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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 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:
· 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:
· 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'.
· 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. A 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:n:n:n:n]/m' is interpreted as a
`[net]/prefixlen' pair. A 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'.
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 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 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> 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.
% EXPANSIONS
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[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 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 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 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 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 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 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 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.
MOSTLY OPEN
Here, access is granted by default; only explicitly specified 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 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 (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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Index for
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Alphabetical
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