<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/REC-html40/loose.dtd">
<HTML>
<HEAD>
<META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<META name="GENERATOR" content="hevea 1.05">
<TITLE>
hosts_access(5)
</TITLE>
</HEAD>
<BODY >
<A HREF="omniORB012.html"><IMG SRC ="previous_motif.gif" ALT="Previous"></A>
<A HREF="index.html"><IMG SRC ="contents_motif.gif" ALT="Contents"></A>
<A HREF="omniORB014.html"><IMG SRC ="next_motif.gif" ALT="Next"></A>
<HR>
<H1>Appendix A hosts_access(5)</H1>
<A NAME="apx:hostsaccess"></A>
<H3>DESCRIPTION</H3>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.<BR>
<BR>
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.<BR>
<BR>
In the following text, <I>daemon</I> is the process name of a network
daemon process, and <I>client</I> is the name and/or address of a host
requesting service. Network daemon process names are specified in the
inetd configuration file.<BR>
<BR>
<H3>ACCESS CONTROL FILES</H3>The access control software consults two files. The search stops
at the first match:<BR>
<BR>
<UL>
<LI>
Access will be granted when a (daemon,client) pair matches an
entry in the <TT>/etc/hosts.allow</TT> file.<BR>
<BR>
<LI> Otherwise, access will be denied when a (daemon,client) pair
matches an entry in the <TT>/etc/hosts.deny</TT> file.<BR>
<BR>
<LI> Otherwise, access will be granted.</UL>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.<BR>
<BR>
<H3>ACCESS CONTROL RULES</H3>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.<BR>
<BR>
<UL>
<LI> 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.<BR>
<BR>
<LI> Blank lines or lines that begin with a <TT>#</TT> character are
ignored. This permits you to insert comments and whitespace so that
the tables are easier to read.<BR>
<BR>
<LI> All other lines should satisfy the following format, things
between [] being optional: <TT>daemon_list : client_list [ :
shell_command ] </TT></UL><TT>daemon_list</TT> is a list of one or more daemon process names
(argv[0] values) or wildcards (see below). <BR>
<BR>
<TT>client_list</TT> 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.<BR>
<BR>
The more complex forms <TT>daemon@host</TT> and <TT>user@host</TT> are
explained in the sections on server endpoint patterns and on client
username lookups, respectively.<BR>
<BR>
List elements should be separated by blanks and/or commas. <BR>
<BR>
With the exception of NIS (YP) netgroup lookups, all access control
checks are case insensitive.<BR>
<BR>
<H3>PATTERNS</H3>The access control language implements the following patterns:<BR>
<BR>
<UL>
<LI> A string that begins with a <TT>.</TT> character. A host name is
matched if the last components of its name match the specified
pattern. For example, the pattern <TT>.tue.nl</TT> matches the host
name <TT>wzv.win.tue.nl</TT>.<BR>
<BR>
<LI> A string that ends with a <TT>.</TT> character. A host address
is matched if its first numeric fields match the given string. For
example, the pattern <TT>131.155.</TT> matches the address of (almost)
every host on the Eindhoven University network (<TT>131.155.x.x</TT>).<BR>
<BR>
<LI> 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.<BR>
<BR>
<LI> An expression of the form <TT>n.n.n.n/m.m.m.m</TT> 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
<TT>131.155.72.0/255.255.254.0</TT> matches every address in the
range <TT>131.155.72.0</TT> to <TT>131.155.73.255</TT>.</UL>
<H3>WILDCARDS</H3>The access control language supports explicit wildcards:<BR>
<BR>
<DL COMPACT=compact>
<DT>
<TT>ALL</TT><DD><BR>The universal wildcard, always matches.<BR>
<BR>
<DT><TT>LOCAL</TT><DD><BR>Matches any host whose name does not contain a dot
character.<BR>
<BR>
<DT><TT>UNKNOWN</TT><DD><BR>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.<BR>
<BR>
<DT><TT>KNOWN</TT><DD><BR>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.<BR>
<BR>
<DT><TT>PARANOID</TT><DD><BR>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.</DL>
<H3>OPERATORS</H3><DL COMPACT=compact>
<DT><TT>EXCEPT</TT><DD><BR>Intended use is of the form: <TT>list_1</TT> <TT>EXCEPT</TT>
<TT>list_2</TT>; this construct matches anything that matches
<TT>list_1</TT> unless it matches <TT>list_2</TT>. The
<TT>EXCEPT</TT> operator can be used in <TT>daemon_lists</TT> and in
<TT>client_lists</TT>. The <TT>EXCEPT</TT> operator can be nested: if
the control language would permit the use of parentheses, <TT>a
EXCEPT b EXCEPT c</TT> would parse as <TT>(a EXCEPT (b EXCEPT c))</TT>.</DL>
<H3>SHELL COMMANDS</H3>If the first-matched access control rule contains a shell command,
that command is subjected to <TT>%<letter></TT> 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
<TT>&</TT> at the end of the command if you do not want to wait until
it has completed.<BR>
<BR>
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 <TT>PATH=whatever</TT> statement.<BR>
<BR>
The hosts_options(5) document describes an alternative language that
uses the shell command field in a different and incompatible way.<BR>
<BR>
<H3>% EXPANSIONS</H3>The following expansions are available within shell commands:<BR>
<BR>
<UL>
<LI><TT>%a (%A)</TT> The client (server) host address.
<LI><TT>%c</TT> Client information: user@host, user@address, a host
name, or just an address, depending on how much information is
available.
<LI><TT>%d</TT> The daemon process name (argv[0] value).
<LI><TT>%h (%H)</TT>
The client (server) host name or address, if the host name is
unavailable.
<LI><TT>%n (%N)</TT> The client (server) host name (or "unknown" or
"paranoid").
<LI><TT>%p</TT> The daemon process id.
<LI><TT>%s</TT> Server information: daemon@host, daemon@address, or
just a daemon name, depending on how much information is available.
<LI><TT>%u</TT> The client user name (or "unknown").
<LI><TT>%%</TT> Expands to a single <TT>%</TT> character.</UL>Characters in % expansions that may confuse the shell are replaced by
underscores.<BR>
<BR>
<H3>SERVER ENDPOINT PATTERNS</H3>In order to distinguish clients by the network address that they
connect to, use patterns of the form:<BR>
<BR>
<TT>process_name@host_pattern : client_list ... </TT><BR>
<BR>
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
organisations. 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 <TT>host_pattern</TT> obeys
the same syntax rules as host names and addresses in
<TT>client_list</TT> context. Usually, server endpoint information is
available only with connection-oriented services.<BR>
<BR>
<H3>CLIENT USERNAME LOOKUP</H3>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:<BR>
<BR>
<TT>daemon_list : ... user_pattern@host_pattern ...</TT><BR>
<BR>
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
<TT>daemon_list</TT> and the <TT>host_pattern</TT> match.<BR>
<BR>
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.<BR>
<BR>
<UL>
<LI> 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.<BR>
<BR>
<LI> 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'.<BR>
<BR>
<LI> 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.<BR>
<BR>
<LI> 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.</UL>Selective username lookups can alleviate the last problem. For
example, a rule like:<BR>
<BR>
<TT>daemon_list : @pcnetgroup ALL@ALL </TT><BR>
<BR>
would match members of the pc netgroup without doing username lookups,
but would perform username lookups with all other systems.<BR>
<BR>
<H3>DETECTING ADDRESS SPOOFING ATTACKS</H3>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.<BR>
<BR>
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 <TT>UNKNOWN@host</TT>) is strong evidence
of a host spoofing attack.<BR>
<BR>
A positive IDENT lookup result (the client matches
<TT>KNOWN@host</TT>) 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.<BR>
<BR>
Note: IDENT lookups don't work with UDP services. <BR>
<BR>
<H3>EXAMPLES</H3>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.<BR>
<BR>
When reading the examples below it is important to realise 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.<BR>
<BR>
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.<BR>
<BR>
<H3>MOSTLY CLOSED</H3>In this case, access is denied by default. Only explicitly authorised
hosts are permitted access.<BR>
<BR>
The default policy (no access) is implemented with a trivial deny
file:<BR>
<BR>
<PRE>
/etc/hosts.deny:
ALL: ALL
</PRE>This denies all service to all hosts, unless they are permitted access
by entries in the allow file.<BR>
<BR>
The explicitly authorised hosts are listed in the allow file. For
example:<BR>
<BR>
<PRE>
/etc/hosts.allow:
ALL: LOCAL @some_netgroup
ALL: .foobar.edu EXCEPT terminalserver.foobar.edu
</PRE>The first rule permits access from hosts in the local domain (no . in
the host name) and from members of the <TT>some_netgroup</TT>
netgroup. The second rule permits access from all hosts in the
<TT>foobar.edu</TT> domain (notice the leading dot), with the
exception of <TT>terminalserver.foobar.edu</TT>.<BR>
<BR>
<H3>MOSTLY OPEN</H3>Here, access is granted by default; only explicitly specified hosts
are refused service.<BR>
<BR>
The default policy (access granted) makes the allow file redundant so
that it can be omitted. The explicitly non-authorised hosts are
listed in the deny file. For example:<BR>
<BR>
<PRE>
/etc/hosts.deny:
ALL: some.host.name, .some.domain
ALL EXCEPT in.fingerd: other.host.name, .other.domain
</PRE>The first rule denies some hosts and domains all services; the second
rule still permits finger requests from other hosts and domains.<BR>
<BR>
<H3>BOOBY TRAPS</H3>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.<BR>
<BR>
<PRE>
/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) &
</PRE>The <TT>safe_finger</TT> 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.<BR>
<BR>
The expansion of the %h (client host) and %d (service name)
sequences is described in the section on shell commands.<BR>
<BR>
Warning: do not booby-trap your finger daemon, unless you are prepared
for infinite finger loops.<BR>
<BR>
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.<BR>
<BR>
<H3>DIAGNOSTICS</H3>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 expansion would overflow an internal buffer; when a system call fails
that shouldnt. All problems are reported via the syslog daemon.<BR>
<BR>
<H3>FILES</H3><TT>/etc/hosts.allow</TT>, (daemon,client) pairs that are granted access.<BR>
<BR>
<TT>/etc/hosts.deny</TT>, (daemon,client) pairs that are denied access.<BR>
<BR>
<H3>SEE ALSO</H3>tcpd(8) tcp/ip daemon wrapper program.<BR>
<BR>
tcpdchk(8), tcpdmatch(8), test programs.<BR>
<BR>
<H3>BUGS</H3>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.<BR>
<BR>
Domain name server lookups are case insensitive; NIS (formerly YP)
netgroup lookups are case sensitive.<BR>
<BR>
<H3>AUTHOR</H3>Wietse Venema (wietse@wzv.win.tue.nl)<BR>Department of Mathematics and Computing Science<BR>Eindhoven University of Technology<BR>Den Dolech 2, P.O. Box 513,<BR>5600 MB Eindhoven, The Netherlands<BR><BR>
<BR>
<HR>
<A HREF="omniORB012.html"><IMG SRC ="previous_motif.gif" ALT="Previous"></A>
<A HREF="index.html"><IMG SRC ="contents_motif.gif" ALT="Contents"></A>
<A HREF="omniORB014.html"><IMG SRC ="next_motif.gif" ALT="Next"></A>
</BODY>
</HTML>
