resolver.5

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  19 .\"     @(#)resolver.5  5.9 (Berkeley) 12/14/89
  20 .\"     $From: resolver.5,v 8.3 1995/12/06 20:34:35 vixie Exp $
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  22 .Dd June 6, 2003
  23 .Dt resolver 5
  24 .Os "Mac OS X"
  25 .Sh NAME
  26 .Nm resolver
  27 .Nd resolver configuration file format
  28 .Sh DESCRIPTION
  29 The
  30 .Nm
  31 is a set of routines in the C library
  32 .Xr resolv(3)
  33 that provide access to the Internet Domain Name System (DNS).
  34 A resolver configuration file contains information used to specify parameters
  35 for a DNS resolver client.
  36 The file contains a list of keywords with values that provide various types
  37 of resolver information.
  38 .Pp
  39 Mac OS X supports a DNS search strategy that may involve multiple
  40 DNS resolver clients.
  41 See the
  42 .Sx SEARCH STRATEGY
  43 section below for an overview of multi-client DNS
  44 search.
  45 .Pp
  46 Each DNS client is configured using the contents of a single configuration
  47 file of the format described below, or from a property list supplied from
  48 some other system configuration database.
  49 Note that the
  50 .Pa /etc/resolv.conf
  51 file, which contains configuration for the default (or "primary") DNS resolver client,
  52 is maintained automatically by Mac OS X and should not be edited manually.
  53 Changes to the DNS configuration should be made by using the Network
  54 Preferences panel.
  55 .Pp
  56 The different configuration options are given below.
  57 .Ss nameserver
  58 Internet address (in dot notation for IPv4 or in colon notation for IPv6)
  59 of a name server that the resolver should query.
  60 The address may optionally have a trailing dot followed by a port number.
  61 For example,
  62 .Li 10.0.0.17.55
  63 specifies that the nameserver at 10.0.0.17
  64 uses port 55.
  65 .Pp
  66 Up to
  67 .Va MAXNS
  68 (currently 3) name servers may be listed,
  69 one per keyword.
  70 If there are multiple servers,
  71 the resolver library queries them in the order listed.
  72 The algorithm used is to try a name server, and if the query times out,
  73 try the next, until out of name servers,
  74 then repeat trying all the name servers
  75 until a maximum number of retries are made.
  76 .Ss port
  77 IP port number to be used for this resolver.
  78 The default port is 53.
  79 The port number for an individual nameserver may be specified as
  80 part of the nameserver address (see
  81 .Sx nameserver
  82 above) to override the default
  83 or the port number specified as a value for this keyword.
  84 .Ss domain
  85 Domain name associated with this resolver configuration.
  86 This option is normally not required by the Mac OS X DNS search system
  87 when the resolver configuration is read from a file in the
  88 .Pa /etc/resolver
  89 directory.
  90 In that case the file name is used as the domain name.
  91 However,
  92 .Sx domain
  93 must be provided when there are
  94 multiple resolver clients for the same domain name, since multiple
  95 files may not exist having the same name.
  96 See the
  97 .Sx SEARCH STRATEGY
  98 section for more details.
  99 .Ss search
 100 Search list for host-name lookup.
 101 This parameter is only used by the "Super" DNS resolver, which
 102 manages the DNS search strategy amongst multiple DNS resolver clients.
 103 Unqualified queries will be attempted using each component
 104 of the search list in turn until a match is found.
 105 Note that this process may be slow and will generate a lot of network
 106 traffic if the servers for the listed domains are not local,
 107 and that queries will time out if no server is available
 108 for one of the domains.
 109 .Pp
 110 The search list is currently limited to six domains
 111 with a total of 256 characters.
 112 .Ss search_order
 113 Only required for those clients that share a domain name with other clients.
 114 Queries will be sent to these clients in order by ascending
 115 .Sx search_order
 116 value.
 117 For example, this allows two clients for the ".local"
 118 domain, which is used by Apple's multicast DNS, but which may
 119 also be used at some sites as private DNS domain name.
 120 .Ss sortlist
 121 Sortlist allows addresses returned by gethostbyname to be sorted.
 122 A sortlist is specified by IP address netmask pairs. The netmask is
 123 optional and defaults to the natural netmask of the net. The IP address
 124 and optional network pairs are separated by slashes. Up to 10 pairs may
 125 be specified. For example:
 126 .Bd -literal -offset indent
 127  sortlist 130.155.160.0/255.255.240.0 130.155.0.0
 128 .Ed
 129 .Ss timeout
 130 Specifies the total amount of time allowed for a name resolution.
 131 This time interval is divided by the number of nameservers and the number
 132 of retries allowed for each nameserver.
 133 .Ss options
 134 Options allows certain internal resolver variables to be modified.
 135 The syntax is:
 136 .Pp
 137 options
 138 .Ar option Li "..."
 139 .Pp
 140 where
 141 .Ar option
 142 is one of the following:
 143 .Bl -tag -width -indent
 144 .It Ar debug
 145 sets
 146 .Va RES_DEBUG
 147 in the resolver options.
 148 .It Ar timeout:n
 149 sets the per-retry timeout for resolver queries.
 150 The total timeout allowed for a query depends on the number of retries and the
 151 number of nameservers.  This value is ignored if a total timeout is specified
 152 using the
 153 .Sx timeout
 154 keyword (see above).
 155 .It Ar ndots:n
 156 Sets a threshold for the number of dots which
 157 must appear in a name given to
 158 .Sx res_query
 159 (see
 160 .Xr resolver(3))
 161 before an initial absolute query will be made.  The default for
 162 .Ar n
 163 is ``1'', meaning that if there are any dots in a name, the name
 164 will be tried first as an absolute name before any
 165 .Sx search
 166 list elements are appended to it.
 167 .Pp
 168 The keyword and value must appear on a single line,
 169 and the keyword must start the line.
 170 The value follows the keyword, separated by white space.
 171 .El
 172 .Sh SEARCH STRATEGY
 173 Mac OS X uses a DNS search strategy that supports multiple DNS
 174 client configurations.
 175 Each DNS client has its own set of nameserver
 176 addresses and its own set of operational parameters.
 177 Each client can perform DNS queries and searches independent of other clients.
 178 Each client has a symbolic name which is of the same format as a
 179 domain name, e.g. "apple.com".
 180 A special meta-client, known as the
 181 "Super" DNS client acts as a router for DNS queries.
 182 The Super client chooses among all available clients by finding a best match
 183 between the domain name given in a query and the names of all known clients.
 184 .Pp
 185 Queries for qualified names
 186 are sent using a client configuration
 187 that best matches the domain name given in the query.
 188 For example, if there is a client named "apple.com", a search for
 189 "www.apple.com" would use the resolver configuration specified for that client.
 190 The matching algorithm chooses the client with the maximum number of matching
 191 domain components.
 192 For example, if there are clients named "a.b.c", and "b.c", a search for
 193 "x.a.b.c" would use the "a.b.c" resolver configuration, while a search
 194 for "x.y.b.c" would use the "b.c" client.
 195 If there are no matches, the configuration settings in the default client,
 196 generally corresponding to the
 197 .Pa /etc/resolv.conf
 198 file or to the "primary" DNS
 199 configuration on the system are used for the query.
 200 .Pp
 201 If multiple clients are available for the same domain name, the clients ordered
 202 according to a
 203 .Sx search_order
 204 value (see above).
 205 Queries are sent to these resolvers in sequence by ascending value of
 206 search_order.
 207 .Pp
 208 The configuration for a particular client may be read from a file
 209 having the format described in this man page.
 210 These are at present located by the system in the
 211 .Pa /etc/resolv.conf
 212 file and
 213 in the files found in the
 214 .Pa /etc/resolver
 215 directory.
 216 However, client configurations are not limited to file storage.
 217 The implementation of the DNS multi-client search strategy may also locate
 218 client configuratins in other data sources, such as the System Configuration
 219 Database.
 220 Users of the DNS system should make no assumptions about the
 221 source of the configuration data.
 222 .Sh FILES
 223 /etc/resolv.conf, /etc/resolver/*
 224 .Sh SEE ALSO
 225 gethostbyname(2), getaddrinfo(3), resolver(3)