telnet wiki

Telnet

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For the packet switched network, see Telenet.

Telnet (Telecommunication network) is a network protocol used on the Internet or local area network (LAN) connections. It was developed in 1969 beginning with RFC 15 and standardized as IETF STD 8, one of the first Internet standards. Typically, telnet provides access to a command-line interface on a remote machine.

The term telnet also refers to software which implements the client part of the protocol. Telnet clients are available for virtually all platforms. Most network equipment and OSes with a TCP/IP stack support some kind of Telnet service server for their remote configuration (including ones based on Windows NT). Because of security issues with Telnet, its use has waned in favor of SSH for remote access.

"To telnet" is also used as a verbmeaning to establish or use a Telnet or other interactive TCPconnection, as in, "To change your password, telnet to the server andrun the passwd command".

Most often, a user will be telnetting to a Unix-likeserver system or a simple network device such as a router. For example,a user might "telnet in from home to check his mail at school". Indoing so, he would be using a telnet client to connect from hiscomputer to one of his servers. Once the connection is established, hewould then log in with his account information and execute operatingsystem commands remotely on that computer, such as ls or cd.

On many systems, the client may also be used to make interactive raw-TCPsessions. It is commonly believed that a telnet session which does notinclude an IAC (character 255) is functionally identical. This is notthe case however due to special NVT (Network Virtual Terminal) rulessuch as the requirement for a bare CR (ASCII 13) to be followed by aNULL (ASCII 0).

The TCP/IP model (RFC 1122)Application LayerBGP · DHCP · DNS · FTP · Gopher · GTP · HTTP · IMAP · IRC · NNTP · NTP · POP · RIP · RPC · RTCP · RTP · RTSP · SDP · SIP · SMTP · SNMP · SOAP · SSH · STUN · Telnet · TIME · TLS/SSL · XMPP · (more)Transport LayerTCP · UDP · DCCP · SCTP · RSVP · ECN · (more)Internet LayerIP (IPv4, IPv6) · ICMP · ICMPv6 · IGMP · IPsec · (more)Link LayerARP · RARP · NDP · OSPF · Tunnels (L2TP)  · Media Access Control (Ethernet, DSL, ISDN, FDDI)  · Device Drivers · (more)

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Contents

[hide]

• 1 Protocol details
• 2 Security
• 3 Telnet 5250
• 4 Current status
  • 4.1 Related RFCs
  • 4.2 Telnet clients
• 5 See also
• 6 References
• 7 External links

[edit] Protocol details

Telnet is a client-server protocol, based on a reliable connection-oriented transport. Typically this protocol is used to establish a connection to TCP port 23, where a getty-equivalent program (telnetd) is listening, although Telnet predates TCP/IP and was originally run on NCP.

Before March 5th, 1973, Telnet was an ad-hoc protocol with no official definition ^[1].Essentially, it used an 8-bit channel to exchange 7-bit ASCII data. Anybyte with the high bit set was a special Telnet character. On March5th, 1973, a meeting was held at UCLA ^[2]where "New Telnet" was defined in two NIC documents: Telnet ProtocolSpecification, NIC #15372, and Telnet Option Specifications, NIC#15373. This new protocol, and not the old telnet protocol is whatcontinues in use today.

The protocol has many extensions, some of which have been adopted as Internet standards.IETF standards STD 27 through STD 32 define various extensions, most ofwhich are extremely common. Other extensions are on the IETF standardstrack as proposed standards.

[edit] Security

When Telnet was initially developed in 1969, most users of networkedcomputers were in the computer departments of academic institutions, orat large private and government research facilities. In thisenvironment, security was not nearly as much of a concern as it becameafter the bandwidth explosion of the 1990s. The rise in the number ofpeople with access to the Internet, and by extension, the number ofpeople attempting to crack other people's servers made encrypted alternatives much more of a necessity.

Experts in computer security, such as SANS Institute, and the members of the comp.os.linux.security newsgrouprecommend that the use of Telnet for remote logins should bediscontinued under all normal circumstances, for the following reasons:

• Telnet, by default, does not encryptany data sent over the connection (including passwords), and so it isoften practical to eavesdrop on the communications and use the passwordlater for malicious purposes; anybody who has access to a router, switch, hub or gatewaylocated on the network between the two hosts where Telnet is being usedcan intercept the packets passing by and obtain login and passwordinformation (and whatever else is typed) with any of several commonutilities like tcpdump and Wireshark.
• Most implementations of Telnet have no authentication that would ensure communication is carried out between the two desired hosts and not intercepted in the middle.
• Commonly used Telnet daemons have several vulnerabilities discovered over the years.

These security-related shortcomings have seen the usage of the Telnet protocol drop rapidly, especially on the public Internet, in favor of the sshprotocol, first released in 1995. SSH provides much of thefunctionality of telnet, with the addition of strong encryption toprevent 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.

As has happened with other early Internet protocols, extensions to the Telnet protocol provide TLS security and SASLauthentication that address the above issues. However, most Telnetimplementations do not support these extensions; and there has beenrelatively little interest in implementing these as SSH is adequate formost purposes. The main advantage of TLS-Telnet would be the ability touse certificate-authority signed server certificates to authenticate aserver host to a client that does not yet have the server key stored.In SSH, there is a weakness in that the user must trust the firstsession to a host when it has not yet acquired the server key.

[edit] Telnet 5250

IBM 5250 or 3270 workstation emulation is supported via custom telnet clients, TN5250/TN3270, and IBM servers. Clients and servers designed to pass IBM 5250 data streams over Telnet generally do support SSL encryption, as SSH does not include 5250 emulation. Under OS/400, port 992 is the default port for secured telnet.

[edit] Current status

As of the mid-2000s, while the Telnet protocol itself has beenmostly superseded for remote login, Telnet clients are still used,often when diagnosing problems, to manually "talk" to other serviceswithout specialized client software. For example, it is sometimes usedin debugging network services such as an SMTP, IRC, HTTP, FTP or POP3 server, by serving as a simple way to send commands to the server and examine the responses.

This approach has limitations as what Telnet clients speak is closeto, but not equivalent to, raw mode (due to terminal controlhandshaking and the special rules regarding /377 and /15). Thus, othersoftware such as nc (netcat) or socat on Unix (or PuTTYon Windows) are finding greater favor with some system administratorsfor testing purposes, as they can be called with arguments not to sendany terminal control handshakingdata. Also netcat does not distort the /377 octet, which allows rawaccess to TCP socket, unlike any standard-compliant Telnet software.

Telnet is popular with:

• enterprise networks to access host applications, e.g. on IBM Mainframes.
• administration of network elements, e.g., in commissioning, integration and maintenance of core network elements in mobile communication networks.
• MUD games played over the Internet, as well as talkers, MUSHes, MUCKs, MOOes, and the resurgent BBS community.
• embedded systems

[edit] Related RFCs

• 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 1041 Telnet 3270 regime 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 1184 Telnet linemode option
• RFC 1205 5250 Telnet interface
• RFC 1372 Telnet remote flow control option
• RFC 2217 Telnet Com Port Control 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 4777 IBM's iSeries Telnet Enhancements

[edit] Telnet clients

• PuTTY is a free, open source SSH, Telnet, rlogin, and raw TCP client for Windows, Linux, and Unix.
• NCSA Telnet

[edit] See also

• Reverse telnet
• Telnet 3270 (TN3270)
• HyTelnet
• Kermit
• NCSA Telnet
• SSH

[edit] References

1. ^ RFC 318 - documentation of old ad-hoc telnet protocol
2. ^ RFC 495 - Announcement of Telnet protocol

[edit] External links

• Telnet Options - The official list of assigned option numbers at iana.org
• Telnet Interactions Described as a Sequence Diagram
• Telnet protocol description, with NVT reference