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espec:man-host-access [2008/06/19 19:40] – created mazieroespec:man-host-access [2008/06/19 19:42] (atual) – maziero
Linha 1: Linha 1:
 +** mudar para português**
  
 +<code>
 +HOSTS_ACCESS(5)                                                HOSTS_ACCESS(5)
 +
 +NAME
 +       hosts_access - 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.
 +
 +       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.
 +
 +       In the following text, daemon is the the process name of a network dae-
 +       mon process, and client is the name and/or address of a host requesting
 +       service.  Network  daemon process names are specified in the inetd con-
 +       figuration 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 processed in order of appearance. The search terminates  when
 +       a match is found.
 +
 +       o      A  newline  character  is ignored when it is preceded by a back-
 +              slash 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 optional:
 +
 +                 daemon_list : client_list [ : shell_command ]
 +
 +       daemon_list is a list of one or more daemon process names (argv[0] val-
 +       ues) or wildcards (see below).
 +
 +       client_list  is  a list of one or more host names, host addresses, pat-
 +       terns 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 insensitive.
 +
 +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 components 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)  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.
 +
 +       o      An  expression of the form `n.n.n.n/m.m.m.m' is interpreted as a
 +              `net/mask' pair. An IPv4 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'.
 +
 +       o      An expression of the form `[n:n:n:n:n:n:n:n]/m'  is  interpreted
 +              as  a `[net]/prefixlen' pair. An IPv6 host address is matched if
 +              `prefixlen' bits of `net' is equal to the  `prefixlen'  bits  of
 +              the   address.   For   example,   the   [net]/prefixlen  pattern
 +              `[3ffe:505:2:1::]/64'  matches  every  address  in   the   range
 +              `3ffe:505:2:1::' through `3ffe:505:2:1:ffff:ffff:ffff:ffff'.
 +
 +       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 for-
 +              mat 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.
 +
 +       o      Wildcards `*' and `?' can be  used  to  match  hostnames  or  IP
 +              addresses.   This  method of matching cannot be used in conjunc-
 +              tion with `net/mask' matching, hostname matching beginning  with
 +              `.' or IP address matching ending with `.'.
 +
 +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 soft-
 +              ware cannot figure out what type of network it is talking to.
 +
 +       PARANOID
 +              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.
 +
 +OPERATORS
 +       EXCEPT Intended use is of the form: `list_1 EXCEPT list_2';  this  con-
 +              struct  matches  anything  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))'.
 +
 +SHELL COMMANDS
 +       If the first-matched access control rule contains a shell command, that
 +       command is subjected 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(5)  document  describes an alternative language that
 +       uses the shell command field in a different and incompatible way.
 +
 +% EXPANSIONS
 +       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 dae-
 +              mon 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 con-
 +       nect 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) 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  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).   One
 +       should not get carried away with username 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 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  (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  ser-
 +       vice 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 spoof-
 +       ing 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  imple-
 +       mented 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  termi-
 +       nates  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 includ-
 +       ing 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 some_netgroup netgroup.  The
 +       second rule permits access from all  hosts  in  the  foobar.edu  domain
 +       (notice  the  leading  dot),  with the exception of terminalserver.foo-
 +       bar.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 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  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 stan-
 +       dard 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 con-
 +       trol rule; when the length of an access control rule exceeds the capac-
 +       ity  of  an  internal buffer; when an access control rule is not termi-
 +       nated 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
 +       tcpd(8) tcp/ip daemon wrapper program.
 +       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) net-
 +       group lookups are case sensitive.
 +
 +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)
 +</code>