CS-328 Rlogin & Telnet Protocols

Rlogin Remote Logon to another Host RFC 1282 Well known port : 513

Rlogin Method Client sends 4 null terminated strings to server –byte of 0s –logon name of user on client; byte of 0s –logon name of user on server; byte of 0s –user’s term type ‘/’ term speed; byte of 0s Server responds by sending a byte of 0s

Rlogin Method (cont) Server can optionally ask for a password –handled as normal data exchange, no special protocol –passwords are sent as clear text; newer versions use Kerberos Server usually requests client to send terminals window size

Client responsibilities send a byte at a time to server; echoing is done by server text compression and decompression; Nagle’s algorithm

Client commands Flow Control –Control-S,Control-Q –sent using TCP Urgent –“Raw” mode Start Stop are sent to server as any other data; server determines the semantics and either uses for flow control or passes to local app (ex EMACS)

Client Commands (cont) –“Cooked” mode Start/Stop are interpreted at client to start and stop server from sending more data May lose many characters before server may implement Stop Control Commands (sent by server) –TCP Urgent 02 - client discards all buffered data 10 - enter “raw” mode 20 - enter “cooked” mode 80 - window size request

Telnet Remote logon to another host RFCs 854 and 855 Well known port: 23

Network Virtual Terminal (NVT) Both Telnet end points implement NVT –can’t rely on option negotiation as one end might be a hardwired terminal (like a TTY) bi-directional character based device conceptionally consists of a –keyboard –printer unspecified line length and page length

NVTs understands ASCII printable codes (32-126) and NULL, LF, CR BELL (7), BS (8), HT(9), VT (11) FF(12) are optional embedded in line with the above chars are telnet commands

Telnet Commands commands are placed in the data stream by first inserting an IAC (Interpret as command) character (255) Commands are of the form: –IAC verb option to have NVT not interpret 255 as IAC, use two consecutive IACs

Telnet Commands SE end of subneg.parameters NOP-241-No Operation DataMark-241-accompanies TCP Urgent Break-243-Break or Attn Key Interrupt Process-244-Suspend,terminate remote process

Telnet Commands (more) Abort Output-245-complete remote process without output AreYouThere-246-rqst remote NVT ot give some sign of life EraseChar-247-tell remote NVY to erase last char EraseLine-248-tell remote NVT to delete last line

Telnet Commands (more) GoAhead-249-Go Ahead signal SB-250-subnegotiation begin WILL-251-desire to begin performing WONT-252-refusal to perform DO-253-reqst other party perform DONT-254-demand remote stop IAC-255-Interpret As Command

Option Negotiation Commands Will-offer to remote to perform –Do-remote agrees –Don’t-remote disagrees to offer Won’t-offer to remote to not perform – Do-remote disagrees to the offer –Don’t-remote agree to offer

Option Negotiation Commands Do - request remote to perform –Will- remote agrees to request –Won’t-remote disagrees to request Don’t-request remote to not perform –Will-remote disagrees to request –Won’t remote agrees to request

Standard Options 0-Binary Transmission 1 - Echo 2 - Reconnection 3 - Supress GA 4- Approx Msg Size 5 - Status 6 - Timing Mark 7 - Rem.Control Trans & Echo 8 - Outout Line Width 9 - Output Line Size 10 - Output CR Disposition 11 - Output Horiz Tab Stops 12 - Output Horiz Tab Disposition 13 - Output FormFeed Disposition 14 - Output Vertical Tabstops 15 - Output Vert Tab Disposition 16 - Output Linefeed Disposition 17 - Extended ASCII 18 - Logout 19 - Byte Macro 20 - Data Entry Terminal 21 - SUPDUP 22 - SUPDUP Output 23 - Send Location 24 - Terminaltype 25 - End of Record 26 - TACACS User Id 27 - Output Marking 28 - Terminal Location Number 29 - Telnet X.3PAD

Suboption Negotiation Some options can’t be negotiated by simple option negotiation... IAC SB option parameters IAC SE