SSHD(8) BSD System Manager's Manual SSHD(8)
sshd -- OpenSSH SSH daemon
sshd [-deiqtD46] [-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 programs replace rlogin and rsh, and provide secure encrypted communica-
tions 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 normally 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 imple-
mentation 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,
.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, rlogind, and rexecd are disabled (thus completely disabling rlogin and rsh into the
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
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-md5).
Protocol version 2 provides a public key based user (PubkeyAuthentication) or client host (HostbasedAuthentication) authentication method,
conventional password authentication and challenge response based methods.
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 command on the server side, and the user terminal in 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 a configuration file. Command-line options override values specified in the configura-
sshd rereads its configuration file when it receives a hangup signal, SIGHUP, by executing itself with the name it was started as, i.e.,
The options are as follows:
Specifies the number of bits in the ephemeral protocol version 1 server key (default 768).
-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 configura-
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 protocol 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. 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 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 give options in the format used in the configuration file. This is useful for specifying options for which there is
no separate command-line flag.
Specifies the port on which the server listens for connections (default 22). Multiple port options are permitted. Ports specified
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 overflow
this field to still be uniquely identified. Specifying -u0 indicates that only dotted decimal addresses should be put into the utmp
file. -u0 is 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 RhostsAuthentication, 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.
-D When this option is specified sshd will not detach and does not become a daemon. This allows easy monitoring of sshd.
-4 Forces sshd to use IPv4 addresses only.
-6 Forces sshd to use IPv6 addresses only.
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 config-
uration file or by $HOME/.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 (unless root).
4. Changes to run with normal user privileges.
5. Sets up basic environment.
6. Reads $HOME/.ssh/environment if it exists and users are allowed to change their environment. See the PermitUserEnvironment option
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 protocol
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). 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 specifications. No spaces are permitted, except within double quotes. The fol-
lowing option specifications are supported (note that option keywords are case-insensitive):
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 secu-
rity: 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).
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 a 8-bit clean channel 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 back-
slash. 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 explic-
itly 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 over-
ride 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 authentication. Any port forward requests by the client will return an error.
This might be used, e.g., 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.
no-pty Prevents tty allocation (a request to allocate a pty will fail).
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 permitopen options may be applied separated by commas. No pattern matching is per-
formed on the specified hostnames, they must be literal domains or addresses.
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
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 maintained 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
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
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.
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,...,126.96.36.199 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,188.8.131.52 ssh-rsa AAAA1234.....=
Contains configuration data for sshd. The file format and configuration 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 conve-
nience 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-writable.
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-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 (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).
/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. /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.
Access controls that should be enforced by tcp-wrappers are defined here. Further details are described in hosts_access(5).
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.
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.
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 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 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. 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.
This is processed exactly as /etc/hosts.equiv. However, this file may be useful in environments that want to run both rsh/rlogin and
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 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(1) because sshd will not run xauth automati-
cally 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 environment.
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 $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.
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.
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), login.conf(5), moduli(5), sshd_config(5), sftp-server(8)
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.
September 25, 1999 BSD