hosts_access - format of host access control files
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.
The extended version of the access control language is described in the
(5) document. Note that this language supersedes
the meaning of shell_command as documented below.
In the following text, 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
The access control software consults two files. The search stops at the first
- 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.
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, things
between  being optional:
daemon_list : client_list [ : shell_command ]
is a list of one or more daemon process names (argv
values) or server port numbers 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
The more complex forms daemon@host
are explained in
the sections on server endpoint patterns and on client username lookups,
List elements should be separated by blanks and/or commas.
With the exception of NIS (YP) netgroup lookups, all access control checks are
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
- 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).
- A string that begins with an `@´ 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. An IPv4 host address is matched
if `net´ is equal to the bitwise AND of the address and the
`mask´. For example, the net/mask pattern
`188.8.131.52/255.255.254.0´ matches every address in the range
`184.108.40.206´ through `220.127.116.11´.
`255.255.255.255´ is not a valid mask value, so a single host can
be matched just by its IP.
- An expression of the form `n.n.n.n/mm' is interpreted as a
`net/masklength' pair, where `mm' is the number of consecutive `1' bits in
the netmask applied to the `n.n.n.n' address.
- 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
- A string that begins with a `/´ character is treated
as a file name. A host name or address is matched if it matches any host
name or address pattern listed in the named file. The file format is zero
or more lines with zero or more host name or address patterns separated by
whitespace. A file name pattern can be used anywhere a host name or
address pattern can be used.
- Wildcards `*´ and `?´ can be used to match
hostnames or IP addresses. This method of matching cannot be used in
conjunction with `net/mask´ matching, hostname matching beginning
with `.´ or IP address matching ending with `.´.
The access control language supports explicit wildcards:
- The universal wildcard, always matches.
- Matches any host whose name does not contain a dot
- 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.
- 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.
- Matches any host whose name does not match its address.
When tcpd is built with -DPARANOID (default mode), it drops requests from
such clients even before looking at the access control tables. Build
without -DPARANOID when you want more control over such requests.
- 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))´.
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
(5) document describes an alternative language that uses
the shell command field in a different and incompatible way.
The following expansions are available within shell commands:
- %a (%A)
- 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 value).
- %h (%H)
- The client (server) host name or address, if the host name
- %n (%N)
- The client (server) host name (or "unknown" or
- %r (%R)
- The clients (servers) port number (or "0").
- 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 may confuse the shell are replaced by
In order to distinguish clients by the network address that they connect to, use
patterns of the form:
process_name@host_pattern : client_list ...
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
the hosts_options(5) document. Some systems (Solaris, FreeBSD) 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 obeys the same syntax rules as host names and addresses in
client_list context. Usually, server endpoint information is available only
with connection-oriented services.
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. 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 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
A user pattern has the same syntax as a daemon process pattern, so the same
wildcards apply (netgroup membership is not supported). One should not get
carried away with username lookups, though.
- 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
- 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 may cause noticeable delays for non-UNIX
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
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.
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 strong evidence of a host spoofing attack.
A positive IDENT lookup result (the client matches `KNOWN@host´) is less
trustworthy. 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 that the client´s
IDENT server is lying.
Note: IDENT lookups don´t work with UDP services.
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.
In this case, access is denied by default. Only explicitly authorized hosts are
The default policy (no access) is implemented with a trivial deny file:
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:
ALL: LOCAL @some_netgroup
ALL: .foobar.edu EXCEPT terminalserver.foobar.edu
The first rule permits access from hosts in the local domain (no `.´ in
the host name) and from members of the some_netgroup
second rule permits access from all hosts in the foobar.edu
(notice the leading dot), with the exception of
Here, access is granted by default; only explicitly specified hosts are refused
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:
ALL: some.host.name, .some.domain
ALL EXCEPT in.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.
The next example permits tftp requests 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.
in.tftpd: LOCAL, .my.domain
in.tftpd: ALL: (/usr/sbin/safe_finger -l @%h | \
/usr/bin/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 (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.
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 %<letter> expansion would overflow an internal
buffer; when a system call fails that shouldn´t. All problems are
reported via the syslog daemon.
/etc/hosts.allow, (daemon,client) pairs that are granted access.
/etc/hosts.deny, (daemon,client) pairs that are denied access.
hosts_options(5) extended syntax.
tcpd(8) tcp/ip daemon wrapper program.
tcpdchk(8), tcpdmatch(8), test programs.
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.
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