sshd [-deiqtD46] [-b bits] [-f config_file] [-g login_grace_time] [-h host_key_file] [-k key_gen_time] [-p port] [-V client_protocol_id]
The sshd (Secure Shell daemon) is the daemon program for ssh(1). Together these programs replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. The programs are intended to be as easy to install and use as possible.
sshd is the daemon that listens for connections from clients. It forks a new daemon for each incoming connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange.
This implementation of sshd supports both SSH protocol versions 1 and 2 simultaneously. Because of security weaknesses in the v1 protocol, sites should run only v2, if possible. In the default configuration, only protocol v2 is enabled for the server. To enable v1 and v2 simultaneously, see the instructions in sshd_config(4).
Support for v1 is provided to help sites with existing ssh v1 clients and servers to transition to v2. v1 might not be supported in a future release.
Each host has a host-specific RSA key (normally 1024 bits) used to identify the host. Additionally, when the daemon starts, it generates a server RSA key (normally 768 bits). This key is normally regenerated every hour if it has been used, and is never stored on disk.
Whenever a client connects the daemon responds with its public host and server keys. The client compares the RSA host key against its own database to verify that it has not changed. The client then generates a 256–bit random number. It encrypts this random number using both the host key and the server key, and sends the encrypted number to the server. Both sides then use this random number as a session key which is used to encrypt all further communications in the session. The rest of the session is encrypted using a conventional cipher, currently Blowfish or 3DES, with 3DES being used by default. The client selects the encryption algorithm to use from those offered by the server.
Next, the server and the client enter an authentication dialog. The client tries to authenticate itself using .rhosts authentication, .rhosts authentication combined with RSA host authentication, RSA challenge-response authentication, or password-based authentication.
Rhosts authentication is normally disabled because it is fundamentally insecure, but can be enabled in the server configuration file if desired. System security is not improved unless rshd(1M), rlogind(1M), rexecd(1M), and rexd(1M) are disabled (thus completely disabling rlogin(1) and rsh(1) into the machine).
Version 2 works similarly to version 1: Each host has a host-specific DSA/RSA key. However, when the daemon starts, it does not generate a server key. Forward security is provided through a Diffie-Hellman key agreement. This key agreement results in a shared session key. The rest of the session is encrypted using a symmetric cipher, currently 128-bit AES, Blowfish, 3DES, or AES. The client selects the encryption algorithm to use from those offered by the server. Additionally, session integrity is provided through a cryptographic message authentication code (hmac-sha1 or hmac-md5).
Protocol version 2 provides a public key based user authentication method (PubKeyAuthentication) GSS–API based user authentication, conventional password authentication, and a generic prompt/reply protocol for password-based authentication.
If the client successfully authenticates itself, a dialog for preparing the session is entered. At this time the client can request things like allocating a pseudo-tty, forwarding X11 connections, forwarding TCP/IP connections, or forwarding the authentication agent connection over the secure channel.
Finally, the client either requests a shell or execution of a command. The sides then enter session mode. In this mode, either side may send data at any time, and such data is forwarded to/from the shell or command on the server side, and the user terminal on the client side.
When the user program terminates and all forwarded X11 and other connections have been closed, the server sends command exit status to the client, and both sides exit.
sshd can be configured using command-line options or the configuration file /etc/ssh/ssh_config, described in ssh_config(4). Command-line options override values specified in the configuration file.
sshd rereads its configuration file when it receives a hangup signal, SIGHUP, by executing itself with the name it was started as, that is, /usr/lib/ssh/sshd.
The sshd daemon uses TCP Wrappers to restrict access to hosts. It uses the service name of sshd for hosts_access(). For more information on TCP Wrappers see tcpd(1M) and hosts_access(3) man pages, which are part of the SUNWsfman package (they are not SunOS man pages). TCP wrappers binaries, including libwrap, are in SUNWtcpd, a required package for SUNWsshdu, the package containing sshd.
The options for sshd are as follows:
Specifies the number of bits in the server key (the default is 768).
Debug mode. The server sends verbose debug output to the system log, and does not put itself in the background. The server also will not fork and will only process one connection. This option is only intended for debugging for the server. Multiple -d options increase the debugging level. Maximum is 3.
When this option is specified, sshd will send the output to standard error instead of to the system log.
Specifies the name of the configuration file. The default is /etc/ssh/sshd_config. sshd refuses to start if there is no configuration file.
Gives the grace time for clients to authenticate themselves (the default is 300 seconds). If the client fails to authenticate the user within this number of seconds, the server disconnects and exits. A value of zero indicates no limit.
Specifies a file from which a host key is read. This option must be given if sshd is not run as root (as the normal host key files are normally not readable by anyone but root). The default is /etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for protocol version 2. It is possible to have multiple host key files for the different protocol versions and host key algorithms.
Specifies that sshd is being run from inetd. sshd is normally not run from inetd because it needs to generate the server key before it can respond to the client, and this may take tens of seconds. Clients would have to wait too long if the key was regenerated every time. However, with small key sizes (for example, 512) using sshd from inetd may be reasonable.
(SSHv1-specific) Specifies how often the server key is regenerated (the default is 3600 seconds, or one hour). The motivation for regenerating the key fairly often is that the key is not stored anywhere, and after about an hour, it becomes impossible to recover the key for decrypting intercepted communications even if the machine is cracked into or physically seized. A value of zero indicates that the key will never be regenerated.
Can be used to specify options in the format used in the configuration file. This is useful for specifying options for which there are no separate command-line flags.
Specifies the port on which the server listens for connections (the default is 22).
Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection is logged.
Test mode. Check only the validity of the configuration file and the sanity of the keys. This is useful for updating sshd reliably as configuration options might change.
When this option is specified sshd does not detach and does not become a daemon. This allows easy monitoring of sshd.
Forces sshd to use IPv4 addresses only.
Forces sshd to use IPv6 addresses only.
The $HOME/.ssh/authorized_keys file lists the public keys that are permitted for RSA authentication in protocol version 1 and for public key authentication (PubkeyAuthentication) in protocol version 2. The AuthorizedKeysFile configuration option can be used to specify an alternative file.
Each line of the file contains one key (empty lines and lines starting with a hash mark [#] are ignored as comments).
For each RSA key for protocol version 1, the file consists of the following space-separated fields:
options bits exponent modulus comment
For the public key for protocol version 2, the file consists of the following space-separated fields:
options key-type base64-encoding-key comment
For protocol version 2, key-type is one of ssh-rsa or ssh-dsa.
The options field is optional; its presence is determined by whether the line starts with a number. (The option field never starts with a number.) The bits, exponent, and modulus fields give the RSA key; the comment field is a convenient place for you to identify the key.
Lines in this file are usually several hundred bytes long (because of the size of the key modulus). You will find it very inconvenient to type them in; instead, copy the public key file and edit it.
Permissions of this file must be set so that it is not world or group writable. See the StrictModes option of sshd_config(4).
The options (if present) consist of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported:
Specifies that, in addition to public key authentication, the canonical name of the remote host must be present in the comma-separated list of patterns (`*' and `?' serve as wildcards). The list can also contain negated patterns by prefixing the patterns with `!'. If the canonical host name matches a negated pattern, the key is not accepted.
The purpose of this option is to give you the option of increasing security: public key authentication by itself does not trust the network or name servers or anything but the key. However, if someone manages to steal the key, possession of the key would permit the intruder to log in from anywhere in the world. This option makes using a stolen key more difficult, because name servers and routers would have to be compromised, in addition to just the key.
Specifies that the command is executed whenever this key is used for authentication. The command supplied by the user (if any) is ignored. The command is run on a pty if the client requests a pty; otherwise it is run without a tty. If an 8-bit clean channel is required, one must not request a pty or should specify no-pty. You can include a quote in the command by escaping it with a backslash. This option might be useful to restrict certain public keys from performing a specific operation. An example is a key that permits remote backups but nothing else. Note that the client can specify TCP/IP and/or X11 forwarding unless they are explicitly prohibited from doing so. Also note that this option applies to shell, command, or subsystem execution.
Specifies that the string NAME=value is to be added to the environment when logging in using this key. Environment variables set this way override other default environment values. Multiple options of this type are permitted. Environment processing is disabled by default and is controlled via the PermitUserEnvironment option.
Forbids TCP/IP forwarding when this key is used for authentication. Any port forward requests by the client will return an error. This might be used, for example, in connection with the command option.
Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error.
Forbids authentication agent forwarding when this key is used for authentication.
Prevents tty allocation (a request to allocate a pty will fail).
Limit local ssh -L port forwarding such that it can connect only to the specified host and port. IPv6 addresses can be specified with an alternative syntax: host/port. You can invoke multiple permitopen options, with each instance separated by a comma. No pattern matching is performed on the specified hostnames. They must be literal domains or addresses.
The /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts files contain host public keys for all known hosts. The global file should be prepared by the administrator (optional), and the per-user file is maintained automatically: whenever the user connects from an unknown host its key is added to the per-user file.
For the RSA key for protocol version 1, these files consist of the following space-separated fields:
hostnames bits exponent modulus comment
For the public key for protocol version 2, these files consist of the following space-separated fields:
hostnames key-type base64-encoding-key comment
For protocol version 2, key-type is one of ssh-rsa or ssh-dsa.
Hostnames is a comma-separated list of patterns (* and ? act as wildcards); each pattern in turn is matched against the canonical host name (when authenticating a client) or against the user-supplied name (when authenticating a server). A pattern can also be preceded by ! to indicate negation: if the host name matches a negated pattern, it is not accepted (by that line) even if it matched another pattern on the line.
Alternately, hostnames can be stored in a hashed form, which hides host names and addresses should the file's contents be disclosed. Hashed hostnames start with a vertical bar (|) character. Only one hashed hostname can appear on a single line and none of the above negation or wildcard operators may be applied.
Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained, for example, from /etc/ssh/ssh_host_rsa_key.pub. The optional comment field continues to the end of the line, and is not used.
Lines starting with a hash mark (#) and empty lines are ignored as comments.
When performing host authentication, authentication is accepted if any matching line has the proper key. It is thus permissible (but not recommended) to have several lines or different host keys for the same names. This will inevitably happen when short forms of host names from different domains are put in the file. It is possible that the files contain conflicting information; authentication is accepted if valid information can be found from either file.
The lines in these files are typically hundreds of characters long. You should definitely not type in the host keys by hand. Rather, generate them by a script or by taking /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the front.
sshd sets the following environment variables for commands executed by ssh users:
Indicates the location of the X11 server. It is automatically set by sshd to point to a value of the form hostname:n, where hostname indicates the host where the shell runs, and n is an integer greater than or equal to 1. ssh uses this special value to forward X11 connections over the secure channel. Unless you have important reasons to do otherwise, you should not set DISPLAY explicitly, as that will render the X11 connection insecure and will require you to manually copy any required authorization cookies.
Set to the path of the user's home directory.
A locale setting. The locale defaults to that of sshd (usually the system-wide default locale), or is negotiated between the client and server during initial key exchange (as per RFC 4253).
Following initial key exchange, each of the variables can be overriden in the following sequence:
If a locale setting is set in a client's environment and that client supports “Environment Variable Passing” (see RFC 4254), then the setting will be passed over to the server side.
If the public key authentication method was used to authenticate the server and the PermitUserEnvironment variable in sshd_config(4) is set to yes on the server side, then the setting can be changed through the use of the environment option in the client's AuthorizedKeysFile file.
The setting can be change in the client's ~/.ssh/environment file on the server.
See PermitUserEnvironment in sshd_config(4) as to when the AuthorizedKeysFile and ~/.ssh/environment files are processed and used for setting the user environment.
Synonym for USER. Set for compatibility with systems that use this variable.
Set to point to the user's mailbox.
Indicates the path of a unix-domain socket used to communicate with the agent.
Identifies the client and server ends of the connection. The variable contains four space-separated values: client IP address, client port number, server IP address and server port number.
Identifies the client end of the connection. The variable contains three space-separated values: client IP address, client port number, and server port number.
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.
Indicates the present timezone, if TIMEZONE is set in /etc/default/login or if TZ was set when the daemon was started.
If set in /etc/default/login, the daemon sets it to the same value.
The user's shell, if ALTSHELL=YES in /etc/default/login.
Set to the value of PATH or SUPATH (see login(1)) in /etc/default/login, or, if not set, to /usr/bin:/bin.
Set to the name of the user logging in.
Additionally, sshd reads $HOME/.ssh/environment and adds lines of the format VARNAME=value to the environment.
In the following examples, certain lines might wrap due to line length limits in your display. You should nevertheless consider the wrapped line as a single line.
The following are examples of authorized_key file entries for protocol 1:
1024 33 12121...312314325 email@example.com from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
The following are examples of authorized_key file entries for protocol 2:
ssh-rsa AAAAB3NzaC1y.....EU88ovYKg4GfclWGCFYTuw8= firstname.lastname@example.org from="*.niksula.hut.fi" ssh-rsa AAAAB3NzaC...uw8= ylo@niksula command="dump /home",no-pty,no-port-forwarding ssh-rsa AA..8= backup.hut.fi
The following are examples of ssh_known_hosts file entries for protocol 1:
closenet,closenet.hut.fi,...,126.96.36.199 1024 37 159...93 closenet.hut.fi
The following are examples of ssh_known_hosts file entries for protocol 2:
closenet,closenet.hut.fi,...,188.8.131.52 ssh-rsa AA..8= closenet.hut.fi
Contains defaults for several sshd_config parameters, environment variables, and other environmental factors.
The following parameters affect environment variables (see login(1) and descriptions of these variables, above):
The following /etc/default/login parameters supply default values for corresponding sshd_config(4) parameters:
CONSOLE (see PermitRootLogin in sshd_config(4))
PASSREQ (see PermitEmptyPasswords in sshd_config(4))
TIMEOUT (see LoginGraceTime in sshd_config(4))
The following /etc/default/login parameters:
...set the umask(2) and file size limit of, respectively, the shells and commands spawned by sshd.
Finally, two /etc/default/login parameters affect the maximum allowed login attempts per-connection using interactive user authentication methods (for example, keyboard-interactive but not publickey), as per login(1):
Contains configuration data for sshd. This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable.
Contains the private part of the host key. This file should only be owned by root, readable only by root, and not accessible to others. sshd does not start if this file is group/world-accessible.
Contains the public part of the host key. This file should be world-readable but writable only by root. Its contents should match the private part. This file is not used for encryption; it is provided only for the convenience of the user so its contents can be copied to known hosts files. These two files are created using ssh-keygen(1).
Contains the process ID of the sshd listening for connections. If there are several daemons running concurrently for different ports, this contains the pid of the one started last. The content of this file is not sensitive; it can be world-readable. You can use the PidFile keyword in sshd_config to specify a file other than /var/run/sshd.pid. See sshd_config(4).
These files are consulted when using rhosts with public key host authentication to check the public key of the host. The key must be listed in one of these files to be accepted. The client uses the same files to verify that the remote host is the one it intended to connect. These files should be writable only by root or the owner. /etc/ssh/ssh_known_hosts should be world-readable, and $HOME/.ssh/known_hosts can but need not be world-readable.
If this file exists, sshd refuses to let anyone except root log in. The contents of the file are displayed to anyone trying to log in, and non-root connections are refused. The file should be world-readable.
Lists the public keys (RSA or DSA) that can be used to log into the user's account. This file must be readable by root. This might, on some machines, imply that it is world-readable if the user's home directory resides on an NFS volume. It is recommended that it not be accessible by others. The format of this file is described above. Users will place the contents of their identity.pub, id_dsa.pub and/or id_rsa.pub files into this file, as described in ssh-keygen(1).
This file contains host-username pairs, separated by a space, one per line. The given user on the corresponding host is permitted to log in without password. The same file is used by rlogind and rshd. The file must be writable only by the user; it is recommended that it not be accessible by others. It is also possible to use netgroups in the file. Either host or user name may be of the form +@groupname to specify all hosts or all users in the group.
For ssh, this file is exactly the same as for .rhosts. However, this file is not used by rlogin and rshd, so using this permits access using SSH only.
This file is used during .rhosts authentication. In its simplest form, this file contains host names, one per line. Users on these hosts are permitted to log in without a password, provided they have the same user name on both machines. The host name can also be followed by a user name; such users are permitted to log in as any user on this machine (except root). Additionally, the syntax +@group can be used to specify netgroups. Negated entries start with a hyphen (-).
If the client host/user is successfully matched in this file, login is automatically permitted, provided the client and server user names are the same. Additionally, successful RSA host authentication is normally required. This file must be writable only by root; it is recommended that it be world-readable.
Warning: It is almost never a good idea to use user names in hosts.equiv. Beware that it really means that the named user(s) can log in as anybody, which includes bin, daemon, adm, and other accounts that own critical binaries and directories. For practical purposes, using a user name grants the user root access. Probably the only valid use for user names is in negative entries. This warning also applies to rsh/rlogin.
A private file.
This file is processed exactly as /etc/hosts.equiv. However, this file might be useful in environments that want to run both rsh/rlogin and ssh.
This file is read into the environment at login (if it exists). It can contain only empty lines, comment lines (that start with #), and assignment lines of the form name=value. The file should be writable only by the user; it need not be readable by anyone else. Environment processing is disabled by default and is controlled by means of the PermitUserEnvironment option.
If this file exists, it is run with /bin/sh after reading the environment files but before starting the user's shell or command. If X11 spoofing is in use, this will receive the proto cookie pair in standard input (and DISPLAY in environment). This must call xauth in that case.
The primary purpose of $HOME/.ssh/rc is to run any initialization routines that might be needed before the user's home directory becomes accessible; AFS is a particular example of such an environment. If this file exists, it is run with /bin/sh after reading the environment files, but before starting the user's shell or command. It must not produce any output on stdout; stderr must be used instead. If X11 forwarding is in use, it will receive the proto cookie pair in its standard input and DISPLAY in its environment. The script must call xauth because sshd will not run xauth automatically to add X11 cookies.
This file will probably contain some initialization code followed by something similar to:
if read proto cookie && [ -n "$DISPLAY" ] then if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ] then # X11UseLocalhost=yes echo add unix:`echo $DISPLAY | cut -c11-` $proto $cookie else # X11UseLocalhost=no echo add $DISPLAY $proto $cookie fi | xauth -q - fi
If this file does not exist, /etc/ssh/sshrc is run, and if that does not exist, xauth is used to store the cookie. $HOME/.ssh/rc should be writable only by the user, and need not be readable by anyone else.
Similar to $HOME/.ssh/rc. This can be used to specify machine-specific login-time initializations globally. This file should be writable only by root, and should be world-readable.
sshd supports the use of several user authentication mechanisms: a public key system where keys are associated with users (through users' authorized_keys files), a public key system where keys are associated with hosts (see the HostbasedAuthentication configuration parameter), a GSS-API based method (see the GssAuthentication and GssKeyEx configuration parameters) and three initial authentication methods: none, password, and a generic prompt/reply protocol, keyboard-interactive.
sshd negotiates the use of the GSS-API with clients only if it has a GSS-API acceptor credential for the “host” service. This means that, for GSS-API based authentication, the server must have a Kerberos V keytab entry (see below) or the equivalent for any other GSS-API mechanism that might be installed.
In order for Kerberos authentication to work, a host/<FQDN> Kerberos principal must exist for each Fully Qualified Domain Name associated with the in.sshd server. Each of these host/<FQDN> principals must have a keytab entry in the /etc/krb5/krb5.keytab file on the in.sshd server. An example principal might be:
GSS-API authorization is covered in gss_auth_rules(5).
sshd uses pam(3PAM) for the three initial authentication methods as well as for account management, session management, and password management for all authentication methods.
Specifically, sshd calls pam_authenticate() for the “none,” “password” and “keyboard-interactive” SSHv2 userauth types, as well as for for the null and password authentication methods for SSHv1. Other SSHv2 authentication methods do not call pam_authenticate(). pam_acct_mgmt() is called for each authentication method that succeeds.
pam_setcred() and pam_open_session() are called when authentication succeeds and pam_close_session() is called when connections are closed.
pam_open_session() and pam_close_session() are also called when SSHv2 channels with ptys are opened and closed.
Each SSHv2 userauth type has its own PAM service name:
PAM Service Name
For SSHv1, sshd-v1 is always used.
If pam_acct_mgmt() returns PAM_NEW_AUTHTOK_REQD (indicating that the user's authentication tokens have expired), then sshd forces the use of “keyboard-interactive” userauth, if version 2 of the protocol is in use. The “keyboard-interactive” userauth will call pam_chauthtok() if pam_acct_mgmt() once again returns PAM_NEW_AUTHTOK_REQD. By this means, administrators are able to control what authentication methods are allowed for SSHv2 on a per-user basis.
To establish host-based authentication, you must perform the following steps:
Configure the client.
Configure the server.
Publish known hosts.
Make appropriate entries in /etc/ssh/shosts.equiv and ~/.shosts.
These steps are expanded in the following paragraphs.
On a client machine, in the system-wide client configuration file, /etc/ssh/ssh_config, you must have the entry:
On the server, in the system-wide server configuration file, /etc/ssh/sshd_config, you must have the entry:
If per-user .shost files are to be allowed (see last step), in the same file, you must have:
See sshd_config(4) for a description of these keywords.
To publish known hosts, you must have entries for the clients from which users will be allowed host-based authentication. Make these entries in either or both of the system-wide file (/etc/ssh/ssh_known_hosts) or the per-user file (~/.ssh/known_hosts).
Note that sshd uses .shosts, not .rhosts. If you want the functionality provided by .rhosts, but do not want to use rlogin or rsh because of their security shortcomings, you can use .shosts in conjunction with sshd. To use this feature, make appropriate entries in /etc/ssh/shosts.equiv and ~/.shosts, in the format specified in rhosts(4).
For the vast majority of network environments, .shosts is preferred over .rhosts.
See attributes(5) for descriptions of the following attributes:
The interface stability of /etc/ssh/moduli is Private.
login(1), scp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), svcs(1), gkadmin(1M), kadmin(1M), sftp-server(1M), ssh-keysign(1M), svcadm(1M), pam(3PAM), rhosts(4), ssh_config(4), sshd_config(4), attributes(5), gss_auth_rules(5), kerberos(5), pam_roles(5), smf(5)
The sshd service is managed by the service management facility, smf(5), under the service identifier:
sshd always sets PAM_RHOST and sets PAM_AUSER in the case of host-based userauth. This behavior allows for remote logins to roles using host-based authentication. See pam_roles(5).