sshd - secure shell daemon

Synopsis

Description

Sshd is the daemon that listens for connections from clients. It is normally started at boot time as a detached process from %ETC%\tcpexit.cmd or %ETC%\tcpstart.cmd or equivalent. It forks a new daemon for each incoming connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange.

Sshd works as follows. Each host has a host-specific RSA key (normally 1024 bits) used to identify this host. Additionally, when the daemon starts, it generates a server RSA key (normally 768 bits). This key is normally regenerated every hour if it has been used, and is never stored on disk.

Whenever a client connects to the daemon, the daemon sends its host and server public keys to the client. The client compares the 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 start to 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, IDEA, DES, 3DES, ARCFOUR, and TSS (a fast home-grown algorithm) are supported. IDEA is used by default. The client selects the encryption algorithm to be used 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, .rhosts authentication combined with RSA host authentication, RSA challenge-response authentication, or password based authentication. Under OS/2, it is recommended to use only RSA challenge-response authentication since there is no user based file protection (erverybody having access to your system could be considered having root permissions).

Rhosts authentication is normally disabled because it is fundamentally insecure, but it can be enabled in the server configuration file if desired. System security is not improved unless rshd and rexecd are disabled (thus completely disabling rsh access into that machine).

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/IP connections, or forwarding the authentication agent connection over the secure channel.

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 can be configured using command-line options or a configuration file. Command-line options override values specified in the configuration file.

Sshd rereads its configuration file if it is sent the hangup signal, SIGHUP.

Since sshd originates in the Unix environment certain "unicisms" also surface in the OS/2 version.

• Filenames reported by verbose output, error messages and the like use forward slashes to separate directories.
• Files use Unix line end conventions separating lines by a line feed instead of the combination carriage return and line feed.
• The ssh programs use a number of files which usually do not exist under OS/2. Many of them are related to the fact that Unix is a multi-user system where many people log in from a number of different sources. The code using these files has not been disabled for the OS/2 version of ssh as long as the absence of these files does not have a negative impact.
• Options and functionality which make no sense under OS/2 are disabled.

Options

-b bits

Specifies the number of bits in the server key (default 768).

-d

Debug mode. The server sends verbose debug output to the system log, and does not put itself in the background and will only process one connection. This option is only intended for debugging for the server.

-f configuration_file

Specifies the name of the configuration file. The default is %ETC%/sshd_config.

-g login_grace_time

Gives the grace time for clients to authenticate themselves (default 600 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.

-h host_key_file

Specifies the file from which the host key is read (default %ETC%/ssh_host_key).

-i

Specifies that sshd is being run from inetd. 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

Specifies how often the 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.

-p port

Specifies the port on which the server listens for connections (default 22).

-q

Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection is logged.

Configuration file

The following keywords are possible:

AllowHosts

This keyword can be followed by any number of host name patterns, separated by spaces. If specified, login is allowed only from hosts whose name matches one of the patterns. `*' and `?' can be used as wildcards in the patterns. Normal name servers are used to map the client's host into a canonical host name. If the name cannot be mapped, its IP-address is used as the host name. By default all hosts are allowed to connect. (Note that sshd can also be configured to use tcp_wrappers using the --with-libwrap compile-time configuration option.)

DenyHosts

This keyword can be followed by any number of host name patterns, separated by spaces. If specified, login is disallowed from the hosts whose name matches any of the patterns.

FascistLogging

Specifies whether to use verbose logging. Verbose logging violates the privacy of users and is not recommended. The argument must be "yes" or "no" (without the quotes). The default is "no".

HostKey

Specifies the file containing the private host key (default %ETC%/ssh_host_key).

IgnoreRhosts

Specifies that rhosts and shosts files will not be used in authentication. %ETC%/hosts.equiv and %ETC%/shosts.equiv are still used. The default is "no". Under OS/2, this should be set to "yes".

KeepAlive

Specifies whether the system should send keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. However, this means that connections will die if the route is down temporarily, and some people find it annoying. On the other hand, if keepalives are not send, sessions may hang indefinitely on the server, leaving "ghost" users and consuming server resources. The default is "yes" (to send keepalives), and the server will notice if the network goes down or the client host reboots. This avoids infinitely hanging sessions.

To disable keepalives, the value should be set to "no" in both the server and the client configuration files.

KeyRegenerationInterval

The server key is automatically regenerated after this many seconds (if it has been used). The purpose of regeneration is to prevent decrypting captured sessions by later breaking into the machine and stealing the keys. The key is never stored anywhere. If the value is 0, the key is never regenerated. The default is 3600 (seconds).

ListenAddress

Specifies the ip address of the interface where the sshd server socket is bound to.

LoginGraceTime

The server disconnects after this time if the user has not successfully logged in. If the value is 0, there is no time limit. The default is 600 (seconds).

PasswordAuthentication

Specifies whether password authentication is allowed. The default is "yes". Under OS/2, this should be set to "no".

PermitEmptyPasswords

When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is "yes". Under OS/2, this should be set to "no".

PermitRootLogin

Specifies whether the root can log in using ssh. Under OS/2 all logins are root logins since OS/2 is a single-user operating system. May be set to "yes", "nopwd", or "no". The default is "yes", allowing root logins through any of the authentication types allowed for other users. The "nopwd" value disables password-authenticated root logins. The "no" value disables root logins through any of the authentication methods. ("nopwd" and "no" are equivalent unless you have a .rhosts, .shosts, or .ssh/authorized_keys file in the root home directory.)

Root login with RSA authentication when the "command" option has been specified will be allowed regardless of the value of this setting (which may be useful for taking remote backups even if root login is normally not allowed).

PidFile

Specifies the location of the file containing the process ID of the master sshd daemon, the default is %ETC%/sshd.pid

Port Specifies the port number that sshd listens to. The default is 22.

PrintMotd

Specifies whether sshd should print %ETC%/motd when a user logs in interactively. The default is "yes".

QuietMode

Specifies whether the system runs in quiet mode. In quiet mode, nothing is logged in the system log, except fatal errors. The default is "no".

RandomSeed

Specifies the file containing the random seed for the server; this file is created automatically and updated regularly. The default is %ETC%/ssh_random_seed.

RhostsAuthentication

Specifies whether authentication using rhosts or %ETC%/hosts.equiv files is sufficient. Normally, this method should not be permitted because it is insecure. RhostsRSAAuthentication should be used instead, because it performs RSA-based host authentication in addition to normal rhosts or %ETC%/hosts.equiv authentication. The default is "no".

RhostsRSAAuthentication

Specifies whether rhosts or %ETC%/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is "yes". Under OS/2, this should be set to "no".

RSAAuthentication

Specifies whether pure RSA authentication is allowed. The default is "yes".

ServerKeyBits

Defines the number of bits in the server key. The minimum value is 512, and the default is 768.

StrictModes

Specifies whether sshd should check file modes and ownership of the user's home directory and rhosts files before accepting login. Under Unix this is normally desirable because novices sometimes accidentally leave their directory or files world-writable, therefore the default is "yes". Under OS/2, however, this must be set to "no".

SyslogFacility

Gives the facility code that is used when logging messages from sshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.

X11Forwarding

Specifies whether X11 forwarding is permitted. The default is "yes". Note that disabling X11 forwarding does not improve security in any way, as users can always install their own forwarders.

Login process

1. Sets up the basic environment.
2. Reads %ETC%/environment if it exists.
3. Reads %HOME%/.ssh/environment if it exists.
4. Changes to user's home directory.
5. If %HOME%/.ssh/rc.cmd exists, runs it using the command interpreter specified in the environment variable COMSPEC; else if %ETC%/sshrc.cmd exists, runs it; otherwise runs xauth. The "rc" files are given the X11 authentication protocol and cookie in standard input.
6. Runs the command interpreter specified in the environment variable COMSPEC or the command specified with ssh.

Authorized_keys file format

Note that lines in this file are usually several hundred bytes long (because of the size of the RSA key modulus). You don't want to type them in; instead, copy the identity.pub file and edit it.

The options (if present) consists of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported:

from="pattern-list"

Specifies that in addition to RSA 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: RSA 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"

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 connection requests a pty; otherwise it is run without a tty. A quote may be included in the command by quoting it with a backslash. This option might be useful to restrict certain RSA keys to perform just a specific operation. An example might be a key that permits remote backups but nothing else. Notice that the client may specify TCP/IP and/or X11 forwardings unless they are explicitly prohibited.

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.

no-port-forwarding

Forbids TCP/IP 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.

no-X11-forwarding

Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error.

no-agent-forwarding

Forbids authentication agent forwarding when this key is used for authentication.

no-pty

Prevents tty allocation (a request to allocate a pty will fail).

Examples

   1024 33 12121...312314325 ylo@foo.bar
   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

ssh_known_hosts file format

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.

Bits, exponent, and modulus are taken directly from the host key; they can be obtained e.g. from %ETC%/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 (see make-ssh-known-hosts) or by taking %ETC%/ssh_host_key.pub and adding the host names at the front.

Example

closenet,closenet.hut.fi,...,130.233.208.41 1024 37 159...93 closenet.hut.fi

Files

%ETC%/sshd_config

Contains configuration data for sshd.

%ETC%/ssh_host_key

Contains the private part of the host key. This file is normally created manually under OS/2 using ssh-keygen. This file must be protected against any access by others.

%ETC%/ssh_host_key.pub

Contains the public part of the host key. This file is normally created automatically by "make install", but can also be created manually. Its contents should match the private part. This file is not really used for anything; it is only provided for the convenience of the user so its contents can be copied to known hosts files. This file must be protected against write access by others.

%ETC%/ssh_random_seed

This file contains a seed for the random number generator. This file must be protected against write access by others

%ETC%/sshd.pid

Contains the process id of the sshd listening for connections (if there are several daemons running concurrently for different ports, this contains the pid of the one started last). The contents of this file are not sensitive; it can be read by others.

%HOME%/.ssh/authorized_keys

Lists the RSA keys that can be used to log into the user's account. It is recommended that this file is not accessible by others. The format of this file is described above.

%ETC%/ssh_known_hosts and %HOME%/.ssh/known_hosts

These files are consulted when using rhosts with RSA host 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 the remote host is the one we intended to connect.) These files must be protected against write access by others. %ETC%/ssh_known_hosts should be readable by others, and %HOME%/.ssh/known_hosts can but need not be readable by others.

%ETC%/nologin

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. Since under OS/2 all logins are root logins the presence of this file does not prevent logins.

%HOME%/.rhosts

This file contains host-username pairs, separated by a space, one per line. The given user on the corresponding host is permitted to log in without password. The same file is used by rlogind and rshd. Ssh differs from rlogind and rshd in that it requires RSA host authentication in addition to validating the host name retrieved from domain name servers (unless compiled with the --with-rhosts configuration option). The file must be writable only by the user; it is recommended that it not be accessible by others.

If 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.

%HOME%/.shosts

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.

%ETC%/hosts.equiv

This file is used during .rhosts 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 RSA host authentication is normally 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.

%ETC%/shosts.equiv

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.

%ETC%/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. This file is processed in all environments (normal rsh/rlogin only process it on AIX and potentially some other systems). The file should be writable only by root, and should be world-readable.

%HOME%/.ssh/environment

This file is read into the environment after %ETC%/environment. It has the same format. The file should be writable only by the user; it need not be readable by anyone else.

%HOME%/.ssh/rc.cmd

If this file exists, it is run with the user's shell after reading the environment files but before starting the user's shell or command. If X11 spoofing is in use, this will receive the "proto cookie" pair in standard input (and DISPLAY in environment). This must call xauth in that case.

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; then echo add $DISPLAY $proto $cookie | xauth -q -; fi".

If this file does not exist, %ETC%/sshrc.cmd is run, and if that does not exist either, xauth is used to store the cookie.

This file should be protected against any access by others.

%ETC%/sshrc.cmd

Like %HOME%/.ssh/rc.cmd. This can be used to specify machine-specific login-time initializations globally. This file must be protected against write access by others.

Installation

Considerable work has been put to making sshd secure. However, if you find a security problem, please report it immediately to <ssh-bugs@cs.hut.fi>.

Security under OS/2 only is provided if you let nobody access your machine, neither locally nor remote using ssh or any other means of access. Remember that there is no user-identity based file protection and that everybody having access to your machine can read and modify all files which should be protected against access - including the ssh files containing private keys and other volatile information.

Authors

Information about new releases, mailing lists, and other related issues can be found on the ssh home page at http://www.cs.hut.fi/ssh/.

See also