DDEESSCCRRIIPPTTIIOONN
The rreessoollvveerr is a set of routines in the C library (resolve(3)) that pro-
vide access to the Internet Domain Name System. The rreessoollvveerr configura-
tion file contains information that is read by the rreessoollvveerr routines the
first time they are invoked by a process. The file is designed to be
human readable and contains a list of keywords with values that provide
various types of rreessoollvveerr information.
On a normally configured system, this file should not be necessary. The
only name server to be queried will be on the local machine, the domain
name is determined from the host name, and the domain search path is con-
structed from the domain name.
The different configuration directives are:
nameserver
Internet address (in dot notation) of a name server that the
rreessoollvveerr should query. Up to MAXNS (see _<_r_e_s_o_l_v_._h_>) name
servers may be listed, one per keyword. If there are multiple
servers, the rreessoollvveerr library queries them in the order listed.
If no nameserver entries are present, the default is to use the
name server on the local machine. (The algorithm used is to try
a name server, and if the query times out, try the next, until
out of name servers, then repeat trying all the name servers
until a maximum number of retries are made).
domain Local domain name. Most queries for names within this domain
can use short names relative to the local domain. If no domain
entry is present, the domain is determined from the local host
name returned by gethostname; the domain part is taken to be
everything after the first `.'. Finally, if the host name does
not contain a domain part, the root domain is assumed.
search Search list for host-name lookup. The search list is normally
determined from the local domain name; by default, it contains
only the local domain name. This may be changed by listing the
desired domain search path following the search keyword with
spaces or tabs separating the names. Most rreessoollvveerr queries will
be attempted using each component of the search path in turn
until a match is found. Note that this process may be slow and
will generate a lot of network traffic if the servers for the
listed domains are not local, and that queries will time out if
no server is available for one of the domains.
The search list is currently limited to six domains with a total
of 256 characters.
sortlist
Allows addresses returned by gethostbyname to be sorted. A
sortlist is specified by IP address netmask pairs. The netmask
is optional and defaults to the natural netmask of the net. The
IP address and optional network pairs are separated by slashes.
Up to 10 pairs may be specified. For example:
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options Allows certain internal rreessoollvveerr variables to be modified. The
syntax is
options _o_p_t_i_o_n _._._.
where _o_p_t_i_o_n is one of the following:
debug sets RES_DEBUG in ___r_e_s_._o_p_t_i_o_n_s.
ndots:_n sets a threshold for the number of dots which must
appear in a name given to rreess__qquueerryy() (see
resolver(3)) before an _i_n_i_t_i_a_l _a_b_s_o_l_u_t_e _q_u_e_r_y will be
made. The default for _n is ``1'', meaning that if
there are _a_n_y dots in a name, the name will be tried
first as an absolute name before any _s_e_a_r_c_h _l_i_s_t ele-
ments are appended to it.
timeout:_n
sets the amount of time the resolver will wait for a
response from a remote name server before retrying the
query via a different name server. Measured in sec-
onds, the default is RES_TIMEOUT (see _<_r_e_s_o_l_v_._h_>).
attempts:_n
sets the number of times the resolver will send a
query to its name servers before giving up and return-
ing an error to the calling application. The default
is RES_DFLRETRY (see _<_r_e_s_o_l_v_._h_>).
rotate sets RES_ROTATE in ___r_e_s_._o_p_t_i_o_n_s, which causes round
robin selection of nameservers from among those
listed. This has the effect of spreading the query
load among all listed servers, rather than having all
clients try the first listed server first every time.
no-check-names
sets RES_NOCHECKNAME in ___r_e_s_._o_p_t_i_o_n_s, which disables
the modern BIND checking of incoming host names and
mail names for invalid characters such as underscore
(_), non-ASCII, or control characters.
inet6 sets RES_USE_INET6 in ___r_e_s_._o_p_t_i_o_n_s. This has the
effect of trying a AAAA query before an A query inside
the _g_e_t_h_o_s_t_b_y_n_a_m_e function, and of mapping IPv4
responses in IPv6 ``tunnelled form'' if no AAAA
records are found but an A record set exists.
no-tld-query
sets RES_NOTLDQUERY in ___r_e_s_._o_p_t_i_o_n_s. This option
causes rreess__nnsseeaarrcchh() to not attempt to resolve a
unqualified name as if it were a top level domain
(TLD). This option can cause problems if the site has
"localhost" as a TLD rather than having localhost on
one or more elements of the search list. This option
has no effect if neither RES_DEFNAMES or RES_DNSRCH is
set.
The domain and search keywords are mutually exclusive. If more than one
instance of these keywords is present, the last instance wins.
The search keyword of a system's _r_e_s_o_l_v_._c_o_n_f file can be overridden on a
per-process basis by setting the environment variable ``LOCALDOMAIN'' to
a space-separated list of search domains.
The options keyword of a system's _r_e_s_o_l_v_._c_o_n_f file can be amended on a
per-process basis by setting the environment variable ``RES_OPTIONS to a
space-separated list of'' rreessoollvveerr options as explained above under
options.
The keyword and value must appear on a single line, and the keyword
(e.g., nameserver) must start the line. The value follows the keyword,
separated by white space.
