Remote Access

http://en.wikipedia.org/wiki/Telnet The old, safe world:

telnet, rlogin, rsh, rcp

Telnet

TelecommunicationsNetwork

TELNET

TELecommunication NETwork A network protocol used on Internet or LAN connections Developed in 1969 beginning with RFC 15 Standardized as IETF STD 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.

TELNET

"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 details

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

Security

Security

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

Security

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.security newsgroup

Security

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

Security

Most implementations of TELNET lack an authentication schemeCannot 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

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

Security

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 status

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 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 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 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

rsh

http://en.wikipedia.org/wiki/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 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 Shell

rsh example: Execute the command mkdir testdir as

user remoteuser on the computer host.example.com:

rsh -l remoteuser host.example.com "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

rlogin

http://en.wikipedia.org/wiki/Rlogin

Remote Login

rlogin

rlogin is a Unix software utility that allows users to log in on another host via a network Communicates via TCP port 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

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

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

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

rlogin

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

rcp

http://en.wikipedia.org/wiki/Rcp_%28Unix%29

Remote Copy

rcp

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.

Summary

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