HOSTS_ACCESS(5) OpenBSD Programmer's Manual HOSTS_ACCESS(5)
hosts_access, hosts.deny, hosts.allow - tcp wrapper format of host access
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 intro-
An extended version of the access control language is described in the
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 ser-
vice. Network daemon process names are specified in the inetd configura-
ACCESS CONTROL FILES
The access control software consults two files. The search stops at the
o Access will be granted when a (daemon,client) pair matches an entry
in the /etc/hosts.allow file.
o Otherwise, access will be denied when a (daemon,client) pair matches
an entry in the /etc/hosts.deny file.
o 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 con-
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.
o 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.
o Blank lines or lines that begin with a `#' character are ignored.
This permits you to insert comments and whitespace so that the ta-
bles are easier to read.
o 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 val-
ues) 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. When a client_list item needs to include colon charac-
ters (for IPv6 addresses), the item needs to be wrapped with square
The more complex forms daemon@host and user@host are explained in the
sections on server endpoint patterns and on client username lookups, re-
List elements should be separated by blanks and/or commas.
With the exception of YP netgroup lookups, all access control checks are
The access control language implements the following patterns:
o 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
o 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).
o A string that begins with an `@' character is treated as a YP net-
group 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 `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
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'. Note that the
`m.m.m.m' portion must always be specified.
o An expression of the form `ipv6-addr/ipv6-mask' is interpreted as a
masked IPv6 address match, just like a masked IPv4 address match
(see above). Note that the `ipv6-mask' portion must always be spec-
o An expression of the form `ipv6-addr/prefixlen' is interpreted as a
masked IPv6 address match (with mask specified by numeric pre-
fixlen), just like a masked IPv4 address match (see above). Note
that the `prefixlen' portion must always be specified.
The access control language supports explicit wildcards:
ALL The universal wildcard, always matches.
LOCAL Matches any host whose name does not contain a dot character.
UNKNOWN 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
KNOWN 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 soft-
ware cannot figure out what type of network it is talking to.
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 EX-
CEPT operator can be used in daemon_lists and in client_lists. The EX-
CEPT 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
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 com-
mand if you do not want to wait until it has completed.
Shell commands should not rely on the PATH setting of the inetd. In-
stead, they should use absolute path names, or they should begin with an
explicit PATH=whatever statement.
The hosts_options(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.
%c Client information: user@host, user@address, a host name, or
just an address, depending on how much information is available.
%d The daemon process name (argv value).
%h (%H) The client (server) host name or address, if the host name is
%n (%N) The client (server) host name (or "unknown" or "paranoid").
%p The daemon process ID.
%s Server information: daemon@host, daemon@address, or just a dae-
mon name, depending on how much information is available.
%u The client user name (or "unknown").
%% Expands to a single `%' character.
Characters in % expansions that may confuse the shell are replaced by un-
SERVER ENDPOINT PATTERNS
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. Many 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 dedi-
cated 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 avail-
able only with connection-oriented services.
CLIENT USERNAME LOOKUP
When the client host supports the RFC 931 protocol or one of its descen-
dants (TAP, IDENT, RFC 1413) the wrapper programs can retrieve additional
information about the owner of a connection. Client username informa-
tion, 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.
o The client username information cannot be trusted when it is needed
most, i.e., when the client system has been compromised. In gener-
al, ALL and (UN)KNOWN are the only user name patterns that make
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 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.
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 (RFC 931 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 at-
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 imple-
mented 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 includ-
ing 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 permitted access.
The default policy (no access) is implemented with a trivial deny file:
This denies all services 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 exam-
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 netgroup. The sec-
ond rule permits access from all hosts in the foobar.edu domain (notice
the leading dot), with the exception of terminalserver.foobar.edu.
Here, access is granted by default; only explicitly specified hosts are
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. In-
stead of the requested file, a finger probe is sent to the offending
host. The result is mailed to the superuser.
tftpd: LOCAL, .my.domain
tftpd: ALL: (/some/where/safe_finger -l @%h | \
/usr/bin/mail -s %d-%h root) &
The safe_finger command comes with the tcpd wrapper and should be in-
stalled in a suitable place. It limits possible damage from data sent by
the remote finger server. It gives better protection than the standard
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.
/etc/hosts.allow Access control table (allow list)
/etc/hosts.deny Access control table (deny list)
An error is reported when a syntax error is found in a host access con-
trol 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 prob-
lems are reported via the syslog daemon.
tcpd(8), tcpdchk(8), tcpdmatch(8)
Wietse Venema (wietseATwzv.nl)
Department of Mathematics and Computing Science
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
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; YP netgroup lookups are
The total length of an entry can be no more than 2047 characters long,
including the final newline.
OpenBSD 3.6 June 23, 1997 6