NAME
sssshhdd - OpenSSH SSH daemon
SYNOPSIS
sssshhdd [-4466DDddeeiiqqTTtt] [-bb bits] [-CC connectionspec] [-ff configfile]
[-gg logingracetime] [-hh hostkeyfile] [-kk keygentime]
[-oo option] [-pp port] [-uu len]
DESCRIPTION
sssshhdd (OpenSSH Daemon) is the daemon program for ssh(1). Together theseprograms replace rlogin(1) and rsh(1), and provide secure encrypted com-
munications between two untrusted hosts over an insecure network. sssshhdd 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, com-
mand execution, and data exchange.sssshhdd can be configured using command-line options or a configuration file
(by default sshdconfig(5)); command-line options override values speci-
fied in the configuration file. sssshhdd rereads its configuration file when it receives a hangup signal, SIGHUP, by executing itself with the nameand options it was started with, e.g. /usr/sbin/sshd.
The options are as follows:-44 Forces sssshhdd to use IPv4 addresses only.
-66 Forces sssshhdd to use IPv6 addresses only.
-bb bits
Specifies the number of bits in the ephemeral protocol version 1 server key (default 1024).-CC connectionspec
Specify the connection parameters to use for the -TT extended test
mode. If provided, any MMaattcchh directives in the configuration file that would apply to the specified user, host, and addresswill be set before the configuration is written to standard out-
put. The connection parameters are supplied as keyword=value pairs. The keywords are ``user'', ``host'', and ``addr''. Allare required and may be supplied in any order, either with multi-
ple -CC options or as a comma-separated list.
-DD When this option is specified, sssshhdd will not detach and does not
become a daemon. This allows easy monitoring of sssshhdd.-dd Debug mode. The server sends verbose debug output to the system
log, and does not put itself in the background. The server also will not fork and will only process one connection. This optionis only intended for debugging for the server. Multiple -dd
options increase the debugging level. Maximum is 3.-ee When this option is specified, sssshhdd will send the output to the
standard error instead of the system log.-ff configfile
Specifies the name of the configuration file. The default is/etc/sshdconfig. sssshhdd refuses to start if there is no configu-
ration file.-gg logingracetime
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.-hh hostkeyfile
Specifies a file from which a host key is read. This option must be given if sssshhdd is not run as root (as the normal host key files are normally not readable by anyone but root). The default is /etc/sshhostkey for protocol version 1, and/etc/sshhostrsakey and /etc/sshhostdsakey for protocol ver-
sion 2. It is possible to have multiple host key files for the different protocol versions and host key algorithms.-ii Specifies that sssshhdd is being run from inetd(8). sssshhdd 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 sssshhdd from inetd may be feasible.-kk keygentime
Specifies how often the ephemeral protocol version 1 server keyis regenerated (default 3600 seconds, or one hour). The motiva-
tion 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 communications even if the machine is cracked into or physically seized. A value of zero indicates that the key will never be regenerated.-oo option
Can be used to give options in the format used in the configura-
tion file. This is useful for specifying options for which thereis no separate command-line flag. For full details of the
options, and their values, see sshdconfig(5).
-pp port
Specifies the port on which the server listens for connections(default 22). Multiple port options are permitted. Ports speci-
fied in the configuration file with the PPoorrtt option are ignoredwhen a command-line port is specified. Ports specified using the
LLiisstteennAAddddrreessss option override command-line ports.
-qq Quiet mode. Nothing is sent to the system log. Normally the
beginning, authentication, and termination of each connection is logged.-TT Extended test mode. Check the validity of the configuration
file, output the effective configuration to stdout and then exit.Optionally, MMaattcchh rules may be applied by specifying the connec-
tion parameters using one or more -CC options.
-tt Test mode. Only check the validity of the configuration file and
sanity of the keys. This is useful for updating sssshhdd reliably as configuration options may change.-uu 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 usedinstead. This allows hosts with very long host names that over-
flow this field to still be uniquely identified. Specifying -uu00
indicates that only dotted decimal addresses should be put intothe utmp file. -uu00 may also be used to prevent sssshhdd from making
DNS requests unless the authentication mechanism or configuration requires it. Authentication mechanisms that may require DNS include RRhhoossttssRRSSAAAAuutthheennttiiccaattiioonn, HHoossttbbaasseeddAAuutthheennttiiccaattiioonn, andusing a ffrroomm==""ppaatttteerrnn-lliisstt"" option in a key file. Configuration
options that require DNS include using a USER@HOST pattern in AAlllloowwUUsseerrss or DDeennyyUUsseerrss. AAUUTTHHEENNTTIICCAATTIIOONN The OpenSSH SSH daemon supports SSH protocols 1 and 2. Both protocols are supported by default, though this can be changed via the PPrroottooccoolloption in sshdconfig(5). Protocol 2 supports both RSA and DSA keys;
protocol 1 only supports RSA keys. For both protocols, each host has ahost-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 itsown database to verify that it has not changed. The client then gener-
ates 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 Blowfishor 3DES, with 3DES being used by default. The client selects the encryp-
tion 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 restof 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 theserver. Additionally, session integrity is provided through a crypto-
graphic message authentication code (hmac-md5, hmac-sha1, umac-64 or
hmac-ripemd160).
Finally, the server and the client enter an authentication dialog. Theclient tries to authenticate itself using host-based authentication, pub-
lic 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 DDeennyyUUsseerrss or its group is listed in DDeennyyGGrroouuppss . 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 accountwhile 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 likeallocating a pseudo-tty, forwarding X11 connections, forwarding TCP con-
nections, 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 connec-
tions have been closed, the server sends command exit status to the client, and both sides exit. LLOOGGIINN PPRROOCCEESSSS When a user successfully logs in, sssshhdd 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 thePPeerrmmiittUUsseerrEEnnvviirroonnmmeenntt option in sshdconfig(5).
7. Changes to user's home directory. 8. If ~/.ssh/rc exists, runs it; else if /etc/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. SSSSHHRRCC If the file ~/.ssh/rc exists, sh(1) runs it after reading the environmentfiles but before starting the user's shell or command. It must not pro-
duce any output on stdout; stderr must be used instead. If X11 forward-
ing 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 sssshhdd 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/sshrc is run, and if that does not exist either, xauth is used to add the cookie. AUTHORIZEDKEYS FILE FORMAT AAuutthhoorriizzeeddKKeeyyssFFiillee specifies the file containing public keys for public key authentication; if none is specified, the default is ~/.ssh/authorizedkeys. Each line of the file contains one key (emptylines 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 ver-
sion 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 kilo-
bytes, 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, iddsa.pub, or the idrsa.pub file and edit it. sssshhdd 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 specifica-
tions. No spaces are permitted, except within double quotes. The fol-
lowing option specifications are supported (note that option keywords arecase-insensitive):
ccoommmmaanndd==""ccoommmmaanndd"" 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 apty; otherwise it is run without a tty. If an 8-bit clean chan-
nel is required, one must not request a pty or should specifynnoo-ppttyy. A quote may be included in the command by quoting it
with a backslash. This option might be useful to restrict cer-
tain public keys to perform just a specific operation. An exam-
ple 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 suppliedby the client is available in the SSHORIGINALCOMMAND environ-
ment variable. Note that this option applies to shell, command or subsystem execution.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 PPeerrmmiittUUsseerrEEnnvviirroonnmmeenntt option. This option is automatically disabled if UUsseeLLooggiinn is enabled.ffrroomm==""ppaatttteerrnn-lliisstt""
Specifies that in addition to public key authentication, either the canonical name of the remote host or its IP address must bepresent in the comma-separated list of patterns. See PATTERNS in
sshconfig(5) for more information on patterns.In addition to the wildcard matching that may be applied to host-
names or addresses, a ffrroomm stanza may match IP addressess 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).nnoo-aaggeenntt-ffoorrwwaarrddiinngg
Forbids authentication agent forwarding when this key is used for authentication.nnoo-ppoorrtt-ffoorrwwaarrddiinngg
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 ccoommmmaanndd option.nnoo-ppttyy Prevents tty allocation (a request to allocate a pty will fail).
nnoo-uusseerr-rrcc
Disables execution of ~/.ssh/rc.nnoo-XX1111-ffoorrwwaarrddiinngg
Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error. ppeerrmmiittooppeenn==""hhoosstt::ppoorrtt""Limit local ``ssh -L'' port forwarding such that it may only con-
nect to the specified host and port. IPv6 addresses can be spec-
ified with an alternative syntax: host/port. Multiple ppeerrmmiittooppeenn options may be applied separated by commas. No pattern matching is performed on the specified hostnames, they must be literal domains or addresses. ttuunnnneell==""nn"" Force a tun(4) device on the server. Without this option, thenext available device will be used if the client requests a tun-
nel. An example authorizedkeys 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.netcommand="dump /home",no-pty,no-port-forwarding ssh-dss
AAAAC3...51R== example.netpermitopen="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 SSSSHHKKNNOOWWNNHHOOSSTTSS FFIILLEE FFOORRMMAATT The /etc/sshknownhosts and ~/.ssh/knownhosts files contain host public keys for all known hosts. The global file should be prepared by theadministrator (optional), and the per-user file is maintained automati-
cally: whenever the user connects from an unknown host, its key is addedto 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 wild-
cards); 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 indi-
cate 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 mayappear on a single line and none of the above negation or wildcard opera-
tors may be applied. Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained, for example, from /etc/sshhostkey.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 anymatching line has the proper key. It is thus permissible (but not recom-
mended) to have several lines or different host keys for the same names. This will inevitably happen when short forms of host names from differentdomains are put in the file. It is possible that the files contain con-
flicting 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/sshhostkey.pub and adding the host names at the front. An example sshknownhosts file:# Comments allowed at start of line
closenet,...,192.0.2.53 1024 37 159...93 closenet.example.netcvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
# A hashed hostname
|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
AAAA1234.....= FILES ~/.hushlogin This file is used to suppress printing the last login time and /etc/motd, if PPrriinnttLLaassttLLoogg and PPrriinnttMMoottdd, respectively, are enabled. It does not suppress printing of the banner specified by BBaannnneerr. ~/.rhostsThis file is used for host-based authentication (see ssh(1) for
more information). On some machines this file may need to beworld-readable if the user's home directory is on an NFS parti-
tion, because sssshhdd 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 allowshost-based authentication without permitting login with
rlogin/rsh. ~/.ssh/This directory is the default location for all user-specific con-
figuration and authentication information. There is no general 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/authorizedkeys Lists the public keys (RSA/DSA) 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 sensitive, 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 directory are writable by other users, then the file could be modified or replaced by unauthorized users. In this case, sssshhdd will not allow it to be used unless the SSttrriiccttMMooddeess option has been set to ``no''. The recommended permissions can be set by executing``chmod go-w ~/ ~/.ssh ~/.ssh/authorizedkeys''.
~/.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 PPeerrmmiittUUsseerrEEnnvviirroonnmmeenntt option. ~/.ssh/knownhosts 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 notbe, 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.denyAccess controls that should be enforced by tcp-wrappers are
defined here. Further details are described in hostsaccess(5). /etc/hosts.equivThis file is for host-based authentication (see ssh(1)). It
should only be writable by root. /etc/moduliContains 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, sssshhdd refuses to let anyone except root log in. The contents of the file are displayed to anyone trying tolog 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, butallows host-based authentication without permitting login with
rlogin/rsh. /etc/sshhostkey /etc/sshhostdsakey /etc/sshhostrsakey These three 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 sssshhdd does not start ifthese files are group/world-accessible.
/etc/sshhostkey.pub /etc/sshhostdsakey.pub /etc/sshhostrsakey.pub These three 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 knownhosts files. These files are created using ssh-keygen(1).
/etc/sshknownhosts 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/theowner and should be world-readable.
/etc/sshdconfig
Contains configuration data for sssshhdd. The file format and con-
figuration options are described in sshdconfig(5).
/etc/sshrcSimilar 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 chroot(2) directory used by sssshhdd during privilege separation inthe 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 sssshhdd 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-read-
able.SEE ALSO
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
ssh-keyscan(1), chroot(2), hostsaccess(5), sshdconfig(5),
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, Theode Raadt and Dug Song removed many bugs, re-added newer features and cre-
ated 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. CCAAVVEEAATTSSSystem security is not improved unless rrsshhdd, rrllooggiinndd, and rreexxeeccdd are dis-
abled (thus completely disabling rlogin and rsh into the machine). BSD December 21, 2019 BSD