HOSTS_ACCESS(5) File Formats Manual HOSTS_ACCESS(5)
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 exten‐
sions 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 configu‐
ration 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 (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.
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 `#´ 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[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 patterns:
· 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 `131.155.72.0/255.255.254.0´ matches every address in the range `131.155.72.0´ through
`131.155.73.255´.
· An expression of the form `n.n.n.n/m´ is interpreted as a `net/prefixlen´ pair, as below, for IPv4
addresses.
· 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 format is zero or
more lines with zero or more host name or address patterns separated by whitespace. A file name pat‐
tern 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 `.´.
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 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 pat‐
tern 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 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> sub‐
%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 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.
CLIENT USERNAME LOOKUP
When the client host supports the RFC 931 protocol or one of its descendants (TAP, IDENT, RFC 1413) the wrap‐
per 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 pattern, so the same wildcards apply (netgroup member‐
ship is not supported). One should not get carried away with username lookups, though.
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 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.
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 ta‐
ble, 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 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
leading dot), with the exception of terminalserver.foobar.edu.
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: spawn (/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 fin‐
ger 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 pro‐
vides 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.
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 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 (formerly YP) netgroup lookups are case sensitive.
AUTHOR
Wietse Venema ([email protected])
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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