sshd.8

C＋＋编写

.Dq ssh-rsa .
.Pp
Note that lines in this file are usually several hundred bytes long
(because of the size of the public key encoding).
You don't want to type them in; instead, copy the
.Pa identity.pub ,
.Pa id_dsa.pub
or the
.Pa id_rsa.pub
file and edit it.
.Pp
.Nm
enforces a minimum RSA key modulus size for protocol 1
and protocol 2 keys of 768 bits.
.Pp
The options (if present) consist of comma-separated option
specifications.
No spaces are permitted, except within double quotes.
The following option specifications are supported (note
that option keywords are case-insensitive):
.Bl -tag -width Ds
.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
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
just the key).
.It Cm command="command"
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
.Cm no-pty .
A quote may be included in the command by quoting it with a backslash.
This option might be useful
to restrict certain public keys to perform just a specific operation.
An example might be a key that permits remote backups but nothing else.
Note that the client may specify TCP/IP and/or X11
forwarding unless they are explicitly prohibited.
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
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
.Cm PermitUserEnvironment
option.
This option is automatically disabled if
.Cm UseLogin
is enabled.
.It Cm no-port-forwarding
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, e.g., in connection with the
.Cm command
option.
.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.
.It Cm no-agent-forwarding
Forbids authentication agent forwarding when this key is used for
authentication.
.It Cm no-pty
Prevents tty allocation (a request to allocate a pty will fail).
.It Cm permitopen="host:port"
Limit local
.Li ``ssh -L''
port forwarding such that it may only connect to the specified host and
port.
IPv6 addresses can be specified with an alternative syntax:
.Ar host/port .
Multiple
.Cm permitopen
options may be applied separated by commas. No pattern matching is
performed on the specified hostnames, they must be literal domains or
addresses.
.El
.Ss Examples
1024 33 12121.\|.\|.\|312314325 [e-mail]
.Pp
from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
.Pp
command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
.Pp
permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
.Pa /etc/ssh/ssh_known_hosts
and
.Pa $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.
.Pp
Each line in these files contains the following fields: hostnames,
bits, exponent, modulus, comment.
The fields are separated by spaces.
.Pp
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 may also be preceded by
.Ql !
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.
.Pp
Bits, exponent, and modulus are taken directly from the RSA host key; they
can be obtained, e.g., from
.Pa /etc/ssh/ssh_host_key.pub .
The optional comment field continues to the end of the line, and is not used.
.Pp
Lines starting with
.Ql #
and empty lines are ignored as comments.
.Pp
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.
.Pp
Note that the lines in these files are typically hundreds of characters
long, and you definitely don't want to type in the host keys by hand.
Rather, generate them by a script
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
.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 $HOME/.ssh/authorized_keys
Lists the public keys (RSA or DSA) that can be used to log into the user's account.
This file must be readable by root (which may on some machines imply
it being world-readable if the user's home directory resides on an NFS
volume).
It is recommended that it not be accessible by others.
The format of this file is described above.
Users will place the contents of their
.Pa identity.pub ,
.Pa id_dsa.pub
and/or
.Pa id_rsa.pub
files into this file, as described in
.Xr ssh-keygen 1 .
.It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.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 $HOME/.ssh/known_hosts
can, but need not be, world-readable.
.It Pa /etc/nologin
If this file exists,
.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 $HOME/.rhosts
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 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
accessible by others.
.Pp
If 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 $HOME/.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
.Pa .rhosts
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 RSA host authentication is normally 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 $HOME/.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 # ) ,
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 via the
.Cm PermitUserEnvironment
option.
.It Pa $HOME/.ssh/rc
If this file exists, it is run with
.Pa /bin/sh
after reading the
environment files but before starting the user's shell or command.
It must not produce any output on stdout; stderr must be used
instead.
If X11 forwarding is in use, it will receive the "proto cookie" pair in
its standard input (and
.Ev DISPLAY
in its environment).
The script must call
.Xr xauth 1
because
.Nm
will not run xauth automatically to add X11 cookies.
.Pp
The primary purpose of this file is to run any initialization routines
which may be needed before the user's home directory becomes
accessible; AFS is a particular example of such an environment.
.Pp
This file will probably contain some initialization code followed by
something similar to:
.Bd -literal
if read proto cookie && [ -n "$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.
.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 $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.
.El
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
Theo de Raadt and Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support
for privilege separation.
.Sh SEE ALSO
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
.Xr login.conf 5 ,
.Xr moduli 5 ,
.Xr sshd_config 5 ,
.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