hosts_access(5) hosts_access(5)
NAME [Toc] [Back]
hosts_access - format of host access control files
DESCRIPTION [Toc] [Back]
The access control facility for internet services uses access control
files to grant or deny access to its services. These files are
defined using a simple access control language based on client (host
name/address, user name) and server (process name, hostname/address)
patterns. See the EXAMPLES section for a quick introduction.
An extended version of the access control language is described in
hosts_options(5).
Access Control Files [Toc] [Back]
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
(/etc/inetd.conf). The access control software searches the contents
of two files: /etc/hosts.allow and /etc/hosts.deny.
The files are searched in the following order. The search stops with
the first match:
+ /etc/hosts.allow file is checked first for a matching (daemon,
client) pair. If one is found, access is granted and the
search stops.
+ /etc/hosts.deny file is checked if no match was found in the
/etc/hosts.allow file and access will be denied if a (daemon,
client) pair match is found.
+ If no (daemon, client) match was found in either access
control file, 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 [Toc] [Back]
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. The following points describe the
format of the access control file:
+ A newline character is ignored when it is preceded by a
backslash ("\"). 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.
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+ All other lines should be in the following format. Contents
in between [] square brackets are 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. NOTE: An IPv6 address should
be enclosed in square brackets [] without any
whitespace.
The more complex forms daemon@host and user@host are explained in the
Server Endpoint Patterns and Client Username Lookups sections
respectively.
List elements must be separated by blanks and/or commas.
With the exception of NIS (YP) netgroup lookups, all access control
checks are case-insensitive.
Patterns [Toc] [Back]
The access control language implements the following patterns:
+ A string that begins with a dot (.) character specifies to
match the components after the dot. A host name is matched if
the last components of its name match the specified pattern.
For example, the pattern .xyz.com matches the host name
abc.def.xyz.com.
+ A string that ends with a dot (.) character specifies to match
the components before the dot. A host address is matched if
its first numeric fields match the given string. For example,
the pattern 192.3. matches the address of (almost) every host
(192.3.x.x) on the 192.3 network.
+ A string that begins with an at (@) 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 host address is matched if "net" is equal
to the bit-wise 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.
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+ An expression of the form [IPv6_address/prefix_length] is
interpreted as an IPv6 network prefix. A host address is
matched if the value of prefix_length bits in IPv6_address and
the host address are equal. For example, the pattern
[3ffe::1111:1234/120] matches every address in the range
3ffe::1111:0 through 3ffe::1111:ffff.
Wildcards [Toc] [Back]
The access control language supports explicit wildcards. They are:
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 is 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 addresses 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.
If the configuration parameter, on_reverselookup_fail
in /etc/tcpd.conf, is set to deny, tcpd drops requests
from such clients even before looking at the access
control tables.
Operators [Toc] [Back]
The access control language supports the following operator:
EXCEPT The format of using this operator is as follows:
list_1 EXCEPT list_2
This construct matches anything that matches list_1, provided it does
not match list_2. The EXCEPT operator can be used in daemon_lists and
client_lists. The EXCEPT operator can be nested. If the control
language permits the use of parentheses, "a EXCEPT b EXCEPT c" parses
as "(a EXCEPT (b EXCEPT c))" .
Shell Commands [Toc] [Back]
If the first-matched access control rule contains a shell command, the
command is subjected to %letter expansions (see next section). The
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result is executed by a /bin/sh child process with standard input,
output, and error connected to /dev/null. Specify an ampersand (&) at
the end of the command if you do not want to wait until the command
has completed.
Shell commands should not rely on the PATH setting of inetd. Instead,
they should use absolute path names or begin with an explicit
PATH=whatever statement.
The hosts_options(5) manual page describes the access control language
that uses the shell command field.
% 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 not available.
%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 do not match any alpha-numeric (A-Za-
z0-9) or !@%-_=+:,./ characters are replaced by underscores.
Server Endpoint Patterns [Toc] [Back]
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 hosts_options(5). Some systems can have
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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 conforms to 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 [Toc] [Back]
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. The 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 (in /etc/tcpd.conf) at run 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). Username
lookup needs to be evaluated carefully because of the following
limitations:
+ 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".
+ Username lookups may cause noticeable delays for non-UNIX
users. The timeout value for username lookups is configurable
through /etc/tcpd.conf. See the tcpd.conf(4) for more
information.
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.
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Detecting Address Spoofing Attacks [Toc] [Back]
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 a strong evidence of
host-spoofing attack.
A positive IDENT lookup result (the client matches KNOWN@host) is less
reliable. 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 possible that the
client's IDENT server is lying.
NOTE: IDENT lookups do not work with UDP services.
EXAMPLES [Toc] [Back]
The language is flexible enough that different types of access control
policy can be expressed with a minimum of effort. 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. The search terminates
when a match is found, and 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 Access Control [Toc] [Back]
In this example, 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 services to all the hosts unless they are permitted
access by entries in the allow file.
The explicitly authorized hosts are listed in the allow file. For
example:
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/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 (there is
no dot "." 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 "." in .foobar.edu), with
the exception of terminalserver.foobar.edu.
Mostly Open Access Control [Toc] [Back]
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 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.
Setting Traps [Toc] [Back]
The next example permits tftp requests from hosts in the local domain
(notice the leading dot). Requests from any other hosts are denied
and instead of the requested file, a finger probe is sent to the
offending host. The result is mailed to the superuser.
/etc/hosts.allow:
tftpd: LOCAL, .my.domain
/etc/hosts.deny:
tftpd: ALL: spawn (/usr/bin/sffinger -l @%h | \
/usr/bin/mailx -s %d-%h root) &
The sffinger command comes with tcp wrappers. 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 earlier in the "Shell Commands" section.
WARNING: Do not set traps on your finger daemon, unless you are
prepared for infinite finger loops.
Service trapping can be especially useful on network firewall systems.
The typical network firewall only provides a limited set of services
to the outer world. All other services can be trapped just like the
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above tftp example. The result is an excellent early-warning system.
DIAGNOSTICS [Toc] [Back]
Problems are reported via syslogd, the syslog daemon, at info, notice,
warning and err levels. An error is reported in the following cases:
+ 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 should not.
WARNINGS [Toc] [Back]
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 not case-sensitive. NIS (formerly YP)
netgroup lookups are case-sensitive.
AUTHOR [Toc] [Back]
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
FILES [Toc] [Back]
/etc/hosts.allow (daemon,client) pairs that are granted
access.
/etc/hosts.deny (daemon,client) pairs that are denied access.
SEE ALSO [Toc] [Back]
tcpd(1M) TCP/IP daemon wrapper program.
tcpdchk(1) and tcpdmatch(1) test programs.
tryfrom(1) and sffinger(1) TCP Wrapper utility programs.
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