.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
-.\" $OpenBSD: sshd.8,v 1.220 2006/02/13 10:21:25 jmc Exp $
-.Dd September 25, 1999
+.\" $OpenBSD: sshd.8,v 1.249 2009/10/08 20:42:13 jmc Exp $
+.Dd $Mdocdate$
.Dt SSHD 8
.Os
.Sh NAME
.Sh SYNOPSIS
.Nm sshd
.Bk -words
-.Op Fl 46Ddeiqt
+.Op Fl 46DdeiqTt
.Op Fl b Ar bits
+.Op Fl C Ar connection_spec
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Op Fl h Ar host_key_file
.Nm
(OpenSSH Daemon) is the daemon program for
.Xr ssh 1 .
-Together these programs replace rlogin and rsh, and
-provide secure encrypted communications between two untrusted hosts
+Together these programs replace
+.Xr rlogin 1
+and
+.Xr rsh 1 ,
+and provide secure encrypted communications between two untrusted hosts
over an insecure network.
.Pp
.Nm
.Nm
rereads its configuration file when it receives a hangup signal,
.Dv SIGHUP ,
-by executing itself with the name and options it was started with, e.g.,
+by executing itself with the name and options it was started with, e.g.\&
.Pa /usr/sbin/sshd .
.Pp
The options are as follows:
to use IPv6 addresses only.
.It Fl b Ar bits
Specifies the number of bits in the ephemeral protocol version 1
-server key (default 768).
+server key (default 1024).
+.It Fl C Ar connection_spec
+Specify the connection parameters to use for the
+.Fl T
+extended test mode.
+If provided, any
+.Cm Match
+directives in the configuration file
+that would apply to the specified user, host, and address will be set before
+the configuration is written to standard output.
+The connection parameters are supplied as keyword=value pairs.
+The keywords are
+.Dq user ,
+.Dq host ,
+and
+.Dq addr .
+All are required and may be supplied in any order, either with multiple
+.Fl C
+options or as a comma-separated list.
.It Fl D
When this option is specified,
.Nm
When this option is specified,
.Nm
will send the output to the standard error instead of the system log.
-.It Fl f Ar configuration_file
+.It Fl f Ar config_file
Specifies the name of the configuration file.
The default is
.Pa /etc/ssh/sshd_config .
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 (e.g., 512) using
+However, with small key sizes (e.g. 512) using
.Nm
from inetd may
be feasible.
Nothing is sent to the system log.
Normally the beginning,
authentication, and termination of each connection is logged.
+.It Fl T
+Extended test mode.
+Check the validity of the configuration file, output the effective configuration
+to stdout and then exit.
+Optionally,
+.Cm Match
+rules may be applied by specifying the connection parameters using one or more
+.Fl C
+options.
.It Fl t
Test mode.
Only check the validity of the configuration file and sanity of the keys.
.El
.Sh AUTHENTICATION
The OpenSSH SSH daemon supports SSH protocols 1 and 2.
-Both protocols are supported by default,
+The default is to use protocol 2 only,
though this can be changed via the
.Cm Protocol
option in
to use from those offered by the server.
Additionally, session integrity is provided
through a cryptographic message authentication code
-(hmac-sha1 or hmac-md5).
+(hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
.Pp
Finally, the server and the client enter an authentication dialog.
The client tries to authenticate itself using
a leading
.Ql \&*LOCKED\&*
on FreeBSD and a leading
-.Ql \&!!
-on Linux). If there is a requirement to disable password authentication
+.Ql \&!
+on most Linuxes).
+If there is a requirement to disable password authentication
for the account while allowing still public-key, then the passwd field
should be set to something other than these values (eg
.Ql NP
.Dq rc
files are given the X11
authentication protocol and cookie in standard input.
+See
+.Sx SSHRC ,
+below.
.It
Runs user's shell or command.
.El
+.Sh SSHRC
+If the file
+.Pa ~/.ssh/rc
+exists,
+.Xr sh 1
+runs it 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
+.Ev DISPLAY
+in its environment).
+The script must call
+.Xr xauth 1
+because
+.Nm
+will not run xauth automatically to add X11 cookies.
+.Pp
+The primary purpose of this file is to run any initialization routines
+which may be needed before the user's home directory becomes
+accessible; AFS is a particular example of such an environment.
+.Pp
+This file will probably contain some initialization code followed by
+something similar to:
+.Bd -literal -offset 3n
+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
+.Ed
+.Pp
+If this file does not exist,
+.Pa /etc/ssh/sshrc
+is run, and if that
+does not exist either, xauth is used to add the cookie.
.Sh AUTHORIZED_KEYS FILE FORMAT
.Cm AuthorizedKeysFile
specifies the file containing public keys for
An example might be a key that permits remote backups but nothing else.
Note that the client may specify TCP and/or X11
forwarding unless they are explicitly prohibited.
+The command originally supplied by the client is available in the
+.Ev SSH_ORIGINAL_COMMAND
+environment variable.
Note that this option applies to shell, command or subsystem execution.
.It Cm environment="NAME=value"
Specifies that the string is to be added to the environment when
.Cm UseLogin
is enabled.
.It Cm from="pattern-list"
-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
-.Pf ( Ql \&*
-and
-.Ql \&?
-serve as wildcards).
-The list may also contain
-patterns negated by prefixing them with
-.Ql \&! ;
-if the canonical host name matches a negated pattern, the key is not accepted.
-The purpose
-of this option is to optionally increase security: public key authentication
-by itself does not trust the network or name servers or anything (but
-the key); however, if somebody somehow steals the key, the key
+Specifies that in addition to public key authentication, either the canonical
+name of the remote host or its IP address must be present in the
+comma-separated list of patterns.
+See
+.Sx PATTERNS
+in
+.Xr ssh_config 5
+for more information on patterns.
+.Pp
+In addition to the wildcard matching that may be applied to hostnames or
+addresses, a
+.Cm from
+stanza may match IP addresses using CIDR address/masklen notation.
+.Pp
+The purpose of this option is to optionally increase security: public key
+authentication by itself does not trust the network or name servers or
+anything (but the key); however, if somebody somehow steals the key, the key
permits an intruder to log in from anywhere in the world.
This additional option makes using a stolen key more difficult (name
servers and/or routers would have to be compromised in addition to
.It Cm no-port-forwarding
Forbids TCP forwarding when this key is used for authentication.
Any port forward requests by the client will return an error.
-This might be used, e.g., in connection with the
+This might be used, e.g. in connection with the
.Cm command
option.
.It Cm no-pty
Prevents tty allocation (a request to allocate a pty will fail).
+.It Cm no-user-rc
+Disables execution of
+.Pa ~/.ssh/rc .
.It Cm no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return an error.
.El
.Pp
An example authorized_keys file:
-.Bd -literal
+.Bd -literal -offset 3n
# Comments allowed at start of line
ssh-rsa AAAAB3Nza...LiPk== user@example.net
-from="*.sales.example.net,!pc.sales.example.net" ssh-rsa AAAAB2...19Q== john@example.net
-command="dump /home",no-pty,no-port-forwarding ssh-dss AAAAC3...51R== example.net
-permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss AAAAB5...21S==
-tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== jane@example.net
+from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
+AAAAB2...19Q== john@example.net
+command="dump /home",no-pty,no-port-forwarding ssh-dss
+AAAAC3...51R== example.net
+permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
+AAAAB5...21S==
+tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
+jane@example.net
.Ed
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
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.
+A hostname or address may optionally be enclosed within
+.Ql \&[
+and
+.Ql \&]
+brackets then followed by
+.Ql \&:
+and a non-standard port number.
.Pp
Alternately, hostnames may be stored in a hashed form which hides host names
and addresses should the file's contents be disclosed.
or by taking
.Pa /etc/ssh/ssh_host_key.pub
and adding the host names at the front.
-.Ss Examples
-.Bd -literal
-closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
-cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
-.Ed
-.Bd -literal
+.Pp
+An example ssh_known_hosts file:
+.Bd -literal -offset 3n
+# Comments allowed at start of line
+closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
+cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
# A hashed hostname
|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
AAAA1234.....=
.Ed
.Sh FILES
-.Bl -tag -width Ds
-.It Pa /etc/ssh/sshd_config
-Contains configuration data for
-.Nm sshd .
-The file format and configuration options are described in
-.Xr sshd_config 5 .
-.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
-These three files contain the private parts of the host keys.
-These files should only be owned by root, readable only by root, and not
-accessible to others.
-Note that
-.Nm
-does not start if this file is group/world-accessible.
-.It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
-These three files contain the public parts of the host keys.
-These files should be world-readable but writable only by
-root.
-Their contents should match the respective private parts.
-These files are not
-really used for anything; they are provided for the convenience of
-the user so their contents can be copied to known hosts files.
-These files are created using
-.Xr ssh-keygen 1 .
-.It Pa /etc/moduli
-Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
-The file format is described in
-.Xr moduli 5 .
-.It Pa /var/empty
-.Xr chroot 2
-directory used by
-.Nm
-during privilege separation in the pre-authentication phase.
-The directory should not contain any files and must be owned by root
-and not group or world-writable.
-.It Pa /var/run/sshd.pid
-Contains the process ID of the
-.Nm
-listening for connections (if there are several daemons running
-concurrently for different ports, this contains the process ID of the one
-started last).
-The content of this file is not sensitive; it can be world-readable.
-.It Pa ~/.ssh/authorized_keys
-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 (which may on some machines imply
-it being 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
-.Pa identity.pub ,
-.Pa id_dsa.pub
-and/or
-.Pa id_rsa.pub
-files into this file, as described in
-.Xr ssh-keygen 1 .
-.It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"
-These files are consulted when using rhosts with RSA host
-authentication or protocol version 2 hostbased 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 it is connecting to the correct remote host.
-These files should be writable only by root/the owner.
-.Pa /etc/ssh/ssh_known_hosts
-should be world-readable, and
-.Pa ~/.ssh/known_hosts
-can, but need not be, world-readable.
-.It Pa /etc/motd
-See
-.Xr motd 5 .
-.It Pa ~/.hushlogin
+.Bl -tag -width Ds -compact
+.It ~/.hushlogin
This file is used to suppress printing the last login time and
.Pa /etc/motd ,
if
are enabled.
It does not suppress printing of the banner specified by
.Cm Banner .
-.It Pa /etc/nologin
-If this file exists,
+.Pp
+.It ~/.rhosts
+This file is used for host-based authentication (see
+.Xr ssh 1
+for more information).
+On some machines this file may need to be
+world-readable if the user's home directory is on an NFS partition,
+because
.Nm
-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.
-.It Pa /etc/hosts.allow, /etc/hosts.deny
-Access controls that should be enforced by tcp-wrappers are defined here.
-Further details are described in
-.Xr hosts_access 5 .
-.It Pa ~/.rhosts
-This file is used during
-.Cm RhostsRSAAuthentication
-and
-.Cm HostbasedAuthentication
-and contains host-username pairs, separated by a space, one per
-line.
-The given user on the corresponding host is permitted to log in
-without a 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
+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
-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.
-.It Pa ~/.shosts
-For ssh,
-this file is exactly the same as for
-.Pa .rhosts .
-However, this file is
-not used by rlogin and rshd, so using this permits access using SSH only.
-.It Pa /etc/hosts.equiv
-This file is used during
-.Cm RhostsRSAAuthentication
-and
-.Cm HostbasedAuthentication
-authentication.
-In the simplest form, this file contains host names, one per line.
-Users on
-those hosts are permitted to log in without a password, provided they
-have the same user name on both machines.
-The host name may also be
-followed by a user name; such users are permitted to log in as
-.Em any
-user on this machine (except root).
-Additionally, the syntax
-.Dq +@group
-can be used to specify netgroups.
-Negated entries start with
-.Ql \&- .
-.Pp
-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 client host key authentication is required.
-This file must be writable only by root; it is recommended
-that it be world-readable.
-.Pp
-.Sy "Warning: It is almost never a good idea to use user names in"
-.Pa hosts.equiv .
-Beware that it really means that the named user(s) can log in as
-.Em anybody ,
-which includes bin, daemon, adm, and other accounts that own critical
-binaries and directories.
-Using a user name practically grants the user root access.
-The only valid use for user names that I can think
-of is in negative entries.
-.Pp
-Note that this warning also applies to rsh/rlogin.
-.It Pa /etc/shosts.equiv
-This is processed exactly as
-.Pa /etc/hosts.equiv .
-However, this file may be useful in environments that want to run both
-rsh/rlogin and ssh.
-.It Pa ~/.ssh/environment
+.It ~/.shosts
+This file is used in exactly the same way as
+.Pa .rhosts ,
+but allows host-based authentication without permitting login with
+rlogin/rsh.
+.Pp
+.It ~/.ssh/
+This directory is the default location for all user-specific configuration
+and authentication information.
+There is no general requirement to keep the entire contents of this directory
+secret, but the recommended permissions are read/write/execute for the user,
+and not accessible by others.
+.Pp
+.It ~/.ssh/authorized_keys
+Lists the public keys (RSA/DSA) that can be used for logging in as this user.
+The format of this file is described above.
+The content of the file is not highly sensitive, but the recommended
+permissions are read/write for the user, and not accessible by others.
+.Pp
+If this file, the
+.Pa ~/.ssh
+directory, or the user's home directory are writable
+by other users, then the file could be modified or replaced by unauthorized
+users.
+In this case,
+.Nm
+will not allow it to be used unless the
+.Cm StrictModes
+option has been set to
+.Dq no .
+.Pp
+.It ~/.ssh/environment
This file is read into the environment at login (if it exists).
It can only contain empty lines, comment lines (that start with
.Ql # ) ,
controlled via the
.Cm PermitUserEnvironment
option.
-.It Pa ~/.ssh/rc
-If this file exists, it is run with
-.Pa /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
-.Ev DISPLAY
-in its environment).
-The script must call
-.Xr xauth 1
-because
+.Pp
+.It ~/.ssh/known_hosts
+Contains a list of host keys for all hosts the user has logged into
+that are not already in the systemwide list of known host keys.
+The format of this file is described above.
+This file should be writable only by root/the owner and
+can, but need not be, world-readable.
+.Pp
+.It ~/.ssh/rc
+Contains initialization routines to be run before
+the user's home directory becomes accessible.
+This file should be writable only by the user, and need not be
+readable by anyone else.
+.Pp
+.It /etc/hosts.allow
+.It /etc/hosts.deny
+Access controls that should be enforced by tcp-wrappers are defined here.
+Further details are described in
+.Xr hosts_access 5 .
+.Pp
+.It /etc/hosts.equiv
+This file is for host-based authentication (see
+.Xr ssh 1 ) .
+It should only be writable by root.
+.Pp
+.It /etc/moduli
+Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
+The file format is described in
+.Xr moduli 5 .
+.Pp
+.It /etc/motd
+See
+.Xr motd 5 .
+.Pp
+.It /etc/nologin
+If this file exists,
.Nm
-will not run xauth automatically to add X11 cookies.
+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.
.Pp
-The primary purpose of this file is to run any initialization routines
-which may be needed before the user's home directory becomes
-accessible; AFS is a particular example of such an environment.
+.It /etc/shosts.equiv
+This file is used in exactly the same way as
+.Pa hosts.equiv ,
+but allows host-based authentication without permitting login with
+rlogin/rsh.
.Pp
-This file will probably contain some initialization code followed by
-something similar to:
-.Bd -literal
-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
-.Ed
+.It /etc/ssh/ssh_host_key
+.It /etc/ssh/ssh_host_dsa_key
+.It /etc/ssh/ssh_host_rsa_key
+These three files contain the private parts of the host keys.
+These files should only be owned by root, readable only by root, and not
+accessible to others.
+Note that
+.Nm
+does not start if these files are group/world-accessible.
.Pp
-If this file does not exist,
-.Pa /etc/ssh/sshrc
-is run, and if that
-does not exist either, xauth is used to add the cookie.
+.It /etc/ssh/ssh_host_key.pub
+.It /etc/ssh/ssh_host_dsa_key.pub
+.It /etc/ssh/ssh_host_rsa_key.pub
+These three files contain the public parts of the host keys.
+These files should be world-readable but writable only by
+root.
+Their contents should match the respective private parts.
+These files are not
+really used for anything; they are provided for the convenience of
+the user so their contents can be copied to known hosts files.
+These files are created using
+.Xr ssh-keygen 1 .
.Pp
-This file should be writable only by the user, and need not be
-readable by anyone else.
-.It Pa /etc/ssh/sshrc
-Like
-.Pa ~/.ssh/rc .
-This can be used to specify
+.It /etc/ssh/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.
+The format of this file is described above.
+This file should be writable only by root/the owner and
+should be world-readable.
+.Pp
+.It /etc/ssh/sshd_config
+Contains configuration data for
+.Nm sshd .
+The file format and configuration options are described in
+.Xr sshd_config 5 .
+.Pp
+.It /etc/ssh/sshrc
+Similar to
+.Pa ~/.ssh/rc ,
+it can be used to specify
machine-specific login-time initializations globally.
This file should be writable only by root, and should be world-readable.
+.Pp
+.It /var/empty
+.Xr chroot 2
+directory used by
+.Nm
+during privilege separation in the pre-authentication phase.
+The directory should not contain any files and must be owned by root
+and not group or world-writable.
+.Pp
+.It /var/run/sshd.pid
+Contains the process ID of the
+.Nm
+listening for connections (if there are several daemons running
+concurrently for different ports, this contains the process ID of the one
+started last).
+The content of this file is not sensitive; it can be world-readable.
.El
.Sh SEE ALSO
.Xr scp 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
+.Xr ssh-keyscan 1 ,
.Xr chroot 2 ,
.Xr hosts_access 5 ,
.Xr login.conf 5 ,
.Xr sshd_config 5 ,
.Xr inetd 8 ,
.Xr sftp-server 8
-.Rs
-.%A T. Ylonen
-.%A T. Kivinen
-.%A M. Saarinen
-.%A T. Rinne
-.%A S. Lehtinen
-.%T "SSH Protocol Architecture"
-.%N draft-ietf-secsh-architecture-12.txt
-.%D January 2002
-.%O work in progress material
-.Re
-.Rs
-.%A M. Friedl
-.%A N. Provos
-.%A W. A. Simpson
-.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
-.%N draft-ietf-secsh-dh-group-exchange-02.txt
-.%D January 2002
-.%O work in progress material
-.Re
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
andersk / openssh.git / blobdiff