SSHD(8)SSHD(8)NAMEsshd - OpenSSH SSH daemon
SYNOPSISsshd [-46Ddeiqt] [-b bits] [-f config_file] [-g login_grace_time] [-h
host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]
DESCRIPTIONsshd (SSH Daemon) is the daemon program for ssh(1). Together these
programs replace rlogin and rsh, and provide secure encrypted communi‐
cations between two untrusted hosts over an insecure network. The pro‐
grams are intended to be as easy to install and use as possible.
sshd is the daemon that listens for connections from clients. It is
normally started at boot from /etc/rc. It forks a new daemon for each
incoming connection. The forked daemons handle key exchange, encryp‐
tion, authentication, command execution, and data exchange. This
implementation of sshd supports both SSH protocol version 1 and 2
simultaneously. sshd works as follows:
SSH protocol version 1
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 com‐
bined with RSA host authentication, RSA challenge-response authentica‐
tion, or password based authentication.
Regardless of the authentication type, the account is checked to ensure
that it is accessible. An account is not accessible if it is locked,
listed in DenyUsers or its group is listed in DenyGroups . The defini‐
tion of a locked account is system dependant. Some platforms have their
own account database (eg AIX) and some modify the passwd field ( `*LK*'
on Solaris, `*' on HP-UX, containing `Nologin' on Tru64 and a leading
`!!' on Linux). If there is a requirement to disable password authen‐
tication for the account while allowing still public-key, then the
passwd field should be set to something other than these values (eg
`NP' or `*NP*' ).
rshd, rlogind, and rexecd are disabled (thus completely disabling
rlogin() and rsh() into the machine).
SSH protocol version 2
Version 2 works similarly: Each host has a host-specific key (RSA or
DSA) used to identify the host. 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, cur‐
rently 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or
256-bit AES. The client selects the encryption algorithm to use from
those offered by the server. Additionally, session integrity is pro‐
vided through a cryptographic message authentication code (hmac-sha1 or
hmac-md5).
Protocol version 2 provides a public key based user (PubkeyAuthentica‐
tion) or client host (HostbasedAuthentication) authentication method,
conventional password authentication and challenge response based meth‐
ods.
Command execution and data forwarding
If the client successfully authenticates itself, a dialog for preparing
the session is entered. At this time the client may 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 com‐
mand on the server side, and the user terminal in the client side.
When the user program terminates and all forwarded X11 and other con‐
nections 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 a configuration
file (by default sshd_config(5)) . Command-line options override val‐
ues specified in the configuration file.
sshd rereads its configuration file when it receives a hangup signal,
SIGHUP, by executing itself with the name and options it was started
with, e.g., /usr/sbin/sshd.
The options are as follows:
-4 Forces sshd to use IPv4 addresses only.
-6 Forces sshd to use IPv6 addresses only.
-b bits
Specifies the number of bits in the ephemeral protocol version 1
server key (default 768).
-D When this option is specified, sshd will not detach and does not
become a daemon. This allows easy monitoring of sshd.
-d 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.
-e When this option is specified, sshd will send the output to the
standard error instead of the system log.
-f configuration_file
Specifies the name of the configuration file. The default is
/etc/ssh/sshd_config. sshd refuses to start if there is no con‐
figuration file.
-g login_grace_time
Gives the grace time for clients to authenticate themselves
(default 120 seconds). If the client fails to authenticate the
user within this many seconds, the server disconnects and exits.
A value of zero indicates no limit.
-h host_key_file
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 pro‐
tocol version 2. It is possible to have multiple host key files
for the different protocol versions and host key algorithms.
-i Specifies that sshd is being run from inetd(8). sshd is nor‐
mally 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 (e.g.,
512) using sshd from inetd may be feasible.
-k key_gen_time
Specifies how often the ephemeral protocol version 1 server key
is regenerated (default 3600 seconds, or one hour). The motiva‐
tion for regenerating the key fairly often is that the key is
not stored anywhere, and after about an hour it becomes impossi‐
ble 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.
-o option
Can be used to give options in the format used in the configura‐
tion file. This is useful for specifying options for which
there is no separate command-line flag. For full details of the
options, and their values, see sshd_config(5).
-p port
Specifies the port on which the server listens for connections
(default 22). Multiple port options are permitted. Ports spec‐
ified in the configuration file are ignored when a command-line
port is specified.
-q Quiet mode. Nothing is sent to the system log. Normally the
beginning, authentication, and termination of each connection is
logged.
-t Test mode. Only check the validity of the configuration file
and sanity of the keys. This is useful for updating sshd reli‐
ably as configuration options may change.
-u len This option is used to specify the size of the field in the utmp
structure that holds the remote host name. If the resolved host
name is longer than len, the dotted decimal value will be used
instead. This allows hosts with very long host names that over‐
flow this field to still be uniquely identified. Specifying -u0
indicates that only dotted decimal addresses should be put into
the utmp file. -u0 may also be used to prevent sshd from making
DNS requests unless the authentication mechanism or configura‐
tion requires it. Authentication mechanisms that may require
DNS include RhostsRSAAuthentication, HostbasedAuthentication and
using a from="pattern-list" option in a key file. Configuration
options that require DNS include using a USER@HOST pattern in
AllowUsers or DenyUsers.
CONFIGURATION FILEsshd reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file format and configura‐
tion options are described in sshd_config(5).
LOGIN PROCESS
When a user successfully logs in, sshd does the following:
1. If the login is on a tty, and no command has been specified,
prints last login time and /etc/motd (unless prevented in the
configuration file or by $HOME/.hushlogin; see the FILES sec‐
tion).
2. If the login is on a tty, records login time.
3. Checks /etc/nologin; if it exists, prints contents and quits
(unless root).
4. Changes to run with normal user privileges.
5. Sets up basic environment.
6. Reads the file $HOME/.ssh/environment, if it exists, and users
are allowed to change their environment. See the PermitUserEn‐
vironment option in sshd_config(5).
7. Changes to user's home directory.
8. If $HOME/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists,
runs it; otherwise runs xauth. The ``rc'' files are given the
X11 authentication protocol and cookie in standard input.
9. Runs user's shell or command.
AUTHORIZED_KEYS FILE FORMAT
$HOME/.ssh/authorized_keys is the default file that lists the public
keys that are permitted for RSA authentication in protocol version 1
and for public key authentication (PubkeyAuthentication) in protocol
version 2. AuthorizedKeysFile may be used to specify an alternative
file.
Each line of the file contains one key (empty lines and lines starting
with a `#' are ignored as comments). Each RSA public key consists of
the following fields, separated by spaces: options, bits, exponent,
modulus, comment. Each protocol version 2 public key consists of:
options, keytype, base64 encoded key, comment. The options field is
optional; its presence is determined by whether the line starts with a
number or not (the options field never starts with a number). The
bits, exponent, modulus and comment fields give the RSA key for proto‐
col version 1; the comment field is not used for anything (but may be
convenient for the user to identify the key). For protocol version 2
the keytype is ``ssh-dss'' or ``ssh-rsa''.
Note that lines in this file are usually several hundred bytes long
(because of the size of the public key encoding) up to a limit of 8
kilobytes, which permits DSA keys up to 8 kilobits and RSA keys up to
16 kilobits. You don't want to type them in; instead, copy the iden‐
tity.pub, id_dsa.pub or the id_rsa.pub file and edit it.
sshd enforces a minimum RSA key modulus size for protocol 1 and proto‐
col 2 keys of 768 bits.
The options (if present) consist of comma-separated option specifica‐
tions. No spaces are permitted, except within double quotes. The fol‐
lowing option specifications are supported (note that option keywords
are case-insensitive):
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 (`*' and `?' serve as wildcards).
The list may also contain patterns negated by prefixing them
with `!' ; 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).
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 spec‐
ify 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 for‐
warding unless they are explicitly prohibited. Note that this
option applies to shell, command or subsystem execution.
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 PermitUserEnvironment option.
This option is automatically disabled if UseLogin is enabled.
no-port-forwarding
Forbids TCP/IP forwarding when this key is used for authentica‐
tion. Any port forward requests by the client will return an
error. This might be used, e.g., in connection with the command
option.
no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return an error.
no-agent-forwarding
Forbids authentication agent forwarding when this key is used
for authentication.
no-pty Prevents tty allocation (a request to allocate a pty will fail).
permitopen="host:port"
Limit local ``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: host/port. Multiple per‐
mitopen options may be applied separated by commas. No pattern
matching is performed on the specified hostnames, they must be
literal domains or addresses.
Examples
1024 33 12121...312314325 ylo@foo.bar
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
permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
SSH_KNOWN_HOSTS FILE FORMAT
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 pre‐
pared by the administrator (optional), and the per-user file is main‐
tained automatically: whenever the user connects from an unknown host
its key is added to the per-user file.
Each line in these files contains the following fields: hostnames,
bits, exponent, modulus, comment. The fields are separated by spaces.
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 `!'
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 may be stored in a hashed form which hides host
names and addresses should the file's contents be disclosed. Hashed
hostnames start with a `|' character. Only one hashed hostname may
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, e.g., from /etc/ssh/ssh_host_key.pub. The
optional comment field continues to the end of the line, and is not
used.
Lines starting with `#' 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 rec‐
ommended) 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.
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
/etc/ssh/ssh_host_key.pub and adding the host names at the front.
Examples
closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
# A hashed hostname
|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
AAAA1234.....=
FILES
/etc/ssh/sshd_config
Contains configuration data for sshd. The file format and con‐
figuration options are described in sshd_config(5).
/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 sshd does not start if
this file is group/world-accessible.
/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 ssh-keygen(1).
/etc/moduli
Contains Diffie-Hellman groups used for the "Diffie-Hellman
Group Exchange". The file format is described in moduli(5).
/var/empty
chroot(2) directory used by sshd 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.
/etc/sshd.pid
Contains the process ID of the sshd 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-read‐
able.
$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 iden‐
tity.pub, id_dsa.pub and/or id_rsa.pub files into this file, as
described in ssh-keygen(1).
/etc/ssh/ssh_known_hosts , $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.
/etc/ssh/ssh_known_hosts should be world-readable, and
$HOME/.ssh/known_hosts can, but need not be, world-readable.
/etc/motd
See motd(5).
$HOME/.hushlogin
This file is used to suppress printing the last login time and
/etc/motd, if PrintLastLog and PrintMotd, respectively, are
enabled. It does not suppress printing of the banner specified
by Banner.
/etc/nologin
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.
/etc/hosts.allow, /etc/hosts.deny
Access controls that should be enforced by tcp-wrappers are
defined here. Further details are described in hosts_access(5).
$HOME/.rhosts
This file is used during RhostsRSAAuthentication and Host‐
basedAuthentication 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 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.
$HOME/.shosts
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.
/etc/hosts.equiv
This file is used during RhostsRSAAuthentication and Host‐
basedAuthentication 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
any user on this machine (except root). Additionally, the syn‐
tax ``+@group'' can be used to specify netgroups. Negated
entries start with `-'.
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.
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.
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.
Note that this warning also applies to rsh/rlogin.
/etc/ssh/shosts.equiv
This is processed exactly as /etc/hosts.equiv. However, this
file may be useful in environments that want to run both
rsh/rlogin and ssh.
$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
`#' ) , 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 PermitUserEnvironment option.
$HOME/.ssh/rc
If this file exists, it is run with /bin/sh after reading the
environment files but before starting the user's shell or com‐
mand. It must not produce any output on stdout; stderr must be
used instead. If X11 forwarding is in use, it will receive the
"proto cookie" pair in its standard input (and DISPLAY in its
environment). The script must call xauth(1) because sshd will
not run xauth automatically to add X11 cookies.
The primary purpose of this file is to run any initialization
routines which may be needed before the user's home directory
becomes accessible; AFS is a particular example of such an envi‐
ronment.
This file will probably contain some initialization code fol‐
lowed by something similar to:
if read proto cookie && [ -n "$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 either, xauth is used to add the cookie.
This file should be writable only by the user, and need not be
readable by anyone else.
/etc/ssh/sshrc
Like $HOME/.ssh/rc. This can be used to specify machine-spe‐
cific login-time initializations globally. This file should be
writable only by root, and should be world-readable.
SEE ALSOscp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
chroot(2), hosts_access(5), login.conf(5), moduli(5), sshd_config(5),
inetd(8), sftp-server(8)
S. Lehtinen, T. Rinne, M. Saarinen, T. Kivinen and T. Ylonen, SSH Pro‐
tocol Architecture, draft-ietf-secsh-architecture-12.txt, January 2002,
work in progress material.
W. A. Simpson, N. Provos and M. Friedl, Diffie-Hellman Group Exchange
for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-group-
exchange-02.txt, January 2002, work in progress material.
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 contrib‐
uted support for privilege separation.
September 25, 1999 SSHD(8)
