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OpenSolaris 2009.06 - man page for hosts.deny (opensolaris section 4)

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       hosts_access - format of host access control files

       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.

       An extended version of the access control language is described	in  the  hosts_options(4)
       document.  The  extensions  are	turned	on  at program build time by building with -DPRO-

       In the following text, daemon is the 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.

       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.

       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.

       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

		 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 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-

       IPv4  client  addresses	can be denoted in their usual dotted notation, i.e.  x.x.x.x, but
       IPv6 addresses require a square brace around them - e.g.  [::1].

       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

       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  `'  matches
	      every address in the range `' through `'.

       o      Prefixes can be specified for IPv6 address, e.g. [fe80]::/10

       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.

	      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.

	      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.

       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))'.

       If the first-matched access control rule contains a shell command, that	command  is  sub-
       jected to %<letter> 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
       `&' 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.

       The hosts_options(4) document describes an alternative language that uses the  shell  com-
       mand field in a different and incompatible way.

       The following expansions are available within shell commands:

       %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.

       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(4)  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 dedi-
       cated 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

       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.

       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 (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 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.

       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

       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-

       In this case, access is denied by default. Only explicitly authorized hosts are	permitted

       The default policy (no access) is implemented with a trivial deny file:


       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:

	  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-

       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:

	  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.

       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.

	  in.tftpd: LOCAL, .my.domain

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

       The  safe_finger 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.

       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

       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

       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.

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

       tcpd(1M) tcp/ip daemon wrapper program.
       tcpdchk(1M), tcpdmatch(1M), test programs.

       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.

       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

       See attributes(5) for descriptions of the following attributes:

       |Availability	    | SUNWtcpd	      |
       |Interface Stability | Committed       |
       Source for tcp_wrappers is available in the SUNWtcpdS package.

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