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Remote Access. The old, safe world: telnet, rlogin, rsh, rcp. Telnet. Telecommunications Network. TELNET. TEL ecommunication NET work A network protocol used on Internet or LAN connections Developed in 1969 beginning with RFC 15

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remote access

Remote Access

The old, safe world:

telnet, rlogin, rsh, rcp





  • TELecommunication NETwork
    • A network protocol used on Internet or LAN connections
    • Developed in 1969 beginning with RFC 15
    • Standardized as IETFSTD 8
      • One of the first Internet standards
  • The term telnet also refers to software which implements the client part of the protocol
    • TELNET clients have been available on most Unix systems for many, many years
      • Available for virtually all platforms
    • Most network equipment and OSs with a TCP/IP stack support some kind of TELNET service server for their remote configuration
  • Secure Shell has begun to dominate remote access for Unix-based machines.
  • "To telnet" sometimes used as a verb
    • Establish or use a TELNET or other interactive TCP connection
      • "To change your password, telnet to the server and run the passwd command"
  • Typically a user will be telneting to a Unix-like server system or a simple network device such as a switch
    • User might "telnet in from home to check his mail at school"
    • Use a telnet client to connect local computer to a server
    • Once the connection is established
      • Log in with his account information
      • Execute commands remotely on that computer
        • E.g. ls or cd
  • Client may also be used to make interactive raw-TCP sessions
    • When that option is not available, telnet sessions are equivalent to raw TCP as long as byte 255 never appears in the data
    • ? What is byte 255 ?
protocol details1
Protocol details
  • TELNET is a client-server protocol
    • Based on a reliable connection-oriented transport.
    • Typically TCP port 23
  • TELNET predates TCP/IP
    • Originally ran on NCP
  • The protocol has many extensions
    • Some adopted as Internet standards
      • IETF standards STD 27 through STD 32
        • Define various extensions
        • Most are extremely common.
      • Other extensions are on the IETF standards track as proposed standards
  • TELNET initially developed in 1969
    • Most networked computers at the time:
      • Computer departments of academic institutions
      • Large private and government research facilities
    • Security originally not as much of a concern
      • Changed after the bandwidth explosion of the 1990s
    • Enencrypted alternatives made necessary
      • Rise in the number of people with access to the Internet
      • Number of people attempting to crack other people's servers
  • Experts in computer security1 recommend that the use of TELNET for remote logins should be discontinued under all normal circumstances for the following reasons:

1SANS Institute, members of the comp.os.linux.securitynewsgroup

  • TELNET, by default, does not encrypt any data sent over the connection (including passwords)
    • It is easy to eavesdrop on the communications
      • Easy to intercept ids and passwords
    • Anybody with access to a router, switch, or gateway located on the network between the two hosts where TELNET is being used:
      • Can intercept the packets
      • Obtain login and password information
        • Any of several common utilities
        • E.g. tcpdump and Wireshark
  • Most implementations of TELNET lack an authentication scheme
    • Cannot ensure that 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
  • Security-related shortcomings have seen the usage of the TELNET protocol drop rapidly
    • Especially on the public Internet,
  • In favor of a the ssh protocol
    • First released in 1995
    • SSH provides much of the functionality of telnet
    • Also has:
      • Strong encryption
        • Prevents sensitive data such as passwords from being intercepted
      • Public key authentication
        • Ensures 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 SASL authentication that address many security issues
  • Most TELNET implementations do not support these extensions
    • Relatively little interest in implementing these
    • SSH is adequate for most purposes.
  • The main advantage of TLS-TELNET
    • Ability to use certificate-authority signed server certificates:
      • to authenticate a server host to a client that does not yet have the server key stored
  • SSH weakness:
    • User must trust the first session to a host when it has not yet acquired the server key
current status1
Current status
  • TELNET clients are still used (as of the mid-2000s)
    • Often when diagnosing problems
    • Manually "talk" to other services without specialized client software
      • Sometimes used in debugging network services
        • an SMTP, IRC or HTTP server
        • Serves as a simple way to send commands to the server and examine the responses
current status2
Current status
  • Other software such as nc (netcat) or socat on Unix (or PuTTY on Windows) are finding greater favor with some system administrators for testing purposes
    • They can be called with arguments not to send any terminal control handshaking data
    • netcat does not distort the \377 octet
      • which allows raw access to TCP socket
      • unlike any standard-compliant TELNET software
current status3
Current status
  • TELNET is still very popular in enterprise networks to access host applications
    • IBM Mainframes
    • Typically in an internal secure environment
  • TELNET is still widely used for administration of network elements
    • Commissioning
    • Integration
    • Maintenance
  • of core network elements in mobile communication networks
current status4
Current status
  • TELNET is also heavily used for
    • MUD games played over the Internet
      • talkers, MUSHes, MUCKs, MOOes
    • Resurgent BBS community
  • Windows Vista
    • Telnet.exe is no longer installed by default
    • Is still included as an installable feature


Remote SHell

remote shell
Remote Shell
  • rsh (remote shell):
    • A command line computer program
      • Can execute shell commands
        • As another user
        • On another computer in a computer network
    • Remote system on which the rsh executes needs to be running the rshd daemon.
    • rsh uses well-known port TCP 514.
  • Note: rsh command shares the same name as another common UNIX utility, the restricted shell
    • First appeared in PWB/UNIX; in System V Release 4
    • Restricted shell is often located at /usr/lib/rsh.
remote shell1
Remote Shell
  • rsh originated as part of the BSD Unix operating system, along with rcp, as part of the rlogin package on 4.2BSD in 1983
    • rsh has been ported to other operating systems
  • rsh protocol is not secure for network use
    • Sends unencrypted information over the network
    • Some implementations also authenticate by sending unencrypted passwords over the network
    • rsh has largely been replaced by the very similar ssh (secure shell) program on untrusted networks like the internet
remote shell2
Remote Shell
  • rsh example:
    • Execute the command mkdir testdir as user remoteuser on the computer
      • rsh -l remoteuser "mkdir testdir"
    • After the command has finished rsh terminates
    • If no command is specified then rsh will log in on the remote system using rlogin
    • Network location of the remote computer is looked up using the Domain Name System


Remote Login

  • rlogin is a Unixsoftware utility that allows users to log in on another host via a network
    • Communicates via TCPport 513
    • First distributed as part of the 4.2BSD release
      • rlogin is also the name of the application layer protocol used by the software
        • part of the TCP/IP protocol suite
    • Authenticated users can act as if physically present at the computer
    • RFC 1258 states:
      • "The rlogin facility provides a remote-echoed, locally flow-controlled virtual terminal with proper flushing of output."
    • rlogin communicates with a daemon, rlogind, on the remote host.
    • rlogin is similar to the Telnet command
      • Not customizable
      • Can connect only to Unix hosts
  • rlogin most commonly deployed on corporate or academic networks
    • user account information is shared between all the Unix machines on the network
      • often using NIS
    • Deployments essentially trust most other machines (and the network infrastructure itself)
      • the rlogin protocol relies on this trust.
    • rlogind allows logins without password (where rlogind trusts a remote rlogin client)
      • if the remote host appears in the /etc/hosts.equiv file
      • if the user in question has a .rhosts file in their home directory
  • rlogin has several serious security problems:
    • All information is transmitted unencrypted
      • Including passwords!
    • .rlogin (or .rhosts) file is easy to misuse
      • Potentially allows anyone to login without a password
      • Many corporate system administrators prohibit .rlogin files
        • actively search their networks for offenders
    • Protocol partly relies on the remote party's rlogin client providing information honestly (including source port and source host name)
      • A corrupt client is able to forge this and gain access
      • rlogin protocol has no means of authenticating other machines' identities, or ensuring that the rlogin client on a trusted machine is the real rlogin client
    • Common practice of mounting users' home directories via NFS exposes rlogin to attack by means of fake .rhosts files
      • Any of NFS' security faults automatically plague rlogin
  • Due to these serious problems rlogin is rarely used across untrusted networks (like the public internet)
    • Even in closed deployments it has fallen into relative disuse
      • many Unix and Linux distributions no longer including it by default
    • Many networks which formerly relied on rlogin and telnet
      • Replaced them with SSH and its rlogin-equivalent slogin
  • Original Berkeley package which provides rlogin also features rcp and rsh
    • Share the hosts.equiv and .rhosts access-control scheme
      • Suffer from the same security problems
      • Do connect to a different daemon, rshd
    • ssh suite contains suitable replacements for both:
      • scp replaces rcp
      • ssh itself replaces both rlogin and rsh


Remote Copy

  • rcp: the Unix ‘Remote CoPy' command
    • Command on the Unix used to remotely copy
      • Copy one or more files from one computer system to another
      • Typically uses
        • TCP/IP protocol
        • .rhosts file for authentication
      • Has been implemented to alternatively support Kerberos.
  • rcp is not secure for network use
    • Sends unencrypted information over the network
    • Largely replaced by the ssh-based utility scp
  • Etymology:
    • rcp is a member of the BSD unix family of 'r' (remote) commands
    • Name is a contraction of 'r' remote and 'cp' copy.
  • Host of insecure remote commands
    • Developed before security was a major concern
  • May be okay for “internal” use
    • On “secure” networks
  • Overall:
    • Use modern secure alternatives