SSHD(8) BSD System Manager's Manual SSHD(8)
NAME
sshd — OpenSSH SSH daemon
SYNOPSIS
sshd [-46DdeiqTt] [-b bits] [-C connection_spec] [-c host_certificate_file] [-E log_file] [-f config_file]
[-g login_grace_time] [-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]
DESCRIPTION
sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these programs replace rlogin(1) and rsh(1),
and provide secure encrypted communications between two untrusted hosts over an insecure network.
sshd 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 exe‐
cution, and data exchange.
sshd can be configured using command-line options or a configuration file (by default sshd_config(5)); command-
line options override values 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 1024).
-C connection_spec
Specify the connection parameters to use for the -T extended test mode. If provided, any Match direc‐
tives 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 key‐
word=value pairs. The keywords are “user”, “host”, “laddr”, “lport”, and “addr”. All are required and
may be supplied in any order, either with multiple -C options or as a comma-separated list.
-c host_certificate_file
Specifies a path to a certificate file to identify sshd during key exchange. The certificate file must
match a host key file specified using the -h option or the HostKey configuration directive.
-D When this option is specified, sshd will not detach and does not become a daemon. This allows easy mon‐
itoring of sshd.
-d Debug mode. The server sends verbose debug output to standard error, 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 log_file
Append debug logs to log_file instead of the system log.
-e Write debug logs to standard error instead of the system log.
-f config_file
Specifies the name of the configuration file. The default is /etc/ssh/sshd_config. sshd refuses to
start if there is no configuration file.
-g login_grace_time
Gives the grace time for clients to authenticate themselves (default 120 seconds). If the client fails
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 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 communica‐
tions 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 configuration 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 specified in the configuration file with the Port option are ignored when a command-
line port is specified. Ports specified using the ListenAddress option override command-line ports.
-q Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination
of each connection is logged.
-T Extended test mode. Check the validity of the configuration file, output the effective configuration to
stdout and then exit. Optionally, Match rules may be applied by specifying the connection parameters
using one or more -C options.
-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 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.
AUTHENTICATION
The OpenSSH SSH daemon supports SSH protocols 1 and 2. The default is to use protocol 2 only, though this can
be changed via the Protocol option in sshd_config(5). Protocol 2 supports DSA, ECDSA, ED25519 and RSA keys;
protocol 1 only supports RSA keys. For both protocols, each host has a host-specific key, normally 2048 bits,
used to identify the host.
Forward security for protocol 1 is provided through an additional server key, normally 768 bits, generated when
the server starts. 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 com‐
pares 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 algo‐
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 definition 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 and UnixWare, ‘*’ on HP-UX, containing ‘Nologin’ on Tru64, a leading ‘*LOCKED*’
on FreeBSD and a leading ‘!’ 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 ‘NP’ or ‘*NP*’ ).
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 connec‐
tions, or forwarding the authentication agent connection over the secure channel.
After this, 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.
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 ~/.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 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. See SSHRC, below.
9. Runs user's shell or command.
SSHRC
If the file ~/.ssh/rc exists, 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 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 environment.
This file will probably contain some initialization code followed by something similar to:
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.
AUTHORIZED_KEYS FILE FORMAT
AuthorizedKeysFile specifies the files containing public keys for public key authentication; if none is speci‐
fied, the default is ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2. Each line of the file contains one key
(empty lines and lines starting with a ‘#’ are ignored as comments). Protocol 1 public keys consist of the fol‐
lowing space-separated fields: options, bits, exponent, modulus, comment. Protocol 2 public key consist 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, expo‐
nent, 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
“ecdsa-sha2-nistp256”, “ecdsa-sha2-nistp384”, “ecdsa-sha2-nistp521”, “ssh-ed25519”, “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 identity.pub, id_dsa.pub, id_ecdsa.pub, id_ed25519.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 following option specifications are supported (note that option keywords are case-
insensitive):
cert-authority
Specifies that the listed key is a certification authority (CA) that is trusted to validate signed cer‐
tificates for user authentication.
Certificates may encode access restrictions similar to these key options. If both certificate restric‐
tions and key options are present, the most restrictive union of the two is applied.
command="command"
Specifies that the command is executed whenever this key is used for authentication. The command sup‐
plied by the user (if any) is ignored. The command is run on a pty if the client requests a pty; other‐
wise it is run without a tty. If an 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 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
and/or X11 forwarding unless they are explicitly prohibited. The command originally supplied by the
client is available in the SSH_ORIGINAL_COMMAND environment variable. Note that this option applies to
shell, command or subsystem execution. Also note that this command may be superseded by either a
sshd_config(5) ForceCommand directive or a command embedded in a certificate.
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.
from="pattern-list"
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 PATTERNS in ssh_config(5)
for more information on patterns.
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 command option.
no-pty Prevents tty allocation (a request to allocate a pty will fail).
no-user-rc
Disables execution of ~/.ssh/rc.
no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client
will return an error.
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 by enclosing the address in square brackets. Multiple permitopen
options may be applied separated by commas. No pattern matching is performed on the specified host‐
names, they must be literal domains or addresses. A port specification of * matches any port.
principals="principals"
On a cert-authority line, specifies allowed principals for certificate authentication as a comma-sepa‐
rated list. At least one name from the list must appear in the certificate's list of principals for the
certificate to be accepted. This option is ignored for keys that are not marked as trusted certificate
signers using the cert-authority option.
tunnel="n"
Force a tun(4) device on the server. Without this option, the next available device will be used if the
client requests a tunnel.
An example authorized_keys file:
# Comments allowed at start of line
ssh-rsa AAAAB3Nza...LiPk== [email protected]
from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
AAAAB2...19Q== [email protected]
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...==
[email protected]
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 automati‐
cally: 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: markers (optional), hostnames, bits, exponent, modulus,
comment. The fields are separated by spaces.
The marker is optional, but if it is present then it must be one of “@cert-authority”, to indicate that the line
contains a certification authority (CA) key, or “@revoked”, to indicate that the key contained on the line is
revoked and must not ever be accepted. Only one marker should be used on a key line.
Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained, for example, 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; either
one that matches exactly or, if the server has presented a certificate for authentication, the key of the certi‐
fication authority that signed the certificate. For a key to be trusted as a certification authority, it must
use the “@cert-authority” marker described above.
The known hosts file also provides a facility to mark keys as revoked, for example when it is known that the
associated private key has been stolen. Revoked keys are specified by including the “@revoked” marker at the
beginning of the key line, and are never accepted for authentication or as certification authorities, but
instead will produce a warning from ssh(1) when they are encountered.
It is 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 possi‐
ble 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, ssh-keyscan(1) or by taking
/etc/ssh/ssh_host_key.pub and adding the host names at the front. ssh-keygen(1) also offers some basic auto‐
mated editing for ~/.ssh/known_hosts including removing hosts matching a host name and converting all host names
to their hashed representations.
An example ssh_known_hosts file:
# 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.....=
# A revoked key
@revoked * ssh-rsa AAAAB5W...
# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
FILES
~/.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.
~/.rhosts
This file is used for host-based authentication (see 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
sshd reads it as root. Additionally, this file must be owned by the user, and must not have write per‐
missions for anyone else. The recommended permission for most machines is read/write for the user, and
not accessible by others.
~/.shosts
This file is used in exactly the same way as .rhosts, but allows host-based authentication without per‐
mitting login with rlogin/rsh.
~/.ssh/authorized_keys
Lists the public keys (DSA, ECDSA, ED25519, RSA) that can be used for logging in as this user. The for‐
mat of this file is described above. The content of the file is not highly sensitive, but the recom‐
mended permissions are read/write for the user, and not accessible by others.
If this file, the ~/.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, sshd will not allow it to be
used unless the StrictModes option has been set to “no”.
~/.ssh/environment
This file is read into the environment at login (if it exists). It can only contain empty lines, com‐
ment 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.
~/.ssh/known_hosts
Contains a list of host keys for all hosts the user has logged into that are not already in the sys‐
temwide 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.
~/.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.
/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).
/etc/hosts.equiv
This file is for host-based authentication (see ssh(1)). It should only be writable by root.
/etc/ssh/moduli
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". The file format is
described in moduli(5).
/etc/motd
See motd(5).
/etc/nologin
If this file exists, sshd refuses to let anyone except root log in. The contents of the file are dis‐
played to anyone trying to log in, and non-root connections are refused. The file should be world-read‐
able.
/etc/ssh/shosts.equiv
This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without
permitting login with rlogin/rsh.
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key
These files contain the private parts of the host keys. These files should only be owned by root, read‐
/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.
/etc/ssh/sshd_config
Contains configuration data for sshd. The file format and configuration options are described in
sshd_config(5).
/etc/ssh/sshrc
Similar to ~/.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.
/var/empty/sshd
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.
/var/run/sshd.pid
Contains the process ID of the sshd listening for connections (if there are several daemons running con‐
currently 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.
ENVIRONMENT
SSH_USE_STRONG_RNG
The reseeding of the OpenSSL random generator is usually done from /dev/urandom. If the
SSH_USE_STRONG_RNG environment variable is set to value other than 0 the OpenSSL random generator is
reseeded from /dev/random. The number of bytes read is defined by the SSH_USE_STRONG_RNG value. Mini‐
mum is 14 bytes. This setting is not recommended on the computers without the hardware random generator
because insufficient entropy causes the connection to be blocked until enough entropy is available.
IPV6
IPv6 address can be used everywhere where IPv4 address. In all entries must be the IPv6 address enclosed in
square brackets. Note: The square brackets are metacharacters for the shell and must be escaped in shell.
SEE ALSO
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), chroot(2), hosts_access(5),
login.conf(5), moduli(5), sshd_config(5), inetd(8), sftp-server(8)
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.
CAVEATS
System security is not improved unless rshd, rlogind, and rexecd are disabled (thus completely disabling rlogin
and rsh into the machine).
BSD June 13, 2017 BSD
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