hosts_access(5)hosts_access(5)NAMEhosts_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 section for a quick introduction.
An extended version of the access control language is described in
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:
· file is checked first for a matching (daemon, client) pair.
If one is found, access is granted and the search stops.
· file is checked if no match was found in the file and access
will be denied if a (daemon, client) pair match is found.
· If no (daemon, client) match was found in either access con‐
trol 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
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 back‐
slash ("\"). 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 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
The more complex forms and are explained in the and sections respec‐
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 matches the host name
· 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 exam‐
ple, the pattern matches the address of (almost) every host
(192.3.x.x) on the 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 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 is interpreted as an IPv6 network
prefix. A host address is matched if the value of pre‐
fix_length bits in IPv6_address and the host address are
equal. For example, the pattern matches every address in the
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 tempo‐
rary 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.
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
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 con‐
nected 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.
The following expansions are available within shell commands:
The client (server) host address.
a host name, or just an address, depending on how much infor‐
mation is available.
The daemon process name
The client (server) host name or address, if the host name is
The client (server) host name (or
The daemon process id.
or just a daemon name, depending on how much information is
The client user name (or
Expands to a single % character.
Characters in % expansions that do not match any alpha-numeric or char‐
acters are replaced by underscores.
Server Endpoint Patterns
In order to distinguish clients by the network address that they con‐
nect 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 net‐
work 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:
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
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 limita‐
· The client username information cannot be trusted when it is
needed most, i.e. when the client system has been compro‐
mised. In general, and are the only user name patterns that
· 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 config‐
urable through See the tcpd.conf(4) for more information.
Selective username lookups can alleviate the last problem. For exam‐
ple, 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
Before accepting a client request, the wrappers can use the IDENT ser‐
vice 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.
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
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 autho‐
rized 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 exam‐
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‐
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.
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‐
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
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 the daemon, at and 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 capac‐
ity of an internal buffer,
· When an access control rule is not terminated by a newline
· When the result of expansion would overflow an internal buf‐
· 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 (firstname.lastname@example.org)
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.
SEE ALSOtcpd(1M) TCP/IP daemon wrapper program.
tcpdchk(1) and tcpdmatch(1) test programs.
tryfrom(1) and sffinger(1) TCP Wrapper utility programs.