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.|
|Specifies the number of bits in the ephemeral protocol version 1 server key (default 768).|
|-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 the system log, 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||When this option is specified, sshd will send the output to the standard error instead of the system log.|
|Specifies the name of the configuration file. The default is /etc/ssh/sshd_config. sshd refuses to start if there is no configuration file.|
|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.|
|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_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.|
|-i||Specifies that sshd 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.|
|Specifies how often the ephemeral protocol version 1 server key is regenerated (default 3600 seconds, or one hour). The motivation 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.|
|Can be used to give options in the format used in the configuration 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).|
|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||Test mode. Only check the validity of the configuration file and sanity of the keys. This is useful for updating sshd reliably as configuration options may change.|
|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 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-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* ).
System security is not improved unless rshd, rlogind, and rexecd are disabled (thus completely disabling rlogin and rsh into the machine).
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.
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 dont 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):
|Specifies that in addition to public key authentication, the canonical name of the remote host must be present in the comma-separated list of patterns (* 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).|
|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 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.|
|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.|
|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 command option.|
|Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error.|
|Forbids authentication agent forwarding when this key is used for authentication.|
|Prevents tty allocation (a request to allocate a pty will fail).|
|Limit local 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.|
|Force a tun(4) device on the server. Without this option, the next available device will be used if the client requests a tunnel.|
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...== firstname.lastname@example.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 files 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 dont 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,...,18.104.22.168 1024 37 159...93 closenet.hut.fi cvs.openbsd.org,22.214.171.124 ssh-rsa AAAA1234.....=
# A hashed hostname |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa AAAA1234.....=
|Contains configuration data for sshd. The file format and configuration options are described in sshd_config(5).|
|/etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key|
|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 sshd does not start if this file is group/world-accessible.|
|/etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.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 known hosts files. These files are created using ssh-keygen(1).|
|Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". The file format is described in moduli(5).|
|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.|
|Contains the process ID of the 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.|
|Lists the public keys (RSA or DSA) that can be used to log into the users account. This file must be readable by root (which may on some machines imply it being world-readable if the users home directory resides on an NFS volume). It is recommended that it not be accessible by others. The format of this file is described above. Users will place the contents of their identity.pub, id_dsa.pub and/or id_rsa.pub files into this file, as described in ssh-keygen(1).|
|These files are consulted when using rhosts with RSA host authentication or protocol version 2 hostbased authentication to check the public key of the host. The key must be listed in one of these files to be accepted. The client uses the same files to verify that it is connecting to the correct remote host. These files should be writable only by root/the owner. /etc/ssh/ssh_known_hosts should be world-readable, and ~/.ssh/known_hosts can, but need not be, world-readable.|
|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.|
|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.|
|Access controls that should be enforced by tcp-wrappers are defined here. Further details are described in hosts_access(5).|
This file is used during
and contains host-username pairs, separated by a space, one per
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.
|For ssh, this file is exactly the same as for .rhosts. However, this file is not used by rlogin and rshd, so using this permits access using SSH only.|
This file is used during
In the simplest form, this file contains host names, one per line.
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
user on this machine (except root).
Additionally, the syntax
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.
|This is processed exactly as /etc/hosts.equiv. However, this file may be useful in environments that want to run both rsh/rlogin and ssh.|
|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.|
If this file exists, it is run with
after reading the
environment files but before starting the users shell or command.
It must not produce any output on stdout; stderr must be used
If X11 forwarding is in use, it will receive the "proto cookie" pair in
its standard input (and
in its environment).
The script must call
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 users 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 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.
|Like ~/.ssh/rc. This can be used to specify machine-specific login-time initializations globally. This file should be writable only by root, and should be world-readable.|