Host Access

Command to get man page : man -M /usr/sfw/man -s 4 hosts_access

File Formats                                     HOSTS_ACCESS(4)

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(4) 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  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 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          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 optional:

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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 insensitive.

PATTERNS The access control language implements the  following  pat- terns:

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  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  inter- preted 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'.

WILDCARDS The access control language supports explicit wildcards:

ALL The universal wildcard, always matches.

LOCAL

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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  unavail- able 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  unavail- able 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. 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 construct  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 % substitu- tions (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 command field in a different and incompatible way.

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% EXPANSIONS The following expansions are available  within  shell  com- mands:

%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 infor- mation 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  dif- ferent 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(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  dedicated  network address space.

The host_pattern obeys the same syntax rules as host  names

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and addresses  in client_list context. Usually, server end- point 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 pat- tern, 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  ser- vices, 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  user- name lookups,  but  would perform username lookups with all other systems.

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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 service  to find out that the client did not send the request at all. When the client host provides  IDENT  ser- vice, 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  per- mitted access by entries in the allow file.

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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.foobar.edu.

MOSTLY OPEN Here, access is granted by default; only explicitly  speci- fied 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: (/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.

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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 lim- ited 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 charac- ter; when the result of % 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(1M) tcp/ip daemon wrapper program. tcpdchk(1M), tcpdmatch(1M), 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 (form-     erly YP) netgroup 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

ATTRIBUTES See attributes(5) for descriptions of the following attri- butes:

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_______________________________________   |   ATTRIBUTE TYPE   |  ATTRIBUTE VALUE| |_______________________________________   | Availability       |  SUNWtcpd       | |_______________________________________   | Interface Stability|  External       | |____________________|_________________|

NOTES Source for tcp_wrappers is available in the SUNWtcpdS pack- age.

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