DNS BIND view Clause

This section describes the view clause available in BIND 9.x. The view
clause 
allows BIND to provide different functionality based on the hosts accessing
it.
The view statement can take a serious number of statements shown below. Full

list of statements. A view clause matches (is invoked) when either or both
of 
its match-clients and match-destinations statements match and when the 
match-recursive-only condition is met. If either or both of match-clients
and 
match-destinations are missing they default to any (all hosts match). All
zones
supported by each view clause must be defined with the view clause allowing
a 
view to respond uniquely for each zone if required.

view Clause Syntax

The following statements are allowed within a view clause.

  additional-from-auth (yes | no) ; [ Opt, View ]
  additional-from-cache (yes | no) ; [ Opt, View ]
  allow-notify { address_match_list }; [ Opt, View, Zone ]
  allow-query { address_match_list }; [ Opt, View, Zone ]
  allow-recursion { address_match_list }; [ Opt, View ]
  allow-transfer { address_match_list }; [ Opt, View, Zone ]
  allow-update-forwarding { address_match_list }; [ Opt, View, Zone ]
  also-notify { ip_addr [port ip_port] ; ... ] }; [ Opt, View, Zone ]
  alt-transfer-source ( ipv4 | * ) [ port ( integer | * )]; [ Opt, View,
Zone ]
  alt-transfer-source-v6 ( ipv6 | * ) [ port ( integer | * ) ]; [ Opt,
View, Zone ]
  auth-nxdomain (yes | no); [ Opt, View ]
  cleaning-interval number; [ Opt, View ]
  dialup dialup_options; [ Opt, View, Zone ]
  disable-algorithms string { string; ... }; [ Opt, View ]
  dnssec-enable ( yes | no ); [ Opt, View ]
  dnssec-lookaside domain trust-anchor domain; [ Opt, View ]
  dnssec-must-be-secure domain ( yes | no); [ Opt, View ]
  dual-stack-servers [ port p_num ] { ( "id" [port p_num] | 
  	ipv4 [port p_num] | ipv6 [port p_num] ); ... }; [ Opt, View ]
  edns-udp-size size_in_bytes; [ Opt, View ]
  files number_of_files ; [ Opt, View ]
  forward ( only | first ); [ Opt, View, Zone ]
  forwarders { ipv4_addr | ipv6_addr [port ip_port] ; ... ] }; [ Opt,
View, Zone ]
  heartbeat-interval minutes; [ Opt, View ]
  hostname hostname_string; ; [ Opt, View ]
  ixfr-from-differences ( yes | no); [ Opt, View, Zone ]
  key-directory path_name; [ Opt, View, Zone ]
  lame-ttl number; [ Opt, View ]
  match-clients { address_match_list } ; [ View ]
  match-destinations { address_match_list } ; [ View ]
  match-recursive-only ( yes | no ) ; [ View ]
  max-cache-size size_in_bytes ; [ Opt, View ]
  max-cache-ttl seconds; [ Opt, View ]
  max-journal-size size_in_bytes; [ Opt, View, Zone ]
  max-ncache-ttl seconds; [ Opt, View ]
  max-refresh-time seconds ; [ Opt, View, Zone ]
  max-retry-time seconds ; [ Opt, View, Zone ]
  max-transfer-idle-in minutes; [ Opt, View, Zone ]
  max-transfer-idle-out minutes; [ Opt, View, Zone ]
  max-transfer-time-in minutes; [ Opt, View, Zone ]
  max-transfer-time-out minutes; [ Opt, View, Zone ]
  min-refresh-time seconds ; [ Opt, View, Zone ]
  min-retry-time seconds ; [ Opt, View, Zone ]
  minimal-responses ( yes | no ) ; [ Opt, View ]
  multi-master ( yes | no ) ; [ Opt, View, Zone ]
  notify ( yes | no | explicit ); [ Opt, View, Zone ]
  notify-source (ip4_addr | *) [port ip_port] ; [ Opt, View, Zone ]
  notify-source-v6 (ip6_addr | *) [port ip_port] ; [ Opt, View, Zone ]
  preferred-glue ( A | AAAA) ; [ Opt, View ]
  provide-ixfr ( yes | no) ; [ Opt, View, server ]
  query-source [ address ( ip_addr | * ) ] [ port ( ip_port | * ) ]; [
Opt, View ]
  query-source-v6 [ address ( ip_addr | * ) ] [ port ( ip_port | * ) ]; [
Opt, View ]
  recursion ( yes | no ); [ Opt, View ]
  request-ixfr ( yes | no ); [ Opt, View, server ]
  root-delegation-only [ exclude { namelist } ] ; [ Opt, View ]
  rrset-order { order_spec ; [ order_spec ; ... ] ); [ Opt, View ]
  sig-validity-interval number ; [ Opt, View, Zone ]
  sortlist { address_match_list }; [ Opt, View ]
  sig-validity-interval days ; [ Opt, View, Zone ]
  transfer-format ( one-answer | many-answers ); [ Opt, View, server ]
  transfer-source (ip4_addr | *) [port ip_port] ; [ Opt, View, Zone ]
  transfer-source-v6 (ip6_addr | *) [port ip_port] ; [ Opt, View, Zone ]
  use-alt-transfer-source ( yes | no ); [ Opt, View, Zone ]
  zone-statistics ( yes | no ) ; [ Opt, View, Zone ]

view Clause Syntax

view "view_name" [class] {
  [ match-clients {  address_match_list } ; ]
  [ match-destinations { address_match_list } ; ]
  [ match-recursive-only { yes | no } ; ]
  // view statements
  // zone clauses
};

view_name (a quoted string) is the arbitrary but unique name of this view. A
view clause matches (is invoked) when either or both of its match-clients
and match-destinations statements match and when the match-recursive-only
condition is met. If either or both of match-clients and match-destinations
are missing they default to any (all hosts match). The zones that will be
serviced by this view must be contained within this view.
view Clause Example

The classic example quoted is an alternate implementation of a split or
stealth DNS configuration on a single server so we will follow in well
trodden steps - see also stealth examples:
'split' DNS using views

view "trusted" {
 match-clients { 192.168.23.0/24; }; // our network
  recursion yes;
  // other view statements as required
  zone "example.com" {
   type master;
   // private zone file including local hosts
   file "internal/master.example.com";
  };
  // add required zones
 };
view "badguys" {
 match-clients {"any"; }; // all other hosts
 // recursion not supported
 recursion no;
 // other view statements as required
 zone "example.com" {
   type master;
   // public only hosts
   file "external/master.example.com";
  };
  // add required zones
 };

Notes:

    Depending on the required level of security the above configuration may
be deemed vulnerable. If the file system is compromised then simple
inspection of 'named.conf' will allow penetration of the 'veil of privacy'.

    view clauses have many interesting uses beyond that illustrated here.
This configuration shows a 'view' clause being used to implement a Split
Horizon DNS whereby different IPs are returned based on geographic (or
other) criteria.

    view clauses are processed in the order in which they appear in the
named.conf file. Thus, in the example above the 'badguys' view clause
matching condition (any) also satisfies the 'trusted' view matching
condition. However, since 'trusted' appears first its matching condition is
the first to be satisfied and view matching stops.

    While the example shows two view clauses any number of view clauses may
be present.

    If none of the matching conditions in view clauses matches then BIND
will return a server error.

    view is class dependent but the default class is IN (or 'in' - not case
dependent) and has been omitted.

    While it may seem like a statement of the obvious, the zone files
defined in each view for the same domain name do not need to be the same,
nor do all zones defined in one view require to be present in all views. For
example, it is possible to have private zones that are only visible within
an Intranet or private network.

    The required zone files may differ in each view, for example, there is
no need to provide localhost zones in the "badguys" view.

    The zone files for "example.com" are different allowing 'hiding' of non
-public hosts in the "trusted" view.

    Recursion has been removed in the "badguys" view for performance and
security reasons.

    'slave' servers for each zone will see a single 'zone' based on their IP
address, for instance, "trusted" or "badguys". However, if you multi-home or
'alias' the IP address on the 'slave' server you could get both views.

    Slave DNS Servers with View Clause When using a Slave server with view
clauses it is important to recall that, even when NOTIFY is used, the Slave
always initiates the zone tranfer operation using an INCOMING DNS operation
(TCP on Port 53 normally). To ensure the correct zone file is transferred
the match-clients and/or match-destinations statements associated with the
views must ensure that the requesting Slave server's IP is directed to the
view containing the zone file that should be tranferred.

Note: Early versions of BIND 9 allowed default zones to be defined outside
the scope of view clauses. Current versions of BIND will refuse to load such
configurations. If incoming queries are not matched to a particular view
then Server Error is typically returned. To avoid such problems the last
view defined should use:

match-clients {any};
// or if you enjoy living dangerously omit the statment 
// (see defaults below)

match-clients

 match-clients { address_match_element; ... };
 match-clients { 10.2.3.0/8;172.16.30.0/16;!192.168.0.0/16; };
 # the above could be re-written as 
 match-clients { 10/8;172.16/16;!192.168/16; };
 # (More info on IP Prefix notation)

A view clause matches when either or both of its match-clients and match
-destinations statements match and when the match-recursive-only condition
is met. If either or both of match-clients and match-destinations are
missing they default to any (all hosts match). The match-clients statement
defines the address_match_list for the source IP address of the incoming
messages. Any IP which matches will use the defined view clause. This
statement may only be used in a view clause.
match-destinations

 match-destinations { address_match_element; ... };
 match-destinations { 192.168.0.3; };

A view clause matches when either or both of its match-clients and match
-destinations statements match and when the match-recursive-only condition
is met. If either or both of match-clients and match-destinations are
missing they default to any (all hosts match). The match-destination
statement defines the address_match_list for the destination IP address of
the incoming messages. Any IP which matches will use the defined view
clause. This statement may only be used in a view clause.
match-recursive-only

 match-recursive-only (yes | no);
 match-recursive-only yes;

A view clause matches when either or both of its match-clients and match
-destinations statements match and when the match-recursive-only condition
is met. If either or both of match-clients and match-destinations are
missing they default to any (all hosts match). The match-recursive-only can
be used in conjunction with match-clients and match-destinations or on its
own if that is sufficient differentiation. The default is no. This statement
may only be used in a view clause.