Network Working Group T. Ylonen INTERNET-DRAFT T. Kivinen draft-ietf-secsh-connect-05.txt M. Saarinen Expires in six months T. Rinne S. Lehtinen SSH 22 February 1999 SSH Connection Protocol Status of This memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract SSH is a protocol for secure remote login and other secure network ser- vices over an insecure network. This document describes the SSH connec- tion protocol. It provides interactive login sessions, remote execution of commands, forwarded TCP/IP connections, and forwarded X11 connec- tions. All of these channels are multiplexed into a single encrypted tunnel. The SSH Connection Protocol has been designed to run on top of the SSH transport layer and user authentication protocols. T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 1] INTERNET-DRAFT 6 August 1998 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Global Requests . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Channel Mechanism . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Opening a Channel . . . . . . . . . . . . . . . . . . . . . 3 3.2. Data Transfer . . . . . . . . . . . . . . . . . . . . . . . 4 3.3. Closing a Channel . . . . . . . . . . . . . . . . . . . . . 5 3.4. Channel-Specific Requests . . . . . . . . . . . . . . . . . 6 4. Interactive Sessions . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Opening a Session . . . . . . . . . . . . . . . . . . . . . 6 4.2. Requesting a Pseudo-Terminal . . . . . . . . . . . . . . . . 7 4.3. X11 Forwarding . . . . . . . . . . . . . . . . . . . . . . . 7 4.3.1. Requesting X11 Forwarding . . . . . . . . . . . . . . . 7 4.3.2. X11 Channels . . . . . . . . . . . . . . . . . . . . . . 8 4.4. Authentication Agent Forwarding . . . . . . . . . . . . . . 8 4.4.1. Requesting Authentication Agent Forwarding . . . . . . . 8 4.4.2. Authentication Agent Channels . . . . . . . . . . . . . 8 4.5. SSH1 Authentication Agent Forwarding . . . . . . . . . . . . 9 4.5.1. Requesting SSH1 Authentication Agent Forwarding . . . . 9 4.5.2. SSH1 Authentication Agent Channels . . . . . . . . . . . 9 4.6. Environment Variable Passing . . . . . . . . . . . . . . . . 9 4.7. Starting a Shell or a Command . . . . . . . . . . . . . . . 10 4.8. Session Data Transfer . . . . . . . . . . . . . . . . . . . 10 4.9. Window Dimension Change Message . . . . . . . . . . . . . . 11 4.10. Local Flow Control . . . . . . . . . . . . . . . . . . . . 11 4.11. Signals . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.12. Returning Exit Status . . . . . . . . . . . . . . . . . . . 12 5. TCP/IP Port Forwarding . . . . . . . . . . . . . . . . . . . . . 12 5.1. Requesting Port Forwarding . . . . . . . . . . . . . . . . . 12 5.2. TCP/IP Forwarding Channels . . . . . . . . . . . . . . . . . 13 6. Encoding of Terminal Modes . . . . . . . . . . . . . . . . . . . 14 7. Summary of Message Numbers . . . . . . . . . . . . . . . . . . . 15 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 16 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17 1. Introduction The SSH Connection Protocol has been designed to run on top of the SSH transport layer and user authentication protocols. It provides interactive login sessions, remote execution of commands, forwarded TCP/IP connections, and forwarded X11 connections. The service name for this protocol (after user authentication) is "ssh-connection". This document should be read only after reading the SSH architecture document [SSH-ARCH]. This document freely uses terminology and notation from the architecture document without reference or further explanation. 2. Global Requests T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 2] INTERNET-DRAFT 6 August 1998 There are several kinds of requests that affect the state of the remote end "globally", independent of any channels. An example is a request to start TCP/IP forwarding for a specific port. All such requests use the following format. byte SSH_MSG_GLOBAL_REQUEST string request name (restricted to US-ASCII) boolean want reply ... request-specific data follows The recipient will respond to this message with SSH_MSG_REQUEST_SUCCESS, SSH_MSG_REQUEST_FAILURE, or some request-specific continuation messages if `want reply' is TRUE. byte SSH_MSG_REQUEST_SUCCESS If the recipient does not recognize or support the request, it simply responds with SSH_MSG_REQUEST_FAILURE. byte SSH_MSG_REQUEST_FAILURE 3. Channel Mechanism All terminal sessions, forwarded connections, etc. are channels. Either side may open a channel. Multiple channels are multiplexed into a single connection. Channels are identified by numbers at each end. The number referring to a channel may be different on each side. Requests to open a channel contain the sender's channel number. Any other channel-related messages contain the recipient's channel number for the channel. Channels are flow-controlled. No data may be sent to a channel until a message is received to indicate that window space is available. 3.1. Opening a Channel When either side wishes to open a new channel, it allocates a local number for the channel. It then sends the following message to the other side, and includes the local channel number and initial window size in the message. byte SSH_MSG_CHANNEL_OPEN string channel type (restricted to US-ASCII) uint32 sender channel uint32 initial window size uint32 maximum packet size ... channel type specific data follows The channel type is a name as described in the SSH architecture document, with similar extension mechanisms. `sender channel' is a local identifier for the channel used by the sender of this message. `initial window size' specifies how many bytes of channel data can be sent to the T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 3] INTERNET-DRAFT 6 August 1998 sender of this message without adjusting the window. `Maximum packet size' specifies the maximum size of an individual data packet that can be sent to the sender (for example, one might want to use smaller packets for interactive connections to get better interactive response on slow links). The remote side then decides whether it can open the channel, and responds with either byte SSH_MSG_CHANNEL_OPEN_CONFIRMATION uint32 recipient channel uint32 sender channel uint32 initial window size uint32 maximum packet size ... channel type specific data follows where `recipient channel' is the channel number given in the original open request, and `sender channel' is the channel number allocated by the other side, or byte SSH_MSG_CHANNEL_OPEN_FAILURE uint32 recipient channel uint32 reason code string additional textual information (ISO-10646 UTF-8 [RFC-2044]) string language tag (as defined in [RFC-1766]) If the recipient of the SSH_MSG_CHANNEL_OPEN message does not support the specified channel type, it simply responds with SSH_MSG_CHANNEL_OPEN_FAILURE. The client MAY show the additional information to the user. If this is done, the client software should take the precautions discussed in [SSH-ARCH]. The following reason codes are defined: #define SSH_OPEN_ADMINISTRATIVELY_PROHIBITED 1 #define SSH_OPEN_CONNECT_FAILED 2 #define SSH_OPEN_UNKNOWN_CHANNEL_TYPE 3 #define SSH_OPEN_RESOURCE_SHORTAGE 4 3.2. Data Transfer The window size specifies how many bytes the other party can send before it must wait for the window to be adjusted. Both parties use the following message to adjust the window. byte SSH_MSG_CHANNEL_WINDOW_ADJUST uint32 recipient channel uint32 bytes to add After receiving this message, the recipient MAY send the given number of bytes more that it was previously allowed to send; the window size is incremented. T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 4] INTERNET-DRAFT 6 August 1998 Data transfer is done with messages of the following type. byte SSH_MSG_CHANNEL_DATA uint32 recipient channel string data The maximum amount of data allowed is the current window size. The window size is decremented by the amount of data sent. Both parties MAY ignore all extra data sent after the allowed window is empty. Additionally, some channels can transfer several types of data. An example of this is stderr data from interactive sessions. Such data can be passed with SSH_MSG_CHANNEL_EXTENDED_DATA messages, where a separate integer specifies the type of the data. The available types and their interpretation depend on the type of the channel. byte SSH_MSG_CHANNEL_EXTENDED_DATA uint32 recipient_channel uint32 data_type_code string data Data sent with these messages consumes the same window as ordinary data. Currently, only the following type is defined. #define SSH_EXTENDED_DATA_STDERR 1 3.3. Closing a Channel When a party will no longer send more data to a channel, it SHOULD send SSH_MSG_CHANNEL_EOF. byte SSH_MSG_CHANNEL_EOF uint32 recipient_channel No explicit response is sent to this message; however, the application may send EOF to whatever is at the other end of the channel. Note that the channel remains open after this message, and more data may still be sent in the other direction. This message does not consume window space and can be sent even if no window space is available. When either party wishes to terminate the channel, it sends SSH_MSG_CHANNEL_CLOSE. Upon receiving this message, a party MUST send back a SSH_MSG_CHANNEL_CLOSE unless it has already sent this message for the channel. The channel is considered closed for a party when it has both sent and received SSH_MSG_CHANNEL_CLOSE, and the party may then reuse the channel number. A party MAY send SSH_MSG_CHANNEL_CLOSE without having sent or received SSH_MSG_CHANNEL_EOF. byte SSH_MSG_CHANNEL_CLOSE uint32 recipient_channel This message does not consume window space and can be sent even if no window space is available. T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 5] INTERNET-DRAFT 6 August 1998 It is recommended that any data sent before this message is delivered to the actual destination, if possible. 3.4. Channel-Specific Requests Many channel types have extensions that are specific to that particular channel type. An example is requesting a pty (pseudo terminal) for an interactive session. All channel-specific requests use the following format. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string request type (restricted to US-ASCII) boolean want reply ... type-specific data If want reply is FALSE, no response will be sent to the request. Otherwise, the recipient responds with either SSH_MSG_CHANNEL_SUCCESS or SSH_MSG_CHANNEL_FAILURE, or request-specific continuation messages. If the request is not recognized or is not supported for the channel, SSH_MSG_CHANNEL_FAILURE is returned. This message does not consume window space and can be sent even if no window space is available. Request types are local to each channel type. The client is allowed to send further messages without waiting for the response to the request. byte SSH_MSG_CHANNEL_SUCCESS uint32 recipient_channel byte SSH_MSG_CHANNEL_FAILURE uint32 recipient_channel These messages do not consume window space and can be sent even if no window space is available. 4. Interactive Sessions A session is a remote execution of a program. The program may be a shell, an application, a system command, or some built-in subsystem. It may or may not have a tty, and may or may not involve X11 forwarding. Multiple sessions can be active simultaneously. 4.1. Opening a Session A session is started by sending the following message. byte SSH_MSG_CHANNEL_OPEN string "session" uint32 sender channel uint32 initial window size uint32 maximum packet size T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 6] INTERNET-DRAFT 6 August 1998 Client implementations SHOULD reject any session channel open requests to make it more difficult for a corrupt server to attack the client. 4.2. Requesting a Pseudo-Terminal A pseudo-terminal can be allocated for the session by sending the following message. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient_channel string "pty-req" boolean want_reply string TERM environment variable value (e.g., vt100) uint32 terminal width, characters (e.g., 80) uint32 terminal height, rows (e.g., 24) uint32 terminal width, pixels (e.g., 480) uint32 terminal height, pixels (e.g., 640) string encoded terminal modes The encoding of terminal modes is described in Section ``Encoding of Terminal Modes''. Zero dimension parameters MUST be ignored. The character/row dimensions override the pixel dimensions (when nonzero). Pixel dimensions refer to the drawable area of the window. The dimension parameters are only informational. The client SHOULD ignore pty requests. 4.3. X11 Forwarding 4.3.1. Requesting X11 Forwarding X11 forwarding may be requested for a session by sending byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "x11-req" boolean want reply boolean single connection string x11 authentication protocol string x11 authentication cookie uint32 x11 screen number It is recommended that the authentication cookie that is sent be a fake, random cookie, and that the cookie is checked and replaced by the real cookie when a connection request is received. X11 connection forwarding should stop when the session channel is closed; however, already opened forwardings should not be automatically closed when the session channel is closed. If `single connection' is TRUE, only a single connection should be forwarded. No more connections will be forwarded after the first, or T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 7] INTERNET-DRAFT 6 August 1998 after the session channel has been closed. `X11 authentication protocol is the name of the X11 authentication method used, i.e. "MIT-MAGIC-COOKIE-1". 4.3.2. X11 Channels X11 channels are opened with a channel open request. The resulting channels are independent of the session, and closing the session channel does not close the forwarded X11 channels. byte SSH_MSG_CHANNEL_OPEN string "x11" uint32 sender channel uint32 initial window size uint32 maximum packet size string originator address (e.g. "192.168.7.38") uint32 originator port The recipient should respond with SSH_MSG_CHANNEL_OPEN_CONFIRMATION or SSH_MSG_CHANNEL_OPEN_FAILURE. Implementations MUST reject any X11 channel open requests if they have not requested X11 forwarding. 4.4. Authentication Agent Forwarding It is RECOMMENDED that authentication agent forwarding is allowed even when either or both parties do not support the SSH authentication agent protocol [SSH-AGENT]. 4.4.1. Requesting Authentication Agent Forwarding Authentication agent forwarding may be requested for a session by sending byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "auth-agent-req" boolean want reply The server responds with either SSH_MSG_CHANNEL_SUCCESS or SSH_MSG_CHANNEL_FAILURE (if `want reply' is TRUE). The client MAY send further messages without waiting for the response to this message. 4.4.2. Authentication Agent Channels When an application requests a connection to the authentication agent, the following message is sent to the originator of the session. byte SSH_MSG_CHANNEL_OPEN string "auth-agent" uint32 sender channel T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 8] INTERNET-DRAFT 6 August 1998 uint32 initial window size uint32 maximum packet size The recipient SHOULD respond with open confirmation or open failure. Implementations MUST reject any agent channel open requests if they have not requested agent forwarding. 4.5. SSH1 Authentication Agent Forwarding Implementations MAY support ssh1 authentication agent forwarding in order to provide compatibility with old ssh versions. 4.5.1. Requesting SSH1 Authentication Agent Forwarding Authentication agent forwarding may be requested for a session by sending byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "auth-ssh1-agent-req" boolean want reply The server responds with either SSH_MSG_CHANNEL_SUCCESS or SSH_MSG_CHANNEL_FAILURE (if `want reply' is TRUE). The client MAY send further messages without waiting for the response to this message. 4.5.2. SSH1 Authentication Agent Channels When an application requests a connection to the authentication agent, the following message is sent to the originator of the session. byte SSH_MSG_CHANNEL_OPEN string "auth-ssh1-agent" uint32 sender channel uint32 initial window size uint32 maximum packet size The recipient SHOULD respond with open confirmation or open failure. Implementations MUST reject any agent channel open requests if they have not requested ssh1 agent forwarding. 4.6. Environment Variable Passing Environment variables may be passed to the shell/command to be started later. Typically, each machine will have a preconfigured set of variables that it will allow. Since uncontrolled setting of environment variables can be very dangerous, it is recommended that implementations allow setting only variables whose names have been explicitly configured to be allowed. byte SSH_MSG_CHANNEL_REQUEST T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 9] INTERNET-DRAFT 6 August 1998 uint32 recipient channel string "env" boolean want reply string variable name string variable value 4.7. Starting a Shell or a Command Once the session has been set up, a program is started at the remote end. Program can be a shell, an application program or a subsystem with a host-independent name. Only one of these requests can succeed per channel. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "shell" boolean want reply This message will request the user's default shell (typically defined in /etc/passwd in UNIX systems) to be started at the other end. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "exec" boolean want reply string command This message will request the server to start the execution of the given command. The command string may contain a path. Normal precautions MUST be taken to prevent the execution of unauthorized commands. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "subsystem" boolean want reply string subsystem name This last form executes a predefined subsystem. It expected that these will include a general file transfer mechanism, and possibly other features. Implementations may also allow configuring more such mechanisms. The server SHOULD not halt the execution of the protocol stack when starting a shell or a program. All input and output from these SHOULD be redirected the the channel or to the encrypted tunnel. It is RECOMMENDED to request and check the reply for these messages. The client SHOULD ignore these messages. 4.8. Session Data Transfer Data transfer for a session is done using SSH_MSG_CHANNEL_DATA and SSH_MSG_CHANNEL_EXTENDED_DATA packets and the window mechanism. The T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 10] INTERNET-DRAFT 6 August 1998 extended data type SSH_EXTENDED_DATA_STDERR has been defined for stderr data. 4.9. Window Dimension Change Message When the window (terminal) size changes on the client side, it MAY send a message to the other side to inform it of the new dimensions. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient_channel string "window-change" boolean FALSE uint32 terminal width, columns uint32 terminal height, rows uint32 terminal width, pixels uint32 terminal height, pixels No response SHOULD be sent to this message. 4.10. Local Flow Control On many systems it is possible to determine if a pseudo-terminal is using control-S control-Q flow control. When flow control is allowed, it is often desirable to do the flow control at the client end to speed up responses to user requests. This is facilitated by the following notification. Initially, the server is responsible for flow control. (Here, again, client means the side originating the session, and server the other side.) The message below is used by the server to inform the client when it can or cannot perform flow control (control-S/control-Q processing). If `client can do' is TRUE, the client is allowed to do flow control using control-S and control-Q. The client MAY ignore this message. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "xon-xoff" boolean FALSE boolean client can do No response is sent to this message. 4.11. Signals A signal can be delivered to the remote process/service using the following message. Some systems may not implement signals, in which case they SHOULD ignore this message. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "signal" boolean FALSE uint32 signal number T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 11] INTERNET-DRAFT 6 August 1998 4.12. Returning Exit Status When the command running at the other end terminates, The following message can be sent to return the exit status of the command. Returning the status is RECOMMENDED. No acknowledgment is sent for this message. The channel needs to be closed with SSH_MSG_CHANNEL_CLOSE after this message. The client MAY ignore these messages. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient_channel string "exit-status" boolean FALSE uint32 exit_status The remote command may also terminate violently due to a signal. Such a condition can be indicated by the following message. A zero exit_status usually means that the command terminated successfully. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "exit-signal" boolean FALSE uint32 signal number boolean core dumped string error message (ISO-10646 UTF-8 [RFC-2044]) string language tag (as defined in [RFC-1766]) The `error message' contains an additional explanation of the error message. The message may consist of multiple lines. The client software MAY display this message to the user. 5. TCP/IP Port Forwarding 5.1. Requesting Port Forwarding A party need not explicitly request forwardings from its own end to the other direction. However, it if wishes to have connections to a port on the other side be forwarded to the local side, it must explicitly request this. byte SSH_MSG_GLOBAL_REQUEST string "tcpip-forward" boolean want reply string address to bind (e.g. "0.0.0.0") uint32 port number to bind `Address to bind' and `port number to bind' specify the IP address and port to which the socket to be listened is bound. The address should be "0.0.0.0" if connections are allowed from anywhere. (Note that the client can still filter connections based on information passed in the open request.) T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 12] INTERNET-DRAFT 6 August 1998 Implementations should only allow forwarding privileged ports if the user has been authenticated as a privileged user. Client implementations SHOULD reject these messages; they are normally only sent by the client. A port forwarding can be cancelled with the following message. Note that channel open requests may be received until a reply to this message is received. byte SSH_MSG_GLOBAL_REQUEST string "cancel-tcpip-forward" boolean want reply string address_to_bind (e.g. "127.0.0.1") uint32 port number to bind Client implementations SHOULD reject these messages; they are normally only sent by the client. 5.2. TCP/IP Forwarding Channels When a connection comes to a port for which remote forwarding has been requested, a channel is opened to forward the port to the other side. byte SSH_MSG_CHANNEL_OPEN string "forwarded-tcpip" uint32 sender channel uint32 initial window size uint32 maximum packet size string address that was connected uint32 port that was connected string originator IP address uint32 originator port Implementations MUST reject these messages unless they have previously requested a remote TCP/IP port forwarding with the given port number. When a connection comes to a locally forwarded TCP/IP port, the following packet is sent to the other side. Note that these messages MAY be sent also for ports for which no forwarding has been explicitly requested. The receiving side must decide whether to allow the forwarding. byte SSH_MSG_CHANNEL_OPEN string "direct-tcpip" uint32 sender channel uint32 initial window size uint32 maximum packet size string host to connect uint32 port to connect string originator IP address uint32 originator port `Host to connect' and `port to connect' specify the TCP/IP host and port T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 13] INTERNET-DRAFT 6 August 1998 where the recipient should connect the channel. `Host to connect' may be either a domain name or a numeric IP address. `Originator IP address' is the numeric IP address of the machine where the connection request comes from, and `originator port' is the port on the originator host from where the connection came from. Forwarded TCP/IP channels are independent of any sessions, and closing a session channel does not in any way imply that forwarded connections should be closed. Client implementations SHOULD reject direct TCP/IP open requests for security reasons. 6. Encoding of Terminal Modes Terminal modes (as passed in a pty request) are encoded into a byte stream. It is intended that the coding be portable across different environments. The tty mode description is a stream of bytes. The stream consists of opcode-argument pairs. It is terminated by opcode TTY_OP_END (0). Opcodes 1-159 have a single uint32 argument. Opcodes 160-255 are not yet defined, and cause parsing to stop (they should only be used after any other data). The client SHOULD put in the stream any modes it knows about, and the server MAY ignore any modes it does not know about. This allows some degree of machine-independence, at least between systems that use a POSIX-like tty interface. The protocol can support other systems as well, but the client may need to fill reasonable values for a number of parameters so the server pty gets set to a reasonable mode (the server leaves all unspecified mode bits in their default values, and only some combinations make sense). The following opcodes have been defined. The naming of opcodes mostly follows the POSIX terminal mode flags. 0 TTY_OP_END Indicates end of options. 1 VINTR Interrupt character; 255 if none. Similarly for the other characters. Not all of these characters are supported on all systems. 2 VQUIT The quit character (sends SIGQUIT signal on POSIX systems). 3 VERASE Erase the character to left of the cursor. 4 VKILL Kill the current input line. 5 VEOF End-of-file character (sends EOF from the terminal). 6 VEOL End-of-line character in addition to carriage return and/or linefeed. 7 VEOL2 Additional end-of-line character. 8 VSTART Continues paused output (normally control-Q). 9 VSTOP Pauses output (normally control-S). 10 VSUSP Suspends the current program. T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 14] INTERNET-DRAFT 6 August 1998 11 VDSUSP Another suspend character. 12 VREPRINT Reprints the current input line. 13 VWERASE Erases a word left of cursor. 14 VLNEXT Enter the next character typed literally, even if it is a special character 15 VFLUSH Character to flush output. 16 VSWTCH Switch to a different shell layer. 17 VSTATUS Prints system status line (load, command, pid etc). 18 VDISCARD Toggles the flushing of terminal output. 30 IGNPAR The ignore parity flag. The parameter SHOULD be 0 if this flag is FALSE set, and 1 if it is TRUE. 31 PARMRK Mark parity and framing errors. 32 INPCK Enable checking of parity errors. 33 ISTRIP Strip 8th bit off characters. 34 INLCR Map NL into CR on input. 35 IGNCR Ignore CR on input. 36 ICRNL Map CR to NL on input. 37 IUCLC Translate uppercase characters to lowercase. 38 IXON Enable output flow control. 39 IXANY Any char will restart after stop. 40 IXOFF Enable input flow control. 41 IMAXBEL Ring bell on input queue full. 50 ISIG Enable signals INTR, QUIT, [D]SUSP. 51 ICANON Canonicalize input lines. 52 XCASE Enable input and output of uppercase characters by preceding their lowercase equivalents with `\'. 53 ECHO Enable echoing. 54 ECHOE Visually erase chars. 55 ECHOK Kill character discards current line. 56 ECHONL Echo NL even if ECHO is off. 57 NOFLSH Don't flush after interrupt. 58 TOSTOP Stop background jobs from output. 59 IEXTEN Enable extensions. 60 ECHOCTL Echo control characters as ^(Char). 61 ECHOKE Visual erase for line kill. 62 PENDIN Retype pending input. 70 OPOST Enable output processing. 71 OLCUC Convert lowercase to uppercase. 72 ONLCR Map NL to CR-NL. 73 OCRNL Translate carriage return to newline (output). 74 ONOCR Translate newline to carriage return-newline (output). 75 ONLRET Newline performs a carriage return (output). 90 CS7 7 bit mode. 91 CS8 8 bit mode. 92 PARENB Parity enable. 93 PARODD Odd parity, else even. 128 TTY_OP_ISPEED Specifies the input baud rate in bits per second. 129 TTY_OP_OSPEED Specifies the output baud rate in bits per second. 7. Summary of Message Numbers T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 15] INTERNET-DRAFT 6 August 1998 #define SSH_MSG_GLOBAL_REQUEST 80 #define SSH_MSG_REQUEST_SUCCESS 81 #define SSH_MSG_REQUEST_FAILURE 82 #define SSH_MSG_CHANNEL_OPEN 90 #define SSH_MSG_CHANNEL_OPEN_CONFIRMATION 91 #define SSH_MSG_CHANNEL_OPEN_FAILURE 92 #define SSH_MSG_CHANNEL_WINDOW_ADJUST 93 #define SSH_MSG_CHANNEL_DATA 94 #define SSH_MSG_CHANNEL_EXTENDED_DATA 95 #define SSH_MSG_CHANNEL_EOF 96 #define SSH_MSG_CHANNEL_CLOSE 97 #define SSH_MSG_CHANNEL_REQUEST 98 #define SSH_MSG_CHANNEL_SUCCESS 99 #define SSH_MSG_CHANNEL_FAILURE 100 8. Security Considerations This protocol is assumed to run on top of a secure, authenticated transport. User authentication and protection against network-level attacks are assumed to be provided by the underlying protocols. This protocol can, however, be used to execute commands on remote machines. The protocol also permits the server to run commands on the client. Implementations may wish to disallow this to prevent an attacker from coming from the server machine to the client machine. X11 forwarding provides major security improvements over normal cookie- based X11 forwarding. The cookie never needs to be transmitted in the clear, and traffic is encrypted and integrity-protected. No useful authentication data will remain on the server machine after the connection has been closed. On the other hand, in some situations a forwarded X11 connection might be used to get access to the local X server across security perimeters. Port forwardings can potentially allow an intruder to cross security perimeters such as firewalls. They do not offer anything fundamentally new that a user couldn't do otherwise; however, they make opening tunnels very easy. Implementations should allow policy control over what can be forwarded. Administrators should be able to deny forwardings where appropriate. Since this protocol normally runs inside an encrypted tunnel, firewalls will not be able to examine the traffic. It is RECOMMENDED that implementations disable all of the potentially dangerous features (e.g. agent forwarding, X11 forwarding, and TCP/IP forwarding) if the host key has changed. 9. References [RFC-1766] Alvestrand, H., "Tags for the Identification of Languages", March 1995. T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 16] INTERNET-DRAFT 6 August 1998 [RFC-1884] Hinden, R., and Deering, S., "IP Version 6 Addressing Architecture", December 1995 [RFC-2044] Yergeau, F., "UTF-8, a Transformation Format of Unicode and ISO 10646", October 1996. [SSH-ARCH] Ylonen, T., et al, "SSH Protocol Architecture", Internet Draft, draft-ietf-secsh-architecture-03.txt [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol", Internet Draft, draft-ietf-secsh-transport-05.txt [SSH-USERAUTH] Ylonen, T., et al, "SSH Authentication Protocol", Internet Draft, draft-ietf-secsh-userauth-05.txt 10. Authors' Addresses Tatu Ylonen SSH Communications Security Ltd. Tekniikantie 12 FIN-02150 ESPOO Finland E-mail: ylo@ssh.fi Tero Kivinen SSH Communications Security Ltd. Tekniikantie 12 FIN-02150 ESPOO Finland E-mail: kivinen@ssh.fi Markku-Juhani O. Saarinen SSH Communications Security Ltd. Tekniikantie 12 FIN-02150 ESPOO Finland E-mail: mjos@ssh.fi Timo J. Rinne SSH Communications Security Ltd. Tekniikantie 12 FIN-02150 ESPOO Finland E-mail: tri@ssh.fi Sami Lehtinen SSH Communications Security Ltd. Tekniikantie 12 FIN-02150 ESPOO Finland E-mail: sjl@ssh.fi T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 17]