resolver, res_ninit, fp_resstat, res_hostalias, res_nquery, res_nsearch,
res_nquerydomain, res_nmkquery, res_nsend, res_nclose, res_nsendsigned,
dn_comp, dn_expand, hstrerror, res_init, res_query, res_search, res_mkquery,
res_send, herror, res_getservers, res_setservers, res_ndestroy - resolver
routines
BIND 8.2.2 Interfaces
cc [ flag ... ] file ... -lresolv -lsocket -lnsl [ library ... ]
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include <netdb.h>
int res_ninit(res_state statp);
void res_ndestroy(res_state statp);
void fp_resstat(const res_state statp, FILE *fp);
const char *res_hostalias(const res_state statp, const char *name,
char * name, char *buf, size_tbuflen);
int res_nquery(res_state statp, const char *dname, int class, int type,
u_char *answer, int datalen, int anslen);
int res_nsearch(res_state statp, const char *dname, int class, int type,
u_char *answer, int anslen);
int res_nquerydomain(res_state statp, const char *name,
const char *domain, int class, int type,
u_char *answer, int anslen);
int res_nmkquery(res_state statp, int op, const char *dname, int class,
int type, u_char *answer, int datalen,
int anslen);
int res_nsend(res_state statp, const u_char *msg, int msglen,
u_char *answer, int anslen);
void res_nclose(res_state statp);
int res_snendsigned(res_state statp, const u_char *msg,
int msglen, ns_tsig_key *key, u_char *answer, int anslen);
int dn_comp(const char *exp_dn, u_char *comp_dn, int length,
u_char **dnptrs, **lastdnptr);
int dn_expand(const u_char *msg, *eomorig, *comp_dn, char *exp_dn,
int length);
const char *hstrerror(int err);
void res_setservers(res_state statp, const union res_sockaddr_union *set,
int cnt);
int res_getservers(res_state statp, union res_sockaddr_union *set,
int cnt);
Deprecated Interfaces
cc [ flag ... ] file ... -lresolv -lsocket -lnsl [ library ... ]
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include <netdb.h>
int res_init(void)
int res_query(const char *dname, int class,
int type, u_char *answer,
int anslen);
int res_search(const char *dname, int class,
int type, u_char *answer, int anslen);
int res_mkquery(int op, const char *dname, int class,
int type, const char *data,int datalen,
struct rrec *newrr, u_char *buf, int buflen);
int res_send(const u_char *msg, int msglen, u_char *answer,
int anslen);
void herror(const char *s);
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. Set statp to all zeros prior to
making the first call to any of these functions.
The res_ndestroy() function should be called to free memory
allocated by res_ninit() after the last use of statp.
The functions res_init(), res_query(),
res_search(), res_mkquery(), res_send(), and
herror() are deprecated. They are supplied for backwards
compatibility. They use global configuration and state information that is
kept in the structure _res rather than state information referenced
through statp.
Most of the values in statp and _res are initialized
to reasonable defaults on the first call to res_ninit() or
res_init() and can be ignored. Options stored in
statp->options or _res.options are defined in
<resolv.h>. They 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, that is, res_init() or res_ninit()
has been called.
RES_DEBUG
Print debugging messages.
RES_AAONLY
Accept authoritative answers only. With this option,
res_send() will continue until it finds an authoritative answer or
finds an error. Currently this option is not implemented.
RES_USEVC
Use TCP connections for queries instead of
UDP datagrams.
RES_STAYOPEN
Use with RES_USEVC to keep the TCP
connection open between queries. This is a useful option for programs that
regularly do many queries. The normal mode used should be UDP.
RES_IGNTC
Ignore truncation errors; that is, do not retry with
TCP.
RES_RECURSE
Set the recursion-desired bit in queries. This is the
default. res_send() and res_nsend() do not do iterative queries
and expect the name server to handle recursion.
RES_DEFNAMES
If set, res_search() and res_nsearch()
append the default domain name to single-component names, that is, names that
do not contain a dot. This option is enabled by default.
RES_DNSRCH
If this option is set,
res_search() and
res_nsearch() search for host names in the current domain and in parent
domains. See
hostname(1). This option is used by the standard host
lookup routine
gethostbyname(3NSL). 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.
RES_BLAST
If the RES_BLAST option is defined,
resolver() queries will be sent to all servers. If the RES_BLAST
option is not defined, but RES_ROTATE is , the list of nameservers are
rotated according to a round-robin scheme. RES_BLAST overrides
RES_ROTATE.
RES_ROTATE
This option causes res_nsend() and
res_send() to rotate the list of nameservers in
statp->nsaddr_list or _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 would be updated.
The res_ninit() and res_init() routines read the configuration
file, if any is present, to get the default domain name, search list and the
Internet address of the local name server(s). See resolv.conf(5). If no
server is configured, res_init() or res_ninit() will try to
obtain name resolution services from the host on which it is running. The
current domain name is defined by domainname(8), or by the hostname if
it is not specified in the configuration file. Use the environment variable
LOCALDOMAIN to override the domain name. 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. You can set the RES_OPTIONS environment variable to
override certain internal resolver options. You can otherwise set them by
changing fields in the statp /_res structure. Alternatively,
they are inherited from the configuration file's options command. See
resolv.conf(5) for information regarding the syntax of the
RES_OPTIONS environment variable. Initialization normally occurs on the
first call to one of the other resolver routines.
The res_nquery() and 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() and res_query() return the length of the
answer, or -1 upon error.
The res_nquery() and res_query() routines return a
length that may be bigger than anslen. In that case, retry the query
with a larger buf. The answer to the second query may be
larger still], so it is recommended that you supply a buf larger than
the answer returned by the previous query. answer must be
large 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 res_nsearch() and res_search() routines make a query and await
a response, just like like res_nquery() and res_query(). 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. On error, they
return -1.
The res_nsearch() and res_search() routines return a
length that may be bigger than anslen. In that case, retry the query
with a larger buf. The answer to the second query may be
larger still], so it is recommended that you supply a buf larger than
the answer returned by the previous query. answer must be
large 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 res_nquerydomain() function calls res_query() on the
concatenation of name and domain, removing a trailing dot from
name if domain is NULL.
These routines are used by res_nquery() and res_query(). The
res_nmkquery() and res_mkquery() functions construct a standard
query message and place it in buf. The routine 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(), and res_nsendsigned() routines
send a pre-formatted query that returns an answer. The routine calls
res_ninit() or res_init(). If RES_INIT is not set, the
routine sends the query to the local name server and handles timeouts and
retries. Additionally, the res_nsendsigned() uses TSIG
signatures to add authentication to the query and verify the response. In this
case, only one name server will be contacted. The routines return the length
of the reply message, or -1 if there are errors.
The res_nsend() and res_send() routines return a
length that may be bigger than anslen. In that case, retry the query
with a larger buf. The answer to the second query may be
larger still], so it is recommended that you supply a buf larger than
the answer returned by the previous query. answer must be
large 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 fp_resstat() prints out the active flag bits in
statp->options preceded by the text ";; res
options:" on file.
The function res_hostalias() looks up name in the file referred to
by the HOSTALIASES environment variable and returns the fully qualified
host name. If name is not found or an error occurs, NULL is
returned. res_hostalias() stores the result in buf.
The res_nclose() function closes any open files referenced through
statp.
The res_ndestroy() function calls res_nclose(), then frees any
memory allocated by res_ninit() referenced through statp.
The dn_comp() function compresses the domain name exp_dn and
stores it in comp_dn. The dn_comp() function returns the size of
the compressed name, or −1 if there were errors. length
is the size of the array pointed to by comp_dn.
The dnptrs parameter is a pointer to the head of the list
of pointers to previously compressed names in the current message. The first
pointer must point to the beginning of the message. The list ends with
NULL. The limit to the array is specified by lastdnptr.
A side effect of calling dn_comp() is to update the list of
pointers for labels inserted into the message by dn_comp() as the
name is compressed. If dnptrs is NULL, names are not
compressed. If lastdnptr is NULL, dn_comp() does not
update the list of labels.
The dn_expand() function 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 that
message. The uncompressed name is placed in the buffer indicated by
exp_dn, which is of size length. The dn_expand() function
returns the size of the compressed name, or −1 if there was an
error.
The variables statp->res_h_errno and _res.res_h_errno and
external variable h_errno are set whenever an error occurs during a
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 herror() function writes a message to the diagnostic
output consisting of the string parameters, the constant string
":", and a message corresponding to the value of
h_errno.
The hstrerror() function returns a string, which is the
message text that corresponds to the value of the err parameter.
The functions res_getservers() and res_setservers() are used to
get and set the list of servers to be queried.
/etc/resolv.conf
resolver configuration file
See attributes(7) for descriptions of the following attributes:
ATTRIBUTE
TYPE |
ATTRIBUTE VALUE |
Interface Stability |
Committed |
MT-Level |
Unsafe for deprecated interfaces; MT-Safe for all others. |
libresolv(3LIB), gethostbyname(3NSL), resolv.conf(5),
attributes(7), domainname(8)
Lottor, M. RFC 1033, Domain Administrators Operations
Guide. Network Working Group. November 1987.
Mockapetris, Paul. RFC 1034, Domain Names - Concepts and
Facilities. Network Working Group. November 1987.
Mockapetris, Paul. RFC 1035, Domain Names - Implementation
and Specification. Network Working Group. November 1987.
Partridge, Craig. RFC 974, Mail Routing and the Domain
System. Network Working Group. January 1986.
Stahl, M. RFC 1032, Domain Administrators Guide. Network
Working Group. November 1987.
Vixie, Paul, Dunlap, Kevin J., Karels, Michael J. Name Server
Operations Guide for BIND. Internet Software Consortium,
1996.
When the caller supplies a work buffer, for example the answer buffer
argument to res_nsend() or res_send(), the buffer should be
aligned on an eight byte boundary. Otherwise, an error such as a SIGBUS
may result.