NAME

hosts_access, hosts.allow, hosts.deny - 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.

Note that in a `stock' installation of the tcp_wrappers package, a program called _t_c_p_d is called from _/_e_t_c_/_i_n_e_t_d_._c_o_n_f, and this program performs the wrapper checks and then executes the daemon. In NetBSD _i_n_e_t_d(8) has been modified to perform this check internally, and so tcpd is neither used nor supplied.

Also note that libwrap under NetBSD uses the extensions to the access control language as described in the _h_o_s_t_s___o_p_t_i_o_n_s(5).

In the following text, _d_a_e_m_o_n is the process name of a network daemon process, and _c_l_i_e_n_t 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 _/_e_t_c_/_h_o_s_t_s_._a_l_l_o_w file.
· Otherwise, access will be denied when a (daemon,client) pair matches an
entry in the _/_e_t_c_/_h_o_s_t_s_._d_e_n_y 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. WWAARRNNIINNGG:: The total length of an entry can be no more than 2047 characters long including the final newline.
· 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 : option : option ...

_d_a_e_m_o_n___l_i_s_t is a list of one or more daemon process names (argv[0] values) or wildcards (see below).

_c_l_i_e_n_t___l_i_s_t 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. When a client_list item needs to include colon character (for IPv6 addresses), the item needs to be wrapped with square bracket.

The more complex forms _d_a_e_m_o_n_@_h_o_s_t and _u_s_e_r_@_h_o_s_t 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 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´. Note that `m.m.m.m´ portion must always be specified.
· An expression of the form `ipv6-addr/ipv6-mask´ is interpreted as
masked IPv6 address match, just like masked IPv4 address match (see above). Note that `ipv6-mask´ portion must always be specified.
· An expression of the form `ipv6-addr/prefixlen´ is interpreted as
masked IPv6 address match (with mask specified by numeric prefixlen), just like masked IPv4 address match (see above). Note that `prefixlen´ portion must always be specified.

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
_o_r 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
_a_n_d 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.
Note that unlike the default mode of _t_c_p_d, NetBSD _i_n_e_t_d does not automatically drop these requests; you must explicitly drop them in your _/_e_t_c_/_h_o_s_t_s_._a_l_l_o_w or _/_e_t_c_/_h_o_s_t_s_._d_e_n_y file.
{RBL}._d_o_m_a_i_n Matches any host whose reversed address appears in the DNS under
_d_o_m_a_i_n. The primary such domain used for blocking unsolicited commercial e-mail (spam) is `.rbl.maps.vix.com´.

OPERATORS

EXCEPT Intended use is of the form: `list_1 EXCEPT list_2´; this construct
matches anything that matches _l_i_s_t___1 unless it matches _l_i_s_t___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))´.

% EXPANSIONS

The following expansions are available within some options:
%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, NetBSD) 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 _d_a_e_m_o_n___l_i_s_t and the _h_o_s_t___p_a_t_t_e_r_n 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 (RFC 931 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 _s_o_m_e___n_e_t_g_r_o_u_p netgroup. The second rule permits access from all hosts in the _f_o_o_b_a_r_._e_d_u domain (notice the leading dot), with the exception of _t_e_r_m_i_n_a_l_s_e_r_v_e_r_._f_o_o_b_a_r_._e_d_u.

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: spawn (/some/where/safe_finger -l @%h | \
                       /usr/ucb/mail -s %d-%h root) &
               

(The safe_finger command can be gotten from the tcp_wrappers package and 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


               hosts_options(5), hosts_access(3)
               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.

The total length of an entry can be no more than 2047 characters long, including the final newline.

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