hosts_access

Format of host access control files for Directory Server Enterprise Edition

Synopsis

instance-path/config/hosts.allow
instance-path/config/hosts.deny

Description

This manual page describes a simple access control language that is based on client (host name/address, user name), and server (process name, host name/address) patterns. Examples are given at the end. The impatient reader is encouraged to skip to the EXAMPLES section for a quick introduction.

In the following text, daemon is the the process name of a network daemon process, and client is the name and/or address of a host requesting service.

Note that the version of hosts_access supplied with Directory Server Enterprise Edition is different from the version delivered with Solaris. The Directory Server Enterprise Edition version of hosts_access has the following characteristics:

Shell commands are not available on Microsoft Windows.
hosts_options are not available on any OS.
IPv6 is supported on all platforms.
PARANOID mode is not available.
You cannot replace the lib with your own lib, as it is statically linked to the server.
There is no support of NIS Netgroups. Any '@' symbols in rules are ignored.
The daemon_list process name is the port number of the server. For example, 3389:eng.example.com or 636:192.168.11.254. Port numbers are server properties: ldap-port, ldap-secure-port, dsml-port, dsml-secure-port. Use the dsconf command to view and modify these properties.
Server instances can share files by pointing the instance name at the same file (instance-path/config/hosts.allow). Use the dsconf command to view and modify the server property host-access-dir-path. For example, to have all server instances pointing to /etc/hosts.{deny,allow}, run the following command on all servers: $ dsconf set-server-prop —h host —p port host-access-dir-path: /etc
You can make changes to the hosts_access or hosts_deny files without needing to restart the server. You can safely ignore the dsconf server restart message.
Two conditions must be met in order for acess to be allowed. Firstly, the permissions of the file(s) must be owned by nsslapd-localuser or root and secondly write permission must be allowed for the owner, but not for group or other. Ensure that you use correct permissions, as incorrect permissons on shared files can cause problems. Note that these conditions are not checked on Windows platforms.

ACCESS CONTROL FILES

The access control software consults two files. The search stops at the first match:

Access will be granted when a (daemon,client) pair matches an entry in the instance-path/config/hosts.allow file.
Otherwise, access will be denied when a (daemon,client) pair matches an entry in the instance-path/config/hosts.deny file.
Otherwise, access will be granted.

A non-existing access control file is treated as if it were an empty file. Thus, access control can be turned off by providing no access control files.

ACCESS CONTROL RULES

Each access control file consists of zero or more lines of text. These lines are processed in order of appearance. The search terminates when a match is found.

A newline character is ignored when it is preceded by a backslash character. This permits you to break up long lines so that they are easier to edit.
Blank lines or lines that begin with a `#' character are ignored. This permits you to insert comments and whitespace so that the tables are easier to read.
All other lines should satisfy the following format:

Hosts are identified by server port numbers. If there is no port number match or wildcard, the access control check skips that line of the file.

List elements should be separated by blanks and/or commas.

All access control checks are case insensitive.

PATTERNS

The access control language implements the following patterns:

A string that begins with a `.' character. A host name is matched if the last components of its name match the specified pattern. For example, the pattern `.tue.nl' matches the host name `wzv.win.tue.nl'.
A string that ends with a `.' character. A host address is matched if its first numeric fields match the given string. For example, the pattern `131.155.' matches the address of (almost) every host on the Eindhoven University network (131.155.x.x).
An expression of the form `n.n.n.n/m.m.m.m' is interpreted as a `net/mask' pair. A host address is matched if `net' is equal to the bitwise AND of the address and the `mask'. For example, the net/mask pattern `131.155.72.0/255.255.254.0' matches every address in the range `131.155.72.0' through `131.155.73.255'.
When using IPv6 for matching, be aware that an expression of the form [n:n:n:n:n:n:n:n/m] is interpreted as a [net/prefixlen] pair. An IPv6 host address is matched if prefixlen bits of net is equal to the prefixlen bits of the address. For example, the [net/prefixlen] pattern [3ffe:505:2:1::/64] matches every address in the range 3ffe:505:2:1:: through 3ffe:505:2:1:ffff:ffff:ffff:ffff.

WILDCARDS

Wildcards `*' and `?' can be used to match hostnames or IP addresses. However, this method of matching cannot be used in conjunction with the following: net/mask matching, hostname matching beginning with `.' , IP address matching ending with `.' or a IPv6 rule (begins with '[').

The access control language supports explicit wildcards:

ALL

The universal wildcard, always matches.

LOCAL

Matches any host whose name does not contain a dot character.

UNKNOWN

Matches any user whose name is unknown, and matches any host whose name or address are unknown. This pattern should be used with care: host names may be unavailable due to temporary name server problems. A network address will be unavailable when the software cannot figure out what type of network it is talking to.

It is not recommended to use the UNKNOWN wildcard. Directory Server always fills in both host and address, so there is never a case when the host name is unknown. The user is unavailable because of no NIS netgroups support.

KNOWN

Matches any user whose name is known, and matches any host whose name and address are known. This pattern should be used with care: host names may be unavailable due to temporary name server problems. A network address will be unavailable when the software cannot figure out what type of network it is talking to.

In Directory Server the user is always marked as unknown and is unavailable because of the NIS Netgroup restriction.

OPERATORS

EXCEPT: Intended use is of the form: `list_1 EXCEPT list_2'; this construct matches anything that matches list_1 unless it matches list_2. The EXCEPT operator can be used in daemon_lists and in client_lists. The EXCEPT operator can be nested: if the control language would permit the use of parentheses, `a EXCEPT b EXCEPT c' would parse as `(a EXCEPT (b EXCEPT c))'.

SHELL COMMANDS

Note that shell commands are not available on Microsoft Windows.

If the first-matched access control rule contains a shell command, that command is subjected to %letter substitutions (see next section). The result is executed by a /bin/sh child process with standard input, output and error connected to /dev/null. Specify an `&' at the end of the command if you do not want to wait until it has completed.
Shell commands should not rely on the PATH setting of the inetd. Instead, they should use absolute path names, or they should begin with an explicit PATH=whatever statement.

% EXPANSIONS

The following expansions are available within shell commands:

%a (%A): The client (server) host address.
%c: Client information: user@host, user@address, a host name, or just an address, depending on how much information is available.
%d: The daemon process name (argv[0] value).
%h (%H): The client (server) host name or address, if the host name is unavailable.
%n (%N): The client (server) host name (or "unknown").
%p: The daemon process id.
%s: Server information: daemon@host, daemon@address, or just a daemon name, depending on how much information is available.
%u: The client user name (or "unknown").
%%: Expands to a single `%' character.

Characters in % expansions that may confuse the shell are replaced by underscores.

Examples

The language is flexible enough that different types of access control policy can be expressed with a minimum of fuss. Although the language uses two access control tables, the most common policies can be implemented with one of the tables being trivial or even empty.

When reading the examples below it is important to realize that the allow table is scanned before the deny table, that the search terminates when a match is found, and that access is granted when no match is found at all.

The examples use host and domain names. They can be improved by including address and/or network/netmask information, to reduce the impact of temporary name server lookup failures.

MOSTLY CLOSED

In this case, access is denied by default. Only explicitly authorized hosts are permitted access.

The default policy (no access) is implemented with a trivial deny file:

instance-path/config/hosts.deny:  ALL: ALL

This denies all service to all hosts, unless they are permitted access by entries in the allow file.

The explicitly authorized hosts are listed in the allow file. For example:

instance-path/config/hosts.allow:  
ALL: LOCAL ALL: .foobar.edu EXCEPT terminalserver.foobar.edu

The first rule permits access from hosts in the local domain (no `.' in the host name). The second rule permits access from all hosts in the .foobar.edu domain (notice the leading period), with the exception of terminalserver.foobar.edu.

MOSTLY OPEN

Here, access is granted by default; only explicitly specified hosts are refused service.

The default policy (access granted) makes the allow file redundant so that it can be omitted. The explicitly non-authorized hosts are listed in the deny file. For example:

instance-path/config/hosts.deny:   
ALL: some.host.name, .some.domain
ALL EXCEPT 1389: other.host.name, .other.domain

The first rule denies some hosts and domains all services; the second rule still permits connections to directory port 1389 from other hosts and domains.

BOOBY TRAPS

The next example permits requests to Directory Server port 1389 from hosts in the local domain (notice the leading dot). Requests from any other hosts are denied. Instead of the requested file, a finger probe is sent to the offending host. The result is mailed to the superuser.

instance-path/config/hosts.allow:
1389: LOCAL, .my.domain 

instance-path/config/hosts.deny:
ALL: (/usr/sfw/sbin/safe_finger -l @%h | \
      /usr/ucb/mail -s %d-%h root) &

The above example assumes that the safe_finger command is installed in /usr/sfw/sbin. For Solaris, the safe_finger command is in the SUNWtcpd package. The default location for the safe_finger command in the SUNWtcpd package is "/usr/sfw/sbin". For other operating systems the safe_finger command should be installed in a suitable place. The safe_finger command limits possible damage from data sent by the remote finger server, and gives better protection than the standard finger command. Shell commands for Windows is not supported, so Windows users should not use this rule.

The expansion of the %h (client host) and %d (service name) sequences is described in the section on shell commands.

Warning: do not booby-trap your finger daemon, unless you are prepared for infinite finger loops.

On network firewall systems this trick can be carried even further. The typical network firewall only provides a limited set of services to the outer world. All other services can be "bugged" just like the above tftp example. The result is an excellent early-warning system.

Diagnostics

An error is reported when a syntax error is found in a host access control rule; when the length of an access control rule exceeds the capacity of an internal buffer (2048); when an access control rule is not terminated by a newline character; when the result of %letter expansion would overflow an internal buffer; when a system call fails that should not. All problems are written to the Directory Server instance access log.

Files

instance-path/config/hosts.allow, (daemon,client) pairs are granted access. instance-path/config/hosts.deny, (daemon,client) pairs are denied access.

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Availability	SUNWdsee7
Stability Level	External