hosts_access(5)

Access Control Files

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 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:

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. The following points describe the format of the access control file:

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

The access control language implements the following patterns:

Wildcards

The access control language supports explicit wildcards. They are:

Operators

The access control language supports the following operator:

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

If the first-matched access control rule contains a shell command, the command is subjected to %letter expansions (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 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:

Server Endpoint Patterns

In order to distinguish clients by the network address that they connect to, use patterns of the form:

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 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

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:

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:

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

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 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.

Mostly Closed Access Control

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:

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:

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

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:

The first rule denies some hosts and domains all services. The second rule still permits finger requests from other hosts and domains.

Setting Traps

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.

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 above tftp example. The result is an excellent early-warning system.