HOSTS_ACCESS(5)

NAME
hosts_access - format of host access control files

DESCRIPTION
This manual page describes a simple access control language that is based on client (host name/address, user name) and server (process name) patterns. Examples are given at the end. The impatient reader can 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. Network daemon process names are specified in the inetd configuration file.

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 /etc/hosts.allow file.

•   Otherwise, access will be denied when a (daemon,client) pair matches an entry in the /etc/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.

•   Blank lines or lines that begin with a `#' character are ignored.

•   All other lines should satisfy the following format, things between [] being optional:

    daemon_list : client_list [ : shell_command ]

daemon_list is a list of one or more daemon process names (argv[0] values) or wildcards (see below).

client_list is a list of one or more host names, host addresses, patterns or wildcards (see below) that will be matched against the remote host name or address.

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

With the exception of NIS (YP) netgroup lookups, all access control checks are case insensitive.

PATTERNS
The access control language implements the following patterns:

•   A string that begins with a `.' character. A client name or address is matched if its last components 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 client name or address is matched if its first 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).

•   A string that begins with a `@' character is treated as a netgroup name. Netgroups are usually supported on systems with NIS (formerly YP) data bases. A client host name is matched if it is a (host) member of the specified netgroup.

•   An expression of the form `n.n.n.n/m.m.m.m' is interpreted as a `net/mask' pair. A client 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'.

WILDCARDS
The access control language supports explicit wildcards:

ALL If this token appears in a daemon_list, it matches all network daemon process names. If the ALL token appears in a client_list, it matches all client names and addresses.

LOCAL
    Matches any string that does not contain a dot character. Typical use is in client_lists.

UNKNOWN
    Matches any host whose name or address are unknown. 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.

KNOWN
    Matches any host whose name and address are known. 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.

FAIL
    Like the ALL wildcard, but causes the software to pretend that the scan of the current access control table fails. FAIL is being phased out; it will become an undocumented feature. The EXCEPT operator (see below) is a much cleaner alternative.

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. This construct 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
If the first-matched access control rule contains a shell command, that command is subjected to the following substitutions:

%a  expands to the remote host address.

%c  expands to client information: user@host, user@address, a host name, or just an address, depending on how much information is available.

%h  expands to the remote host name (or address, if the host name is unavailable).

%d  expands to the daemon process name (argv[0] value).

%p  expands to the daemon process id.

%u  expands to the remote user name (or "unknown").

%%  expands to a single `%' character.

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

REMOTE USERNAME LOOKUP
When the client host supports the RFC 931 protocol or one of its descendants (TAP, IDENT) the wrapper programs can retrieve additional information about the owner of a connection. Remote username information, when available, is logged together with the client host name, and can be used to match patterns like:

daemon_list : ... user_pattern@host_pattern ...

The daemon wrappers can be configured at compile time to perform rule-driven username lookups (default) or to always interrogate the client host. In the case of rule-driven username lookups, the above rule would cause username lookup only when both the daemon_list and the host_pattern match.

A user pattern has the same syntax as a daemon process name, host name or host address pattern, so the same wildcards etc. apply (but netgroup membership of users is not supported). One should not get carried away with username lookups, though.

•   The remote username information cannot be trusted when it is needed most, i.e. when the remote system has been compromised. In general, ALL and (UN)KNOWN are the only user name patterns that make sense.

•   Username lookups are possible only with TCP-based services, and only when the client host runs a suitable daemon; in all other cases the result is "unknown".

•   A well-known UNIX kernel bug may cause loss of service when username lookups are blocked by a firewall. The wrapper README document describes a procedure to find out if your kernel has this bug.

•   Username lookups cause noticeable delays for PC users. The default timeout for username lookups is 10 seconds: too short to cope with slow networks, but long enough to irritate PC users.

Selective username lookups can alleviate the last problem. For example, a rule like:

daemon_list : @pcnetgroup ALL@ALL

would match members of the pc netgroup without doing username lookups, but would perform username lookups with all other systems.

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:

/etc/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:

/etc/hosts.allow:
ALL: LOCAL @some_netgroup
ALL: .foobar.edu EXCEPT terminalserver.foobar.edu

The first rule permits access to all services from hosts in the local domain (no `.' in the host name) and from members of the some_netgroup netgroup. The second rule permits access to all services from all hosts in the .foobar.edu domain, 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:

/etc/hosts.deny:
ALL: some.host.name, .some.domain
ALL EXCEPT in.fingerd: other.host.name, .other.domain

The first rule denies some hosts all services; the second rule still permits finger requests from other hosts.

BOOBY TRAPS
The next example permits tftp requests from hosts in the local domain. 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.

/etc/hosts.allow:
in.tftpd: LOCAL, .my.domain

/etc/hosts.deny:
in.tftpd: ALL: (/some/where/safe_finger -l @%h | \
	/usr/ucb/mail -s %d-%h root) &

The safe_finger command comes with the tcpd wrapper and should be installed in a suitable place. It limits possible damage from data sent by the remote finger server. It gives better protection than the standard finger command.

The expansion of the %h (remote 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; when an access control rule is not terminated by a newline character; when the result of %<character> expansion would overflow an internal buffer; when a system call fails that shouldn't. All problems are reported via the syslog daemon.

FILES
/etc/hosts.allow, (daemon,client) pairs that are granted access.
/etc/hosts.deny, (daemon,client) pairs that are denied access.

SEE ALSO
tcpd(8) tcp/ip daemon wrapper program.

BUGS
If a name server lookup times out, the host name will not be available to the access control software, even though the host is registered.

Domain name server lookups are case insensitive; NIS (formerly YP) netgroup lookups are case sensitive.

AUTHOR
Wietse Venema (wietse@wzv.win.tue.nl)
Department of Mathematics and Computing Science
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513, 5600 MB Eindhoven, The Netherlands