| .\" -*- 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: openssh.1,v 1.1 1999/10/28 04:03:14 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/openssh/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/openssh/shosts.equiv , |
| and if additionally the server can verify the client's |
| host key (see |
| .Pa /etc/openssh/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/openssh/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/openssh/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/openssh/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/openssh/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/openssh/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/openssh/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/openssh/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/openssh/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/openssh/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/openssh/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 |