From 2056e015b79f9ac007dbdcd54b09fbb9b13541b7 Mon Sep 17 00:00:00 2001 From: basney Date: Wed, 30 Jul 2008 20:42:58 +0000 Subject: [PATCH] Removed in OpenSSH 5.1p1. --- openssh/RFC.nroff | 1780 --------------------------------------------- 1 file changed, 1780 deletions(-) delete mode 100644 openssh/RFC.nroff diff --git a/openssh/RFC.nroff b/openssh/RFC.nroff deleted file mode 100644 index d6baed6..0000000 --- a/openssh/RFC.nroff +++ /dev/null @@ -1,1780 +0,0 @@ -.\" -*- nroff -*- -.\" -.\" $OpenBSD: RFC.nroff,v 1.2 2000/10/16 09:38:44 djm Exp $ -.\" -.pl 10.0i -.po 0 -.ll 7.2i -.lt 7.2i -.nr LL 7.2i -.nr LT 7.2i -.ds LF Ylonen -.ds RF FORMFEED[Page %] -.ds CF -.ds LH Internet-Draft -.ds RH 15 November 1995 -.ds CH SSH (Secure Shell) Remote Login Protocol -.na -.hy 0 -.in 0 -Network Working Group T. Ylonen -Internet-Draft Helsinki University of Technology -draft-ylonen-ssh-protocol-00.txt 15 November 1995 -Expires: 15 May 1996 - -.in 3 - -.ce -The SSH (Secure Shell) Remote Login Protocol - -.ti 0 -Status of This Memo - -This document is an Internet-Draft. 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 docu- -ments at any time. It is inappropriate to use Internet-Drafts as -reference material or to cite them other than as ``work in pro- -gress.'' - -To learn the current status of any Internet-Draft, please check the -``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow -Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), -munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or -ftp.isi.edu (US West Coast). - -The distribution of this memo is unlimited. - -.ti 0 -Introduction - -SSH (Secure Shell) is a program to log into another computer over a -network, to execute commands in a remote machine, and to move files -from one machine to another. It provides strong authentication and -secure communications over insecure networks. Its features include -the following: -.IP o -Closes several security holes (e.g., IP, routing, and DNS spoofing). -New authentication methods: .rhosts together with RSA [RSA] based host -authentication, and pure RSA authentication. -.IP o -All communications are automatically and transparently encrypted. -Encryption is also used to protect integrity. -.IP o -X11 connection forwarding provides secure X11 sessions. -.IP o -Arbitrary TCP/IP ports can be redirected over the encrypted channel -in both directions. -.IP o -Client RSA-authenticates the server machine in the beginning of every -connection to prevent trojan horses (by routing or DNS spoofing) and -man-in-the-middle attacks, and the server RSA-authenticates the client -machine before accepting .rhosts or /etc/hosts.equiv authentication -(to prevent DNS, routing, or IP spoofing). -.IP o -An authentication agent, running in the user's local workstation or -laptop, can be used to hold the user's RSA authentication keys. -.RT - -The goal has been to make the software as easy to use as possible for -ordinary users. The protocol has been designed to be as secure as -possible while making it possible to create implementations that -are easy to use and install. The sample implementation has a number -of convenient features that are not described in this document as they -are not relevant for the protocol. - - -.ti 0 -Overview of the Protocol - -The software consists of a server program running on a server machine, -and a client program running on a client machine (plus a few auxiliary -programs). The machines are connected by an insecure IP [RFC0791] -network (that can be monitored, tampered with, and spoofed by hostile -parties). - -A connection is always initiated by the client side. The server -listens on a specific port waiting for connections. Many clients may -connect to the same server machine. - -The client and the server are connected via a TCP/IP [RFC0793] socket -that is used for bidirectional communication. Other types of -transport can be used but are currently not defined. - -When the client connects the server, the server accepts the connection -and responds by sending back its version identification string. The -client parses the server's identification, and sends its own -identification. The purpose of the identification strings is to -validate that the connection was to the correct port, declare the -protocol version number used, and to declare the software version used -on each side (for debugging purposes). The identification strings are -human-readable. If either side fails to understand or support the -other side's version, it closes the connection. - -After the protocol identification phase, both sides switch to a packet -based binary protocol. The server starts by sending its host key -(every host has an RSA key used to authenticate the host), server key -(an RSA key regenerated every hour), and other information to the -client. The client then generates a 256 bit session key, encrypts it -using both RSA keys (see below for details), and sends the encrypted -session key and selected cipher type to the server. Both sides then -turn on encryption using the selected algorithm and key. The server -sends an encrypted confirmation message to the client. - -The client then authenticates itself using any of a number of -authentication methods. The currently supported authentication -methods are .rhosts or /etc/hosts.equiv authentication (disabled by -default), the same with RSA-based host authentication, RSA -authentication, and password authentication. - -After successful authentication, the client makes a number of requests -to prepare for the session. Typical requests include allocating a -pseudo tty, starting X11 [X11] or TCP/IP port forwarding, starting -authentication agent forwarding, and executing the shell or a command. - -When a shell or command is executed, the connection enters interactive -session mode. In this mode, data is passed in both directions, -new forwarded connections may be opened, etc. The interactive session -normally terminates when the server sends the exit status of the -program to the client. - - -The protocol makes several reservations for future extensibility. -First of all, the initial protocol identification messages include the -protocol version number. Second, the first packet by both sides -includes a protocol flags field, which can be used to agree on -extensions in a compatible manner. Third, the authentication and -session preparation phases work so that the client sends requests to -the server, and the server responds with success or failure. If the -client sends a request that the server does not support, the server -simply returns failure for it. This permits compatible addition of -new authentication methods and preparation operations. The -interactive session phase, on the other hand, works asynchronously and -does not permit the use of any extensions (because there is no easy -and reliable way to signal rejection to the other side and problems -would be hard to debug). Any compatible extensions to this phase must -be agreed upon during any of the earlier phases. - -.ti 0 -The Binary Packet Protocol - -After the protocol identification strings, both sides only send -specially formatted packets. The packet layout is as follows: -.IP o -Packet length: 32 bit unsigned integer, coded as four 8-bit bytes, msb -first. Gives the length of the packet, not including the length field -and padding. The maximum length of a packet (not including the length -field and padding) is 262144 bytes. -.IP o -Padding: 1-8 bytes of random data (or zeroes if not encrypting). The -amount of padding is (8 - (length % 8)) bytes (where % stands for the -modulo operator). The rationale for always having some random padding -at the beginning of each packet is to make known plaintext attacks -more difficult. -.IP o -Packet type: 8-bit unsigned byte. The value 255 is reserved for -future extension. -.IP o -Data: binary data bytes, depending on the packet type. The number of -data bytes is the "length" field minus 5. -.IP o -Check bytes: 32-bit crc, four 8-bit bytes, msb first. The crc is the -Cyclic Redundancy Check, with the polynomial 0xedb88320, of the -Padding, Packet type, and Data fields. The crc is computed before -any encryption. -.RT - -The packet, except for the length field, may be encrypted using any of -a number of algorithms. The length of the encrypted part (Padding + -Type + Data + Check) is always a multiple of 8 bytes. Typically the -cipher is used in a chained mode, with all packets chained together as -if it was a single data stream (the length field is never included in -the encryption process). Details of encryption are described below. - -When the session starts, encryption is turned off. Encryption is -enabled after the client has sent the session key. The encryption -algorithm to use is selected by the client. - - -.ti 0 -Packet Compression - -If compression is supported (it is an optional feature, see -SSH_CMSG_REQUEST_COMPRESSION below), the packet type and data fields -of the packet are compressed using the gzip deflate algorithm [GZIP]. -If compression is in effect, the packet length field indicates the -length of the compressed data, plus 4 for the crc. The amount of -padding is computed from the compressed data, so that the amount of -data to be encrypted becomes a multiple of 8 bytes. - -When compressing, the packets (type + data portions) in each direction -are compressed as if they formed a continuous data stream, with only the -current compression block flushed between packets. This corresponds -to the GNU ZLIB library Z_PARTIAL_FLUSH option. The compression -dictionary is not flushed between packets. The two directions are -compressed independently of each other. - - -.ti 0 -Packet Encryption - -The protocol supports several encryption methods. During session -initialization, the server sends a bitmask of all encryption methods -that it supports, and the client selects one of these methods. The -client also generates a 256-bit random session key (32 8-bit bytes) and -sends it to the server. - -The encryption methods supported by the current implementation, and -their codes are: -.TS -center; -l r l. -SSH_CIPHER_NONE 0 No encryption -SSH_CIPHER_IDEA 1 IDEA in CFB mode -SSH_CIPHER_DES 2 DES in CBC mode -SSH_CIPHER_3DES 3 Triple-DES in CBC mode -SSH_CIPHER_TSS 4 An experimental stream cipher -SSH_CIPHER_RC4 5 RC4 -.TE - -All implementations are required to support SSH_CIPHER_DES and -SSH_CIPHER_3DES. Supporting SSH_CIPHER_IDEA, SSH_CIPHER_RC4, and -SSH_CIPHER_NONE is recommended. Support for SSH_CIPHER_TSS is -optional (and it is not described in this document). Other ciphers -may be added at a later time; support for them is optional. - -For encryption, the encrypted portion of the packet is considered a -linear byte stream. The length of the stream is always a multiple of -8. The encrypted portions of consecutive packets (in the same -direction) are encrypted as if they were a continuous buffer (that is, -any initialization vectors are passed from the previous packet to the -next packet). Data in each direction is encrypted independently. -.IP SSH_CIPHER_DES -The key is taken from the first 8 bytes of the session key. The least -significant bit of each byte is ignored. This results in 56 bits of -key data. DES [DES] is used in CBC mode. The iv (initialization vector) is -initialized to all zeroes. -.IP SSH_CIPHER_3DES -The variant of triple-DES used here works as follows: there are three -independent DES-CBC ciphers, with independent initialization vectors. -The data (the whole encrypted data stream) is first encrypted with the -first cipher, then decrypted with the second cipher, and finally -encrypted with the third cipher. All these operations are performed -in CBC mode. - -The key for the first cipher is taken from the first 8 bytes of the -session key; the key for the next cipher from the next 8 bytes, and -the key for the third cipher from the following 8 bytes. All three -initialization vectors are initialized to zero. - -(Note: the variant of 3DES used here differs from some other -descriptions.) -.IP SSH_CIPHER_IDEA -The key is taken from the first 16 bytes of the session key. IDEA -[IDEA] is used in CFB mode. The initialization vector is initialized -to all zeroes. -.IP SSH_CIPHER_TSS -All 32 bytes of the session key are used as the key. - -There is no reference available for the TSS algorithm; it is currently -only documented in the sample implementation source code. The -security of this cipher is unknown (but it is quite fast). The cipher -is basically a stream cipher that uses MD5 as a random number -generator and takes feedback from the data. -.IP SSH_CIPHER_RC4 -The first 16 bytes of the session key are used as the key for the -server to client direction. The remaining 16 bytes are used as the -key for the client to server direction. This gives independent -128-bit keys for each direction. - -This algorithm is the alleged RC4 cipher posted to the Usenet in 1995. -It is widely believed to be equivalent with the original RSADSI RC4 -cipher. This is a very fast algorithm. -.RT - - -.ti 0 -Data Type Encodings - -The Data field of each packet contains data encoded as described in -this section. There may be several data items; each item is coded as -described here, and their representations are concatenated together -(without any alignment or padding). - -Each data type is stored as follows: -.IP "8-bit byte" -The byte is stored directly as a single byte. -.IP "32-bit unsigned integer" -Stored in 4 bytes, msb first. -.IP "Arbitrary length binary string" -First 4 bytes are the length of the string, msb first (not including -the length itself). The following "length" bytes are the string -value. There are no terminating null characters. -.IP "Multiple-precision integer" -First 2 bytes are the number of bits in the integer, msb first (for -example, the value 0x00012345 would have 17 bits). The value zero has -zero bits. It is permissible that the number of bits be larger than the -real number of bits. - -The number of bits is followed by (bits + 7) / 8 bytes of binary data, -msb first, giving the value of the integer. -.RT - - -.ti 0 -TCP/IP Port Number and Other Options - -The server listens for connections on TCP/IP port 22. - -The client may connect the server from any port. However, if the -client wishes to use any form of .rhosts or /etc/hosts.equiv -authentication, it must connect from a privileged port (less than -1024). - -For the IP Type of Service field [RFC0791], it is recommended that -interactive sessions (those having a user terminal or forwarding X11 -connections) use the IPTOS_LOWDELAY, and non-interactive connections -use IPTOS_THROUGHPUT. - -It is recommended that keepalives are used, because otherwise programs -on the server may never notice if the other end of the connection is -rebooted. - - -.ti 0 -Protocol Version Identification - -After the socket is opened, the server sends an identification string, -which is of the form -"SSH-.-\\n", where - and are integers and specify the -protocol version number (not software distribution version). - is server side software version string (max 40 characters); -it is not interpreted by the remote side but may be useful for -debugging. - -The client parses the server's string, and sends a corresponding -string with its own information in response. If the server has lower -version number, and the client contains special code to emulate it, -the client responds with the lower number; otherwise it responds with -its own number. The server then compares the version number the -client sent with its own, and determines whether they can work -together. The server either disconnects, or sends the first packet -using the binary packet protocol and both sides start working -according to the lower of the protocol versions. - -By convention, changes which keep the protocol compatible with -previous versions keep the same major protocol version; changes that -are not compatible increment the major version (which will hopefully -never happen). The version described in this document is 1.3. - -The client will - -.ti 0 -Key Exchange and Server Host Authentication - -The first message sent by the server using the packet protocol is -SSH_SMSG_PUBLIC_KEY. It declares the server's host key, server public -key, supported ciphers, supported authentication methods, and flags -for protocol extensions. It also contains a 64-bit random number -(cookie) that must be returned in the client's reply (to make IP -spoofing more difficult). No encryption is used for this message. - -Both sides compute a session id as follows. The modulus of the server -key is interpreted as a byte string (without explicit length field, -with minimum length able to hold the whole value), most significant -byte first. This string is concatenated with the server host key -interpreted the same way. Additionally, the cookie is concatenated -with this. Both sides compute MD5 of the resulting string. The -resulting 16 bytes (128 bits) are stored by both parties and are -called the session id. - -The client responds with a SSH_CMSG_SESSION_KEY message, which -contains the selected cipher type, a copy of the 64-bit cookie sent by -the server, client's protocol flags, and a session key encrypted -with both the server's host key and server key. No encryption is used -for this message. - -The session key is 32 8-bit bytes (a total of 256 random bits -generated by the client). The client first xors the 16 bytes of the -session id with the first 16 bytes of the session key. The resulting -string is then encrypted using the smaller key (one with smaller -modulus), and the result is then encrypted using the other key. The -number of bits in the public modulus of the two keys must differ by at -least 128 bits. - -At each encryption step, a multiple-precision integer is constructed -from the data to be encrypted as follows (the integer is here -interpreted as a sequence of bytes, msb first; the number of bytes is -the number of bytes needed to represent the modulus). - -The most significant byte (which is only partial as the value must be -less than the public modulus, which is never a power of two) is zero. - -The next byte contains the value 2 (which stands for public-key -encrypted data in the PKCS standard [PKCS#1]). Then, there are -non-zero random bytes to fill any unused space, a zero byte, and the -data to be encrypted in the least significant bytes, the last byte of -the data in the least significant byte. - -This algorithm is used twice. First, it is used to encrypt the 32 -random bytes generated by the client to be used as the session key -(xored by the session id). This value is converted to an integer as -described above, and encrypted with RSA using the key with the smaller -modulus. The resulting integer is converted to a byte stream, msb -first. This byte stream is padded and encrypted identically using the -key with the larger modulus. - -After the client has sent the session key, it starts to use the -selected algorithm and key for decrypting any received packets, and -for encrypting any sent packets. Separate ciphers are used for -different directions (that is, both directions have separate -initialization vectors or other state for the ciphers). - -When the server has received the session key message, and has turned -on encryption, it sends a SSH_SMSG_SUCCESS message to the client. - -The recommended size of the host key is 1024 bits, and 768 bits for -the server key. The minimum size is 512 bits for the smaller key. - - -.ti 0 -Declaring the User Name - -The client then sends a SSH_CMSG_USER message to the server. This -message specifies the user name to log in as. - -The server validates that such a user exists, checks whether -authentication is needed, and responds with either SSH_SMSG_SUCCESS or -SSH_SMSG_FAILURE. SSH_SMSG_SUCCESS indicates that no authentication -is needed for this user (no password), and authentication phase has -now been completed. SSH_SMSG_FAILURE indicates that authentication is -needed (or the user does not exist). - -If the user does not exist, it is recommended that this returns -failure, but the server keeps reading messages from the client, and -responds to any messages (except SSH_MSG_DISCONNECT, SSH_MSG_IGNORE, -and SSH_MSG_DEBUG) with SSH_SMSG_FAILURE. This way the client cannot -be certain whether the user exists. - - -.ti 0 -Authentication Phase - -Provided the server didn't immediately accept the login, an -authentication exchange begins. The client sends messages to the -server requesting different types of authentication in arbitrary order as -many times as desired (however, the server may close the connection -after a timeout). The server always responds with SSH_SMSG_SUCCESS if -it has accepted the authentication, and with SSH_SMSG_FAILURE if it has -denied authentication with the requested method or it does not -recognize the message. Some authentication methods cause an exchange -of further messages before the final result is sent. The -authentication phase ends when the server responds with success. - -The recommended value for the authentication timeout (timeout before -disconnecting if no successful authentication has been made) is 5 -minutes. - -The following authentication methods are currently supported: -.TS -center; -l r l. -SSH_AUTH_RHOSTS 1 .rhosts or /etc/hosts.equiv -SSH_AUTH_RSA 2 pure RSA authentication -SSH_AUTH_PASSWORD 3 password authentication -SSH_AUTH_RHOSTS_RSA 4 .rhosts with RSA host authentication -.TE -.IP SSH_AUTH_RHOSTS - -This is the authentication method used by rlogin and rsh [RFC1282]. - -The client sends SSH_CMSG_AUTH_RHOSTS with the client-side user name -as an argument. - -The server checks whether to permit authentication. On UNIX systems, -this is usually done by checking /etc/hosts.equiv, and .rhosts in the -user's home directory. The connection must come from a privileged -port. - -It is recommended that the server checks that there are no IP options -(such as source routing) specified for the socket before accepting -this type of authentication. The client host name should be -reverse-mapped and then forward mapped to ensure that it has the -proper IP-address. - -This authentication method trusts the remote host (root on the remote -host can pretend to be any other user on that host), the name -services, and partially the network: anyone who can see packets coming -out from the server machine can do IP-spoofing and pretend to be any -machine; however, the protocol prevents blind IP-spoofing (which used -to be possible with rlogin). - -Many sites probably want to disable this authentication method because -of the fundamental insecurity of conventional .rhosts or -/etc/hosts.equiv authentication when faced with spoofing. It is -recommended that this method not be supported by the server by -default. -.IP SSH_AUTH_RHOSTS_RSA - -In addition to conventional .rhosts and hosts.equiv authentication, -this method additionally requires that the client host be -authenticated using RSA. - -The client sends SSH_CMSG_AUTH_RHOSTS_RSA specifying the client-side -user name, and the public host key of the client host. - -The server first checks if normal .rhosts or /etc/hosts.equiv -authentication would be accepted, and if not, responds with -SSH_SMSG_FAILURE. Otherwise, it checks whether it knows the host key -for the client machine (using the same name for the host that was used -for checking the .rhosts and /etc/hosts.equiv files). If it does not -know the RSA key for the client, access is denied and SSH_SMSG_FAILURE -is sent. - -If the server knows the host key of the client machine, it verifies -that the given host key matches that known for the client. If not, -access is denied and SSH_SMSG_FAILURE is sent. - -The server then sends a SSH_SMSG_AUTH_RSA_CHALLENGE message containing -an encrypted challenge for the client. The challenge is 32 8-bit -random bytes (256 bits). When encrypted, the highest (partial) byte -is left as zero, the next byte contains the value 2, the following are -non-zero random bytes, followed by a zero byte, and the challenge put -in the remaining bytes. This is then encrypted using RSA with the -client host's public key. (The padding and encryption algorithm is -the same as that used for the session key.) - -The client decrypts the challenge using its private host key, -concatenates this with the session id, and computes an MD5 checksum -of the resulting 48 bytes. The MD5 output is returned as 16 bytes in -a SSH_CMSG_AUTH_RSA_RESPONSE message. (MD5 is used to deter chosen -plaintext attacks against RSA; the session id binds it to a specific -session). - -The server verifies that the MD5 of the decrypted challenge returned by -the client matches that of the original value, and sends SSH_SMSG_SUCCESS if -so. Otherwise it sends SSH_SMSG_FAILURE and refuses the -authentication attempt. - -This authentication method trusts the client side machine in that root -on that machine can pretend to be any user on that machine. -Additionally, it trusts the client host key. The name and/or IP -address of the client host is only used to select the public host key. -The same host name is used when scanning .rhosts or /etc/hosts.equiv -and when selecting the host key. It would in principle be possible to -eliminate the host name entirely and substitute it directly by the -host key. IP and/or DNS [RFC1034] spoofing can only be used -to pretend to be a host for which the attacker has the private host -key. -.IP SSH_AUTH_RSA - -The idea behind RSA authentication is that the server recognizes the -public key offered by the client, generates a random challenge, and -encrypts the challenge with the public key. The client must then -prove that it has the corresponding private key by decrypting the -challenge. - -The client sends SSH_CMSG_AUTH_RSA with public key modulus (n) as an -argument. - -The server may respond immediately with SSH_SMSG_FAILURE if it does -not permit authentication with this key. Otherwise it generates a -challenge, encrypts it using the user's public key (stored on the -server and identified using the modulus), and sends -SSH_SMSG_AUTH_RSA_CHALLENGE with the challenge (mp-int) as an -argument. - -The challenge is 32 8-bit random bytes (256 bits). When encrypted, -the highest (partial) byte is left as zero, the next byte contains the -value 2, the following are non-zero random bytes, followed by a zero -byte, and the challenge put in the remaining bytes. This is then -encrypted with the public key. (The padding and encryption algorithm -is the same as that used for the session key.) - -The client decrypts the challenge using its private key, concatenates -it with the session id, and computes an MD5 checksum of the resulting -48 bytes. The MD5 output is returned as 16 bytes in a -SSH_CMSG_AUTH_RSA_RESPONSE message. (Note that the MD5 is necessary -to avoid chosen plaintext attacks against RSA; the session id binds it -to a specific session.) - -The server verifies that the MD5 of the decrypted challenge returned -by the client matches that of the original value, and sends -SSH_SMSG_SUCCESS if so. Otherwise it sends SSH_SMSG_FAILURE and -refuses the authentication attempt. - -This authentication method does not trust the remote host, the -network, name services, or anything else. Authentication is based -solely on the possession of the private identification keys. Anyone -in possession of the private keys can log in, but nobody else. - -The server may have additional requirements for a successful -authentiation. For example, to limit damage due to a compromised RSA -key, a server might restrict access to a limited set of hosts. -.IP SSH_AUTH_PASSWORD - -The client sends a SSH_CMSG_AUTH_PASSWORD message with the plain text -password. (Note that even though the password is plain text inside -the message, it is normally encrypted by the packet mechanism.) - -The server verifies the password, and sends SSH_SMSG_SUCCESS if -authentication was accepted and SSH_SMSG_FAILURE otherwise. - -Note that the password is read from the user by the client; the user -never interacts with a login program. - -This authentication method does not trust the remote host, the -network, name services or anything else. Authentication is based -solely on the possession of the password. Anyone in possession of the -password can log in, but nobody else. -.RT - -.ti 0 -Preparatory Operations - -After successful authentication, the server waits for a request from -the client, processes the request, and responds with SSH_SMSG_SUCCESS -whenever a request has been successfully processed. If it receives a -message that it does not recognize or it fails to honor a request, it -returns SSH_SMSG_FAILURE. It is expected that new message types might -be added to this phase in future. - -The following messages are currently defined for this phase. -.IP SSH_CMSG_REQUEST_COMPRESSION -Requests that compression be enabled for this session. A -gzip-compatible compression level (1-9) is passed as an argument. -.IP SSH_CMSG_REQUEST_PTY -Requests that a pseudo terminal device be allocated for this session. -The user terminal type and terminal modes are supplied as arguments. -.IP SSH_CMSG_X11_REQUEST_FORWARDING -Requests forwarding of X11 connections from the remote machine to the -local machine over the secure channel. Causes an internet-domain -socket to be allocated and the DISPLAY variable to be set on the server. -X11 authentication data is automatically passed to the server, and the -client may implement spoofing of authentication data for added -security. The authentication data is passed as arguments. -.IP SSH_CMSG_PORT_FORWARD_REQUEST -Requests forwarding of a TCP/IP port on the server host over the -secure channel. What happens is that whenever a connection is made to -the port on the server, a connection will be made from the client end -to the specified host/port. Any user can forward unprivileged ports; -only the root can forward privileged ports (as determined by -authentication done earlier). -.IP SSH_CMSG_AGENT_REQUEST_FORWARDING -Requests forwarding of the connection to the authentication agent. -.IP SSH_CMSG_EXEC_SHELL -Starts a shell (command interpreter) for the user, and moves into -interactive session mode. -.IP SSH_CMSG_EXEC_CMD -Executes the given command (actually " -c " or -equivalent) for the user, and moves into interactive session mode. -.RT - - -.ti 0 -Interactive Session and Exchange of Data - -During the interactive session, any data written by the shell or -command running on the server machine is forwarded to stdin or -stderr on the client machine, and any input available from stdin on -the client machine is forwarded to the program on the server machine. - -All exchange is asynchronous; either side can send at any time, and -there are no acknowledgements (TCP/IP already provides reliable -transport, and the packet protocol protects against tampering or IP -spoofing). - -When the client receives EOF from its standard input, it will send -SSH_CMSG_EOF; however, this in no way terminates the exchange. The -exchange terminates and interactive mode is left when the server sends -SSH_SMSG_EXITSTATUS to indicate that the client program has -terminated. Alternatively, either side may disconnect at any time by -sending SSH_MSG_DISCONNECT or closing the connection. - -The server may send any of the following messages: -.IP SSH_SMSG_STDOUT_DATA -Data written to stdout by the program running on the server. The data -is passed as a string argument. The client writes this data to -stdout. -.IP SSH_SMSG_STDERR_DATA -Data written to stderr by the program running on the server. The data -is passed as a string argument. The client writes this data to -stderr. (Note that if the program is running on a tty, it is not -possible to separate stdout and stderr data, and all data will be sent -as stdout data.) -.IP SSH_SMSG_EXITSTATUS -Indicates that the shell or command has exited. Exit status is passed -as an integer argument. This message causes termination of the -interactive session. -.IP SSH_SMSG_AGENT_OPEN -Indicates that someone on the server side is requesting a connection -to the authentication agent. The server-side channel number is passed -as an argument. The client must respond with either -SSH_CHANNEL_OPEN_CONFIRMATION or SSH_CHANNEL_OPEN_FAILURE. -.IP SSH_SMSG_X11_OPEN -Indicates that a connection has been made to the X11 socket on the -server side and should be forwarded to the real X server. An integer -argument indicates the channel number allocated for this connection on -the server side. The client should send back either -SSH_MSG_CHANNEL_OPEN_CONFIRMATION or SSH_MSG_CHANNEL_OPEN_FAILURE with -the same server side channel number. -.IP SSH_MSG_PORT_OPEN -Indicates that a connection has been made to a port on the server side -for which forwarding has been requested. Arguments are server side -channel number, host name to connect to, and port to connect to. The -client should send back either -SSH_MSG_CHANNEL_OPEN_CONFIRMATION or SSH_MSG_CHANNEL_OPEN_FAILURE with -the same server side channel number. -.IP SSH_MSG_CHANNEL_OPEN_CONFIRMATION -This is sent by the server to indicate that it has opened a connection -as requested in a previous message. The first argument indicates the -client side channel number, and the second argument is the channel number -that the server has allocated for this connection. -.IP SSH_MSG_CHANNEL_OPEN_FAILURE -This is sent by the server to indicate that it failed to open a -connection as requested in a previous message. The client-side -channel number is passed as an argument. The client will close the -descriptor associated with the channel and free the channel. -.IP SSH_MSG_CHANNEL_DATA -This packet contains data for a channel from the server. The first -argument is the client-side channel number, and the second argument (a -string) is the data. -.IP SSH_MSG_CHANNEL_CLOSE -This is sent by the server to indicate that whoever was in the other -end of the channel has closed it. The argument is the client side channel -number. The client will let all buffered data in the channel to -drain, and when ready, will close the socket, free the channel, and -send the server a SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message for the -channel. -.IP SSH_MSG_CHANNEL_CLOSE_CONFIRMATION -This is send by the server to indicate that a channel previously -closed by the client has now been closed on the server side as well. -The argument indicates the client channel number. The client frees -the channel. -.RT - -The client may send any of the following messages: -.IP SSH_CMSG_STDIN_DATA -This is data to be sent as input to the program running on the server. -The data is passed as a string. -.IP SSH_CMSG_EOF -Indicates that the client has encountered EOF while reading standard -input. The server will allow any buffered input data to drain, and -will then close the input to the program. -.IP SSH_CMSG_WINDOW_SIZE -Indicates that window size on the client has been changed. The server -updates the window size of the tty and causes SIGWINCH to be sent to -the program. The new window size is passed as four integer arguments: -row, col, xpixel, ypixel. -.IP SSH_MSG_PORT_OPEN -Indicates that a connection has been made to a port on the client side -for which forwarding has been requested. Arguments are client side -channel number, host name to connect to, and port to connect to. The -server should send back either SSH_MSG_CHANNEL_OPEN_CONFIRMATION or -SSH_MSG_CHANNEL_OPEN_FAILURE with the same client side channel number. -.IP SSH_MSG_CHANNEL_OPEN_CONFIRMATION -This is sent by the client to indicate that it has opened a connection -as requested in a previous message. The first argument indicates the -server side channel number, and the second argument is the channel -number that the client has allocated for this connection. -.IP SSH_MSG_CHANNEL_OPEN_FAILURE -This is sent by the client to indicate that it failed to open a -connection as requested in a previous message. The server side -channel number is passed as an argument. The server will close the -descriptor associated with the channel and free the channel. -.IP SSH_MSG_CHANNEL_DATA -This packet contains data for a channel from the client. The first -argument is the server side channel number, and the second argument (a -string) is the data. -.IP SSH_MSG_CHANNEL_CLOSE -This is sent by the client to indicate that whoever was in the other -end of the channel has closed it. The argument is the server channel -number. The server will allow buffered data to drain, and when ready, -will close the socket, free the channel, and send the client a -SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message for the channel. -.IP SSH_MSG_CHANNEL_CLOSE_CONFIRMATION -This is send by the client to indicate that a channel previously -closed by the server has now been closed on the client side as well. -The argument indicates the server channel number. The server frees -the channel. -.RT - -Any unsupported messages during interactive mode cause the connection -to be terminated with SSH_MSG_DISCONNECT and an error message. -Compatible protocol upgrades should agree about any extensions during -the preparation phase or earlier. - - -.ti 0 -Termination of the Connection - -Normal termination of the connection is always initiated by the server -by sending SSH_SMSG_EXITSTATUS after the program has exited. The -client responds to this message by sending SSH_CMSG_EXIT_CONFIRMATION -and closes the socket; the server then closes the socket. There are -two purposes for the confirmation: some systems may lose previously -sent data when the socket is closed, and closing the client side first -causes any TCP/IP TIME_WAIT [RFC0793] waits to occur on the client side, not -consuming server resources. - -If the program terminates due to a signal, the server will send -SSH_MSG_DISCONNECT with an appropriate message. If the connection is -closed, all file descriptors to the program will be closed and the -server will exit. If the program runs on a tty, the kernel sends it -the SIGHUP signal when the pty master side is closed. - -.ti 0 -Protocol Flags - -Both the server and the client pass 32 bits of protocol flags to the -other side. The flags are intended for compatible protocol extension; -the server first announces which added capabilities it supports, and -the client then sends the capabilities that it supports. - -The following flags are currently defined (the values are bit masks): -.IP "1 SSH_PROTOFLAG_SCREEN_NUMBER" -This flag can only be sent by the client. It indicates that the X11 -forwarding requests it sends will include the screen number. -.IP "2 SSH_PROTOFLAG_HOST_IN_FWD_OPEN" -If both sides specify this flag, SSH_SMSG_X11_OPEN and -SSH_MSG_PORT_OPEN messages will contain an additional field containing -a description of the host at the other end of the connection. -.RT - -.ti 0 -Detailed Description of Packet Types and Formats - -The supported packet types and the corresponding message numbers are -given in the following table. Messages with _MSG_ in their name may -be sent by either side. Messages with _CMSG_ are only sent by the -client, and messages with _SMSG_ only by the server. - -A packet may contain additional data after the arguments specified -below. Any such data should be ignored by the receiver. However, it -is recommended that no such data be stored without good reason. (This -helps build compatible extensions.) -.IP "0 SSH_MSG_NONE" -This code is reserved. This message type is never sent. -.IP "1 SSH_MSG_DISCONNECT" -.TS -; -l l. -string Cause of disconnection -.TE -This message may be sent by either party at any time. It causes the -immediate disconnection of the connection. The message is intended to -be displayed to a human, and describes the reason for disconnection. -.IP "2 SSH_SMSG_PUBLIC_KEY" -.TS -; -l l. -8 bytes anti_spoofing_cookie -32-bit int server_key_bits -mp-int server_key_public_exponent -mp-int server_key_public_modulus -32-bit int host_key_bits -mp-int host_key_public_exponent -mp-int host_key_public_modulus -32-bit int protocol_flags -32-bit int supported_ciphers_mask -32-bit int supported_authentications_mask -.TE -Sent as the first message by the server. This message gives the -server's host key, server key, protocol flags (intended for compatible -protocol extension), supported_ciphers_mask (which is the -bitwise or of (1 << cipher_number), where << is the left shift -operator, for all supported ciphers), and -supported_authentications_mask (which is the bitwise or of (1 << -authentication_type) for all supported authentication types). The -anti_spoofing_cookie is 64 random bytes, and must be sent back -verbatim by the client in its reply. It is used to make IP-spoofing -more difficult (encryption and host keys are the real defense against -spoofing). -.IP "3 SSH_CMSG_SESSION_KEY" -.TS -; -l l. -1 byte cipher_type (must be one of the supported values) -8 bytes anti_spoofing_cookie (must match data sent by the server) -mp-int double-encrypted session key -32-bit int protocol_flags -.TE -Sent by the client as the first message in the session. Selects the -cipher to use, and sends the encrypted session key to the server. The -anti_spoofing_cookie must be the same bytes that were sent by the -server. Protocol_flags is intended for negotiating compatible -protocol extensions. -.IP "4 SSH_CMSG_USER" -.TS -; -l l. -string user login name on server -.TE -Sent by the client to begin authentication. Specifies the user name -on the server to log in as. The server responds with SSH_SMSG_SUCCESS -if no authentication is needed for this user, or SSH_SMSG_FAILURE if -authentication is needed (or the user does not exist). [Note to the -implementator: the user name is of arbitrary size. The implementation -must be careful not to overflow internal buffers.] -.IP "5 SSH_CMSG_AUTH_RHOSTS" -.TS -; -l l. -string client-side user name -.TE -Requests authentication using /etc/hosts.equiv and .rhosts (or -equivalent mechanisms). This authentication method is normally -disabled in the server because it is not secure (but this is the -method used by rsh and rlogin). The server responds with -SSH_SMSG_SUCCESS if authentication was successful, and -SSH_SMSG_FAILURE if access was not granted. The server should check -that the client side port number is less than 1024 (a privileged -port), and immediately reject authentication if it is not. Supporting -this authentication method is optional. This method should normally -not be enabled in the server because it is not safe. (However, not -enabling this only helps if rlogind and rshd are disabled.) -.IP "6 SSH_CMSG_AUTH_RSA" -.TS -; -l l. -mp-int identity_public_modulus -.TE -Requests authentication using pure RSA authentication. The server -checks if the given key is permitted to log in, and if so, responds -with SSH_SMSG_AUTH_RSA_CHALLENGE. Otherwise, it responds with -SSH_SMSG_FAILURE. The client often tries several different keys in -sequence until one supported by the server is found. Authentication -is accepted if the client gives the correct response to the challenge. -The server is free to add other criteria for authentication, such as a -requirement that the connection must come from a certain host. Such -additions are not visible at the protocol level. Supporting this -authentication method is optional but recommended. -.IP "7 SSH_SMSG_AUTH_RSA_CHALLENGE" -.TS -; -l l. -mp-int encrypted challenge -.TE -Presents an RSA authentication challenge to the client. The challenge -is a 256-bit random value encrypted as described elsewhere in this -document. The client must decrypt the challenge using the RSA private -key, compute MD5 of the challenge plus session id, and send back the -resulting 16 bytes using SSH_CMSG_AUTH_RSA_RESPONSE. -.IP "8 SSH_CMSG_AUTH_RSA_RESPONSE" -.TS -; -l l. -16 bytes MD5 of decrypted challenge -.TE -This message is sent by the client in response to an RSA challenge. -The MD5 checksum is returned instead of the decrypted challenge to -deter known-plaintext attacks against the RSA key. The server -responds to this message with either SSH_SMSG_SUCCESS or -SSH_SMSG_FAILURE. -.IP "9 SSH_CMSG_AUTH_PASSWORD" -.TS -; -l l. -string plain text password -.TE -Requests password authentication using the given password. Note that -even though the password is plain text inside the packet, the whole -packet is normally encrypted by the packet layer. It would not be -possible for the client to perform password encryption/hashing, -because it cannot know which kind of encryption/hashing, if any, the -server uses. The server responds to this message with -SSH_SMSG_SUCCESS or SSH_SMSG_FAILURE. -.IP "10 SSH_CMSG_REQUEST_PTY" -.TS -; -l l. -string TERM environment variable value (e.g. vt100) -32-bit int terminal height, rows (e.g., 24) -32-bit int terminal width, columns (e.g., 80) -32-bit int terminal width, pixels (0 if no graphics) (e.g., 480) -32-bit int terminal height, pixels (0 if no graphics) (e.g., 640) -n bytes tty modes encoded in binary -.TE -Requests a pseudo-terminal to be allocated for this command. This -message can be used regardless of whether the session will later -execute the shell or a command. If a pty has been requested with this -message, the shell or command will run on a pty. Otherwise it will -communicate with the server using pipes, sockets or some other similar -mechanism. - -The terminal type gives the type of the user's terminal. In the UNIX -environment it is passed to the shell or command in the TERM -environment variable. - -The width and height values give the initial size of the user's -terminal or window. All values can be zero if not supported by the -operating system. The server will pass these values to the kernel if -supported. - -Terminal modes are encoded into a byte stream in a portable format. -The exact format is described later in this document. - -The server responds to the request with either SSH_SMSG_SUCCESS or -SSH_SMSG_FAILURE. If the server does not have the concept of pseudo -terminals, it should return success if it is possible to execute a -shell or a command so that it looks to the client as if it was running -on a pseudo terminal. -.IP "11 SSH_CMSG_WINDOW_SIZE" -.TS -; -l l. -32-bit int terminal height, rows -32-bit int terminal width, columns -32-bit int terminal width, pixels -32-bit int terminal height, pixels -.TE -This message can only be sent by the client during the interactive -session. This indicates that the size of the user's window has -changed, and provides the new size. The server will update the -kernel's notion of the window size, and a SIGWINCH signal or -equivalent will be sent to the shell or command (if supported by the -operating system). -.IP "12 SSH_CMSG_EXEC_SHELL" - -(no arguments) - -Starts a shell (command interpreter), and enters interactive session -mode. -.IP "13 SSH_CMSG_EXEC_CMD" -.TS -; -l l. -string command to execute -.TE -Starts executing the given command, and enters interactive session -mode. On UNIX, the command is run as " -c ", where - is the user's login shell. -.IP "14 SSH_SMSG_SUCCESS" - -(no arguments) - -This message is sent by the server in response to the session key, a -successful authentication request, and a successfully completed -preparatory operation. -.IP "15 SSH_SMSG_FAILURE" - -(no arguments) - -This message is sent by the server in response to a failed -authentication operation to indicate that the user has not yet been -successfully authenticated, and in response to a failed preparatory -operation. This is also sent in response to an authentication or -preparatory operation request that is not recognized or supported. -.IP "16 SSH_CMSG_STDIN_DATA" -.TS -; -l l. -string data -.TE -Delivers data from the client to be supplied as input to the shell or -program running on the server side. This message can only be used in -the interactive session mode. No acknowledgement is sent for this -message. -.IP "17 SSH_SMSG_STDOUT_DATA" -.TS -; -l l. -string data -.TE -Delivers data from the server that was read from the standard output of -the shell or program running on the server side. This message can -only be used in the interactive session mode. No acknowledgement is -sent for this message. -.IP "18 SSH_SMSG_STDERR_DATA" -.TS -; -l l. -string data -.TE -Delivers data from the server that was read from the standard error of -the shell or program running on the server side. This message can -only be used in the interactive session mode. No acknowledgement is -sent for this message. -.IP "19 SSH_CMSG_EOF" - -(no arguments) - -This message is sent by the client to indicate that EOF has been -reached on the input. Upon receiving this message, and after all -buffered input data has been sent to the shell or program, the server -will close the input file descriptor to the program. This message can -only be used in the interactive session mode. No acknowledgement is -sent for this message. -.IP "20 SSH_SMSG_EXITSTATUS" -.TS -; -l l. -32-bit int exit status of the command -.TE -Returns the exit status of the shell or program after it has exited. -The client should respond with SSH_CMSG_EXIT_CONFIRMATION when it has -received this message. This will be the last message sent by the -server. If the program being executed dies with a signal instead of -exiting normally, the server should terminate the session with -SSH_MSG_DISCONNECT (which can be used to pass a human-readable string -indicating that the program died due to a signal) instead of using -this message. -.IP "21 SSH_MSG_CHANNEL_OPEN_CONFIRMATION" -.TS -; -l l. -32-bit int remote_channel -32-bit int local_channel -.TE -This is sent in response to any channel open request if the channel -has been successfully opened. Remote_channel is the channel number -received in the initial open request; local_channel is the channel -number the side sending this message has allocated for the channel. -Data can be transmitted on the channel after this message. -.IP "22 SSH_MSG_CHANNEL_OPEN_FAILURE" -.TS -; -l l. -32-bit int remote_channel -.TE -This message indicates that an earlier channel open request by the -other side has failed or has been denied. Remote_channel is the -channel number given in the original request. -.IP "23 SSH_MSG_CHANNEL_DATA" -.TS -; -l l. -32-bit int remote_channel -string data -.TE -Data is transmitted in a channel in these messages. A channel is -bidirectional, and both sides can send these messages. There is no -acknowledgement for these messages. It is possible that either side -receives these messages after it has sent SSH_MSG_CHANNEL_CLOSE for -the channel. These messages cannot be received after the party has -sent or received SSH_MSG_CHANNEL_CLOSE_CONFIRMATION. -.IP "24 SSH_MSG_CHANNEL_CLOSE" -.TS -; -l l. -32-bit int remote_channel -.TE -When a channel is closed at one end of the connection, that side sends -this message. Upon receiving this message, the channel should be -closed. When this message is received, if the channel is already -closed (the receiving side has sent this message for the same channel -earlier), the channel is freed and no further action is taken; -otherwise the channel is freed and SSH_MSG_CHANNEL_CLOSE_CONFIRMATION -is sent in response. (It is possible that the channel is closed -simultaneously at both ends.) -.IP "25 SSH_MSG_CHANNEL_CLOSE_CONFIRMATION" -.TS -; -l l. -32-bit int remote_channel -.TE -This message is sent in response to SSH_MSG_CHANNEL_CLOSE unless the -channel was already closed. When this message is sent or received, -the channel is freed. -.IP "26 (OBSOLETED; was unix-domain X11 forwarding) -.IP "27 SSH_SMSG_X11_OPEN" -.TS -; -l l. -32-bit int local_channel -string originator_string (see below) -.TE -This message can be sent by the server during the interactive session -mode to indicate that a client has connected the fake X server. -Local_channel is the channel number that the server has allocated for -the connection. The client should try to open a connection to the -real X server, and respond with SSH_MSG_CHANNEL_OPEN_CONFIRMATION or -SSH_MSG_CHANNEL_OPEN_FAILURE. - -The field originator_string is present if both sides -specified SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags. It -contains a description of the host originating the connection. -.IP "28 SSH_CMSG_PORT_FORWARD_REQUEST" -.TS -; -l l. -32-bit int server_port -string host_to_connect -32-bit int port_to_connect -.TE -Sent by the client in the preparatory phase, this message requests -that server_port on the server machine be forwarded over the secure -channel to the client machine, and from there to the specified host -and port. The server should start listening on the port, and send -SSH_MSG_PORT_OPEN whenever a connection is made to it. Supporting -this message is optional, and the server is free to reject any forward -request. For example, it is highly recommended that unless the user -has been authenticated as root, forwarding any privileged port numbers -(below 1024) is denied. -.IP "29 SSH_MSG_PORT_OPEN" -.TS -; -l l. -32-bit int local_channel -string host_name -32-bit int port -string originator_string (see below) -.TE -Sent by either party in interactive session mode, this message -indicates that a connection has been opened to a forwarded TCP/IP -port. Local_channel is the channel number that the sending party has -allocated for the connection. Host_name is the host the connection -should be be forwarded to, and the port is the port on that host to -connect. The receiving party should open the connection, and respond -with SSH_MSG_CHANNEL_OPEN_CONFIRMATION or -SSH_MSG_CHANNEL_OPEN_FAILURE. It is recommended that the receiving -side check the host_name and port for validity to avoid compromising -local security by compromised remote side software. Particularly, it -is recommended that the client permit connections only to those ports -for which it has requested forwarding with SSH_CMSG_PORT_FORWARD_REQUEST. - -The field originator_string is present if both sides -specified SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags. It -contains a description of the host originating the connection. -.IP "30 SSH_CMSG_AGENT_REQUEST_FORWARDING" - -(no arguments) - -Requests that the connection to the authentication agent be forwarded -over the secure channel. The method used by clients to contact the -authentication agent within each machine is implementation and machine -dependent. If the server accepts this request, it should arrange that -any clients run from this session will actually contact the server -program when they try to contact the authentication agent. The server -should then send a SSH_SMSG_AGENT_OPEN to open a channel to the agent, -and the client should forward the connection to the real -authentication agent. Supporting this message is optional. -.IP "31 SSH_SMSG_AGENT_OPEN" -.TS -; -l l. -32-bit int local_channel -.TE -Sent by the server in interactive session mode, this message requests -opening a channel to the authentication agent. The client should open -a channel, and respond with either SSH_MSG_CHANNEL_OPEN_CONFIRMATION -or SSH_MSG_CHANNEL_OPEN_FAILURE. -.IP "32 SSH_MSG_IGNORE" -.TS -; -l l. -string data -.TE -Either party may send this message at any time. This message, and the -argument string, is silently ignored. This message might be used in -some implementations to make traffic analysis more difficult. This -message is not currently sent by the implementation, but all -implementations are required to recognize and ignore it. -.IP "33 SSH_CMSG_EXIT_CONFIRMATION" - -(no arguments) - -Sent by the client in response to SSH_SMSG_EXITSTATUS. This is the -last message sent by the client. -.IP "34 SSH_CMSG_X11_REQUEST_FORWARDING" -.TS -; -l l. -string x11_authentication_protocol -string x11_authentication_data -32-bit int screen number (if SSH_PROTOFLAG_SCREEN_NUMBER) -.TE -Sent by the client during the preparatory phase, this message requests -that the server create a fake X11 display and set the DISPLAY -environment variable accordingly. An internet-domain display is -preferable. The given authentication protocol and the associated data -should be recorded by the server so that it is used as authentication -on connections (e.g., in .Xauthority). The authentication protocol -must be one of the supported X11 authentication protocols, e.g., -"MIT-MAGIC-COOKIE-1". Authentication data must be a lowercase hex -string of even length. Its interpretation is protocol dependent. -The data is in a format that can be used with e.g. the xauth program. -Supporting this message is optional. - -The client is permitted (and recommended) to generate fake -authentication information and send fake information to the server. -This way, a corrupt server will not have access to the user's terminal -after the connection has terminated. The correct authorization codes -will also not be left hanging around in files on the server (many -users keep the same X session for months, thus protecting the -authorization data becomes important). - -X11 authentication spoofing works by initially sending fake (random) -authentication data to the server, and interpreting the first packet -sent by the X11 client after the connection has been opened. The -first packet contains the client's authentication. If the packet -contains the correct fake data, it is replaced by the client by the -correct authentication data, and then sent to the X server. -.IP "35 SSH_CMSG_AUTH_RHOSTS_RSA" -.TS -; -l l. -string clint-side user name -32-bit int client_host_key_bits -mp-int client_host_key_public_exponent -mp-int client_host_key_public_modulus -.TE -Requests authentication using /etc/hosts.equiv and .rhosts (or -equivalent) together with RSA host authentication. The server should -check that the client side port number is less than 1024 (a privileged -port), and immediately reject authentication if it is not. The server -responds with SSH_SMSG_FAILURE or SSH_SMSG_AUTH_RSA_CHALLENGE. The -client must respond to the challenge with the proper -SSH_CMSG_AUTH_RSA_RESPONSE. The server then responds with success if -access was granted, or failure if the client gave a wrong response. -Supporting this authentication method is optional but recommended in -most environments. -.IP "36 SSH_MSG_DEBUG" -.TS -; -l l. -string debugging message sent to the other side -.TE -This message may be sent by either party at any time. It is used to -send debugging messages that may be informative to the user in -solving various problems. For example, if authentication fails -because of some configuration error (e.g., incorrect permissions for -some file), it can be very helpful for the user to make the cause of -failure available. On the other hand, one should not make too much -information available for security reasons. It is recommended that -the client provides an option to display the debugging information -sent by the sender (the user probably does not want to see it by default). -The server can log debugging data sent by the client (if any). Either -party is free to ignore any received debugging data. Every -implementation must be able to receive this message, but no -implementation is required to send these. -.IP "37 SSH_CMSG_REQUEST_COMPRESSION" -.TS -; -l l. -32-bit int gzip compression level (1-9) -.TE -This message can be sent by the client in the preparatory operations -phase. The server responds with SSH_SMSG_FAILURE if it does not -support compression or does not want to compress; it responds with -SSH_SMSG_SUCCESS if it accepted the compression request. In the -latter case the response to this packet will still be uncompressed, -but all further packets in either direction will be compressed by gzip. -.RT - - -.ti 0 -Encoding of Terminal Modes - -Terminal modes (as passed in SSH_CMSG_REQUEST_PTY) 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 1-127 have one-byte arguments. Opcodes 128-159 have 32-bit -integer arguments (stored msb first). Opcodes 160-255 are not yet -defined, and cause parsing to stop (they should only be used after any -other data). - -The client puts in the stream any modes it knows about, and the server -ignores any modes it does not know about. This allows some degree of -machine-independence, at least between systems that use a POSIX-like -[POSIX] 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. -.IP "0 TTY_OP_END" -Indicates end of options. -.IP "1 VINTR" -Interrupt character; 255 if none. Similarly for the other characters. -Not all of these characters are supported on all systems. -.IP "2 VQUIT" -The quit character (sends SIGQUIT signal on UNIX systems). -.IP "3 VERASE" -Erase the character to left of the cursor. -.IP "4 VKILL" -Kill the current input line. -.IP "5 VEOF " -End-of-file character (sends EOF from the terminal). -.IP "6 VEOL " -End-of-line character in addition to carriage return and/or linefeed. -.IP "7 VEOL2" -Additional end-of-line character. -.IP "8 VSTART" -Continues paused output (normally ^Q). -.IP "9 VSTOP" -Pauses output (^S). -.IP "10 VSUSP" -Suspends the current program. -.IP "11 VDSUSP" -Another suspend character. -.IP "12 VREPRINT" -Reprints the current input line. -.IP "13 VWERASE" -Erases a word left of cursor. -.IP "14 VLNEXT" -More special input characters; these are probably not supported on -most systems. -.IP "15 VFLUSH" -.IP "16 VSWTCH" -.IP "17 VSTATUS" -.IP "18 VDISCARD" - -.IP "30 IGNPAR" -The ignore parity flag. The next byte should be 0 if this flag is not -set, and 1 if it is set. -.IP "31 PARMRK" -More flags. The exact definitions can be found in the POSIX standard. -.IP "32 INPCK" -.IP "33 ISTRIP" -.IP "34 INLCR" -.IP "35 IGNCR" -.IP "36 ICRNL" -.IP "37 IUCLC" -.IP "38 IXON" -.IP "39 IXANY" -.IP "40 IXOFF" -.IP "41 IMAXBEL" - -.IP "50 ISIG" -.IP "51 ICANON" -.IP "52 XCASE" -.IP "53 ECHO" -.IP "54 ECHOE" -.IP "55 ECHOK" -.IP "56 ECHONL" -.IP "57 NOFLSH" -.IP "58 TOSTOP" -.IP "59 IEXTEN" -.IP "60 ECHOCTL" -.IP "61 ECHOKE" -.IP "62 PENDIN" - -.IP "70 OPOST" -.IP "71 OLCUC" -.IP "72 ONLCR" -.IP "73 OCRNL" -.IP "74 ONOCR" -.IP "75 ONLRET" - -.IP "90 CS7" -.IP "91 CS8" -.IP "92 PARENB" -.IP "93 PARODD" - -.IP "192 TTY_OP_ISPEED" -Specifies the input baud rate in bits per second. -.IP "193 TTY_OP_OSPEED" -Specifies the output baud rate in bits per second. -.RT - - -.ti 0 -The Authentication Agent Protocol - -The authentication agent is a program that can be used to hold RSA -authentication keys for the user (in future, it might hold data for -other authentication types as well). An authorized program can send -requests to the agent to generate a proper response to an RSA -challenge. How the connection is made to the agent (or its -representative) inside a host and how access control is done inside a -host is implementation-dependent; however, how it is forwarded and how -one interacts with it is specified in this protocol. The connection -to the agent is normally automatically forwarded over the secure -channel. - -A program that wishes to use the agent first opens a connection to its -local representative (typically, the agent itself or an SSH server). -It then writes a request to the connection, and waits for response. -It is recommended that at least five minutes of timeout are provided -waiting for the agent to respond to an authentication challenge (this -gives sufficient time for the user to cut-and-paste the challenge to a -separate machine, perform the computation there, and cut-and-paste the -result back if so desired). - -Messages sent to and by the agent are in the following format: -.TS -; -l l. -4 bytes Length, msb first. Does not include length itself. -1 byte Packet type. The value 255 is reserved for future extensions. -data Any data, depending on packet type. Encoding as in the ssh packet -protocol. -.TE - -The following message types are currently defined: -.IP "1 SSH_AGENTC_REQUEST_RSA_IDENTITIES" - -(no arguments) - -Requests the agent to send a list of all RSA keys for which it can -answer a challenge. -.IP "2 SSH_AGENT_RSA_IDENTITIES_ANSWER" -.TS -; -l l. -32-bit int howmany -howmany times: -32-bit int bits -mp-int public exponent -mp-int public modulus -string comment -.TE -The agent sends this message in response to the to -SSH_AGENTC_REQUEST_RSA_IDENTITIES. The answer lists all RSA keys for -which the agent can answer a challenge. The comment field is intended -to help identify each key; it may be printed by an application to -indicate which key is being used. If the agent is not holding any -keys, howmany will be zero. -.IP "3 SSH_AGENTC_RSA_CHALLENGE -.TS -; -l l. -32-bit int bits -mp-int public exponent -mp-int public modulus -mp-int challenge -16 bytes session_id -32-bit int response_type -.TE -Requests RSA decryption of random challenge to authenticate the other -side. The challenge will be decrypted with the RSA private key -corresponding to the given public key. - -The decrypted challenge must contain a zero in the highest (partial) -byte, 2 in the next byte, followed by non-zero random bytes, a zero -byte, and then the real challenge value in the lowermost bytes. The -real challenge must be 32 8-bit bytes (256 bits). - -Response_type indicates the format of the response to be returned. -Currently the only supported value is 1, which means to compute MD5 of -the real challenge plus session id, and return the resulting 16 bytes -in a SSH_AGENT_RSA_RESPONSE message. -.IP "4 SSH_AGENT_RSA_RESPONSE" -.TS -; -l l. -16 bytes MD5 of decrypted challenge -.TE -Answers an RSA authentication challenge. The response is 16 bytes: -the MD5 checksum of the 32-byte challenge. -.IP "5 SSH_AGENT_FAILURE" - -(no arguments) - -This message is sent whenever the agent fails to answer a request -properly. For example, if the agent cannot answer a challenge (e.g., -no longer has the proper key), it can respond with this. The agent -also responds with this message if it receives a message it does not -recognize. -.IP "6 SSH_AGENT_SUCCESS" - -(no arguments) - -This message is sent by the agent as a response to certain requests -that do not otherwise cause a message be sent. Currently, this is -only sent in response to SSH_AGENTC_ADD_RSA_IDENTITY and -SSH_AGENTC_REMOVE_RSA_IDENTITY. -.IP "7 SSH_AGENTC_ADD_RSA_IDENTITY" -.TS -; -l l. -32-bit int bits -mp-int public modulus -mp-int public exponent -mp-int private exponent -mp-int multiplicative inverse of p mod q -mp-int p -mp-int q -string comment -.TE -Registers an RSA key with the agent. After this request, the agent can -use this RSA key to answer requests. The agent responds with -SSH_AGENT_SUCCESS or SSH_AGENT_FAILURE. -.IP "8 SSH_AGENT_REMOVE_RSA_IDENTITY" -.TS -; -l l. -32-bit int bits -mp-int public exponent -mp-int public modulus -.TE -Removes an RSA key from the agent. The agent will no longer accept -challenges for this key and will not list it as a supported identity. -The agent responds with SSH_AGENT_SUCCESS or SSH_AGENT_FAILURE. -.RT - -If the agent receives a message that it does not understand, it -responds with SSH_AGENT_FAILURE. This permits compatible future -extensions. - -It is possible that several clients have a connection open to the -authentication agent simultaneously. Each client will use a separate -connection (thus, any SSH connection can have multiple agent -connections active simultaneously). - - -.ti 0 -References - -.IP "[DES] " -FIPS PUB 46-1: Data Encryption Standard. National Bureau of -Standards, January 1988. FIPS PUB 81: DES Modes of Operation. -National Bureau of Standards, December 1980. Bruce Schneier: Applied -Cryptography. John Wiley & Sons, 1994. J. Seberry and J. Pieprzyk: -Cryptography: An Introduction to Computer Security. Prentice-Hall, -1989. -.IP "[GZIP] " -The GNU GZIP program; available for anonymous ftp at prep.ai.mit.edu. -Please let me know if you know a paper describing the algorithm. -.IP "[IDEA] " -Xuejia Lai: On the Design and Security of Block Ciphers, ETH Series in -Information Processing, vol. 1, Hartung-Gorre Verlag, Konstanz, -Switzerland, 1992. Bruce Schneier: Applied Cryptography, John Wiley & -Sons, 1994. See also the following patents: PCT/CH91/00117, EP 0 482 -154 B1, US Pat. 5,214,703. -.IP [PKCS#1] -PKCS #1: RSA Encryption Standard. Version 1.5, RSA Laboratories, -November 1993. Available for anonymous ftp at ftp.rsa.com. -.IP [POSIX] -Portable Operating System Interface (POSIX) - Part 1: Application -Program Interface (API) [C language], ISO/IEC 9945-1, IEEE Std 1003.1, -1990. -.IP [RFC0791] -J. Postel: Internet Protocol, RFC 791, USC/ISI, September 1981. -.IP [RFC0793] -J. Postel: Transmission Control Protocol, RFC 793, USC/ISI, September -1981. -.IP [RFC1034] -P. Mockapetris: Domain Names - Concepts and Facilities, RFC 1034, -USC/ISI, November 1987. -.IP [RFC1282] -B. Kantor: BSD Rlogin, RFC 1258, UCSD, December 1991. -.IP "[RSA] " -Bruce Schneier: Applied Cryptography. John Wiley & Sons, 1994. See -also R. Rivest, A. Shamir, and L. M. Adleman: Cryptographic -Communications System and Method. US Patent 4,405,829, 1983. -.IP "[X11] " -R. Scheifler: X Window System Protocol, X Consortium Standard, Version -11, Release 6. Massachusetts Institute of Technology, Laboratory of -Computer Science, 1994. -.RT - - -.ti 0 -Security Considerations - -This protocol deals with the very issue of user authentication and -security. - -First of all, as an implementation issue, the server program will have -to run as root (or equivalent) on the server machine. This is because -the server program will need be able to change to an arbitrary user -id. The server must also be able to create a privileged TCP/IP port. - -The client program will need to run as root if any variant of .rhosts -authentication is to be used. This is because the client program will -need to create a privileged port. The client host key is also usually -stored in a file which is readable by root only. The client needs the -host key in .rhosts authentication only. Root privileges can be -dropped as soon as the privileged port has been created and the host -key has been read. - -The SSH protocol offers major security advantages over existing telnet -and rlogin protocols. -.IP o -IP spoofing is restricted to closing a connection (by encryption, host -keys, and the special random cookie). If encryption is not used, IP -spoofing is possible for those who can hear packets going out from the -server. -.IP o -DNS spoofing is made ineffective (by host keys). -.IP o -Routing spoofing is made ineffective (by host keys). -.IP o -All data is encrypted with strong algorithms to make eavesdropping as -difficult as possible. This includes encrypting any authentication -information such as passwords. The information for decrypting session -keys is destroyed every hour. -.IP o -Strong authentication methods: .rhosts combined with RSA host -authentication, and pure RSA authentication. -.IP o -X11 connections and arbitrary TCP/IP ports can be forwarded securely. -.IP o -Man-in-the-middle attacks are deterred by using the server host key to -encrypt the session key. -.IP o -Trojan horses to catch a password by routing manipulation are deterred -by checking that the host key of the server machine matches that -stored on the client host. -.RT - -The security of SSH against man-in-the-middle attacks and the security -of the new form of .rhosts authentication, as well as server host -validation, depends on the integrity of the host key and the files -containing known host keys. - -The host key is normally stored in a root-readable file. If the host -key is compromised, it permits attackers to use IP, DNS and routing -spoofing as with current rlogin and rsh. It should never be any worse -than the current situation. - -The files containing known host keys are not sensitive. However, if an -attacker gets to modify the known host key files, it has the same -consequences as a compromised host key, because the attacker can then -change the recorded host key. - -The security improvements obtained by this protocol for X11 are of -particular significance. Previously, there has been no way to protect -data communicated between an X server and a client running on a remote -machine. By creating a fake display on the server, and forwarding all -X11 requests over the secure channel, SSH can be used to run any X11 -applications securely without any cooperation with the vendors of the -X server or the application. - -Finally, the security of this program relies on the strength of the -underlying cryptographic algorithms. The RSA algorithm is used for -authentication key exchange. It is widely believed to be secure. Of -the algorithms used to encrypt the session, DES has a rather small key -these days, probably permitting governments and organized criminals to -break it in very short time with specialized hardware. 3DES is -probably safe (but slower). IDEA is widely believed to be secure. -People have varying degrees of confidence in the other algorithms. -This program is not secure if used with no encryption at all. - - -.ti 0 -Additional Information - -Additional information (especially on the implementation and mailing -lists) is available via WWW at http://www.cs.hut.fi/ssh. - -Comments should be sent to Tatu Ylonen or the SSH -Mailing List . - -.ti 0 -Author's Address - -.TS -; -l. -Tatu Ylonen -Helsinki University of Technology -Otakaari 1 -FIN-02150 Espoo, Finland - -Phone: +358-0-451-3374 -Fax: +358-0-451-3293 -EMail: ylo@cs.hut.fi -.TE -- 2.45.1