Telnet

Even though Telnet was an ad hoc protocol with no official definition until March 5, 1973,[11] the name actually referred to Teletype Over Network Protocol as the RFC 206 (NIC 7176) on Telnet makes the connection clear:[12]

The TELNET protocol is based upon the notion of a virtual teletype, employing a 7-bit ASCII character set. The primary function of a User TELNET, then, is to provide the means by which its users can 'hit' all the keys on that virtual teletype.[13]

Essentially, it used an 8-bit channel to exchange 7-bit ASCII data. Any byte with the high bit set was a special Telnet character. On March 5, 1973, a Telnet protocol standard was defined at UCLA[14] with the publication of two NIC documents: Telnet Protocol Specification, NIC 15372, and Telnet Option Specifications, NIC 15373.

Many extensions were made for Telnet because of its negotiable options protocol architecture. Some of these extensions have been adopted as Internet standards, IETF documents STD 27 through STD 32. Some extensions have been widely implemented and others are proposed standards on the IETF standards track (see below).

The Telnet service is the application providing services over the Telnet protocol. Most operating systems provide a service that can be installed or enabled to provide Telnet services to clients.[15]

Historically, Telnet provided access to a command-line interface on a remote host. However, because of serious security concerns when using Telnet over an open network such as the Internet, its use for this purpose has waned significantly in favor of SSH.[19] The usage of Telnet for remote management has declined rapidly, especially on the public Internet, in favor of the Secure Shell (SSH) protocol.[3][20] SSH provides much of the functionality of telnet, with the addition of strong encryption to prevent sensitive data such as passwords from being intercepted, and public key authentication, to ensure that the remote computer is actually who it claims to be.

Telnet may be used in debugging network services such as SMTP, IRC, HTTP, FTP or POP3, to issue commands to a server and examine the responses.[15][21][22] For example, Telnet client applications can establish an interactive TCP session to a port other than the Telnet server port. For example, a command line telnet client could make an HTTP request to a web server on TCP port 80 as follows:[22]

$ telnet www.example.com 80
GET /path/to/file.html HTTP/1.1
Host: www.example.com
Connection: close

The older protocol is used these days only in rare cases to access decades-old legacy equipment that does not support more modern protocols.[23] For example, a large number of industrial and scientific devices only have Telnet available as a communication option. Some are built with only a standard RS-232 port and use a serial server hardware appliance to provide the translation between the TCP/Telnet data and the RS-232 serial data. In such cases, SSH is not an option unless the interface appliance can be configured for SSH (or is replaced with one supporting SSH).

Telnet is commonly used by amateur radio operators for providing public information.[21]

Although recommended against, security researchers estimated that 7,096,465 devices connected to the Internet continue to use Telnet, however, much less are often estimated because most estimates only scan for TCP port 23.[9]

Telnet commands consist of at least two bytes.[7] The first byte is the IAC escape character (typically byte 255) followed by the byte code for a given command:

All data octets except 0xff are transmitted over Telnet as is. (0xff, or 255 in decimal, is the IAC byte (Interpret As Command) which signals that the next byte is a telnet command. The command to insert 0xff into the stream is 0xff, so 0xff must be escaped by doubling it when sending data over the telnet protocol.)[7]

Telnet also has a variety of options that terminals implementing Telnet should support.

• RFC 854, Telnet Protocol Specification
• RFC 855, Telnet Option Specifications
• RFC 856, Telnet Binary Transmission
• RFC 857, Telnet Echo Option
• RFC 858, Telnet Suppress Go Ahead Option
• RFC 859, Telnet Status Option
• RFC 860, Telnet Timing Mark Option
• RFC 861, Telnet Extended Options: List Option

• RFC 885, Telnet End of Record Option
• RFC 1073, Telnet Window Size Option
• RFC 1079, Telnet Terminal Speed Option
• RFC 1091, Telnet Terminal-Type Option
• RFC 1096, Telnet X Display Location Option
• RFC 1123, Requirements for Internet Hosts - Application and Support
• RFC 1184, Telnet Linemode Option
• RFC 1372, Telnet Remote Flow Control Option
• RFC 1572, Telnet Environment Option
• RFC 2941, Telnet Authentication Option
• RFC 2942, Telnet Authentication: Kerberos Version 5
• RFC 2943, TELNET Authentication Using DSA
• RFC 2944, Telnet Authentication: SRP
• RFC 2946, Telnet Data Encryption Option
• RFC 4248, The telnet URI Scheme

• RFC 1143, The Q Method of Implementing TELNET Option Negotiation
• RFC 1571, Telnet Environment Option Interoperability Issues

• RFC 1041, Telnet 3270 Regime Option
• RFC 2355, TN3270 Enhancements
• RFC 1205, 5250 Telnet Interface
• RFC 2217, Telnet Com Port Control Option
• RFC 4777, IBM's iSeries Telnet Enhancements