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HP-UX 11i Version 3: February 2007

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hosts_access — format of host access control files


The access control facility for internet services uses access control files to grant or deny access to its services. These files are defined using a simple access control language based on client (host name/address, user name) and server (process name, hostname/address) patterns. See the EXAMPLES section for a quick introduction.

An extended version of the access control language is described in hosts_options(5).

Access Control Files

daemon is 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 (/etc/inetd.conf). The access control software searches the contents of two files: /etc/hosts.allow and /etc/hosts.deny.

The files are searched in the following order. The search stops with the first match:

  • /etc/hosts.allow file is checked first for a matching (daemon, client) pair. If one is found, access is granted and the search stops.

  • /etc/hosts.deny file is checked if no match was found in the /etc/hosts.allow file and access will be denied if a (daemon, client) pair match is found.

  • If no (daemon, client) match was found in either access control file, 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. The following points describe the format of the access control file:

  • A newline character is ignored when it is preceded by a backslash ("\"). 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 be in the following format. Contents in between [] square brackets are optional:

    daemon_list : client_list [ : shell_command ]


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


    is a list of one or more host names, host addresses, patterns or wildcards (see below) that will be matched against the client host name or address. NOTE: An IPv6 address should be enclosed in square brackets [] without any whitespace.

The more complex forms daemon@host and user@host are explained in the Server Endpoint Patterns and Client Username Lookups sections respectively.

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

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


The access control language implements the following patterns:

  • A string that begins with a dot (.) character specifies to match the components after the dot. A host name is matched if the last components of its name match the specified pattern. For example, the pattern .xyz.com matches the host name abc.def.xyz.com.

  • A string that ends with a dot (.) character specifies to match the components before the dot. A host address is matched if its first numeric fields match the given string. For example, the pattern 192.3. matches the address of (almost) every host (192.3.x.x) on the 192.3 network.

  • A string that begins with an at (@) character is treated as an NIS (formerly YP) netgroup name. A host name is matched if it is a host member of the specified netgroup. Netgroup matches are not supported for daemon process names or for client user names.

  • 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 bit-wise AND of the address and the "mask". For example, the net/mask pattern matches every address in the range through

  • An expression of the form [IPv6_address/prefix_length] is interpreted as an IPv6 network prefix. A host address is matched if the value of prefix_length bits in IPv6_address and the host address are equal. For example, the pattern [3ffe::1111:1234/120] matches every address in the range 3ffe::1111:0 through 3ffe::1111:ffff.


The access control language supports explicit wildcards. They are:


The universal wildcard, always matches.


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


Matches any user whose name is unknown, and matches any host whose name or address is 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.


Matches any user whose name is known, and matches any host whose name and addresses 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.


Matches any host whose name does not match its address. If the configuration parameter, on_reverselookup_fail in /etc/tcpd.conf, is set to deny, tcpd drops requests from such clients even before looking at the access control tables.


The access control language supports the following operator:


The format of using this operator is as follows:

list_1 EXCEPT list_2

This construct matches anything that matches list_1, provided it does not match list_2. The EXCEPT operator can be used in daemon_lists and client_lists. The EXCEPT operator can be nested. If the control language permits the use of parentheses, "a EXCEPT b EXCEPT c" parses as "(a EXCEPT (b EXCEPT c))" .

Shell Commands

If the first-matched access control rule contains a shell command, the command is subjected to %letter expansions (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 ampersand (&) at the end of the command if you do not want to wait until the command has completed.

Shell commands should not rely on the PATH setting of inetd. Instead, they should use absolute path names or begin with an explicit PATH=whatever statement.

The hosts_options(5) manual page describes the access control language that uses the shell command field.

% Expansions

The following expansions are available within shell commands:


The client (server) host address.


Client information: user@host, user@address, a host name, or just an address, depending on how much information is available.


The daemon process name (argv[0] value).


The client (server) host name or address, if the host name is not available.


The client (server) host name (or unknown or paranoid).


The daemon process id.


Server information: daemon@host, daemon@address, or just a daemon name, depending on how much information is available.


The client user name (or unknown).


Expands to a single % character.

Characters in % expansions that do not match any alpha-numeric (A-Za-z0-9) or !@%-_=+:,./ characters are replaced by underscores.

Server Endpoint Patterns

In order to distinguish clients by the network address that they connect to, use patterns of the form:


Patterns like these can be used when the machine has different Internet addresses with different Internet hostnames. Service providers can use this facility to offer FTP, GOPHER or WWW archives with Internet names that may even belong to different organizations. See also the twist option in hosts_options(5). Some systems can have more than one Internet address on one physical interface. With other systems you may have to resort to SLIP or PPP pseudo interfaces that live in a dedicated network address space.

The host_pattern conforms to the same syntax rules as host names and addresses in client_list context. Usually, server endpoint information is available only with connection-oriented services.

Client Username Lookup

When the client host supports the RFC 931 protocol or one of its descendants (TAP, IDENT, RFC 1413), the wrapper programs can retrieve additional information about the owner of a connection. The client 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 (in /etc/tcpd.conf) at run 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 pattern, so the same wildcards apply (netgroup membership is not supported). Username lookup needs to be evaluated carefully because of the following limitations:

  • The client username information cannot be trusted when it is needed most, i.e. when the client 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".

  • Username lookups may cause noticeable delays for non-UNIX users. The timeout value for username lookups is configurable through /etc/tcpd.conf. See the tcpd.conf(4) for more information.

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.

Detecting Address Spoofing Attacks

A flaw in the sequence number generator of many TCP/IP implementations allows intruders to easily impersonate trusted hosts and to break in via; for example, the remote shell service. The IDENT (RFC931 etc.) service can be used to detect such and other host address spoofing attacks.

Before accepting a client request, the wrappers can use the IDENT service to find out that the client did not send the request at all. When the client host provides IDENT service, a negative IDENT lookup result (the client matches UNKNOWN@host) is a strong evidence of host-spoofing attack.

A positive IDENT lookup result (the client matches KNOWN@host) is less reliable. It is possible for an intruder to spoof both the client connection and the IDENT lookup, although doing so is much harder than spoofing just a client connection. It may also be possible that the client's IDENT server is lying.

NOTE: IDENT lookups do not work with UDP services.


The language is flexible enough that different types of access control policy can be expressed with a minimum of effort. 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. The search terminates when a match is found, and 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 Access Control

In this example, 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 services to all the 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 from hosts in the local domain (there is no dot "." in the host name) and from members of the some_netgroup netgroup. The second rule permits access from all hosts in the foobar.edu domain (notice the leading dot "." in .foobar.edu), with the exception of terminalserver.foobar.edu.

Mostly Open Access Control

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 fingerd: other.host.name, .other.domain

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

Setting Traps

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

/etc/hosts.allow: tftpd: LOCAL, .my.domain /etc/hosts.deny: tftpd: ALL: spawn (/usr/bin/sffinger -l @%h | \ /usr/bin/mailx -s %d-%h root) &

The sffinger command comes with tcp wrappers. 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 (client host) and %d (service name) sequences is described earlier in the "Shell Commands" section.

WARNING: Do not set traps on your finger daemon, unless you are prepared for infinite finger loops.

Service trapping can be especially useful on network firewall systems. The typical network firewall only provides a limited set of services to the outer world. All other services can be trapped just like the above tftp example. The result is an excellent early-warning system.


Problems are reported via syslogd, the syslog daemon, at info, notice, warning and err levels. An error is reported in the following cases:

  • 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 %letter expansion would overflow an internal buffer,

  • When a system call fails that should not.


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 not case-sensitive. NIS (formerly YP) netgroup lookups are case-sensitive.


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



(daemon,client) pairs that are granted access.


(daemon,client) pairs that are denied access.


tcpd(1M) TCP/IP daemon wrapper program.

tcpdchk(1) and tcpdmatch(1) test programs.

tryfrom(1) and sffinger(1) TCP Wrapper utility programs.

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