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RESOLVER(3) BSD Programmer's Manual RESOLVER(3)
NNAAMMEE
rreess__qquueerryy, rreess__sseeaarrcchh, rreess__mmkkqquueerryy, rreess__sseenndd, rreess__iinniitt, ddnn__ccoommpp,
ddnn__eexxppaanndd - resolver routines
SSYYNNOOPPSSIISS
##iinncclluuddee <<ssyyss//ttyyppeess..hh>>
##iinncclluuddee <<nneettiinneett//iinn..hh>>
##iinncclluuddee <<aarrppaa//nnaammeesseerr..hh>>
##iinncclluuddee <<rreessoollvv..hh>>
rreess__qquueerryy(_c_o_n_s_t _c_h_a_r _*_d_n_a_m_e, _i_n_t _c_l_a_s_s_, _t_y_p_e, _u___c_h_a_r _*_a_n_s_w_e_r,
_i_n_t _a_n_s_l_e_n);
rreess__sseeaarrcchh(_c_o_n_s_t _c_h_a_r _*_d_n_a_m_e, _i_n_t _c_l_a_s_s_, _t_y_p_e, _u___c_h_a_r _*_a_n_s_w_e_r,
_i_n_t _a_n_s_l_e_n);
rreess__mmkkqquueerryy(_i_n_t _o_p, _c_o_n_s_t _c_h_a_r _*_d_n_a_m_e, _i_n_t _c_l_a_s_s_, _t_y_p_e, _c_o_n_s_t _c_h_a_r _*_d_a_t_a,
_i_n_t _d_a_t_a_l_e_n, _s_t_r_u_c_t _r_r_e_c _*_n_e_w_r_r, _u___c_h_a_r _*_b_u_f, _i_n_t _b_u_f_l_e_n);
rreess__sseenndd(_c_o_n_s_t _u___c_h_a_r _*_m_s_g, _i_n_t _m_s_g_l_e_n, _u___c_h_a_r _*_a_n_s_w_e_r, _i_n_t _a_n_s_l_e_n);
rreess__iinniitt();
ddnn__ccoommpp(_c_o_n_s_t _c_h_a_r _*_e_x_p___d_n, _u___c_h_a_r _*_c_o_m_p___d_n, _i_n_t _l_e_n_g_t_h,
_u___c_h_a_r _*_*_d_n_p_t_r_s_, _*_*_l_a_s_t_d_n_p_t_r);
ddnn__eexxppaanndd(_c_o_n_s_t _u___c_h_a_r _*_m_s_g_, _*_e_o_m_o_r_i_g_, _*_c_o_m_p___d_n, _c_h_a_r _*_e_x_p___d_n,
_i_n_t _l_e_n_g_t_h);
hheerrrroorr(_c_o_n_s_t _c_h_a_r _*_s);
hhssttrreerrrroorr(_i_n_t _e_r_r);
DDEESSCCRRIIPPTTIIOONN
These routines are used for making, sending and interpreting query and
reply messages with Internet domain name servers.
Global configuration and state information that is used by the resolver
routines is kept in the structure ___r_e_s _. Most of the values have reason-
able defaults and can be ignored. Options stored in ___r_e_s_._o_p_t_i_o_n_s are de-
fined in _r_e_s_o_l_v_._h and are as follows. Options are stored as a simple bit
mask containing the bitwise ``OR'' of the options enabled.
RES_INIT
True if the initial name server address and default domain name
are initialized (i.e., rreess__iinniitt() has been called).
RES_DEBUG
Print debugging messages.
RES_AAONLY
Accept authoritative answers only. With this option, rreess__sseenndd()
should continue until it finds an authoritative answer or finds
an error. Currently this is not implemented.
RES_USEVC
Use TCP connections for queries instead of UDP datagrams.
RES_STAYOPEN
Used with RES_USEVC to keep the TCP connection open between
queries. This is useful only in programs that regularly do many
queries. UDP should be the normal mode used.
RES_IGNTC
Unused currently (ignore truncation errors, i.e., don't retry
with TCP).
RES_RECURSE
Set the recursion-desired bit in queries. This is the default.
(rreess__sseenndd() does not do iterative queries and expects the name
server to handle recursion.)
RES_DEFNAMES
If set, rreess__sseeaarrcchh() will append the default domain name to sin-
gle-component names (those that do not contain a dot). This op-
tion is enabled by default.
RES_DNSRCH
If this option is set, rreess__sseeaarrcchh() will search for host names
in the current domain and in parent domains; see hostname(7).
This is used by the standard host lookup routine
gethostbyname(3). This option is enabled by default.
RES_NOALIASES
This option turns off the user level aliasing feature controlled
by the HOSTALIASES environment variable. Network daemons should
set this option.
The rreess__iinniitt() routine reads the configuration file (if any; see
resolver(5)) to get the default domain name, search list and the Inter-
net address of the local name server(s). If no server is configured, the
host running the resolver is tried. The current domain name is defined
by the hostname if not specified in the configuration file; it can be
overridden by the environment variable LOCALDOMAIN. This environment
variable may contain several blank-separated tokens if you wish to over-
ride the ``search list'' on a per-process basis. This is similar to the
sseeaarrcchh command in the configuration file. Another environment variable
(``RES_OPTIONS'') can be set to override certain internal resolver op-
tions which are otherwise set by changing fields in the ___r_e_s structure or
are inherited from the configuration file's ooppttiioonnss command. The syntax
of the ``RES_OPTIONS'' environment variable is explained in resolver(5).
Initialization normally occurs on the first call to one of the other re-
solver routines.
The rreess__qquueerryy() function provides an interface to the server query mecha-
nism. It constructs a query, sends it to the local server, awaits a re-
sponse, and makes preliminary checks on the reply. The query requests
information of the specified _t_y_p_e and _c_l_a_s_s for the specified fully-qual-
ified domain name _d_n_a_m_e. The reply message is left in the _a_n_s_w_e_r buffer
with length _a_n_s_l_e_n supplied by the caller.
The rreess__sseeaarrcchh() routine makes a query and awaits a response like
rreess__qquueerryy(), but in addition, it implements the default and search rules
controlled by the RES_DEFNAMES and RES_DNSRCH options. It returns the
first successful reply.
The remaining routines are lower-level routines used by rreess__qquueerryy(). The
rreess__mmkkqquueerryy() function constructs a standard query message and places it
in _b_u_f. It returns the size of the query, or -1 if the query is larger
than _b_u_f_l_e_n. The query type _o_p is usually QUERY, but can be any of the
query types defined in _<_a_r_p_a_/_n_a_m_e_s_e_r_._h_>. The domain name for the query is
given by _d_n_a_m_e. _N_e_w_r_r is currently unused but is intended for making up-
date messages.
The rreess__sseenndd() routine sends a pre-formatted query and returns an answer.
It will call rreess__iinniitt() if RES_INIT is not set, send the query to the lo-
cal name server, and handle timeouts and retries. The length of the re-
ply message is returned, or -1 if there were errors.
The ddnn__ccoommpp() function compresses the domain name _e_x_p___d_n and stores it in
_c_o_m_p___d_n. The size of the compressed name is returned or -1 if there were
errors. The size of the array pointed to by _c_o_m_p___d_n is given by _l_e_n_g_t_h.
The compression uses an array of pointers _d_n_p_t_r_s to previously-compressed
names in the current message. The first pointer points to to the begin-
ning of the message and the list ends with NULL. The limit to the array
is specified by _l_a_s_t_d_n_p_t_r. A side effect of ddnn__ccoommpp() is to update the
list of pointers for labels inserted into the message as the name is com-
pressed. If _d_n_p_t_r is NULL, names are not compressed. If _l_a_s_t_d_n_p_t_r is
NULL, the list of labels is not updated.
The ddnn__eexxppaanndd() entry expands the compressed domain name _c_o_m_p___d_n to a
full domain name. The compressed name is contained in a query or reply
message; _m_s_g is a pointer to the beginning of the message. The uncom-
pressed name is placed in the buffer indicated by _e_x_p___d_n which is of size
_l_e_n_g_t_h. The size of compressed name is returned or -1 if there was an er-
ror.
The external variable _h___e_r_r_n_o is set whenever an error occurs during re-
solver operation. The following definitions are given in _<_n_e_t_d_b_._h_>:
#define NETDB_INTERNAL -1 /* see errno */
#define NETDB_SUCCESS 0 /* no problem */
#define HOST_NOT_FOUND 1 /* Authoritative Answer Host not found */
#define TRY_AGAIN 2 /* Non-Authoritive not found, or SERVFAIL */
#define NO_RECOVERY 3 /* Nonrecoverable: FORMERR, REFUSED, NOTIMP */
#define NO_DATA 4 /* Valid name, no data for requested type */
The hheerrrroorr() function writes a message to the diagnostic output consist-
ing of the string parameter _s, the constant string ": ", and a message
corresponding to the value of _h___e_r_r_n_o _.
The hhssttrreerrrroorr() function returns a string which is the message text cor-
responding to the value of the _e_r_r parameter.
FFIILLEESS
/etc/resolv.conf See resolver(5).
SSEEEE AALLSSOO
gethostbyname(3), hostname(7), named(8), resolver(5); RFC1032,
RFC1033, RFC1034, RFC1035, RFC974; SMM:11, ``Name Server Operations Guide
for BBIINNDD''
4th Berkeley Distribution December 11, 1995 3