Ident protocol

Ident
Communication protocol
Purpose	Identification
Developer(s)	Michael C. St. Johns at US Department of Defense
Introduction	February 1993;31 years ago
Based on	RFC 931
OSI layer	Application layer (Layer 7)
Port(s)	TCP/113
RFC(s)	RFC 1413

Last updated May 05, 2024

The Ident Protocol (Identification Protocol, Ident), specified in RFC 1413, is an Internet protocol that helps identify the user of a particular TCP connection. One popular daemon program for providing the ident service is identd.

Function

The Ident Protocol is designed to work as a server daemon, on a user's computer, where it receives requests to a specified TCP port, generally 113. In the query, a client specifies a pair of TCP ports (a local and a remote port), encoded as ASCII decimals and separated by a comma (,). The server then sends a response that identifies the username of the user who runs the program that uses the specified pair of TCP ports, or specifies an error.

Suppose host A wants to know the name of the user who is connecting to its TCP port 23 (Telnet) from the client's (host B) port 6191. Host A would then open a connection to the ident service on host B, and issue the following query:

6191, 23

As TCP connections generally use one unique local port (6191 in this case), host B can unambiguously identify the program that has initiated the specified connection to host A's port 23, should it exist. Host B would then issue a response, identifying the user ("stjohns" in this example) who owns the program that initiated this connection and the name of its local operating system:

6193, 23 : USERID : UNIX : stjohns

But if it would turn out that no such connection exists on host B, it would instead issue an error response:

6195, 23 : ERROR : NO-USER

All ident messages should be delimited by an end of line sequence consisting of the carriage return and linefeed characters (CR+LF).^[1]

Usefulness of ident

Dialup hosts or shared shell servers often provide ident to enable abuse to be tracked back to specific users. In the case that abuse is handled on this host the concern about trusting the ident daemon is mostly irrelevant. Spoofing of the service and privacy concerns can be avoided by providing varying cryptographically strong tokens instead of real usernames.

If abuse is to be handled by the administrators of the service that users connect to using the ident providing host, then the ident service must provide information identifying each user. Usually it is impossible for the administrators of the remote service to know whether specific users are connecting via a trustable server or from a computer they themselves control. In the latter case the ident service provides no reliable information.

The usefulness of Ident for proving of a known identity to a remote host is limited to circumstances when:

The user connecting is not the administrator of the machine. This is only likely for hosts providing Unix shell access, shared servers using a suEXEC-like construction and the like.
One trusts the administrators of the machine and knows their user policy. This is most likely for hosts in a common security domain such as within a single organization.
One trusts that the machine is the machine it claims to be and knows that machine. This is only easily arranged for hosts on a local area network or virtual network where all hosts on the network are trusted and new hosts cannot easily be added due to physical protection. On remote and normal local networks false ident replies can be accomplished by ip spoofing and, if DNS is used, by all kinds of DNS trickery. The ident daemon may provide cryptographically signed replies which, if they can be confirmed, solves these last, but not the first, concerns.
There exist no intermediate obstacles to connecting to identd such as firewall, NAT, or proxy (such as if you were using ident with Apache httpd). These are common occurrences when going between security domains (as with public HTTP or FTP servers).

Security

The ident protocol is considered dangerous because it allows crackers to gain a list of usernames on a computer system which can later be used for attacks. A generally accepted solution to this is to set up a generic/generated identifier, returning node information or even gibberish (from the requesters point of view) rather than usernames. This gibberish may be turned into real usernames by the ident administrator, when he or she is contacted about possible abuse, which means the usefulness for tracking abuse is preserved.

Uses

Ident is important on IRC as a large number of people connect to IRC from a server shared by multiple users, often using a bouncer. Without Ident there would be no way to ban a single user without banning the entire host. The server administrator may also use this information to identify the abusive user.

On most IRC networks, when the server fails to get an Ident response it falls back to the username given by client, but marks it as "not verified", usually by prefixing with a tilde; e.g., ~josh. Some IRC servers even go as far as blocking clients without an ident response,^[2] the main reason being that it makes it much harder to connect via an "open proxy" or a system where you have compromised a single account of some form but do not have root (on Unix-like systems, only root can listen for network connections on ports below 1024).

However, Ident provides no additional authentication when the user is connecting directly from their personal computer, on which they have enough privileges to control the Ident daemon as well.^[1]

Software

oidentd (for Unix-like systems)
Retina Scan Identd (for Windows; supports multiple users in a way similar to Unix identd)
Windows Ident Server.

Related Research Articles

IRC is a text-based chat system for instant messaging. IRC is designed for group communication in discussion forums, called channels, but also allows one-on-one communication via private messages as well as chat and data transfer, including file sharing.

The Lightweight Directory Access Protocol is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network. Directory services play an important role in developing intranet and Internet applications by allowing the sharing of information about users, systems, networks, services, and applications throughout the network. As examples, directory services may provide any organized set of records, often with a hierarchical structure, such as a corporate email directory. Similarly, a telephone directory is a list of subscribers with an address and a phone number.

The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC 8314. For retrieving messages, IMAP is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.

The Secure Shell Protocol (SSH) is a cryptographic network protocol for operating network services securely over an unsecured network. Its most notable applications are remote login and command-line execution.

Telnet is a client/server application protocol that provides access to virtual terminals of remote systems on local area networks or the Internet. It is a protocol for bidirectional 8-bit communications. Its main goal was to connect terminal devices and terminal-oriented processes.

An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email.

The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data connections between the client and the server. FTP users may authenticate themselves with a plain-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure transmission that protects the username and password, and encrypts the content, FTP is often secured with SSL/TLS (FTPS) or replaced with SSH File Transfer Protocol (SFTP).

In computer networking, the Name/Finger protocol and the Finger user information protocol are simple network protocols for the exchange of human-oriented status and user information.

Remote Authentication Dial-In User Service (RADIUS) is a networking protocol that provides centralized authentication, authorization, and accounting (AAA) management for users who connect and use a network service. RADIUS was developed by Livingston Enterprises in 1991 as an access server authentication and accounting protocol. It was later brought into IEEE 802 and IETF standards.

Extensible Messaging and Presence Protocol is an open communication protocol designed for instant messaging (IM), presence information, and contact list maintenance. Based on XML, it enables the near-real-time exchange of structured data between two or more network entities. Designed to be extensible, the protocol offers a multitude of applications beyond traditional IM in the broader realm of message-oriented middleware, including signalling for VoIP, video, file transfer, gaming and other uses.

SOCKS is an Internet protocol that exchanges network packets between a client and server through a proxy server. SOCKS5 optionally provides authentication so only authorized users may access a server. Practically, a SOCKS server proxies TCP connections to an arbitrary IP address, and provides a means for UDP packets to be forwarded. A SOCKS server accepts incoming client connection on TCP port 1080, as defined in RFC 1928.

Terminal Access Controller Access-Control System refers to a family of related protocols handling remote authentication and related services for network access control through a centralized server. The original TACACS protocol, which dates back to 1984, was used for communicating with an authentication server, common in older UNIX networks including but not limited to the ARPANET, MILNET and BBNNET. It spawned related protocols:

An authentication protocol is a type of computer communications protocol or cryptographic protocol specifically designed for transfer of authentication data between two entities. It allows the receiving entity to authenticate the connecting entity as well as authenticate itself to the connecting entity by declaring the type of information needed for authentication as well as syntax. It is the most important layer of protection needed for secure communication within computer networks.

The Berkeley r-commands are a suite of computer programs designed to enable users of one Unix system to log in or issue commands to another Unix computer via TCP/IP computer network. The r-commands were developed in 1982 by the Computer Systems Research Group at the University of California, Berkeley, based on an early implementation of TCP/IP.

An IRCd, short for Internet Relay Chat daemon, is server software that implements the IRC protocol, enabling people to talk to each other via the Internet. It is distinct from an IRC bot that connects outbound to an IRC channel.

Port triggering is a configuration option on a NAT-enabled router that controls communication between internal and external host machines in an IP network. It is similar to port forwarding in that it enables incoming traffic to be forwarded to a specific internal host machine, although the forwarded port is not open permanently and the target internal host machine is chosen dynamically.

The following tables compare general and technical information between a number of notable IRC client programs which have been discussed in independent, reliable prior published sources.

In computer networking, a port or port number is a number assigned to uniquely identify a connection endpoint and to direct data to a specific service. At the software level, within an operating system, a port is a logical construct that identifies a specific process or a type of network service. A port at the software level is identified for each transport protocol and address combination by the port number assigned to it. The most common transport protocols that use port numbers are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP); those port numbers are 16-bit unsigned numbers.

References

1 2 Johns, Michael (February 1993). Identification Protocol. IETF. doi: 10.17487/RFC1413 . RFC 1413 . Retrieved 1 April 2013.
↑ "News für IRCNet-Nutzer bei T-Online". german IRCnet opers. Retrieved 2011-12-26.