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NAME
hosts_access - format of host access control files
DESCRIPTION
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.
An extended version of the access control language is
described in the hosts_options(5) document. The extensions
are turned on at program build time by building with
-DPROCESS_OPTIONS.
In the following text, daemon is the 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 file.
ACCESS CONTROL FILES
The access control software consults two files. The search
stops at the first match:
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 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.
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 tables 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[0] 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.
The more complex forms daemon@host and user@host are
explained in the sections on server endpoint patterns and on
client username lookups, respectively.
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.
PATTERNS
The access control language implements the following pat-
terns:
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 `wzv.win.tue.nl'.
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 an NIS (formerly YP) netgroup name. A host name is
matched if it is a host member of the specified net-
group. 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 inter-
preted 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
`131.155.72.0/255.255.254.0' matches every address in
the range `131.155.72.0' through `131.155.73.255'.
WILDCARDS
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 unavail-
able 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.
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 unavail-
able 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.
PARANOID
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.
OPERATORS
EXCEPT
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))'.
SHELL COMMANDS
If the first-matched access control rule contains a shell
command, that command is subjected to %<letter> substitu-
tions (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.
% EXPANSIONS
The following expansions are available within shell com-
mands:
%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 infor-
mation is available.
%d The daemon process name (argv[0] value).
%h (%H)
The client (server) host name or address, if the host
name is unavailable.
%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 daemon 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 underscores.
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 dif-
ferent 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 end-
point 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 host_pattern match.
A user pattern has the same syntax as a daemon process pat-
tern, so the same wildcards apply (netgroup membership is
not supported). One should not get carried away with user-
name 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 general, ALL and (UN)KNOWN are the
only user name patterns that make sense.
o Username lookups are possible only with TCP-based ser-
vices, and only when the client host runs a suitable
daemon; in all other cases the result is "unknown".
o 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.
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 user-
name 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 ser-
vice, 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.
EXAMPLES
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.
MOSTLY CLOSED
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:
/etc/hosts.deny:
ALL: ALL
This denies all service to all hosts, unless they are per-
mitted access by entries in the allow file.
The explicitly authorized hosts are listed in the allow
file. For example:
/etc/hosts.allow:
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.foobar.edu.
MOSTLY OPEN
Here, access is granted by default; only explicitly speci-
fied 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:
/etc/hosts.deny:
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.
BOOBY TRAPS
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.
/etc/hosts.allow:
in.tftpd: LOCAL, .my.domain
/etc/hosts.deny:
in.tftpd: ALL: (/some/where/safe_finger -l @%h | \
/usr/ucb/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 lim-
ited 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.
DIAGNOSTICS
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 charac-
ter; 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.
FILES
/etc/hosts.allow, (daemon,client) pairs that are granted access.
/etc/hosts.deny, (daemon,client) pairs that are denied access.
SEE ALSO
tcpd(8) tcp/ip daemon wrapper program.
tcpdchk(8), tcpdmatch(8), test programs.
BUGS
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 (form-
erly 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
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