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|Internet protocol suite|
It was 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 they were physically present at the computer. RFC 1282, in which it was defined, states that: "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, but has the disadvantage of not being as customizable and being able to connect only to Unix hosts.
rlogin is most commonly deployed on corporate or academic networks, where user account information is shared between all the Unix machines on the network (often using NIS). These deployments essentially trust ALL other machines (and the network infrastructure).
rlogin has several serious security problems:
- All information, including passwords, is transmitted unencrypted (making it vulnerable to interception).
- The .rlogin (or .rhosts) file is easy to misuse (potentially allowing anyone to log in without a password) - for this reason many corporate system administrators prohibit .rlogin files and actively search their networks for offenders.
- The protocol partly relies on the remote party's rlogin client providing 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 authenticating other machines' identities, or ensuring that the rlogin 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 NFS's security faults automatically plague rlogin.
Due to these serious problems rlogin was rarely used across untrusted networks (like the public internet) and even in closed deployments it has fallen into relative disuse (with many Unix and Linux distributions no longer including it by default). Many networks which formerly relied on rlogin and telnet have replaced it with SSH and its rlogin-equivalent slogin. 
The original Berkeley package which provides rlogin also features rcp (remote-copy, allowing files to be copied over the network) and 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 problems. The ssh suite contains suitable replacements for both: scp replaces rcp, and ssh itself replaces both rlogin and rsh.