5. Configuring the Secure Shell (SSH) Server v2

This chapter describes how to configure and maintain the SSH for OpenVMS Secure Shell (SSH) server v2.

This is the server side of the software that allows secure interactive connections to other computers in the manner of rlogin/rshell/telnet. The SSH server has been developed to discriminate between SSH v1 and SSH v2 protocols, so the two protocols can coexist simultaneously on the same system.

SSH1 and SSH2 Differences

SSH1 and SSH2 are different, and incompatible, protocols. The SSH1 implementation is based on the version 1.5 protocol, and the SSH2 implementation is based on the V2. While SSH2 is generally regarded to be more secure than SSH1, both protocols are offered by SSH for OpenVMS, and although they are incompatible, they may exist simultaneously on an OpenVMS system. The SSH for OpenVMS server front-end identifies what protocol a client desires to use and will create an appropriate server for that client.

The cryptographic library used by the SSH2 server (this does not apply to SSH1 sessions) is FIPS 140-2 level 2 compliant, as determined by the Computer Security Division of the National Institute of Science and Technology (NIST).

Note: You must install the DEC C 6.0 backport library on all OpenVMS VAX v6.0 and earlier systems prior to using SSH. This is the AACRT060.A file. You can find the ECO on the MultiNet CD in the following directory: VAX55_DECC_RTL.DIR.

Restrictions

When using SSH2 to connect to a VMS server, if the VMS account is set up with a secondary password, SSH2 does not prompt the user for the secondary password. If the VMS primary password entered is valid, the user is logged in, bypassing the secondary password.

When using SSH2 to execute single commands (in the same manner as RSHELL), some keystrokes like CTRL+Y are ignored. In addition, some interactive programs such as HELP may not function as expected. This is a restriction of SSH2. If this behavior poses a problem, log into the remote system using SSH2 in interactive mode to execute the program.

Understanding the Secure Shell Server

Secure Shell daemon (SSHD) is the daemon program for SSH2 that listens for connections from clients. The server program replaces the rshell and telnet programs. The server/client programs provide secure encrypted communications between two untrusted hosts over an insecure network. A new daemon is created for each incoming connection. These daemons handle key exchange, encryption, authentication, command execution, and data exchange.

Servers and Clients

An SSH server is an OpenVMS system that acts as a host for executing interactive commands or for conducting an interactive session. The server software consists of two pieces of software (for future reference, “SSHD” will refer to both SSHD_MASTER and SSHD, unless otherwise specified):

· SSHD_MASTER, recognizes the differences between SSH v1 and SSH v2 and starts the appropriate server. If the request is for SSH v1, then the existing SSH v1 server is run; if the request is for SSH v2, then the SSH v2 server is run.

· SSHD, a copy of which is spawned for each connection instance. SSHD handles all the interaction with the SSH client.

A client is any system that accesses the server. A client program (SSH) is provided with SSH for OpenVMS, but any SSH client that uses SSH version 2 protocol may be used to access the server. Examples of such programs are MultiNet SSH, TCPware SSH, SecureCRT, and puTTY for Windows, MacSSH for Apple systems, and other SSH programs on Linux and UNIX-based systems.

Each host has a key using DSA encryption and is usually 1024 bits long (although, the user may create a different-sized key, if desired). The same key may be used on multiple machines. For example, each machine in a VMScluster could use the same key.

When a client connects to the SSHD daemon:

· The client and server together, using the Diffie-Hellman key-exchange method, determine a 256-bit random number to use as the "session key". This key is used to encrypt all further communications in the session.

Note that this key may be renegotiated between the client and the server on a periodic basis by including the RekeyIntervalSeconds keyword in the server configuration file (SSH2_DIR:SSHD2_CONFIG). This is desirable because during long sessions, the more data that is exchanged using the same encryption key, the more likely it is that an attacker who is watching the encrypted traffic could deduce the session key.

· The server informs the client which encryption methods it supports. See the description of the CIPHERS configuration keyword for the encryption methods supported.

· The client selects the encryption algorithm from those offered by the server.

· The client and the server then enter a user authentication dialog. The server informs the client which authentication methods it supports, and the client then attempts to authenticate the user by using some or all of the authentication methods. The following authentication algorithms are supported:

o public-key (DSA keys)

o host-based

o password keyboard-interactive

o Kerberos V5 (password, kerberos-tgt, kerberos-1, kerberos-tgt-1, kerberos-2, kerberos-tgt-2)

o Certificate

System security is not improved unless the RLOGIN and RSHELL services are disabled.

If the client authenticates itself successfully, a dialog is entered for preparing the session. At this time the client may request things like:

· forwarding X11 connections

· forwarding TCP/IP connections

· forwarding the authentication agent connection over the secure channel

Finally, the client either requests an interactive session or execution of a command. The client and the server enter session mode. In this mode, either the client or the server may send data at any time, and such data is forwarded to/from the virtual terminal 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.

Expired Password Handling

The SSH2 server supports expired password changing for interactive accounts without the CAPTIVE or RESTRICTED flags set and, via the DCL SET PASSWORD command. When an expired password is detected, the server will behave as if a SET PASSWORD command was specified by the user as a remotely executed command (e.g., $ ssh host set password), and the user will be logged out after changing the password. The user may then log in again using the changed password.

For CAPTIVE or RESTRICTED accounts, or for those accounts where LGICMD is set in the UAF record, the scenario is different. In these cases, the server can't directly execute SET PASSWORD command, because the command procedure specified in the LGICMD field of the UAF record will override the SSH server attempting to do a SET PASSWORD command. For these types of accounts, the system manager and/or user can use the value of the LOGIN_FLAGS for the process (normal interactive sessions may also examine these flags). For SSH logins, these flags will reflect:

· new mail has been received (JPI$M_NEW_MAIL_AT_LOGIN)

· the password is about to expire (JPI$M_PASSWORD_WARNING)

· the password has expired (JPI$M_PASSWORD_EXPIRED)

The DCL lexical function F$GETJPI may be used to examine these flags, as can the $GETJPI(W) system service or LIB$GETJPI RTL function. When an expired password value is detected, the user may then execute a SET PASSWORD command in the command procedure run for the account.

For example:

$! Login_flags:

$! 1 = new mail messages waiting (JPI$M_NEW_MAIL_AT_LOGIN)

$! 4 = password expired during login (JPI$M_PASSWORD_EXPIRED)

$! 5 = password expires within 5 days (JPI$M_PASSWORD_WARNING)

$ flags = f$getjpi("", "LOGIN_FLAGS")

$ new_flags = (flags/2)*2

$ if new_flags .ne. flags then write sys$output "New mail waiting"

$! Note - new_flags is used below because it has the NEW_MAIL_AT_LOGIN$

$! bit stripped. The rest of the possible values are all

$! discrete; i.e., you can't have combinations of them at the

$! same time.

$ if new_flags .eq. 4 then write sys$output "Password expired during login"

$ if new_flags .eq. 5 then write sys$output "Password expires within 5 days" $!

When an account in the SYSUAF has an expired password and the system syslogin.com or user’s login.com has a SET TERM command, a warning message will be displayed prior to prompting to change the password as shown in the following example:

Your password has expired; you must set a new password to log in

% SET-W-NOTSET, error modifying DKA0:

-SET-E-INVDEV, device is invalid for requested operation

Old password:

The way to suppress these warning messages would be to check for the appropriate login flag, ignoring any SET TERM commands. For example:

$ flags = $getjpi(““, “LOGIN_FLAGS”)

$ new_flags = (flags/2)*2

$ if new_flags.eq.4 then goto skip_the_inquiry

Break-In and Intrusion Detection

Care must be exercised when configuring the SSH clients and server to minimize problems due to intrusion records created by OpenVMS security auditing. The SSH user should consult the system manager to determine the authentication methods offered by the SSH server. The client should then be configured to not attempt any authentication method that is not offered by the server.

If a client attempts authentication methods not offered by the server, the OpenVMS security auditing system may log several intrusion records for each attempt to create a session to that server. The result being that the user could be locked out and prevented from accessing the server system without intervention from the server's system manager.

The authentication methods to be offered by the server are determined by the configuration keywords AllowedAuthentications and RequiredAuthentications. The number of intrusion records to be logged for any attempted SSH session is determined by the StrictIntrusionLogging configuration keyword.

When StrictIntrusionLogging is set to YES (the default), each method that is tried and fails causes an intrusion record to be logged. The following rules apply:

· When HostBased or PublicKey authentications are attempted and fail, one intrusion record is logged for each failed method.

· When password authentication is attempted, one intrusion record is logged for each failed password.

Example 1:

The server is set up to allow HostBased and password authentication; also, up to three password attempts are allowed. If all methods fail, four intrusion records are logged:

1 for the failed HostBased

3 for the failed password attempts, one per attempt

When StrictIntrusionLogging is set to NO, it has the effect of relaxing the number of intrusions logged. Overall failure of all authentication methods simply counts as a single failure, except for password authentication. The following rules apply:

· When password authentication is attempted, one intrusion record is logged for each failed password.

· When any of HostBased or PublicKey authentication fails, and password authentication is not attempted, exactly one intrusion record is logged, as opposed to one for each failed method.

· When any of HostBased or PublicKey authentication fails, but password authentication is attempted and succeeds, the only intrusion record(s) logged is one for each failed password attempt.

Example 2:

The server is set up to allow HostBased and password authentication; also, up to three password attempts are allowed. If all methods fail, three intrusion records are logged:

0 for the failed HostBased

3 for the failed password attempts, one per attempt

Example 3:

The server is set up to allow HostBased and password authentication; also, up to three password attempts are allowed. HostBased and RSA fail, but password authentication is successful after 1 failed password. Therefore, one intrusion record is logged:

0 for the failed HostBased

1 for the failed password attempt

Example 4:

The server is set up to allow HostBased and PublicKey authentication, but not password authentication. If all methods fail, one intrusion record is logged.

Example 5:

The server is set up to allow HostBased and PublicKey authentication, but not password authentication. HostBased authentication fails, but PublicKey succeeds. No intrusion records are logged.

Configuring SSHD Master

The SSHD Master is configured via CNFSSH. See Chapter 3 of the SSH for OpenVMS Administration and User’s Guide for details on using CNFSSH to configure SSH.

Note: The only supported methods for starting SSH are to use the

@SYS$STARTUP:PSCSSH$STARTUP command if SSH isn’t running, or to use the SSHCTRL RESTART command if SSH is currently running.

SSH2 Configuration File

SSHD reads configuration data from its configuration file. By default, this file is SSH2_DIR:SSHD2_CONFIG. The file contains keyword value pairs, one per line. Lines starting with # and empty lines are interpreted as comments. The following keywords are possible.

Keywords are case insensitive.

Keyword	Value	Default	Description
AllowedAuthentications	List	Publickey, Password	Permitted techniques. Valid values are: keyboard- interactive, password, publickey, hostbased, kerberos-1, kerberos-tgt-1, kerberos-2, kerberos-tgt-2. Along with Required Authentications, the system administrator can force the users to complete several authentications before they are considered authenticated.
AllowedPasswordAuthentications	List	kerberos, local	Specifies the different password authentication schemes that are allowed. Only kerberos and local are acceptable.
AllowGroups	List		Access control by UAF rights list entries
AllowHosts	Host list		Access control by hostname
AllowShosts	Host list		Access control by hostname
AllowTcpForwarding	Y/N	Y	Enable TCP port forwarding
AllowTcpForwardingForUsers	User list		Per-User forwarding
AllowTcpForwardingForGroups	Rights list		Per-rights list ID forwarding
AllowUsers	User list		Access control by username
AllowX11Forwarding	Y/N	Y	Enable X11 forwarding
AuthInteractiveFailureTimeout	Seconds	2	Delay, in seconds, that the server delays after a failed attempt to log in using keyboard-interactive and password authentication.
AuthKbdInt.NumOptional	Number	0	Specifies how many optional sub methods must be passed before the authentication is considered a success. (Note that all reported sub methods must always be passed.) See AuthKbdInt.Optional for specifying optional sub methods, and AuthKbdInt.Required for required sub methods. The default is 0, although if no required sub methods are specified, the client must always pass at least one optional sub method.
AuthKbdint.Optional	List	None	Specifies the optional sub methods keyboard-interactive will use. Currently only the sub method password is defined. AuthKbdInt.NumOptional specifies how many optional sub methods must be passed. The keyboard-interactive authentication method is considered a success when the specified amount of optional sub methods and all required sub methods are passed.
AuthKbdInt.Required			Specifies the required sub methods that must be passed before the keyboard-interactive authentication method can succeed.
AuthKbdInt.Retries	Number	3	Specifies how many times the user can retry keyboard-interactive.
AuthorizationFile	Filename	Authorization	Authorization file for public key authentication.
AuthPublicKey.MaxSize	Number	0	Specifies the maximum size of a public key that can be used to log in. Value of 0 disables the check.
AuthPublicKey.MinSize	Number	0	Specifies the minimum size of a public key that can be used to log in. Value of 0 disables the check.
Cert.RSA.Compat.HashScheme	md5 or sha	md5	Previous clients and servers may use hashes in RSA certificates incoherently (sometimes SHA-1 and sometimes MD5). This specifies the hash used when a signature is sent to old versions during the initial key exchanges.
BannerMessageFile	Filename	SYS$ANNOUNCE	Message sent to the client before authentication begins.
CheckMail	Y/N	Y	Display information about new mail messages when logging in
Ciphers	Cipher list		Encryption ciphers offered
DenyGroups	Rights list		Deny access for UAF rights list identifiers
DenyHosts	Host list		Deny access for hosts
DenySHosts	Host list		Deny access for hosts
DenyTcpForwardingForUsers	User list		Forbid forwarding for listed users
DenyTcpForwardingForGroups	Rights list		Forbid forwarding for listed rights list names
DenyUsers	User list		Access control by username
FascistLogging	Y/N	Y	Verbose logging
ForwardACL	Pattern	None	With this option, you can have more fine-grained control over what the client is allowed to forward, and to where. See ForwardACL Notes below.
ForwardAgent	Y/N	Y	Enable agent forwarding
HostCA	Certificate	None	Specifies the CA certificate (in binary or PEM format) to be used when authenticating remote hosts. The certificate received from the host must be issued by the specified CA and must contain a correct alternate name of type DNS (FQDN). If no CA certificates are specified in the configuration file, the protocol tries to do key exchange with ordinary public keys. Otherwise, certificates are preferred. Multiple CAs are permitted.
HostCANoCRLs	Certificate	None	Similar to HostCA, but disables CRL checking for the given ca-certificate.
HostCertificateFile	Filename	None	This keyword works very much like PublicHostKeyFile, except that the file is assumed to contain an X.509 certificate in binary format. The keyword must be paired with a corresponding HostKeyFile option. If multiple certificates with the same public key type (DSS or RSA) are specified, only the first one is used.
HostbasedAuthForceClient HostnameDNSMatch	Y/N	N	Host name given by client.
Hostkeyfile	Filename	Hostkey	Host key filename
HostSpecificConfig	Pattern	None	Specifies a sub-configuration file for this server, based on the hostname of the client system.
IdentityFile	Filename	Identification	Identity filename
IdleTimeout	Time	0 = none	Set idle timeout (in seconds)
IgnoreRhosts	Y/N	N	Don’t use rhosts and shosts for host-based authentication for all users
IgnoreRootRhosts	Y/N	Y	Don’t use rhosts and shosts files for authentication of SYSTEM
KeepAlive	Y/N	Y	Send keepalives
LdapServers	Server URL	None	Specified as ldap:// server.domainname:389 CRLs are automatically retrieved from the CRL distribution point defined in the certificate to be checked if the point exists. Otherwise, the comma-separated server list given by option LdapServers is used. If intermediate CA certificates are needed in certificate validity checking, this option must be used or retrieving the certificates will fail.
ListenAddress	IP address	0.0.0.0	Listen on given interface
Macs	Algorithm		Select MAC (Message Authentication Code) algorithm
MapFile	Filename	None	This keyword specifies a mapping file for the preceding Pki keyword. Multiple mapping files are permitted per one Pki keyword. The mapping file format is described below.
MaxBroadcastsPerSecond	#broadcasts	0	Listen for UDP broadcasts
NoDelay	Y/N	N	Enable Nagel Algorithm
PasswordAuthentication	Y/N	Y	Permit password authentication
PasswordGuesses	#guesses	3	Limit number of password tries to specified number
PermitEmptyPasswords	Y/N	N	Permit empty (blank) passwords
PermitRootLogin	Y/N	N	SYSTEM can log in
Pki	Filename	None	This keyword enables user authentication using certificates. Cacertificate must be an X.509 certificate in binary format. This keyword must be followed by one or more MapFile keywords. The validity of a received certificate is checked separately using each of the defined Pki keywords in turn until they are exhausted (in which case the authentication fails), or a positive result is achieved. If the certificate is valid, the mapping files are examined to determine whether the certificate allows the user to log in. A correct signature generated by a matching private key is always required.
PkiDisableCrls	Y/N	Y	This keyword disables CRL checking for the Pki keyword, if set to “Y”.
PrintMotd	Y/N	Y	Display SYS$WELCOME when logging in
PublicHostKeyFile	Filename	Hostkey.pub	Host key file location
QuietMode	Y/N	N	Quiet mode
RandomSeedFile	Filename	Random_seed	Random seed file
RekeyIntervalSeconds	#seconds	0	Frequency of rekeying
RequiredAuthentication	Authentication list		Authentications client must support
RequireReverseMapping	Y/N	N	Remote IP address must map to hostname
ResolveClientHostName	Y/N	Y	Controls whether the server will try to resolve the client IP address at all, or not. This is useful when you know that the DNS cannot be reached, and the query would cause additional delay in logging in. Note that if you set this to “no”, you should not set RequireReverseMapping to “Y”.
RSAAuthentication	Y/N	Y	Enable RSA authentication
SendKeyGuess	Y/N	Y	This parameter controls whether the server will try to guess connection parameters during key exchange, or not. Some clients do not support key exchange guesses and may fail when they are present.
SftpSysLogFacility	log facility	None	Defines the log facility the SFTP server will use
StrictIntrusionLogging	Y/N	Y	Determine how intrusion records are created by failed authentication attempts.
StrictModes	Y/N	N	Strict checking for directory and file protection.
SyslogFacility	Facility	AUTH	Defines what log facility to be used when logging server messages.
Terminal.AllowUsers	pattern	All users	List users that are allowed terminal (interactive) access to the server.
Terminal.DenyUsers	pattern	None	List users that are denied terminal (interactive) access to the server.
Terminal.AllowGroups	pattern	All groups	Similar to Terminal.AllowUsers but matches groups instead of usernames.
Terminal.DenyGroups	pattern	None	Similar to Terminal.DenyUsers but matches groups instead of usernames
UserConfigDirectory	Directory	SYS$LOGIN:	Location of user SSH2 directories
UserKnownHosts	Y/N	Y	Respect user [.ssh2] known hosts keys
UserSpecificConfig	Pattern	None	Specifies a sub-configuration file for this server, based on user logging in.
VerboseMode	Y/N	N	Verbose mode

The keywords MACS and CIPHERS have discrete values, plus there are values that denote a grouping of 2 or more of the discrete values. Each of these values may be put in the configuration file SSH2_DIR:SSHD2_CONFIG.

MACs	discrete values: hmac-sha1, hmac-sha256, hmac-md5, hmac-ripemd160, none
	group ANYMAC consists of: hmac-sha1, hmac-sha256, hmac-md5, hmac-ripemd160
	group ANY consists of: hmac-sha1, hmac-sha256, hmac-md5, hmac-ripemd160, none
	group ANYSTD consists of: rhmac-sha1, hmac-md5, none
	group ANYSTDMAC consists of: hmac-sha1, hmac-md5
Ciphers	discrete values: 3des, aes, blowfish, aes128-ctr, aes128-cbc, aes192-ctr, aes192-cbc, aes256-ctr, aes256-cbc, 3des-ctr, 3des-cbc, blowfish-ctr, blowfish-cbc, des-cbc@ssh.com, rc2-cbc@ssh.com, none
	group ANYSTDCIPHER consists of: aes128-ctr, aes128-cbc, aes192-ctr, aes192-cbc, aes256-ctr, aes256-cbc, 3des-ctr, 3des-cbc, blowfish-ctr, blowfish-cbc
	group ANY consists of: aes128-ctr, aes128-cbc, aes192-ctr, aes192-cbc, aes256-ctr, aes256-cbc, 3des-ctr, 3des-cbc, blowfish-ctr, blowfish-cbc, des-cbc@ssh.com, rc2-cbc@ssh.com, none
	group ANYCIPHER aes128-cbc, 3des-cbc, twofish128-cbc, cast128-cbc, twofish-cbc, blowfish-cbc, aes192-cbc, aes256-cbc, twofish192-cbc, twofish256-cbc, arcfour, des-cbc@ssh.com, rc2-cbc@ssh.com
	group ANYSTD aes128-cbc, 3des-cbc, twofish128-cbc, cast128-cbc, twofish-cbc, blowfish-cbc, aes192-cbc, aes256-cbc, twofish192-cbc, twofish256-cbc, arcfour, none

A discrete value or a group identifier may be used with MACs and Ciphers. For example, in the configuration file, the following examples could be used:

Ciphers ANYCIPHER

Ciphers 3des, aes128-cbc

MACs ANYMAC

MACs hmac-sha1

Aliases may be used for some standard ciphers:

Alias	Value
aes	aes128-cbc
3des	3des-cbc
blowfish	blowfish-cbc

HostSpecificConfig Notes:

The global server file (SSH_DIR:SSHD2_CONFIG) now can use the keyword HostSpecificConfig to allow the specification of a configuration file based on the client system. These lines are specified as:

HostSpecificConfig hostname subconfig-file

hostname will be used to match the client host, as specified under option AllowHosts. The file subconfig-file will then be read, and configuration data amended accordingly. The file is read before any actual protocol transactions begin, and you can specify most of the options allowed in the main configuration file. You can specify more than one sub-configuration file; in which case the patterns are matched and the files read in the order specified. Later defined values of configuration options will either override or amend the previous value, depending on which option it is. The effect of redefining an option is described in the documentation for that option. For example, setting Ciphers in the sub-configuration file will override the old value, but setting AllowUsers will amend the value.

The subconfig-file will be assumed by default to exist in the SSH2_DIR directory. However, this may be overridden by specifying a complete directory/file specification. For example:

HostSpecificConfig foo.example.com dka0:[sshconfigs]fooconfig.dat

HostSpecificConfig lima.example.com limaconfig.dat

In the first instance, an incoming connection from foo.example.com will use the sub-configuration file dka0:[sshconfigs]fooconfig.dat. In the second example, an incoming connection from lima.beans.com will use ssh2_dir:limaconfig.dat.

Unlike ssh2_config, the sub-configuration files may have configuration blocks, or stanzas, in them. They are used per-host. The sub-configuration heading is interpreted identically to what is described above (i.e, with UserSpecificConfig, the pattern is of the format hostname.)

Note: If the sub-configuration file cannot be found or cannot be parsed successfully for any reason, access to the system will be denied for the system to which the sub-configuration file applies.

UserSpecificConfig Notes:

The global server file (SSH2_DIR:SSHD2_CONFIG) can use the keyword UserSpecificConfig to allow the specification of a configuration file based on the username of the user who’s logging into the server. These keywords are of the form:

UserSpecificConfig user[@host] subconfig-file

The user and host fields will be used to match the username, as specified under the option AllowUsers. The file subconfig-file will then be read, and configuration data amended accordingly. The file is read before any actual protocol transactions begin, and you can specify most of the options allowed in the main configuration file. You can specify more than one sub-configuration file, in which case the patterns are matched and the files read in the order specified. Later defined values of configuration options will either override or amend the previous value, depending on which option it is. The effect of redefining an option is described in the documentation for that option. For example, setting Ciphers in the sub-configuration file will override the old value, but setting AllowUsers will amend the value.

Unlike sshd2_config, the sub-configuration files may have configuration blocks, or stanzas, in them. They are used per user. The sub-configuration heading is interpreted identically to what is described above (i.e., with UserSpecificConfig, the pattern is of the format user[@host].

The subconfig-file will be assumed by default to exist in the SSH2_DIR directory. However, this may be overridden by specifying a complete directory/file specification. For example:

UserSpecificConfig dilbert dka0:[sshconfigs]dilbert.dat

UserSpecificConfig boss@lima.example.com pointyhair.dat

In the first instance, an incoming connection for user dilbert will use the sub-configuration file dka0:[sshconfigs]dilbert.dat. In the second example, an incoming connection from user boss at system lima.example.com will use ssh2:dir:pointyhair.dat.

Note: If the sub-configuration file cannot be found or cannot be parsed successfully for any reason, access to the system will be denied for the user to which the sub-configuration file applies.

KEYBOARD-INTERACTIVE Notes:

KEYBOARD-INTERACTIVE mode is simply another form of password authentication. The user won’t notice anything different with this mode.

ForwardACL Notes

With this option, you can have more fine-grained control over what the client is allowed to forward, and to where. Format for this option is:

[allow|deny] [local|remote] user-pat forward-pat [originator-pat]

user-pat will be used to match the client-user, as specified under the option UserSpecificConfig. forward-pat is a pattern of format host-id[%port]. This has different interpretations, depending on whether the ACL is specified for local or remote forwards. For local forwards, the host-id will match with the target host of the forwarding, as specified under the option AllowHosts. port will match with the target port. Also, if the client sent a host name, the IP address will be looked up from the DNS, which will be used to match the pattern. For remote forwardings, where the forward target is not known (the client handles that end of the connection); this will be used to match with the listen address specified by the user (and as such is not as usable as with local forwards). port will match the port the server is supposed to be listening to with this forward. With local forwards, originator-pat will match with the originator address that the client has reported. Remember, if you do not administer the client machine, users on that machine may use a modified copy of ssh that can be used to lie about the originator address. Also, with NATs (Network Address Translation), the originator address will not be meaningful (it will probably be an internal network address). Therefore, you should not rely on the originator address with local forwards, unless you know exactly what you are doing. With remote forwards, originator-pat will match with the IP address of the host connecting to the forwarded port. This will be valid information, as it is the server that is checking that information.

If you specify any allow directives, all forwards in that class (local or remote) not specifically allowed will be denied (note that local and remote forwards are separate in this respect, e.g., if you have one “allow remote” definition, local forwards are still allowed, pending other restrictions). If a forward matches with both allow and deny directives, the forwarding will be denied. Also, if you have specified any of the options [Allow.Deny]TcpForwardingForUsers.Groups] or AllowTcpForwarding, and the forwarding for the user is disabled with those, an allow directive will not re-enable the forwarding for the user. Forwarding is enabled by default.

Mapping File Format

When certificates are used in user authentication, one or more mapping files determine whether the user can log to an account with a certificate. The mapping file must contain one or more lines in the following format:

account-id keyword arguments

Keyword must be one of the following: Email, EmailRegex, Subject, SerialAndIssuer, or SubjectRegex.

Arguments are different for each keyword. The following list describes each variation:

arguments: an email address in standard format. If the certificate contains the email address as an alternate name, it is good for logging in as user account-id.

Subject

arguments: a subject name in DN notation (LDAP style). If the name matches the one in the certificate, the certificate is good for logging in as user account-id.

SerialAndIssuer

arguments: a number and an issuer name in DN notation (LDAP style), separated by whitespace. If the issuer name and serial number match those in the certificate, the certificate is good for logging in as user account-id.

EmailRegex

arguments: a regular expression (egrep syntax). If it matches an alternate name (of type email-address) in the certificate, the certificate is good lor logging in as user account-id. As a special feature, if account-id contains a string %subst%, it is replaced by the first parenthesized substring of the regular expression before comparing it with the account the user is trying to log into.

SubjectRegex

Works identically to EmailRegex, except it matches the regular expression to the canonical subject name in the received certificate.

Empty lines and lines beginning with # are ignored.

Example MAPPINGFILE

guest email guest@domain.org guest subject C=Fl,O=Company Ltd., CN-Guest User guest SerialAndUser 123 C=Fl, O=Foo\Ltd., CN=Test CA %subst% EmailRegex ([a-z]+)@domain.\org

%subst% Subjectregex ^C=Fl,O=Company,CN=([a-z]+)$

The example EmailRegex permits in users with email addresses with domain domain.org and usernames that contain only letters, each user to the account that corresponds to the username part of the email address.

The example SubjectRegex lets in all users with fields C=Fl and O=Company in the subject name if their CN field contains only letters and is the account name they are trying to log into.

Note the ^ and $ at the beginning and end of the regular expression; they are required to prevent the regular expression from matching less than the whole string (subject name).

Note also that all characters interpreted by the regular expression parser as special characters must be escaped with a backslash if they are a part of the subject name. This also means that the backslash in the SerialAndIssuer example would have to be escaped with another backslash if the same subject name was used in a SubjectRegex rule.

Starting the SSH Server for the First Time

Follow these instructions to configure the SSH server. If SSH isn’t currently running, you must define the MULTINET logicals by using:

$ @SYS$STARTUP:PSCSSH$STARTUP LOGICALS

1. Use the CNFSSH utility to enable the SSH2 server. It is recommended that the host keys be generated when executing the CNFSSH procedure, by answering Y to the question “Do you want to generate the SSH2 host key now?” For more information, see Chapter 3 in this manual.

2. Use SSHKEYGEN /SSH2/HOST to generate an SSH2 key and to create the server key in the MULTINET_SSH2_HOSTKEY_DIR directory if it has not previously been created as part of the CNFSSH configuration:

$ DEFINE MULTINET_SSH2_HOSTKEY_DIR -

_$ MULTINET_SPECIFIC_ROOT:[MULTINET.PSCSSH.SSH2.HOSTKEYS]

$ MULTINET SSHKEYGEN /SSH2/HOST

Generating 1024-bit dsa key pair

8 .oOo.oOoo.oO Key generated.

1024-bit dsa, lillies@flower.example.com, Mon Mar 03 2022 09:19:47 Private key saved to multinet_ssh2_hostkey_dir:hostkey. Public key saved to multinet_ssh2_hostkey_dir:hostkey.pub

3. Copy the template server configuration file to the ssh2_dir: directory renaming it SSHD2_CONFIG.:

$ COPY MULTINET_SPECIFIC_ROOT:[MULTINET.PSCSSH.SSH2]SSHD2_CONFIG.TEMPLATE -_$ MULTINET_SPECIFIC_ROOT:[MULTINET.PSCSSH.SSH2]SSHD2_CONFIG.

4. Copy the template client configuration file to the ssh2_dir: directory renaming it SSH2_CONFIG.:

$ COPY MULTINET_SPECIFIC_ROOT:[MULTINET.PSCSSH.SSH2]SSH2_CONFIG.TEMPLATE - _$ MULTINET_SPECIFIC_ROOT:[MULTINET.PSCSSH.SSH2]SSH2_CONFIG.

Note: As delivered, the template files provide a reasonably secure SSH environment. However, Process Software recommends these files be examined and modified appropriately to reflect the security policies of your organization.

5. Start SSH. This creates the SSH server process and defines the SSH logical names.

$ @SYS$STARTUP:PSCSSH$STARTUP

$ SHOW PROCESS "SSHD Master"

3-MAR-2022 09:03:06.42 User: SYSTEM Process ID: 00000057 Node: PANTHR Process name: "SSHD Master"

Terminal:

User Identifier: [SYSTEM]

Base priority: 4

Default file spec: Not available

Number of Kthreads: 1

Devices allocated: BG1:

BG2:

$ SHOW LOGICAL/SYSTEM *SSH*

"MULTINET_SSH2_HOSTKEY_DIR" ="MULTINET_SPECIFIC_ROOT:

[MULTINET.PSCSSH.SSH2.HOSTKEYS]"

"MULTINET_SSH2_KNOWNHOSTS_DIR"=

"MULTINET_SPECIFIC_ROOT:[MULTINET.PSCSSH.SSH2.KNOWNHOSTS]"

"MULTINET_SSH_ALLOW_EXPIRED_PW"="1"

"MULTINET_SSH_ALLOW_PREEXPIRED_PW"="1"

"MULTINET_SSH_DISPLAY_SYS$ANNOUNCE"="1"

"MULTINET_SSH_ENABLE_SSH1_CONNECTIONS"="1"

"MULTINET_SSH_ENABLE_SSH2_CONNECTIONS"="1"

"MULTINET_SSH_LOG_MBX" = "MBA37"

"MULTINET_SSH_PARAMETERS_0"="/BITS=768/VERBOSE/QUIET/PORT=22"

"MULTINET_SSH_PARAMETERS_1"="/KEY_GEN_TIME=3600"

"MULTINET_SSH_PARAMETERS_2"=""

"MULTINET_SSH_PARAMETERS_3" =""

"SSH2_DIR"=MULTINET__SPECIFIC_ROOT:[MULTINET.PSCSSH.SSH2]"

"SSH_DIR"="MULTINET_SPECIFIC_ROOT:[MULTINET.PSCSSH.SSH]"

"SSH_EXE"= MULTINET_COMMON_ROOT:[MULTINET.PSCSSH]"

"SSH_LOG"= MULTINET_SPECIFIC_ROOT:[MULTINET.PSCSSH.LOG"

"SSH_MAX_SESSIONS"="100"

"SSH_TERM_MBX"="MBA36:"

Modifying the SSH2 Configuration File

If you make a change to the SSH configuration file after you have enabled SSH, you must restart SSH for these changes to take effect.

$ SSHCTRL RESTART

Note: When issuing the RESTART command for SSH, all active SSH server sessions are terminated. Active client sessions are not affected.

Connection and Login Process

To create a session, SSHD does the following:

1. SSHD_MASTER sees the connection attempt. It creates an SSHD process, passing the operating parameters to it. SSHD performs validation for the user.

2. Assuming the login is successful, SSHD creates a pseudo terminal for the user (an _FTAnn: device). This device is owned by the user attempting to log in.

3. SSHD creates an interactive process on the pseudo terminal, using the username, priority, and privileges of the user who is attempting to log in. If a command was specified, it is executed and the session is terminated.

4. SSH generates the file SSHD.LOG for each connection to the SSH server. Many connections result in many log files. Instead of purging the files on a regular basis, use the following DCL command to limit the number of versions:

$ SET FILE /VERSION_LIMIT=x MULTINET_ROOT:[MULTINET.SSH]SSHD.LOG

Note: The value for /VERSION_LIMIT must not be smaller than the maximum number of simultaneous SSH sessions anticipated. If the value is smaller, SSH users may be prevented from establishing sessions with the server.

Files

MULTINET:HOSTS.EQUIV

Contains host names, one per line. This file is used during .rhosts authentication. Users on those hosts are permitted to log in without a password, provided they have the same username on both machines. The hostname may also be followed by a username. Such users are permitted to log in as any user on the remote machine (except SYSTEM). Additionally, the syntax +@group can be used to specify netgroups. Negated entries start with dash (-). If the client host/user is matched in this file, login is permitted provided the client and server usernames are the same. Successful RSA host authentication is required. This file should be world-readable but writable only by SYSTEM.

As with the RSHELL and RLOGIN services, it’s never a good idea to use usernames in hosts.equiv. It means the named user(s) can log in as anybody, which includes accounts that own critical programs and directories. Using a username grants the user SYSTEM access. The only valid use for usernames is in negative entries.

MULTINET:SHOSTS.EQUIV

Processed as MULTINET:HOSTS.EQUIV. May be useful in environments that want to run both RSHELL/RLOGIN and SSH.

MULTINET_SSH2_HOSTKEY_DIR:HOSTKEY

Contains the private part of the host key. This file does not exist when SSH for OpenVMS is installed. The SSH server starts only with this file. This file must be created manually using the command:

$ MULTINET SSHKEYGEN /SSH2 /HOST.

This file should be owned by SYSTEM, readable only by SYSTEM, and not accessible to others.

To create a host key with a name that is different than what SSHKEYGEN creates, do one of the following:

· Generate with SSHKEYGEN /SSH2/HOST and simply rename the file(s).

· Generate without the /HOST switch and then name the file(s) whatever you want.

By default the logical name SSH2_DIR points to the MULTINET_SPECIFIC_ROOT:[MULTINET.SSH2] directory.

Refer to the MultiNet User’s Guide, Chapter 8, for more details about SSHKEYGEN.

MULTINET_SSH2_HOSTKEY_DIR:HOSTKEY.PUB

Contains the public part of the host key. This file should be world-readable but writable only by SYSTEM. Its contents should match the private part. This file is not used for anything; it is only provided for the convenience of the user so its contents can be copied to known hosts files.

SSH2:SSH_RANDOM_SEED

SYS$LOGIN:[.SSH]RANDOM_SEED

Contains a seed for the random number generator. This file should only be accessible by SYSTEM.

SSH2_DIR:SSHD2_CONFIG

Contains configuration data for the SSHv2 server. This file should be writable by SYSTEM only, but it is recommended (though not necessary) that it be world-readable.

SYS$LOGIN:[.SSH2].SHOSTS

Permits access using SSH2 only. For SSH2, this file is the same as for .rhosts. However, this file is not used by RLOGIN and RSHELL daemon.

SYS$LOGIN:.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 a password. SSH2 requires RSA host authentication in addition to validating the hostname retrieved from domain name servers. The file must be writable only by the user. It is recommended that it not be accessible by others. It is possible to use netgroups in the file. Either host or username may be of the form +@groupname to specify all hosts or all users in the group.

SYS$LOGIN:[.SSH2]AUTHORIZATION

This file contains information on how the server verifies the identity of a user.

SYS$LOGIN:[.SSH2.KNOWNHOSTS]xxxxyyyy.pub

These are the public host keys of hosts that a user wants to log in from using host-based authentication (equivalent to SSH1's RhostsRSAAuthentication). Also, a user must set up their individual .SHOSTS or .RHOSTS file. If the username is the same in both hosts, it is adequate to put the public host key in SSH2_DIR:KNOWNHOSTS and add the host's name to the systemwide SHOSTS.EQUIV or RHOSTS.EQUIV file.

xxxx is the hostname (FQDN) and yyyy denotes the public key algorithm of the key (ssh-dss or ssh-rsa).

For example flower.example.com's host key algorithm is ssh-dss. The host key would then be flower_example_com_ssh-dss.pub in the [.SSH2.KNOWNHOSTS] directory.

SSH2 AUTHORIZATION File Format

The authorization file contains information on how the server verifies the identity of a user. This file has the same general syntax as the SSH2 configuration files. The following keywords may be used:

Keyword	Description
KEY	The filename of a public key in the [.SSH2] directory in the user's SYS$LOGIN directory. This key is used for identification when contacting the host. If there are multiple KEY lines, all are acceptable for login.
COMMAND	This keyword, if used, must follow the KEY keyword above. This is used to specify a "forced command" that executes on the server side instead of anything else when the user is authenticated. This option might be useful for restricting certain public keys to perform certain operations.

SSH2 Logicals

These logicals are used with the SSH server in the system logical name table.

SSH_DIR

Points to the directory where the master server log file is kept. Normally, this is MULTINET_SPECIFIC_ROOT:[MULTINET]. It is defined in START_SSH.COM.

SSH_EXE

Points to the directory where SSH executables are kept. Normally, this is MULTINET_COMMON_ROOT:[MULTINET]. It is defined in START_SSH.COM.

SSH_LOG

Points to the directory where the log files are kept. Normally, this is MULTINET_SPECIFIC_ROOT:[MULTINET.SSH]. It is defined in START_SSH.COM.

MULTINET_LOG_MBX

Points to the OpenVMS mailbox used to log connection accept and reject messages. This must not be modified by the user.

MULTINET_SSH_ACC_REJ_LOG_FILE

If the user has set a log file to log connection accept and reject messages, this logical will be defined and will provide the name of the log file. This logical is set by using the SET LOG-FILE keyword in MULTINET CONFIGURE/SERVER, and should not be modified directly by the user.

MULTINET_SSH_LOG_ACCEPTS

When set, causes the server to log successful connection requests as either an OPCOM message or a line in a log file. Specified by the SET LOG-ACCEPT command in MULTINET CONFIGURE/ SERVER. Note that the server does not use the information set in the ACCEPT-HOSTS keyword in CONFIGURE/SERVER. Rather, it uses the AllowHosts and DenyHosts keywords in the

SSH server configuration file. Also, a successful connection request doesn't equate to a successful authentication request. This logical should not be modified directly by the user.

MULTINET_SSH_LOG_REJECTS

When set, causes the server to log rejected connection requests as either an OPCOM message or a line in a log file. Specified by the SET LOG-REJECT command in MULTINET CONFIGURE/ SERVER. Note that the server does not use the information set in the REJECT-HOSTS keyword in CONFIGURE/SERVER. Rather, it uses the AllowHosts and DenyHosts keywords in the SSH server configuration file. This logical should not be modified directly by the user.

MULTINET_SSH_MAX_SESSIONS

Set this to the maximum number of concurrent SSH sessions you want to allow on the server system. If MULTINET_SSH_MAX_SESSIONS is not defined, the default is 1000. Setting MULTINET_SSH_MAX_SESSIONS to zero (0) will cause an error. The value must be between 1 and 1000. The suggested place to set this is in START_SSH.COM. SSH must be restarted to use the new value if it is changed.

SSH_TERM_MBX

Mailbox used by SSHD_MASTER to receive termination messages from SSHD daemon processes. Do not change this logical name. This is created by the SSHD_MASTER process.

MULTINET_SSH_KEYGEN_MIN_PW_LEN

Defines the minimum passphrase length when one is to be set in SSHKEYGEN. If not defined, defaults to zero.

MULTINET_SSH_PARAMETERS_n

These values are set by MultiNet and must not be modified by the user.

MULTINET_SSH_USE_SYSGEN_LGI

If defined, causes SSHD to use the VMS SYSGEN value of LGI_PWD_TMO to set the login grace time, overriding anything specified in the command line or the configuration file.

MULTINET_SSH_ENABLE_SSH2_CONNECTIONS

Enables SSHD Master to accept SSH2 sessions.

MULTINET_SSH2_HOSTKEY_DIR

Directory containing the host keys for the SSH2 server. Normally set to MULTINET_SPECIFIC_ROOT:[MULTINET.SSH2.HOSTKEYS]

MULTINET_SSH2_KNOWNHOSTS_DIR

Directory containing the public keys for known systems. Normally set to MULTINET_SPECIFIC_ROOT:[MULTINET.SSH2.KNOWNHOSTS].

SSH2_DIR

Contains all SSH V2-specific files, such as configuration files. Normally set to MULTINET_SPECIFIC_ROOT:[MULTINET.SSH2]

SSH daemon Files

These files are used by or created by SSH when you log into a daemon. These files are not to be altered in any way.

SSH_LOG:SSHD.LOG

This log file is created by each SSHD daemon.

SSHD_MASTER.LOG

This log file is created by SSHD_MASTER.

SSH_START.COM

This file is used to start SSH.