SSHD(8) BSD System Manager’s Manual SSHD(8)
sshd - OpenSSH SSH daemon
sshd [-46Ddeiqt] [-b bits] [-f config_file] [-g login_grace_time]
[-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]
sshd (SSH Daemon) is the daemon program for ssh(1). Together these pro-
grams 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 is nor-
mally started at boot from /etc/rc. It forks a new daemon for each
incoming connection. The forked daemons handle key exchange, encryption,
authentication, command execution, and data exchange. This implementa-
tion 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 2048 bits) used to iden-
tify 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 encryp-
tion 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 combined
with RSA host authentication, RSA challenge-response authentication, 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 authenti-
cation for the account while allowing still public-key, then the passwd
field should be set to something other than these values (eg ‘NP’ or
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
The rest of the session is encrypted using a symmetric cipher, currently
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 provided
through a cryptographic message authentication code (hmac-sha1 or hmac-
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-
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
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 in the client side.
When the user program terminates and all forwarded X11 and other connec-
tions 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 values speci-
fied 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.
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.
Specifies the name of the configuration file. The default is
/etc/ssh/sshd_config. sshd refuses to start if there is no con-
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.
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 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 (e.g.,
512) using sshd from inetd may be feasible.
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 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 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).
Specifies the port on which the server listens for connections
(default 22). Multiple port options are permitted. Ports speci-
fied 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
-t Test mode. Only check the validity of the configuration file and
sanity of the keys. This is useful for updating sshd reliably 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 configuration
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.
sshd 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).
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 ~/.hushlogin; see the FILES section).
2. If the login is on a tty, records login time.
3. Checks /etc/nologin; if it exists, prints contents and quits
4. Changes to run with normal user privileges.
5. Sets up basic environment.
6. Reads the file ~/.ssh/environment, if it exists, and users are
allowed to change their environment. See the
PermitUserEnvironment option in sshd_config(5).
7. Changes to user’s home directory.
8. If ~/.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
~/.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, key-
type, 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, mod-
ulus and comment fields give the RSA key for protocol version 1; the com-
ment 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
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 kilo-
bytes, 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
identity.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 protocol
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
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); how-
ever, 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
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 chan-
nel is required, one must not request a pty or should specify
no-pty. A quote may be included in the command by quoting it
with a backslash. This option might be useful to restrict cer-
tain public keys to perform just a specific operation. An exam-
ple 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.
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.
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
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
no-pty Prevents tty allocation (a request to allocate a pty will fail).
Limit local ‘‘ssh -L’’ port forwarding such that it may only con-
nect to the specified host and port. IPv6 addresses can be spec-
ified with an alternative syntax: host/port. Multiple permitopen
options may be applied separated by commas. No pattern matching
is performed on the specified hostnames, they must be literal
domains or addresses.
1024 33 12121...312314325 firstname.lastname@example.org
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
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 ~/.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 auto-
matically: 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 wild-
cards); 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 indi-
cate 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 opera-
tors 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 recom-
mended) 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 con-
flicting 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.
closenet,...,220.127.116.11 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,18.104.22.168 ssh-rsa AAAA1234.....=
# A hashed hostname
Contains configuration data for sshd. The file format and con-
figuration options are described in sshd_config(5).
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.
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).
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
Exchange". The file format is described in moduli(5).
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-
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-
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
identity.pub, id_dsa.pub and/or id_rsa.pub files into this file,
as described in ssh-keygen(1).
These files are consulted when using rhosts with RSA host authen-
tication 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
~/.ssh/known_hosts can, but need not be, world-readable.
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
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
Access controls that should be enforced by tcp-wrappers are
defined here. Further details are described in hosts_access(5).
This file is used during RhostsRSAAuthentication and
HostbasedAuthentication and contains host-username pairs, sepa-
rated by a space, one per line. The given user on the corre-
sponding 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
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 RhostsRSAAuthentication and
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
any user on this machine (except root). Additionally, the syntax
“+@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
Note that this warning also applies to rsh/rlogin.
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.
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.
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-
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
echo add unix:‘echo $DISPLAY |
cut -c11-‘ $proto $cookie
echo add $DISPLAY $proto $cookie
fi | xauth -q -
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.
Like ~/.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.
scp(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),
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January
2002, work in progress material.
M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman Group Exchange
for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-group-
exchange-02.txt, January 2002, work in progress material.
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 cre-
ated 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.
BSD September 25, 1999 BSD
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