RESOLVER(3) BSD 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 buflen);
res_pquery(const res_state statp, const u_char *msg, int msglen, FILE *fp);
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, int anslen);
res_nupdate(res_state statp, ns_updrec *rrecp_in);
res_nmkupdate(res_state statp, ns_updrec *rrecp_in, u_char *buf, int buflen);
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, int cnt);
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 anslen);
res_search(const char *dname, int class, int type, u_char *answer, int anslen);
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 contain-
ing 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 been
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 used.
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 exam-
ple, 10.1.1.1 will be returned as ::ffff:10.1.1.1. The option is meaningful with certain kernel configuration only.
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.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 configu-
ration 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 mem-
ory 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 answer.
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 512.
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
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 pre-
viously-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 mes-
sage 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
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 resolv.conf(5).
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.
July 4, 2000 BSD