SSHD

SSHD(8) BSD System Manager’s Manual SSHD(8)

NAME top

sshd — OpenSSH SSH daemon

SYNOPSIS top

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 top

sshd (OpenSSH 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.

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 execution, 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 directives in the configura‐
tion 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
keyword=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-sepa‐
rated 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 monitoring 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 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_dsa_key, /etc/ssh/ssh_host_ecdsa_key.
/etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for
protocol version 2. It is possible to have multiple host key
files for the different protocol versions and host key algo‐
rithms.

-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 moti‐
vation for regenerating the key fairly often is that the key is
not stored anywhere, and after about an hour it becomes impos‐
sible 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 regener‐
ated.

-o option
Can be used to give options in the format used in the configu‐
ration 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 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 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 pre‐
vent sshd from making DNS requests unless the authentication
mechanism or configuration requires it. Authentication mecha‐
nisms 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 top

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 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.

For protocol 2, 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-md5,
hmac-sha1, umac-64, umac-128, hmac-ripemd160, hmac-sha2-256 or hmac-
sha2-512).

Finally, the server and the client enter an authentication dialog. The
client tries to authenticate itself using host-based authentication,
public key authentication, challenge-response authentication, or pass‐
word 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 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 connections, or forwarding the authentication agent connection over
the secure channel.

After this, the client either requests a shell or execution of a com‐
mand. 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 con‐
nections have been closed, the server sends command exit status to the
client, and both sides exit.

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 and the sshd_config(5) PermitUserRC
option is set, 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 top

If the file ~/.ssh/rc exists, sh(1) runs it after reading the environ‐
ment 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 cook‐
ies.

The primary purpose of this file is to run any initialization routines
which may be needed before the user’s home directory becomes accessi‐
ble; 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
# 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.

AUTHORIZED_KEYS FILE FORMAT top

AuthorizedKeysFile specifies the files containing public keys for pub‐
lic key authentication; if none is specified, 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 following
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, 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 “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 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):

cert-authority
Specifies that the listed key is a certification authority (CA)
that is trusted to validate signed certificates for user
authentication.

Certificates may encode access restrictions similar to these
key options. If both certificate restrictions 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 supplied by the user (if
any) is ignored. The command is run on a pty if the client
requests a pty; otherwise it is run without a tty. If an 8-bit
clean channel is required, one must not request a pty or should
specify 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 opera‐
tion. 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 com‐
mand 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.

In addition to the wildcard matching that may be applied to
hostnames or addresses, a from stanza may match IP addresses
using CIDR address/masklen notation.

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).

no-agent-forwarding
Forbids authentication agent forwarding when this key is used
for authentication.

no-port-forwarding
Forbids TCP 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-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 authentica‐
tion. 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. Multi‐
ple permitopen options may be applied separated by commas. No
pattern matching is performed on the specified hostnames, 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 cer‐
tificate authentication as a comma-separated 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 top

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: 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.

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. A hostname or address may optionally be enclosed within ‘[’ and
‘]’ brackets then followed by ‘:’ and a non-standard port number.

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, 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 certification 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 authenti‐
cation 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 dif‐
ferent 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; authen‐
tication 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 automated 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 top

~/.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 parti‐
tion, because sshd reads it as root. Additionally, this file
must be owned by the user, and must not have write permissions
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 permitting login with
rlogin/rsh.

~/.ssh/
This directory is the default location for all user-specific
configuration and authentication information. There is no gen‐
eral requirement to keep the entire contents of this directory
secret, but the recommended permissions are read/write/execute
for the user, and not accessible by others.

~/.ssh/authorized_keys
Lists the public keys (DSA, ECDSA, ED25519, RSA) that can be
used for logging in as this user. The format of this file is
described above. The content of the file is not highly sensi‐
tive, but the recommended permissions are read/write for the
user, and not accessible by others.

If this file, the ~/.ssh directory, or the user’s home direc‐
tory are writable by other users, then the file could be modi‐
fied 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, 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.

~/.ssh/known_hosts
Contains a list of host keys for all hosts the user has logged
into that are not already in the systemwide 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.equiv
This file is for host-based authentication (see ssh(1)). It
should only be writable by root.

/etc/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 displayed to anyone trying to
log in, and non-root connections are refused. The file should
be world-readable.

/etc/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, readable only by root, and
not accessible to others. Note that sshd does not start if
these files are group/world-accessible.

/etc/ssh/ssh_host_key.pub
/etc/ssh/ssh_host_dsa_key.pub
/etc/ssh/ssh_host_ecdsa_key.pub
/etc/ssh/ssh_host_ed25519_key.pub
/etc/ssh/ssh_host_rsa_key.pub
These 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/ssh/ssh_known_hosts
Systemwide list of known host keys. This file should be pre‐
pared 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 con‐
figuration options are described in sshd_config(5).

/etc/ssh/sshrc
Similar to ~/.ssh/rc, it 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.

/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.

/var/run/sshd.pid
Contains the process ID of the sshd listening for connections
(if there are several daemons running concurrently for differ‐
ent ports, this contains the process ID of the one started
last). The content of this file is not sensitive; it can be
world-readable.

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