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Diffstat (limited to 'ssh.1')
| -rw-r--r-- | ssh.1 | 966 |
1 files changed, 966 insertions, 0 deletions
@@ -0,0 +1,966 @@ +.\" -*- nroff -*- +.\" +.\" ssh.1.in +.\" +.\" Author: Tatu Ylonen <ylo@cs.hut.fi> +.\" +.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland +.\" All rights reserved +.\" +.\" Created: Sat Apr 22 21:55:14 1995 ylo +.\" +.\" $Id: ssh.1,v 1.1 1999/10/27 03:42:45 damien Exp $ +.\" +.Dd September 25, 1999 +.Dt SSH 1 +.Os +.Sh NAME +.Nm ssh +.Nd OpenSSH secure shell client (remote login program) +.Sh SYNOPSIS +.Nm ssh +.Op Fl l Ar login_name +.Op Ar hostname | user@hostname +.Op Ar command +.Pp +.Nm ssh +.Op Fl afgknqtvxCPX +.Op Fl c Ar blowfish | 3des +.Op Fl e Ar escape_char +.Op Fl i Ar identity_file +.Op Fl l Ar login_name +.Op Fl o Ar option +.Op Fl p Ar port +.Oo Fl L Xo +.Sm off +.Ar host : +.Ar port : +.Ar hostport +.Sm on +.Xc +.Oc +.Oo Fl R Xo +.Sm off +.Ar host : +.Ar port : +.Ar hostport +.Sm on +.Xc +.Oc +.Op Ar hostname | user@hostname +.Op Ar command +.Sh DESCRIPTION +.Nm +(Secure Shell) is a program for logging into a remote machine and for +executing commands on a remote machine. It is intended to replace +rlogin and rsh, and provide secure encrypted communications between +two untrusted hosts over an insecure network. X11 connections and +arbitrary TCP/IP ports can also be forwarded over the secure channel. +.Pp +.Nm +connects and logs into the specified +.Ar hostname . +The user must prove +his/her identity to the remote machine using one of several methods. +.Pp +First, if the machine the user logs in from is listed in +.Pa /etc/hosts.equiv +or +.Pa /etc/shosts.equiv +on the remote machine, and the user names are +the same on both sides, the user is immediately permitted to log in. +Second, if +.Pa \&.rhosts +or +.Pa \&.shosts +exists in the user's home directory on the +remote machine and contains a line containing the name of the client +machine and the name of the user on that machine, the user is +permitted to log in. This form of authentication alone is normally not +allowed by the server because it is not secure. +.Pp +The second (and primary) authentication method is the +.Pa rhosts +or +.Pa hosts.equiv +method combined with RSA-based host authentication. It +means that if the login would be permitted by +.Pa \&.rhosts , +.Pa \&.shosts , +.Pa /etc/hosts.equiv , +or +.Pa /etc/shosts.equiv , +and if additionally the server can verify the client's +host key (see +.Pa /etc/ssh_known_hosts +in the +.Sx FILES +section), only then login is +permitted. This authentication method closes security holes due to IP +spoofing, DNS spoofing and routing spoofing. [Note to the +administrator: +.Pa /etc/hosts.equiv , +.Pa \&.rhosts , +and the rlogin/rsh protocol in general, are inherently insecure and should be +disabled if security is desired.] +.Pp +As a third authentication method, +.Nm +supports RSA based authentication. +The scheme is based on public-key cryptography: there are cryptosystems +where encryption and decryption are done using separate keys, and it +is not possible to derive the decryption key from the encryption key. +RSA is one such system. The idea is that each user creates a public/private +key pair for authentication purposes. The +server knows the public key, and only the user knows the private key. +The file +.Pa $HOME/.ssh/authorized_keys +lists the public keys that are permitted for logging +in. When the user logs in, the +.Nm +program tells the server which key pair it would like to use for +authentication. The server checks if this key is permitted, and if +so, sends the user (actually the +.Nm +program running on behalf of the user) a challenge, a random number, +encrypted by the user's public key. The challenge can only be +decrypted using the proper private key. The user's client then decrypts the +challenge using the private key, proving that he/she knows the private +key but without disclosing it to the server. +.Pp +.Nm +implements the RSA authentication protocol automatically. The user +creates his/her RSA key pair by running +.Xr ssh-keygen 1 . +This stores the private key in +.Pa \&.ssh/identity +and the public key in +.Pa \&.ssh/identity.pub +in the user's home directory. The user should then +copy the +.Pa identity.pub +to +.Pa \&.ssh/authorized_keys +in his/her home directory on the remote machine (the +.Pa authorized_keys +file corresponds to the conventional +.Pa \&.rhosts +file, and has one key +per line, though the lines can be very long). After this, the user +can log in without giving the password. RSA authentication is much +more secure than rhosts authentication. +.Pp +The most convenient way to use RSA authentication may be with an +authentication agent. See +.Xr ssh-agent 1 +for more information. +.Pp +If other authentication methods fail, +.Nm +prompts the user for a password. The password is sent to the remote +host for checking; however, since all communications are encrypted, +the password cannot be seen by someone listening on the network. +.Pp +When the user's identity has been accepted by the server, the server +either executes the given command, or logs into the machine and gives +the user a normal shell on the remote machine. All communication with +the remote command or shell will be automatically encrypted. +.Pp +If a pseudo-terminal has been allocated (normal login session), the +user can disconnect with +.Ic ~. , +and suspend +.Nm +with +.Ic ~^Z . +All forwarded connections can be listed with +.Ic ~# +and if +the session blocks waiting for forwarded X11 or TCP/IP +connections to terminate, it can be backgrounded with +.Ic ~& +(this should not be used while the user shell is active, as it can cause the +shell to hang). All available escapes can be listed with +.Ic ~? . +.Pp +A single tilde character can be sent as +.Ic ~~ +(or by following the tilde by a character other than those described above). +The escape character must always follow a newline to be interpreted as +special. The escape character can be changed in configuration files +or on the command line. +.Pp +If no pseudo tty has been allocated, the +session is transparent and can be used to reliably transfer binary +data. On most systems, setting the escape character to +.Dq none +will also make the session transparent even if a tty is used. +.Pp +The session terminates when the command or shell in on the remote +machine exists and all X11 and TCP/IP connections have been closed. +The exit status of the remote program is returned as the exit status +of +.Nm ssh . +.Pp +If the user is using X11 (the +.Ev DISPLAY +environment variable is set), the connection to the X11 display is +automatically forwarded to the remote side in such a way that any X11 +programs started from the shell (or command) will go through the +encrypted channel, and the connection to the real X server will be made +from the local machine. The user should not manually set +.Ev DISPLAY . +Forwarding of X11 connections can be +configured on the command line or in configuration files. +.Pp +The +.Ev DISPLAY +value set by +.Nm +will point to the server machine, but with a display number greater +than zero. This is normal, and happens because +.Nm +creates a +.Dq proxy +X server on the server machine for forwarding the +connections over the encrypted channel. +.Pp +.Nm +will also automatically set up Xauthority data on the server machine. +For this purpose, it will generate a random authorization cookie, +store it in Xauthority on the server, and verify that any forwarded +connections carry this cookie and replace it by the real cookie when +the connection is opened. The real authentication cookie is never +sent to the server machine (and no cookies are sent in the plain). +.Pp +If the user is using an authentication agent, the connection to the agent +is automatically forwarded to the remote side unless disabled on +command line or in a configuration file. +.Pp +Forwarding of arbitrary TCP/IP connections over the secure channel can +be specified either on command line or in a configuration file. One +possible application of TCP/IP forwarding is a secure connection to an +electronic purse; another is going trough firewalls. +.Pp +.Nm +automatically maintains and checks a database containing RSA-based +identifications for all hosts it has ever been used with. The +database is stored in +.Pa \&.ssh/known_hosts +in the user's home directory. Additionally, the file +.Pa /etc/ssh_known_hosts +is automatically checked for known hosts. Any new hosts are +automatically added to the user's file. If a host's identification +ever changes, +.Nm +warns about this and disables password authentication to prevent a +trojan horse from getting the user's password. Another purpose of +this mechanism is to prevent man-in-the-middle attacks which could +otherwise be used to circumvent the encryption. The +.Cm StrictHostKeyChecking +option (see below) can be used to prevent logins to machines whose +host key is not known or has changed. +.Sh OPTIONS +.Bl -tag -width Ds +.It Fl a +Disables forwarding of the authentication agent connection. This may +also be specified on a per-host basis in the configuration file. +.It Fl c Ar blowfish|3des +Selects the cipher to use for encrypting the session. +.Ar 3des +is used by default. It is believed to be secure. +.Ar 3des +(triple-des) is an encrypt-decrypt-encrypt triple with three different keys. +It is presumably more secure than the +.Ar des +cipher which is no longer supported in ssh. +.Ar blowfish +is a fast block cipher, it appears very secure and is much faster than +.Ar 3des . +.It Fl e Ar ch|^ch|none +Sets the escape character for sessions with a pty (default: +.Ql ~ ) . +The escape character is only recognized at the beginning of a line. The +escape character followed by a dot +.Pq Ql \&. +closes the connection, followed +by control-Z suspends the connection, and followed by itself sends the +escape character once. Setting the character to +.Dq none +disables any escapes and makes the session fully transparent. +.It Fl f +Requests +.Nm +to go to background after authentication. This is useful +if +.Nm +is going to ask for passwords or passphrases, but the user +wants it in the background. This implies +.Fl n . +The recommended way to start X11 programs at a remote site is with +something like +.Ic ssh -f host xterm . +.It Fl i Ar identity_file +Selects the file from which the identity (private key) for +RSA authentication is read. Default is +.Pa \&.ssh/identity +in the user's home directory. Identity files may also be specified on +a per-host basis in the configuration file. It is possible to have +multiple +.Fl i +options (and multiple identities specified in +configuration files). +.It Fl g +Allows remote hosts to connect to local forwarded ports. +.It Fl k +Disables forwarding of Kerberos tickets and AFS tokens. This may +also be specified on a per-host basis in the configuration file. +.It Fl l Ar login_name +Specifies the user to log in as on the remote machine. This may also +be specified on a per-host basis in the configuration file. +.It Fl n +Redirects stdin from +.Pa /dev/null +(actually, prevents reading from stdin). +This must be used when +.Nm +is run in the background. A common trick is to use this to run X11 +programs in a remote machine. For example, +.Ic ssh -n shadows.cs.hut.fi emacs & +will start an emacs on shadows.cs.hut.fi, and the X11 +connection will be automatically forwarded over an encrypted channel. +The +.Nm +program will be put in the background. +(This does not work if +.Nm +needs to ask for a password or passphrase; see also the +.Fl f +option.) +.It Fl o Ar option +Can be used to give options in the format used in the config file. +This is useful for specifying options for which there is no separate +command-line flag. The option has the same format as a line in the +configuration file. +.It Fl p Ar port +Port to connect to on the remote host. This can be specified on a +per-host basis in the configuration file. +.It Fl P +Use a non-privileged port for outgoing connections. +This can be used if your firewall does +not permit connections from privileged ports. +Note that this option turns of +.Cm RhostsAuthentication +and +.Cm RhostsRSAAuthentication . +.It Fl q +Quiet mode. Causes all warning and diagnostic messages to be +suppressed. Only fatal errors are displayed. +.It Fl t +Force pseudo-tty allocation. This can be used to execute arbitary +screen-based programs on a remote machine, which can be very useful +e.g. when implementing menu services. +.It Fl v +Verbose mode. Causes +.Nm +to print debugging messages about its progress. This is helpful in +debugging connection, authentication, and configuration problems. +The verbose mode is also used to display +.Xr skey 1 +challenges, if the user entered "s/key" as password. +.It Fl x +Disables X11 forwarding. This can also be specified on a per-host +basis in a configuration file. +.It Fl X +Enables X11 forwarding. +.It Fl C +Requests compression of all data (including stdin, stdout, stderr, and +data for forwarded X11 and TCP/IP connections). The compression +algorithm is the same used by gzip, and the +.Dq level +can be controlled by the +.Cm CompressionLevel +option (see below). Compression is desirable on modem lines and other +slow connections, but will only slow down things on fast networks. +The default value can be set on a host-by-host basis in the +configuration files; see the +.Cm Compress +option below. +.It Fl L Ar port:host:hostport +Specifies that the given port on the local (client) host is to be +forwarded to the given host and port on the remote side. This works +by allocating a socket to listen to +.Ar port +on the local side, and whenever a connection is made to this port, the +connection is forwarded over the secure channel, and a connection is +made to +.Ar host:hostport +from the remote machine. Port forwardings can also be specified in the +configuration file. Only root can forward privileged ports. +.It Fl R Ar port:host:hostport +Specifies that the given port on the remote (server) host is to be +forwarded to the given host and port on the local side. This works +by allocating a socket to listen to +.Ar port +on the remote side, and whenever a connection is made to this port, the +connection is forwarded over the secure channel, and a connection is +made to +.Ar host:hostport +from the local machine. Port forwardings can also be specified in the +configuration file. Privileged ports can be forwarded only when +logging in as root on the remote machine. +.El +.Sh CONFIGURATION FILES +.Nm +obtains configuration data from the following sources (in this order): +command line options, user's configuration file +.Pq Pa $HOME/.ssh/config , +and system-wide configuration file +.Pq Pa /etc/ssh_config . +For each parameter, the first obtained value +will be used. The configuration files contain sections bracketed by +"Host" specifications, and that section is only applied for hosts that +match one of the patterns given in the specification. The matched +host name is the one given on the command line. +.Pp +Since the first obtained value for each parameter is used, more +host-specific declarations should be given near the beginning of the +file, and general defaults at the end. +.Pp +The configuration file has the following format: +.Pp +Empty lines and lines starting with +.Ql # +are comments. +.Pp +Otherwise a line is of the format +.Dq keyword arguments . +The possible +keywords and their meanings are as follows (note that the +configuration files are case-sensitive): +.Bl -tag -width Ds +.It Cm Host +Restricts the following declarations (up to the next +.Cm Host +keyword) to be only for those hosts that match one of the patterns +given after the keyword. +.Ql \&* +and +.Ql ? +can be used as wildcards in the +patterns. A single +.Ql \&* +as a pattern can be used to provide global +defaults for all hosts. The host is the +.Ar hostname +argument given on the command line (i.e., the name is not converted to +a canonicalized host name before matching). +.It Cm AFSTokenPassing +Specifies whether to pass AFS tokens to remote host. The argument to +this keyword must be +.Dq yes +or +.Dq no . +.It Cm BatchMode +If set to +.Dq yes , +passphrase/password querying will be disabled. This +option is useful in scripts and other batch jobs where you have no +user to supply the password. The argument must be +.Dq yes +or +.Dq no . +.It Cm Cipher +Specifies the cipher to use for encrypting the session. Currently, +.Dq blowfish , +and +.Dq 3des +are supported. The default is +.Dq 3des . +.It Cm Compression +Specifies whether to use compression. The argument must be +.Dq yes +or +.Dq no . +.It Cm CompressionLevel +Specifies the compression level to use if compression is enable. The +argument must be an integer from 1 (fast) to 9 (slow, best). The +default level is 6, which is good for most applications. The meaning +of the values is the same as in GNU GZIP. +.It Cm ConnectionAttempts +Specifies the number of tries (one per second) to make before falling +back to rsh or exiting. The argument must be an integer. This may be +useful in scripts if the connection sometimes fails. +.It Cm EscapeChar +Sets the escape character (default: +.Ql ~ ) . +The escape character can also +be set on the command line. The argument should be a single +character, +.Ql ^ +followed by a letter, or +.Dq none +to disable the escape +character entirely (making the connection transparent for binary +data). +.It Cm FallBackToRsh +Specifies that if connecting via +.Nm +fails due to a connection refused error (there is no +.Xr sshd 8 +listening on the remote host), +.Xr rsh 1 +should automatically be used instead (after a suitable warning about +the session being unencrypted). The argument must be +.Dq yes +or +.Dq no . +.It Cm ForwardAgent +Specifies whether the connection to the authentication agent (if any) +will be forwarded to the remote machine. The argument must be +.Dq yes +or +.Dq no . +.It Cm ForwardX11 +Specifies whether X11 connections will be automatically redirected +over the secure channel and +.Ev DISPLAY +set. The argument must be +.Dq yes +or +.Dq no . +.It Cm GatewayPorts +Specifies whether remote hosts are allowed to connect to local +forwarded ports. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +.It Cm GlobalKnownHostsFile +Specifies a file to use instead of +.Pa /etc/ssh_known_hosts . +.It Cm HostName +Specifies the real host name to log into. This can be used to specify +nicnames or abbreviations for hosts. Default is the name given on the +command line. Numeric IP addresses are also permitted (both on the +command line and in +.Cm HostName +specifications). +.It Cm IdentityFile +Specifies the file from which the user's RSA authentication identity +is read (default +.Pa .ssh/identity +in the user's home directory). +Additionally, any identities represented by the authentication agent +will be used for authentication. The file name may use the tilde +syntax to refer to a user's home directory. It is possible to have +multiple identity files specified in configuration files; all these +identities will be tried in sequence. +.It Cm KeepAlive +Specifies whether the system should send keepalive messages to the +other side. If they are sent, death of the connection or crash of one +of the machines will be properly noticed. However, this means that +connections will die if the route is down temporarily, and some people +find it annoying. +.Pp +The default is +.Dq yes +(to send keepalives), and the client will notice +if the network goes down or the remote host dies. This is important +in scripts, and many users want it too. +.Pp +To disable keepalives, the value should be set to +.Dq no +in both the server and the client configuration files. +.It Cm KerberosAuthentication +Specifies whether Kerberos authentication will be used. The argument to +this keyword must be +.Dq yes +or +.Dq no . +.It Cm KerberosTgtPassing +Specifies whether a Kerberos TGT will be forwarded to the server. This +will only work if the Kerberos server is actually an AFS kaserver. The +argument to this keyword must be +.Dq yes +or +.Dq no . +.It Cm LocalForward +Specifies that a TCP/IP port on the local machine be forwarded over +the secure channel to given host:port from the remote machine. The +first argument must be a port number, and the second must be +host:port. Multiple forwardings may be specified, and additional +forwardings can be given on the command line. Only the root can +forward privileged ports. +.It Cm PasswordAuthentication +Specifies whether to use password authentication. The argument to +this keyword must be +.Dq yes +or +.Dq no . +.It Cm NumberOfPasswordPrompts +Specifies the number of password prompts before giving up. The +argument to this keyword must be an integer. Default is 3. +.It Cm Port +Specifies the port number to connect on the remote host. Default is +22. +.It Cm ProxyCommand +Specifies the command to use to connect to the server. The command +string extends to the end of the line, and is executed with /bin/sh. +In the command string, %h will be substituted by the host name to +connect and %p by the port. The command can be basically anything, +and should read from its stdin and write to its stdout. It should +eventually connect an +.Xr sshd 8 +server running on some machine, or execute +.Ic sshd -i +somewhere. Host key management will be done using the +HostName of the host being connected (defaulting to the name typed by +the user). +.Pp +.It Cm RemoteForward +Specifies that a TCP/IP port on the remote machine be forwarded over +the secure channel to given host:port from the local machine. The +first argument must be a port number, and the second must be +host:port. Multiple forwardings may be specified, and additional +forwardings can be given on the command line. Only the root can +forward privileged ports. +.It Cm RhostsAuthentication +Specifies whether to try rhosts based authentication. Note that this +declaration only affects the client side and has no effect whatsoever +on security. Disabling rhosts authentication may reduce +authentication time on slow connections when rhosts authentication is +not used. Most servers do not permit RhostsAuthentication because it +is not secure (see RhostsRSAAuthentication). The argument to this +keyword must be +.Dq yes +or +.Dq no . +.It Cm RhostsRSAAuthentication +Specifies whether to try rhosts based authentication with RSA host +authentication. This is the primary authentication method for most +sites. The argument must be +.Dq yes +or +.Dq no . +.It Cm RSAAuthentication +Specifies whether to try RSA authentication. The argument to this +keyword must be +.Dq yes +or +.Dq no . +RSA authentication will only be +attempted if the identity file exists, or an authentication agent is +running. +.It Cm CheckHostIP +If this flag is set to +.Dq yes , +ssh will additionally check the host ip address in the +.Pa known_hosts +file. This allows ssh to detect if a host key changed due to DNS spoofing. +If the option is set to +.Dq no , +the check will not be executed. +.It Cm StrictHostKeyChecking +If this flag is set to +.Dq yes , +.Nm +ssh will never automatically add host keys to the +.Pa $HOME/.ssh/known_hosts +file, and refuses to connect hosts whose host key has changed. This +provides maximum protection against trojan horse attacks. However, it +can be somewhat annoying if you don't have good +.Pa /etc/ssh_known_hosts +files installed and frequently +connect new hosts. Basically this option forces the user to manually +add any new hosts. Normally this option is disabled, and new hosts +will automatically be added to the known host files. The host keys of +known hosts will be verified automatically in either case. The +argument must be +.Dq yes +or +.Dq no . +.It Cm User +Specifies the user to log in as. This can be useful if you have a +different user name in different machines. This saves the trouble of +having to remember to give the user name on the command line. +.It Cm UserKnownHostsFile +Specifies a file to use instead of +.Pa $HOME/.ssh/known_hosts . +.It Cm UsePrivilegedPort +Specifies whether to use a privileged port for outgoing connections. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq yes . +Note that setting this option to +.Dq no +turns of +.Cm RhostsAuthentication +and +.Cm RhostsRSAAuthentication . +.It Cm UseRsh +Specifies that rlogin/rsh should be used for this host. It is +possible that the host does not at all support the +.Nm +protocol. This causes +.Nm +to immediately exec +.Xr rsh 1 . +All other options (except +.Cm HostName ) +are ignored if this has been specified. The argument must be +.Dq yes +or +.Dq no . +.Sh ENVIRONMENT +.Nm +will normally set the following environment variables: +.Bl -tag -width Ds +.It Ev DISPLAY +The +.Ev DISPLAY +variable indicates the location of the X11 server. It is +automatically set by +.Nm +to point to a value of the form +.Dq hostname:n +where hostname indicates +the host where the shell runs, and n is an integer >= 1. Ssh uses +this special value to forward X11 connections over the secure +channel. The user should normally not set DISPLAY explicitly, as that +will render the X11 connection insecure (and will require the user to +manually copy any required authorization cookies). +.It Ev HOME +Set to the path of the user's home directory. +.It Ev LOGNAME +Synonym for +.Ev USER ; +set for compatibility with systems that use this variable. +.It Ev MAIL +Set to point the user's mailbox. +.It Ev PATH +Set to the default +.Ev PATH , +as specified when compiling +.Nm ssh . +.It Ev SSH_AUTH_SOCK +indicates the path of a unix-domain socket used to communicate with the +agent. +.It Ev SSH_CLIENT +Identifies the client end of the connection. The variable contains +three space-separated values: client ip-address, client port number, +and server port number. +.It Ev SSH_TTY +This is set to the name of the tty (path to the device) associated +with the current shell or command. If the current session has no tty, +this variable is not set. +.It Ev TZ +The timezone variable is set to indicate the present timezone if it +was set when the daemon was started (e.i., the daemon passes the value +on to new connections). +.It Ev USER +Set to the name of the user logging in. +.El +.Pp +Additionally, +.Nm +reads +.Pa $HOME/.ssh/environment , +and adds lines of the format +.Dq VARNAME=value +to the environment. +.Sh FILES +.Bl -tag -width $HOME/.ssh/known_hosts +.It Pa $HOME/.ssh/known_hosts +Records host keys for all hosts the user has logged into (that are not +in +.Pa /etc/ssh_known_hosts ) . +See +.Xr sshd 8 . +.It Pa $HOME/.ssh/random_seed +Used for seeding the random number generator. This file contains +sensitive data and should read/write for the user and not accessible +for others. This file is created the first time the program is run +and updated automatically. The user should never need to read or +modify this file. +.It Pa $HOME/.ssh/identity +Contains the RSA authentication identity of the user. This file +contains sensitive data and should be readable by the user but not +accessible by others (read/write/execute). +Note that +.Nm +ignores this file if it is accessible by others. +It is possible to specify a passphrase when +generating the key; the passphrase will be used to encrypt the +sensitive part of this file using 3DES. +.It Pa $HOME/.ssh/identity.pub +Contains the public key for authentication (public part of the +identity file in human-readable form). The contents of this file +should be added to +.Pa $HOME/.ssh/authorized_keys +on all machines +where you wish to log in using RSA authentication. This file is not +sensitive and can (but need not) be readable by anyone. This file is +never used automatically and is not necessary; it is only provided for +the convenience of the user. +.It Pa $HOME/.ssh/config +This is the per-user configuration file. The format of this file is +described above. This file is used by the +.Nm +client. This file does not usually contain any sensitive information, +but the recommended permissions are read/write for the user, and not +accessible by others. +.It Pa $HOME/.ssh/authorized_keys +Lists the RSA keys that can be used for logging in as this user. The +format of this file is described in the +.Xr sshd 8 +manual page. In the simplest form the format is the same as the .pub +identity files (that is, each line contains the number of bits in +modulus, public exponent, modulus, and comment fields, separated by +spaces). This file is not highly sensitive, but the recommended +permissions are read/write for the user, and not accessible by others. +.It Pa /etc/ssh_known_hosts +Systemwide list of known host keys. This file should be prepared by the +system administrator to contain the public host keys of all machines in the +organization. This file should be world-readable. This file contains +public keys, one per line, in the following format (fields separated +by spaces): system name, number of bits in modulus, public exponent, +modulus, and optional comment field. When different names are used +for the same machine, all such names should be listed, separated by +commas. The format is described on the +.Xr sshd 8 +manual page. +.Pp +The canonical system name (as returned by name servers) is used by +.Xr sshd 8 +to verify the client host when logging in; other names are needed because +.Nm +does not convert the user-supplied name to a canonical name before +checking the key, because someone with access to the name servers +would then be able to fool host authentication. +.It Pa /etc/ssh_config +Systemwide configuration file. This file provides defaults for those +values that are not specified in the user's configuration file, and +for those users who do not have a configuration file. This file must +be world-readable. +.It Pa $HOME/.rhosts +This file is used in +.Pa \&.rhosts +authentication to list the +host/user pairs that are permitted to log in. (Note that this file is +also used by rlogin and rsh, which makes using this file insecure.) +Each line of the file contains a host name (in the canonical form +returned by name servers), and then a user name on that host, +separated by a space. One some machines this file may need to be +world-readable if the user's home directory is on a NFS partition, +because +.Xr sshd 8 +reads it as root. Additionally, this file must be owned by the user, +and must not have write permissions for anyone else. The recommended +permission for most machines is read/write for the user, and not +accessible by others. +.Pp +Note that by default +.Xr sshd 8 +will be installed so that it requires successful RSA host +authentication before permitting \s+2.\s0rhosts authentication. If your +server machine does not have the client's host key in +.Pa /etc/ssh_known_hosts , +you can store it in +.Pa $HOME/.ssh/known_hosts . +The easiest way to do this is to +connect back to the client from the server machine using ssh; this +will automatically add the host key inxi +.Pa $HOME/.ssh/known_hosts . +.It Pa $HOME/.shosts +This file is used exactly the same way as +.Pa \&.rhosts . +The purpose for +having this file is to be able to use rhosts authentication with +.Nm +without permitting login with +.Xr rlogin 1 +or +.Xr rsh 1 . +.It Pa /etc/hosts.equiv +This file is used during +.Pa \&.rhosts authentication. It contains +canonical hosts names, one per line (the full format is described on +the +.Xr sshd 8 +manual page). If the client host is found in this file, login is +automatically permitted provided client and server user names are the +same. Additionally, successful RSA host authentication is normally +required. This file should only be writable by root. +.It Pa /etc/shosts.equiv +This file is processed exactly as +.Pa /etc/hosts.equiv . +This file may be useful to permit logins using +.Nm +but not using rsh/rlogin. +.It Pa /etc/sshrc +Commands in this file are executed by +.Nm +when the user logs in just before the user's shell (or command) is started. +See the +.Xr sshd 8 +manual page for more information. +.It Pa $HOME/.ssh/rc +Commands in this file are executed by +.Nm +when the user logs in just before the user's shell (or command) is +started. +See the +.Xr sshd 8 +manual page for more information. +.It Pa libcrypto.so.X.1 +A version of this library which includes support for the RSA algorithm +is required for proper operation. +.Sh AUTHOR +Tatu Ylonen <ylo@cs.hut.fi> +.Pp +Issues can be found from the SSH WWW home page: +.Pp +.Dl http://www.cs.hut.fi/ssh +.Pp +OpenSSH +is a derivative of the original (free) ssh 1.2.12 release, but with bugs +removed and newer features re-added. Rapidly after the 1.2.12 release, +newer versions bore successively more restrictive licenses. This version +of OpenSSH +.Bl -bullet +.It +has all components of a restrictive nature (ie. patents, see +.Xr ssl 8 ) +directly removed from the source code; any licensed or patented components +are chosen from +external libraries. +.It +has been updated to support ssh protocol 1.5. +.It +contains added support for +.Xr kerberos 8 +authentication and ticket passing. +.It +supports one-time password authentication with +.Xr skey 1 . +.El +.Pp +The libraries described in +.Xr ssl 8 +are required for proper operation. +.Sh SEE ALSO +.Xr rlogin 1 , +.Xr rsh 1 , +.Xr scp 1 , +.Xr ssh-add 1 , +.Xr ssh-agent 1 , +.Xr ssh-keygen 1 , +.Xr telnet 1 , +.Xr sshd 8 , +.Xr ssl 8 |