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NetBSD 6.1.5 - man page for hosts.deny (netbsd section 5)

HOSTS_ACCESS(5) 		       File Formats Manual			  HOSTS_ACCESS(5)

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 tcpd  is
       called  from  /etc/inetd.conf,  and this program performs the wrapper checks and then exe-
       cutes the daemon.  In NetBSD inetd(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 hosts_options(5).

       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 the inetd configuration file.

ACCESS CONTROL FILES
       The access control software consults two files.	The search stops at the first match:

       o      Access will be granted  when  a  (daemon,client)	pair  matches  an  entry  in  the
	      /etc/hosts.allow file.

       o      Otherwise,  access  will	be denied when a (daemon,client) pair matches an entry in
	      the /etc/hosts.deny file.

       o      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 pro-
       cessed in order of appearance.  The search terminates when a match is found.

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

       o      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.

       o      All other lines should satisfy  the  following  format,  things  between	[]  being
	      optional:

		 daemon_list : client_list : option : option ...

       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.  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 daemon@host and user@host 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 insen-
       sitive.

PATTERNS
       The access control language implements the following patterns:

       o      A string that begins with a `.' character.  A host name is matched if the last com-
	      ponents of its  name  match  the	specified  pattern.   For  example,  the  pattern
	      `.tue.nl' matches the host name `wzv.win.tue.nl'.

       o      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).

       o      A string that begins with an `@' character is treated as an NIS (formerly YP)  net-
	      group  name.   A	host name is matched if it is a host member of the specified net-
	      group.  Netgroup matches are not supported for daemon process names or  for  client
	      user names.

       o      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.

       o      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.

       o      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.

       o      A string that begins with a `/' character is treated as a file name. A host name or
	      address is matched if it matches any host name or address  pattern  listed  in  the
	      named  file.  The  file format is zero or more lines with zero or more host name or
	      address patterns separated by whitespace.  A file name pattern can be used anywhere
	      a host name or address pattern can be used.

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 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.

       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: 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 tcpd, NetBSD inetd does not automatically drop these requests;  you
	      must explicitly drop them in your /etc/hosts.allow or /etc/hosts.deny file.

       {RBL}.domain
	      Matches  any host whose reversed address appears in the DNS under domain.  The pri-
	      mary 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 any-
	      thing 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 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	organiza-
       tions.	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  inter-
       face; 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 dae-
       mon_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 user-
       name lookups, though.

       o      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.

       o      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".

       o      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.

       o      Username lookups may cause noticeable delays for non-UNIX users.	The default time-
	      out 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 ser-
       vice.  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 fail-
       ures.

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 some_netgroup netgroup.  The second rule permits access from all hosts
       in the  foobar.edu  domain  (notice  the  leading  dot),  with  the  exception  of  termi-
       nalserver.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 omit-
       ted.  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-warn-
       ing 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  con-
       trol 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

										  HOSTS_ACCESS(5)


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