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Remote Access The old, “safe” world: telnet, rlogin, rsh, rcp

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Presentation on theme: "Remote Access The old, “safe” world: telnet, rlogin, rsh, rcp"— Presentation transcript:

1 Remote Access The old, “safe” world: telnet, rlogin, rsh, rcp
The old, “safe” world: telnet, rlogin, rsh, rcp

2 Telecommunications Network
Telnet Telecommunications Network

3 TELNET TELecommunication NETwork
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 Note: Secure Shell has begun to dominate remote access for Unix-based machines

4 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 value 255 never appears in the data ? What is byte 255 ?

5 Protocol details

6 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 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 proposed standards

7 Security

8 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 Most connections in a “secure” environment Changed after the bandwidth explosion of the 1990s Encrypted alternatives made necessary with: Rise in the number of people with access to the Internet Number of people attempting to crack other people's servers

9 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 newsgroup

10 Security TELNET, by default, does not encrypt any data sent over the connection including passwords Easy to eavesdrop on 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

11 Security 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

12 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

13 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 Main advantage of TLS-TELNET Ability to use certificate-authority signed server certificates: 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

14 Current status

15 Current status TELNET clients are still used (as of 2010ish)
Often when diagnosing problems Manually "talk" to other services without specialized client software Sometimes used in debugging network services SMTP, IRC or HTTP servers Serves as a simple way to send commands to the server and examine the responses

16 Current status Other software is finding greater favor with some system administrators for testing purposes Examples: nc (netcat) socat on Unix PuTTY on Windows Can be called with arguments to not 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

17 Current TELNET status Still very popular in enterprise networks to access host applications IBM Mainframes Typically within a secure internal environment Still widely used for administration of network elements Commissioning Integration Maintenance Core network elements in mobile communication networks

18 Current status TELNET is also heavily used for
MUD games played over the Internet talkers, MUSHes, MUCKs, MOOes Resurgent BBS community Windows Vista and after: Telnet.exe no longer installed by default Still available as an installable feature

19 Remote SHell
rsh Remote SHell

20 Remote Shell rsh (remote shell): Command line computer program Executes shell commands As another user On another computer in a computer network One command at a time Remote system which rsh accesses Must have the rshd daemon running 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 System V Release 4 Restricted shell is often located at /usr/lib/rsh

21 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

22 Remote Shell rsh example: To 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 via the Domain Name System

23 Remote Login
rlogin Remote Login

24 rlogin rlogin: Unix software utility that allows users to log in on another host via a network TCP port 513 First distributed as part of the 4.2BSD release Note: 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 From RFC 1258: "The rlogin facility provides a remote-echoed, locally flow-controlled virtual terminal with proper flushing of output." rlogin communicates with a daemon on the remote host rlogind rlogin Similar to the Telnet command Not customizable Can connect only to Unix hosts

25 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) Remote host must be in the /etc/hosts.equiv file If user has a .rhosts file in their home directory

26 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 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

27 rlogin Due to these serious problems rlogin is rarely used across untrusted networks e.g. like the public internet Even in closed deployments it has fallen into relative disuse many Unix and Linux distributions no longer include it by default Many networks which formerly relied on rlogin and telnet Replaced them with SSH and its rlogin-equivalent slogin

28 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 Connects to a different daemon: rshd Note: ssh suite contains suitable replacements for both: scp replaces rcp ssh itself replaces both rlogin and rsh

29 Resume 10/26

30 Remote Copy
rcp Remote Copy

31 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

32 ftp

33 ftp File Transfer Protocol Syntax Platform “independent”
FTP runs exclusively over TCP Listens on port 21 (default) Incoming connections from FTP clients Responds on a dynamic port Sent by client Usually port 20 Syntax ftp [-options] [-s:filename] [-w:buffer] [host]

34 Typical use Connect to an FTP server Login
ftp machineid Login Enter a valid user id an password Interact with the FPT server At the ftp> prompt

35 FTP Options -s:filename host -g -n -i -v -w:buffer -d -a
Run a text file containing FTP commands host Host name or IP address of the remote host. -g Disable filename wildcards. -n No auto-login. -i No interactive prompts during ftp. -v Hide remote server responses. -w:buffer Set buffer size to buffer (default=4096) -d Debug -a Use any local interface when binding data connection.

36 Commonly Used FTP Commands
? request help or information about the FTP commands append local-file [remote-file] Append a local file to a file on the remote computer. ascii set the mode of file transfer to ASCII the default transmits seven bits per character binary set the mode of file transfer to binary binary mode transmits all eight bits per byte provides less chance of a transmission error must be used to transmit files other than ASCII files bye exit the FTP environment same as quit

37 Commonly Used Commands
cd change directory on the remote machine close terminate a connection with another computer   close userid closes the current FTP connection with userid still leaves you within the FTP environment delete remote-file delete a file in the current remote directory same as rm in UNIX dir [remote-directory] [local-file] List a remote directory's files and subdirectories. (or save the listing to local-file) disconnect Disconnect from the remote host, retaining the ftp prompt

38 Commonly Used FTP Commands
get remote-file [local-file] copy one file from the remote machine to the local machine   get ABC DEF copies file ABC in the current remote directory to (or on top of) a file named DEF in your current local directory get ABC copies file ABC in the current remote directory to (or on top of) a file with the same name, ABC, in your current local directory help [command] request a list of all available FTP commands lcd change directory on your local machine same as UNIX cd ls list the names of the files in the current remote directory mdelete remote-files [ ...] Delete files on remote host

39 Commonly Used Commands
mdir remote-files [ ...] local-file Display a list of a remote directory's files and subdirectories (or save the listing to local-file) Mdir allows you to specify multiple files. mget remote-files [ ...] copy multiple files from the remote machine to the local machine you are prompted for a y/n answer before transferring each file   mget * copies all the files in the current remote directory to your current local directory, using the same filenames Notice the use of the wild card character, *. mkdir directory make a new directory within the current remote directory mls remote-files [ ...] local-file List a remote directory's files and folders. (short format)

40 Commonly Used Commands
mput local-files [ ...] copy multiple files from the local machine to the remote machine you are prompted for a y/n answer before transferring each file open computer [port] open a connection with another computer   open userid opens a new FTP connection with userid must enter a username and password for the useid account unless it is to be an anonymous connection prompt Toggle prompting. Ftp prompts during multiple file transfers to allow you to selectively retrieve or store files; mget and mput transfer all files if prompting is turned off. By default, prompting is on. put copy one file from the local machine to the remote machine pwd find out the pathname of the current directory on the remote machine

41 Commonly Used FTP Commands
pwd find out the pathname of the current directory on the remote machine quit exit the FTP environment (same as bye) remotehelp [command] Display help for remote commands. rename filename newfilename Rename remote files. rmdir remove a directory in the current remote directory

42 Commonly Used FTP Commands
send local-file [remote-file] Copy a local file to the remote host. status Display the current status of FTP connections and toggles. trace Toggles packet tracing; trace displays the route of each packet type [type-name] Set or display the file transfer type: `binary' or `ASCII' (the default) If type-name is not specified, the current type is displayed. ASCII should be used when transferring text files. In ASCII text mode, character-set and end-of-line characters are converted as necessary. Use `Binary' for transferring executable files. user user-name [password] [account] Specifies a user to the remote host. verbose Toggle verbose mode. By default, verbose is on.

43 Telnet should never be use because it is unsafe:
True False

44 ftp allows both the uploading and downloading of files to a remote computer
True False

45 Summary Host of insecure remote commands
Developed before security was a major concern May be okay for “internal” use On “secure” networks May be better for speed Where security is not a concern Overall: Use modern secure alternatives

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