Berkeley r-commands: Difference between revisions

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The '''Berkeley r-commands''' are a [[Software suite|suite]] of [[computer program]]s designed to enable computer users to access various [[System resource|computer resources]] available on a [[Computer network|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]] (the [[protocol stack]] of the [[Internet]]).<ref name="McKusick">{{cite book |title=[[Open Sources: Voices from the Open Source Revolution]] |last=McKusick |first=Marshall Kirk |authorlink=Marshall Kirk McKusick |year=1999 |isbn=978-1-56592-582-3 |chapter=Twenty Years of Berkeley Unix: From AT&T-Owned to Freely Redistributable |chapter-url=http://www.oreilly.com/openbook/opensources/book/kirkmck.html |publisher=O'Reilly & Associates |at=Section: "4.2BSD" |access-date=2018-03-03}}</ref>

'''rlogin''' is a [[software utility]] for [[Unix-like]] computer [[operating system]]s that was first distributed as part of the [[Berkeley Software Distribution|4.2BSD]] release. rlogin allows users to log in on another [[Server (computing)|host]] via a [[computer network|network]], using [[Transmission Control Protocol|TCP]] [[TCP and UDP port|port]] 513.

The CSRG incorporated the r-commands into their [[Unix]] [[operating system]], the [[Berkeley Software Distribution]] (BSD). The r-commands premiered in BSD v4.1.<ref name="McKusick"/> Among the programs in the suite are: {{code|rcp}} (remote copy), {{code|rexec}} (remote [[Execution (computing)|execution]]), {{code|rlogin}} (remote [[login]]), {{code|rsh}} (remote [[Shell (computing)|shell]]), and {{code|rwho}} (remote [[who (Unix)|who]]).

rlogin is also the name of the [[application layer]] [[Communications protocol|protocol]] used by the software, part of the [[TCP/IP]] [[protocol suite]]. Authenticated users can act as if they were physically present at the computer. RFC 1282, in which it was defined, states: "The rlogin facility provides a remote-echoed, locally flow-controlled virtual terminal with proper flushing of output." rlogin communicates with a [[daemon (computer software)|daemon]], '''rlogind''', on the remote host. rlogin is similar to the [[Telnet]] command, but is not as customizable and is able to connect only to Unix-like hosts.

== Use rlogin==

{{code|rlogin}} enables a user to log in on another [[Server (computing)|server]] via [[computer network]], using [[Transmission Control Protocol|TCP]] [[network port]] 513.

rlogin is also the name of the [[application layer]] [[Communications protocol|protocol]] used by the software, part of the [[TCP/IP]] [[protocol suite]]. Authenticated users can act as if they were physically present at the computer. RFC 1282, in which it was defined, states: "The rlogin facility provides a remote-echoed, locally flow-controlled virtual terminal with proper flushing of output." rlogin communicates with a [[daemon (computer software)|daemon]], '''rlogind''', on the remote host. rlogin is similar to the [[Telnet]] command, but is not as customizable and is able to connect only to Unix-like hosts.

rlogin is most commonly deployed on corporate and academic networks, where user account information is shared among all the Unix machines on the network (often using [[Network Information Service|NIS]]). These deployments essentially trust all other machines (and the network infrastructure).

==rcp, rexec, rsh, and rwho==

== Security ==

{{Further|Remote Shell}}

The original Berkeley package that provides rlogin also features [[rcp (Unix)|rcp]] (remote-copy, allowing files to be copied over the network) and [[Remote Shell|rsh]] (remote-shell, allowing commands to be run on a remote machine without the user logging into it). These share the hosts.equiv and .rhosts access-control scheme (although they connect to a different daemon, rshd), and as such suffer from the same security exposures. The ssh suite contains suitable replacements for both: scp replaces rcp, and ssh itself replaces both rlogin and rsh.

rlogin has several serious security problems:

Like rsh, rexec enables the user to run shell commands on a remote computer. However, unlike the rsh server, the rexec server (rexecd) authenticates users by reading the username and password (unencrypted) from the socket.<ref>{{cite web |url=http://www.manpagez.com/man/8/rexecd/ |title=rexecd(8) |website=manpagez.com |access-date=2018-03-03}}</ref> rexec uses TCP port 512.

* All information, including passwords, is transmitted unencrypted (making it vulnerable to interception).

* The .rlogin (or .rhosts) file is easy to misuse (potentially allowing logins without a [[password]]) - for this reason many corporate system administrators prohibit .rlogin files and actively scrutinize their networks for offenders.

* The protocol partly relies on the remote party's rlogin client to provide information honestly, including source port and source host name. A corrupt client is thus able to forge this and gain access, as the rlogin protocol has no means of [[Authentication|authenticating]] other machines' identities, or ensuring that the requesting client on a trusted machine is the real rlogin client.

* The common practice of mounting users' home directories via NFS exposes rlogin to attack by means of fake .rhosts files - this means that any of [[Network File System|NFS's]] security faults automatically plague rlogin.

With rwho, a user can display a list of users who are logged into a remote, [[multi-user]] computer.

Due to these problems, rlogin was rarely used across untrusted networks (like the public internet) and even in closed deployments it fell into relative disuse (with many Unix and [[Linux]] distributions no longer including it by default). Many networks that formerly relied on rlogin and telnet have replaced it with [[Secure shell|SSH]] and its rlogin-equivalent ''slogin''.<ref name="Sobell">{{cite book|last=Sobell|first=Mark|title=A Practical Guide to Linux Commands, Editors, and Shell Programming|ISBN=978-0-13-136736-4|date=2010|publisher=Pearson Education, Inc}}</ref><ref name="iu">{{cite web|title=Unix job control command list|publisher=Indiana University|url=https://kb.iu.edu/d/afnw|accessdate=20 December 2014}}</ref>

{{anchor|rcp}}

== Replacements ==

The original Berkeley package that provides rlogin also features [[rcp (Unix)|rcp]] (remote-copy, allowing files to be copied over the network) and [[Remote Shell|rsh]] (remote-shell, allowing commands to be run on a remote machine without the user logging into it). These share the hosts.equiv and .rhosts access-control scheme (although they connect to a different daemon, rshd), and as such suffer from the same security exposures. The ssh suite contains suitable replacements for both: scp replaces rcp, and ssh itself replaces both rlogin and rsh.

== See also ==

* [[Secure Shell]]

* [[Telnet]]

* [[who (Unix)]]

==Notes==

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== References ==

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==Further reading==