| 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.This implementation of sshd supports both
SSH protocol version 1 and 2 simultaneously. sshd
works as follows:
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 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 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*’ ).
System security is not improved unless
rshd, rlogind, and
rexecd are disabled (thus completely disabling
rlogin and rsh into the machine).
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.
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 reads configuration data from
/etc/ssh/sshd_config (or the file specified with
-f on the command line). The file format and
configuration options are described in sshd_config(5).
sshd does the
following:
PermitUserEnvironment option in
sshd_config(5).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) 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):
from="pattern-list"*’ 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); 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).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. 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.no-port-forwardingcommand option.no-X11-forwardingno-agent-forwardingno-ptypermitopen="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"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
tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org
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.
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, 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,...,130.233.208.41 1024 37 159...93 closenet.hut.fi cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
# A hashed hostname |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa AAAA1234.....=
sshd. The file
format and configuration options are described in
sshd_config(5).sshd does not start if this file
is group/world-accessible.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.PrintLastLog
and PrintMotd, respectively, are enabled. It does
not suppress printing of the banner specified by
Banner.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.RhostsRSAAuthentication
and HostbasedAuthentication and contains
host-username pairs, separated by a space, one per line. The given user on
the corresponding 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 others.
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.
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 entries.
Note that this warning also applies to rsh/rlogin.
#’), 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.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.
This file should be writable only by the user, and need not be readable by anyone else.
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 |