legacy protocol that provides remote command-line access over TCP port 23. Telnet transmits data—including usernames and passwords—in plaintext, making it vulnerable to interception. Due to lack of encryption, Telnet is mostly replaced by SSH in secure environments. |
Remote management of legacy devices and network equipment, mainly in trusted or isolated networks. |
rlogin (Remote Login) |
A legacy protocol that provides remote terminal access over TCP port 513. It enables a user to log in to another UNIX host. Authentication is based on trusted hosts or .rhosts files, making it insecure for untrusted networks. Often replaced by SSH due to lack of encryption. |
Remote shell access between UNIX systems in trusted networks. |
rsh (Remote Shell) |
Allows remote command execution without interactive login. Uses TCP port 514. Like rlogin, it depends on .rhosts trust, which poses security risks. Replaced by SSH in most modern systems. |
Automating commands across trusted UNIX systems. |
mosh (Mobile Shell) |
A modern remote terminal application designed for intermittent and roaming network conditions. Unlike SSH, it maintains session state when IP addresses or connectivity change. Uses UDP and provides encryption. |
Remote shell access over unreliable or mobile networks. |
Main Features
Provides remote command-line access over TCP port 23
Transmits data, including usernames and passwords, in plaintext
Simple and widely supported on legacy devices
Does not provide encryption or strong authentication
RFC References
RFC 854 – Telnet Protocol Specification
RFC 855 – Telnet Options
Use Cases
Remote management of legacy network devices
Access in trusted or isolated network environments
Basic command-line access where security is not a concern
Alternative Protocols
SSH – Encrypted remote shell access
Console access via serial or out-of-band management
Remote desktop protocols for GUI access (RDP, VNC)
Let us learn more about Telnet:
Main Features
Provides remote login over TCP port 513
Relies on .rhosts for host/user trust relationships
No encryption; data is sent in plaintext
Interactive login shell on remote systems
RFC References
Not formalized in a standalone RFC but described in BSD UNIX documentation
Use Cases
Logging into UNIX systems across trusted LANs
Administrative access in non-critical environments
Alternative Protocols
SSH – Secure replacement with encrypted communication
Telnet – Similar legacy protocol, also insecure
Main Features
Executes commands on a remote machine over TCP port 514
Does not require interactive login
Insecure: relies on .rhosts for access control
Sends credentials and data unencrypted
RFC References
No official RFC; defined in BSD UNIX specifications
Use Cases
Running batch jobs or commands remotely
Scripts requiring remote execution on trusted systems
Alternative Protocols
SSH – Secure remote command execution
SCP/SFTP – For secure file transfers and scripts
Main Features
Designed for mobile/roaming environments
Uses UDP for communication (default port 60000+)
Supports intermittent connectivity and IP changes
Encrypted sessions using AES-CTR and client-side prediction
RFC References
No RFC; open-source protocol developed by MIT
Use Cases
Connecting to remote systems from mobile networks
Stable terminal access in poor or changing connectivity
SSH alternative for mobile workers
Alternative Protocols
SSH – Reliable but less tolerant to IP changes
Tmate – For collaborative remote sessions