hosts_access(5) File Formats Manual hosts_access(5)
NAME
hosts_access - format of host access control files
DESCRIPTION
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 section for a quick introduction.
An extended version of the access control language is described in hosts_options(5).
Access Control Files
is the process name of a network daemon process, and is the name and/or address of a host requesting service. Network daemon process names
are specified in the configuration file The access control software searches the contents of two files: and
The files are searched in the following order. The search stops with the first match:
o file is checked first for a matching (daemon, client) pair. If one is found, access is granted and the search stops.
o file is checked if no match was found in the file and access will be denied if a (daemon, client) pair match is found.
o 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:
o 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.
o 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.
o All other lines should be in the following format. Contents in between square brackets are optional:
daemon_list is a list of one or more daemon process names 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 client host name or address. NOTE: An IPv6 address should be enclosed in square brackets without any
whitespace.
The more complex forms and are explained in the and 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.
Patterns
The access control language implements the following patterns:
o 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 matches the host name
o 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 matches the address of (almost) every host (192.3.x.x) on the
network.
o 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.
o An expression of the form 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
o An expression of the form 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 matches every address in the range through
Wildcards
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 is set to drops requests from such clients even before looking at the
access control tables.
Operators
The access control language supports the following operator:
The format of using this operator is as follows:
This construct matches anything that matches list_1, provided it does not match list_2. The operator can be used in daemon_lists and
client_lists. The operator can be nested. If the control language permits the use of parentheses, parses as
Shell Commands
If the first-matched access control rule contains a shell command, the command is subjected to expansions (see next section). The result
is executed by a child process with standard input, output, and error connected to 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 Instead, they should use absolute path names or begin with an explicit
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:
a host name, or just an address, depending on how much information is available.
The daemon process name
value).
The client (server) host name or address, if the host name is
not available.
The client (server) host name (or
or
The daemon process id.
Server information:
or just a daemon name, depending on how much information is available.
The client user name (or
Expands to a single % character.
Characters in % expansions that do not match any alpha-numeric 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
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 informa-
tion 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:
The daemon wrappers can be configured (in 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). User-
name lookup needs to be evaluated carefully because of the following limitations:
o The client username information cannot be trusted when it is needed most, i.e. when the client system has been compromised. In
general, and are the only user name patterns that make sense.
o 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".
o Username lookups may cause noticeable delays for non-UNIX users. The timeout value for username lookups is configurable through
See the tcpd.conf(4) for more information.
Selective username lookups can alleviate the last problem. For example, a rule like:
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 is a strong evidence of host-spoofing
attack.
A positive IDENT lookup result (the client matches 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.
EXAMPLES
The language is flexible enough that different types of access control policy can be expressed with a minimum of effort. Although the lan-
guage 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:
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:
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 domain (notice the leading dot "." in with the exception of termi-
nalserver.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:
The first rule denies some hosts and domains all services. The second rule still permits requests from other hosts and domains.
Setting Traps
The next example permits 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 probe is sent to the offending host. The result is mailed to the superuser.
The command comes with wrappers. It limits possible damage from data sent by the remote server. It gives better protection than the stan-
dard command.
The expansion of the (client host) and (service name) sequences is described earlier in the "Shell Commands" section.
WARNING: Do not set traps on your daemon, unless you are prepared for infinite loops.
Service trapping can be especially useful on network firewall systems. The typical network firewall only provides a limited set of ser-
vices to the outer world. All other services can be trapped just like the above tftp example. The result is an excellent early-warning
system.
DIAGNOSTICS
Problems are reported via the daemon, at and levels. An error is reported in the following cases:
o When a syntax error is found in a host access control rule,
o When the length of an access control rule exceeds the capacity of an internal buffer,
o When an access control rule is not terminated by a newline character,
o When the result of expansion would overflow an internal buffer,
o When a system call fails that should not.
WARNINGS
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.
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
FILES
(daemon,client) pairs that are granted access.
(daemon,client) pairs that are denied access.
SEE ALSO
tcpd(1M) TCP/IP daemon wrapper program.
tcpdchk(1) and tcpdmatch(1) test programs.
tryfrom(1) and sffinger(1) TCP Wrapper utility programs.
hosts_access(5)