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RESOLVER(3) NetBSD Library Functions Manual RESOLVER(3)
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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)
DNS Resolver Library (libresolv, -lresolv)
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,
res_pquery(const res_state statp, const u_char *msg, int msglen,
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, int buflen);
res_nsend(res_state statp, const u_char *msg, int msglen, u_char *answer,
res_nupdate(res_state statp, ns_updrec *rrecp_in);
res_nmkupdate(res_state statp, ns_updrec *rrecp_in, u_char *buf,
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,
res_getservers(res_state statp, union res_sockaddr_union *set, int cnt);
res_setservers(res_state statp, const union res_sockaddr_union *set,
dn_comp(const char *exp_dn, u_char *comp_dn, int length, u_char **dnptrs,
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,
res_search(const char *dname, int class, int type, u_char *answer,
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,
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
compatibility 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. Cur-
rently 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 con-
trolled by the HOSTALIASES environment variable. Network
daemons should set this option.
RES_ROTATE This options causes 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 are set.
The res_ninit() / res_init() routines read 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 con-
figured, 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 environ-
ment 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 syn-
tax 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.
In NetBSD the initialization code also sets up a kqueue(2) and creates a
kevent(2) watching a file descriptor that points to the resolver file.
Every resolver function calls the internal function __res_check() which
checks for a new kevent(2) related to the resolv.conf(5) file, and
reloads the file if necessary. This does not work if the file is
accessed through a symlink and the symlink changes to point to a differ-
ent file. To fix the symlink issue one could add a system call per
resolver call to get the current time, and reload every so often. This
is not done currently, but it is under consideration.
The memory referred to by statp must be set to all zeros prior to the
first call to res_ninit(). res_ndestroy() should be called to free mem-
ory allocated by res_ninit() after last use.
The res_nquery() / res_query() functions provide interfaces to the server
query mechanism. They construct a query, send it to the local server,
await a response, and make 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 answer.
The res_nsearch() / res_search() routines make a query and awaits a
response like res_nquery() / res_query(), but in addition, they implement
the default and search rules controlled by the RES_DEFNAMES and
RES_DNSRCH options. They return 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 construct a
standard query message and place it in buf. They return 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 send a pre-for-
matted query and return an answer. They 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 preceded by the text ";; res options:" on file.
The functions res_hostalias() / hostalias() look up name in the file
referred to by the HOSTALIASES files and return a fully qualified host-
name 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. They identify the containing zone for each record and group
the records according to containing zone maintaining in zone order then
send an 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
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 an 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 socket file
descriptors referenced through statp / _res. These functions were
designed to be used to emulate endhostent(3), and don't release other
resources held in res_state; to free all_resources, call res_ndestroy().
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 com-
pressed. 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 are 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 */
The following functions are only in libresolv: res_findzonecut(),
res_nmkupdate(), res_nsendsigned(), and res_nupdate(). All the rest are
in both libc and libresolv.
/etc/resolv.conf The configuration file, see resolv.conf(5).
getaddrinfo(3), gethostbyaddr(3), gethostbyname(3), getnameinfo(3),
resolv.conf(5), hostname(7), 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 9.2 February 7, 2018 NetBSD 9.2