sshd
listens for connections from clients.
It is normally started at boot from
/etc/rc.d/sshd
.
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:
/etc/ssh/sshd_config
.
sshd
refuses to start if there is no configuration file.
/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.
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 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.
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-md5, hmac-sha1, umac-64 or hmac-ripemd160).
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.
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.
/etc/motd
(unless prevented in the configuration file or by
~/.hushlogin
;
see the
FILES
section).
/etc/nologin
;
if it exists, prints contents and quits
(unless root).
~/.ssh/environment
,
if it exists, and users are allowed to change their environment.
See the
PermitUserEnvironment
option in
sshd_config(5).
~/.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.
~/.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 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.
~/.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):
SSH_ORIGINAL_COMMAND
environment variable.
Note that this option applies to shell, command or subsystem execution.
See PATTERNS in ssh_config(5) for more information on patterns.
~/.ssh/rc
.
``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.
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
/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 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 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.....=
/etc/motd
,
if
PrintLastLog
and
PrintMotd,
respectively,
are enabled.
It does not suppress printing of the banner specified by
Banner.
.rhosts
,
but allows host-based authentication without permitting login with
rlogin/rsh.
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''.
The recommended permissions can be set by executing
``chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys''.
hosts.equiv
,
but allows host-based authentication without permitting login with
rlogin/rsh.
~/.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.
rlogin
and
rsh
into the machine).