RESOLVER(3) Library Functions Manual RESOLVER(3)
res_ninit, res_ourserver_p, fp_resstat, res_hostalias, res_pquery,
res_nquery, res_nsearch, res_nquerydomain, res_nmkquery, res_nsend,
res_nupdate, res_nmkupdate, res_nclose, res_nsendsigned, res_findzonecut,
res_getservers, res_setservers, res_ndestroy, dn_comp, dn_expand,
res_init, res_isourserver, fp_nquery, p_query, hostalias, res_query,
res_search, res_querydomain, res_mkquery, res_send, res_update,
res_close, -- resolver routines
Standard C Library (libc, -lc)
typedef struct __res_state *res_state;
res_ourserver_p(const res_state statp, const struct sockaddr_in *addr);
fp_resstat(const res_state statp, FILE *fp);
const char *
res_hostalias(const res_state statp, const char *name, char *buf, size_t
res_pquery(const res_state statp, const u_char *msg, int msglen, FILE
res_nquery(res_state statp, const char *dname, int class, int
type, u_char *answer, int anslen);
res_nsearch(res_state statp, const char *dname, int class, int
type, u_char * answer, int anslen);
res_nquerydomain(res_state statp, const char *name, const char
*domain, int class, int type, u_char *answer, int anslen);
res_nmkquery(res_state statp, int op, const char *dname, int class, int
type, const u_char *data, int datalen, const u_char *newrr, u_char *buf,
res_nsend(res_state statp, const u_char *msg, int msglen, u_char
*answer, int anslen);
res_nupdate(res_state statp, ns_updrec *rrecp_in);
res_nmkupdate(res_state statp, ns_updrec *rrecp_in, u_char *buf, int
res_nsendsigned(res_state statp, const u_char *msg, int
msglen, ns_tsig_key *key, u_char *answer, int anslen);
res_findzonecut(res_state statp, const char *dname, ns_class class, int
options, char *zname, size_t zsize, struct in_addr *addrs, int naddrs);
res_getservers(res_state statp, union res_sockaddr_union *set, int cnt);
res_setservers(res_state statp, const union res_sockaddr_union *set, int
dn_comp(const char *exp_dn, u_char *comp_dn, int length, u_char
**dnptrs, u_char **lastdnptr);
dn_expand(const u_char *msg, const u_char *eomorig, const u_char
*comp_dn, char *exp_dn, int length);
res_isourserver(const struct sockaddr_in *addr);
fp_nquery(const u_char *msg, int msglen, FILE *fp);
p_query(const u_char *msg, FILE *fp);
const char *
hostalias(const char *name);
res_query(const char *dname, int class, int type, u_char *answer, int
res_search(const char *dname, int class, int type, u_char *answer, int
res_querydomain(const char *name, const char *domain, int class, int
type, u_char *answer, int anslen);
res_mkquery(int op, const char *dname, int class, int type, const char
*data, int datalen, struct rrec *newrr, u_char *buf, int buflen);
res_send(const u_char *msg, int msglen, u_char *answer, int anslen);
These routines are used for making, sending and interpreting query and
reply messages with Internet domain name servers.
State information is kept in statp and is used to control the behavior of
these functions. statp should be set to all zeros prior to the first
call to any of these functions.
The functions res_init(), res_isourserver(), fp_nquery(), p_query(),
hostalias(), res_query(), res_search(), res_querydomain(), res_mkquery(),
res_send(), res_update(), res_close() are deprecated and are supplied for
compatability with old source code. They use global configuration and
state information that is kept in the structure _res rather than that
referenced through statp.
Most of the values in statp and _res are initialized on the first call to
res_ninit() / res_init() to reasonable defaults and can be ignored.
Options stored in statp->options / _res.options are defined in resolv.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., res_ninit() / res_init() has
RES_DEBUG Print debugging messages.
RES_AAONLY Accept authoritative answers only. 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
RES_IGNTC Ignore truncation errors, i.e., don't retry with TCP.
RES_RECURSE Set the recursion-desired bit in queries. This is the
default. (res_nsend() / res_send() does not do iterative
queries and expects the name server to handle recursion.)
RES_DEFNAMES If set, res_nsearch() / res_search() will append the
default domain name to single-component names (those that
do not contain a dot). This option is enabled by default.
RES_DNSRCH If this option is set, res_nsearch() / res_search() 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_USE_INET6 Enables support for IPv6-only applications. This causes
IPv4 addresses to be returned as an IPv4 mapped address.
For example, 10.1.1.1 will be returned as ::ffff:10.1.1.1.
The option is meaningful with certain kernel configuration
RES_USE_EDNS0 Enables support for OPT pseudo-RR for EDNS0 extension.
With the option, resolver code will attach OPT pseudo-RR
into DNS queries, to inform of our receive buffer size.
The option will allow DNS servers to take advantage of
non-default receive buffer size, and to send larger
replies. DNS query packets with EDNS0 extension is not
compatible with non-EDNS0 DNS servers.
RES_NOALIASES This option turns off the user level aliasing feature
controlled by the HOSTALIASES environment variable.
Network daemons should set this option.
RES_ROTATE This options causes the res_nsend() / res_send() to rotate
the list of nameservers in statp->nsaddr_list /
RES_KEEPTSIG This option causes res_nsendsigned() to leave the message
unchanged after TSIG verification; otherwise the TSIG
record would be removed and the header updated.
This option causes res_nsearch() to not attempt to resolve
an 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 res_ninit() / res_init() routine reads the configuration file (if
any; see resolv.conf(5)) to get the default domain name, search list and
the Internet 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 override the search list on a per-process basis. This is similar
to the search command in the configuration file. Another environment
variable RES_OPTIONS can be set to override certain internal resolver
options which are otherwise set by changing fields in the statp / _res
structure or are inherited from the configuration file's options command.
The syntax of the RES_OPTIONS environment variable is explained in
resolv.conf(5). Initialization normally occurs on the first call to one
of the other resolver routines.
The memory referred to by statp must be set to all zeros prior to the
first call to res_ninit(). res_ndestroy() should be call to free memory
allocated by res_ninit() after last use.
The res_nquery() / res_query() functions provides interfaces to the
server query mechanism. They constructs a query, sends it to the local
server, awaits a response, and makes preliminary checks on the reply.
The query requests information of the specified type and class for the
specified fully-qualified domain name dname. The reply message is left
in the answer buffer with length anslen supplied by the caller.
res_nquery() / res_query() return -1 on error or the length of the
The res_nsearch() / res_search() routines make a query and awaits a
response like res_nquery() / res_query(), but in addition, it implements
the default and search rules controlled by the RES_DEFNAMES and
RES_DNSRCH options. It returns the length of the first successful reply
which is stored in answer or -1 on error.
The remaining routines are lower-level routines used by res_nquery() /
res_query(). The res_nmkquery() / res_mkquery() functions constructs a
standard query message and places it in buf. It returns the size of the
query, or -1 if the query is larger than buflen. The query type op is
usually QUERY, but can be any of the query types defined in
<arpa/nameser.h>. The domain name for the query is given by dname.
newrr is currently unused but is intended for making update messages.
The res_nsend() / res_send() / res_nsendsigned() routines sends a pre-
formatted query and returns an answer. It will call res_ninit() /
res_init() if RES_INIT is not set, send the query to the local name
server, and handle timeouts and retries. Additionally, res_nsendsigned()
will use TSIG signatures to add authentication to the query and verify
the response. In this case, only one nameserver will be contacted. The
length of the reply message is returned, or -1 if there were errors.
res_nquery() / res_query(), res_nsearch() / res_search() and res_nsend()
/ res_send() return a length that may be bigger than anslen. In that
case the query should be retried with a bigger buffer. NOTE the answer
to the second query may be larger still so supplying a buffer that bigger
that the answer returned by the previous query is recommended.
answer MUST be big enough to receive a maximum UDP response from the
server or parts of the answer will be silently discarded. The default
maximum UDP response size is 512 bytes.
The function res_ourserver_p() returns true when inp is one of the
servers in statp->nsaddr_list / _res.nsaddr_list.
The functions fp_nquery() / p_query() print out the query and any answer
in msg on fp. p_query() is equivalent to fp_nquery() with msglen set to
The function fp_resstat() prints out the active flag bits in
statp->options preceeded by the text ";; res options:" on file.
The functions res_hostalias() / hostalias() lookup up name in the file
referred to by the HOSTALIASES files return a fully qualified hostname if
found or NULL if not found or an error occurred. res_hostalias() uses
buf to store the result in, hostalias() uses a static buffer.
The functions res_getservers() and res_setservers() are used to get and
set the list of server to be queried.
The functions res_nupdate() / res_update() take a list of ns_updrec
rrecp_in. Identifies the containing zone for each record and groups the
records according to containing zone maintaining in zone order then sends
and update request to the servers for these zones. The number of zones
updated is returned or -1 on error. Note that res_nupdate() will perform
TSIG authenticated dynamic update operations if the key is not NULL.
The function res_findzonecut() discovers the closest enclosing zone cut
for a specified domain name, and finds the IP addresses of the zone's
The functions res_nmkupdate() / res_mkupdate() take a linked list of
ns_updrec rrecp_in and construct a UPDATE message in buf.
res_nmkupdate() / res_mkupdate() return the length of the constructed
message on no error or one of the following error values.
-1 An error occurred parsing rrecp_in.
-2 The buffer buf was too small.
-3 The first record was not a zone section or there was a section order
problem. The section order is S_ZONE, S_PREREQ and S_UPDATE.
-4 A number overflow occurred.
-5 Unknown operation or no records.
The functions res_nclose() / res_close() close any open files referenced
through statp / _res.
The function res_ndestroy() calls res_nclose() then frees any memory
allocated by res_ninit().
The dn_comp() function compresses the domain name exp_dn and stores it in
comp_dn. The size of the compressed name is returned or -1 if there were
errors. The size of the array pointed to by comp_dn is given by length.
The compression uses an array of pointers dnptrs to previously-compressed
names in the current message. The first pointer points to the beginning
of the message and the list ends with NULL. The limit to the array is
specified by lastdnptr. A side effect of dn_comp() is to update the list
of pointers for labels inserted into the message as the name is
compressed. If dnptr is NULL, names are not compressed. If lastdnptr is
NULL, the list of labels is not updated.
The dn_expand() entry expands the compressed domain name comp_dn to a
full domain name. The compressed name is contained in a query or reply
message; msg is a pointer to the beginning of the message. eomorig is a
pointer to the first location after the message. The uncompressed name
is placed in the buffer indicated by exp_dn which is of size length. The
size of compressed name is returned or -1 if there was an error.
The variables statp->res_h_errno / _res.res_h_errno and external variable
h_errno is set whenever an error occurs during resolver operation. The
following definitions are given in <netdb.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-Authoritative not found, or SERVFAIL */
#define NO_RECOVERY 3
/* Non-Recoverable: FORMERR, REFUSED, NOTIMP */
#define NO_DATA 4
/* Valid name, no data for requested type */
~ /etc/resolv.conf The configuration file, see
gethostbyname(3), hostname(7), resolv.conf(5), named(8)
RFC 974, RFC 1032, RFC 1033, RFC 1034, RFC 1035, RFC 1535
Name Server Operations Guide for BIND.
The res_ninit function appeared in 4.3BSD.
NetBSD 6.1.5 July 4, 2000 NetBSD 6.1.5