HOSTS_ACCESS(5)HOSTS_ACCESS(5)NAMEhosts_access - format of host access control files
DESCRIPTION
This manual page describes a simple access control lan
guage that is based on client (host name/address, user
name), and server (process name, host name/address) pat
terns. 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 pro
cess 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:
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 (dae
mon,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.
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 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 `# charac
ter are ignored. This permits you to insert com
ments and whitespace so that the tables are easier
to read.
All other lines should satisfy the following for
mat, things between [] being optional:
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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:
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 pat
tern `.tue.nl matches the host name
`wzv.win.tue.nl.
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 sup
ported 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. 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.
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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 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
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.
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 paren
theses, `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 state
ment.
The hosts_options(5) document describes an alternative
language that uses the shell command field in a different
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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
information 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 informa
tion 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 host
names. 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 sys
tems (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
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and addresses in client_list context. Usually, server end
point information is available only with connection-ori
ented services.
CLIENT USERNAME LOOKUP
When the client host supports the RFC 931 protocol or one
of its descendants (TAP, IDENT, RFC 1413) the wrapper pro
grams can retrieve additional information about the owner
of a connection. Client username information, when avail
able, 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 user
name lookup only when both the daemon_list and the
host_pattern match.
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 ser
vice when username lookups are blocked by a fire
wall. The wrapper README document describes a pro
cedure 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 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
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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
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 clients
IDENT server is lying.
Note: IDENT lookups dont 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 triv
ial deny file:
/etc/hosts.deny:
ALL: ALL
This denies all service to all hosts, unless they are
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permitted 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 lead
ing 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 possi
ble 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 com
mands.
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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 ser
vices 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
character; when the result of %<letter> expansion would
overflow an internal buffer; when a system call fails that
shouldnt. All problems are reported via the syslog dae
mon.
FILES
/etc/hosts.allow, (daemon,client) pairs that are granted access.
/etc/hosts.deny, (daemon,client) pairs that are denied access.
SEE ALSOtcpd(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 (for
merly 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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