hosts_access(SFF)

hosts_access -- host access control files

Description

The /etc/hosts.allow and /etc/hosts.deny files are read by the tcpd(1Mtcp) server to decide which hosts will or will not be allowed access to services that are configured in the inetd configuration file, inetd.conf(SFF). These files use a simple access control language that is based on client (host name/address, user name), and server (process name, host name/address) patterns.

Extensions to the access control language are enabled in the supplied version of tcpd. These extensions were enabled by building tcpd with PROCESS_OPTIONS defined.

In the following text, daemon is 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 /etc/inetd.conf.

Access control files

The access control software consults two files. The search stops at the first match in the following steps:

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:

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 ``Server endpoint patterns'' and ``Client username lookup'', respectively.

List elements should be separated by blanks and/or commas.

With the exception of NIS 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 network 131.155.*.*.
A string that begins with an ``@'' character is treated as an NIS 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 ``n1.n2.n3.n4/m1.m2.m3.m4'' is interpreted as a ``network_address/netmask'' pair. A host address is matched if the network address is equal to the bitwise AND of its address and the netmask. For example, the 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 the following 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 as 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 as 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. As the supplied version of tcpd was built with PARANOID defined (default mode), it drops requests from such clients even before looking at the access control tables.

Operators

The EXCEPT operator is intended for use in 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 be parsed 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 ``Letter expansions''). 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.

``Control language extensions'' describes an alternative language that uses the shell command field in a different and incompatible way.

Letter expansions

The following expansions are available within shell commands:

%a: The client host address.
%A: The 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: The client host name or address, if the host name is unavailable.
%H: The server host name or address, if the host name is unavailable.
%n: The client host name (or unknown or paranoid).
%N: The 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 ``Control language extensions''. You can use ifconfig(1Mtcp), to define more than one IP address (alias) for a network interface.

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 were configured at compile time to perform 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.

Note the following problems that might occur:

The client username information cannot be trusted when it is needed most, that is, 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 default timeout for username lookups is 10 seconds: too short to cope with slow networks, but long enough to irritate some users.

Selective username lookups can alleviate the last problem. For example, a rule like:

   daemon_list : @pcnetgroup ALL@ALL

This would match members of the pcnetgroup without doing username lookups, but it would perform username lookups with all other systems.

Detecting address spoofing attacks

The IDENT (RFC 931) service can be used to detect host address spoofing attacks where intruders impersonate trusted hosts and, for example, try to break in via the remote shell service. Before accepting a client request, the wrappers can use the IDENT service to find out if the client really did send the request. 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 that IDENT lookups do not work with UDP services.

Control language extensions

The extensions to the control language use the following format:

daemon_list : client_list : option : option ...

Zero or more options may be specified, and must be separated by colons (:). Any ``:'' characters within options should be protected with a backslash.

An option takes the form keyword, keyword value, or keyword=value. Options are processed in the specified order. Some options are subjected to %letter substitutions. For the sake of backwards compatibility with earlier versions, an ``='' character is permitted between the keyword and its value.

Logging

The following option keyword controls event logging:

severity severity
severity facility.severity: Change the severity level (such as info and notice) at which the event will be logged. Facility names (such as mail) are optional. The severity option can be used to emphasize or to ignore specific events.

Access control

The following option keywords control access:

allow: Grant service. This option must appear at the end of a rule.
deny: Deny service. This option must appear at the end of a rule.

The allow and deny keywords make it possible to keep all access control rules within a single file such as hosts.allow.

For example, to permit access from specific hosts only:

   ALL: .friendly.domain: ALLOW
   ALL: ALL: DENY

To permit access from all hosts except a few trouble makers:

   ALL: .bad.domain: DENY
   ALL: ALL: ALLOW

Notice the leading dot on the domain name patterns.

Running other commands

The following option keywords allows you to run other commands:

spawn shell_command

Execute the specified shell command in a child process after performing any %letter expansions. The command is executed with stdin, stdout and stderr connected to the null device, so that it will not mess up the conversation with the client host. For example, the following use of spawn executes the shell command safe_finger -l @%h | mail root in a background child process after replacing %h by the name or address of the remote host.

   spawn (/usr/sbin/safe_finger -l @%h | /usr/bin/mail root) &

The example uses the safe_finger command instead of the regular finger command, to limit possible damage from data sent by the finger server. The safe_finger command is part of the daemon wrapper package; it is a wrapper around the regular finger command that filters the data sent by the remote host.

twist shell_command

Replace the current process by an instance of the specified shell command, after performing any %letter expansions. stdin, stdout and stderr are connected to the client process. This option must appear at the end of a rule.

This example shows twist being used to send a customized bounce message to the client instead of running the real FTP daemon:

   in.ftpd : ... : twist /bin/echo 421 Some bounce message

For an alternative way to talk to client processes, see the description of the banners keyword below.

To run /some/other/in.telnetd without polluting its command-line array or its process environment:

   in.telnetd : ... : twist PATH=/some/other; exec in.telnetd

NOTE: In case of UDP services, do not twist to commands that use standard I/O or the read(2) and write(2) routines to communicate with the client process; UDP requires other I/O primitives.

Network options

The following option keywords control networking behavior:

keepalive: Causes the server to periodically send a message to the client. The connection is considered broken when the client does not respond. The keepalive option can be useful when users turn off their machine while it is still connected to a server. The keepalive option is not useful for datagram (UDP) services.
linger number_of_seconds: Specifies how long the kernel will try to deliver as-yet undelivered data after the server process closes a connection.

Username lookup

The following option keyword controls lookup of usernames:

rfc931 [ timeout_in_seconds ]: Look up the client user name with the RFC 931 (TAP, IDENT, RFC 1413) protocol. This option is silently ignored in case of services based on transports other than TCP. The client system must run a daemon that is compliant with RFC 931 (IDENT, and so on). It may cause noticeable delays with connections from non-UNIX clients. The timeout period is optional. If no timeout is specified, the default value is 10 seconds.

Miscellaneous

The following are miscellaneous option keywords:

banners directory

Look for a file in directory with the same name as the daemon process (for example in.telnetd for the telnet service), and copy its contents to the client. Newline characters are replaced by carriage-return newline, and %letter sequences are expanded.

The tcp wrappers source code distribution provides a sample makefile (Banners.Makefile) for convenient banner maintenance.

NOTE: Banners are supported for connection-oriented (TCP) network services only.

nice [ number ]

Change the nice value of the process (default 10). Specify a positive value to spend more CPU resources on other processes.

setenv name value

Place a (name, value) pair into the process environment. The value is subjected to %letter expansions and may contain whitespace (but leading and trailing blanks are stripped off).

NOTE: Many network daemons reset their environment before spawning a login or shell process.

umask mask

Like the umask command that is built into the shell. The mask argument should be an octal number. An mask of 022 would prevent the creation of files with group and world write permission.

user name

user name.group

Assume the privileges of the userid name (and group if required). The first form is useful with inetd implementations that run all services with root privilege. The second form is useful for services that need special group privileges.

Files

/etc/hosts.allow: (daemon,client) pairs that are granted access.
/etc/hosts.deny: (daemon,client) pairs that are denied access.

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 should not. All problems are reported via the syslog daemon and service is denied.

References

inetd.conf(4tcp), tcpd(ADMN)

Notices

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 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

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 /etc/hosts.deny file:

   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 /etc/hosts.allow file:

   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 /etc/hosts.deny file:

   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. The /etc/hosts.allow file contains:

   in.tftpd: LOCAL .my.domain

The /etc/hosts.deny file contains:

   in.tftpd: ALL: spawn (/usr/sbin/safe_finger -l @%h | \
   	/usr/bin/mail -s %d-%h root) &

The safe_finger command limits possible damage from data sent by the remote finger server. It gives better protection than the standard finger command.

WARNING: Do not booby-trap your finger daemon (fingerd), unless you are prepared to accept 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.