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
The extended version of the access control language is described in the
hosts_options(5) document. Note that this language supersedes the mean‐
ing 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 con‐
ACCESS CONTROL FILES
The access control software consults two files. The search stops at the
· 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
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.
· A newline character is ignored when it is preceded by a back‐
slash 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 ]
daemon_list is a list of one or more daemon process names (argv val‐
ues) or server port numbers or wildcards (see below).
client_list is a list of one or more host names, host addresses, pat‐
terns or wildcards (see below) that will be matched against the client
host name or address.
The more complex forms daemon@host and user@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 case insensitive.
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
· 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 `18.104.22.168/255.255.254.0´
matches every address in the range `22.214.171.124´ through
`126.96.36.199´. `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
`3ffe:505:2:1::´ through `3ffe:505:2:1:ffff:ffff:ffff:ffff´.
· 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 for‐
mat 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 conjunc‐
tion with `net/mask´ matching, hostname matching beginning with
`.´ or IP address matching ending with `.´.
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.
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.
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.
EXCEPT Intended use is of the form: `list_1 EXCEPT list_2´; this con‐
struct 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 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:
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).
The client (server) host name or address, if the host name is
The client (server) host name (or "unknown" or "paranoid").
The clients (servers) port number (or "0").
%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
SERVER ENDPOINT PATTERNS
In order to distinguish clients by the network address that they con‐
nect 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, Free‐
BSD) can have more than one internet address on one physical interface;
with other systems you may have to resort to SLIP or PPP pseudo inter‐
faces 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.
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. 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
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 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".
· 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
· 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
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
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 `UNKNOWN@host´) is strong evidence of a host spoof‐
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 termi‐
nates 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 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
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
second rule permits access from all hosts in the foobar.edu domain
(notice the leading dot), with the exception of terminalserver.foo‐
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.
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 stan‐
dard 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 con‐
trol rule; when the length of an access control rule exceeds the capac‐
ity of an internal buffer; when an access control rule is not termi‐
nated 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.
SEE ALSOhosts_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) net‐
group lookups are case sensitive.
Wietse Venema (email@example.com)
Department of Mathematics and Computing Science
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands