| SSHD(8) | System Manager's Manual | SSHD(8) |
sshd —
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 (OpenSSH Daemon) is the daemon program for
ssh(1). Together these programs replace rlogin and rsh, 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 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:
-4sshd to use IPv4 addresses only.-6sshd to use IPv6 addresses only.-b
bits-Dsshd will not
detach and does not become a daemon. This allows easy monitoring of
sshd.-d-d options
increase the debugging level. Maximum is 3.-esshd will send the
output to the standard error instead of the system log.-f
configuration_filesshd
refuses to start if there is no configuration file.-g
login_grace_time-h
host_key_filesshd 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.-isshd 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.-k
key_gen_time-o
option-p
portPort option are
ignored when a command-line port is specified. Ports specified using the
ListenAddress option override command-line
ports.-q-tsshd
reliably as configuration options may change.-u
lenutmp 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 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.Protocol option in sshd_config(5).
Protocol 2 supports both RSA and DSA 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, 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).
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 password 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 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 Linux). 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 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 does the
following:
PermitUserEnvironment option in
sshd_config(5).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 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.
AuthorizedKeysFile specifies the file containing public
keys for public key authentication; if none is specified, the default is
~/.ssh/authorized_keys. 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 “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, 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):
command="command"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.environment="NAME=value"PermitUserEnvironment option. This option is
automatically disabled if UseLogin is
enabled.from="pattern-list"See PATTERNS in ssh_config(5) for more information on patterns.
no-agent-forwardingno-port-forwardingcommand option.no-ptyno-X11-forwardingpermitopen="host:port"``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 performed on the specified hostnames, they
must be literal domains or addresses.tunnel="n"An example authorized_keys file:
# Comments allowed at start of line ssh-rsa AAAAB3Nza...LiPk== user@example.net from="*.sales.example.net,!pc.sales.example.net" ssh-rsa AAAAB2...19Q== john@example.net 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...== jane@example.net
Each line in these files contains the following fields: hostnames, bits, exponent, modulus, comment. The fields are separated by spaces.
Hostnames is a comma-separated list of patterns
(‘*’ and
‘?’ act as wildcards); each pattern in
turn is matched against the canonical host name (when authenticating a
client) or against the user-supplied name (when authenticating a server). A
pattern may also be preceded by ‘!’ to
indicate negation: if the host name matches a negated pattern, it is not
accepted (by that line) even if it matched another pattern on the line. 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. 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.
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.....=
PrintLastLog
and PrintMotd, respectively, are enabled. It does
not suppress printing of the banner specified by
Banner.
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.
#’), 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.
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.
sshd does not start if these
files are group/world-accessible.
sshd. The file
format and configuration options are described in
sshd_config(5).
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.
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.rshd,
rlogind, and rexecd are
disabled (thus completely disabling rlogin and
rsh into the machine).
| September 25, 1999 | BSD |