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 been called).
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 example, 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.
RES_NOTLDQUERY
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 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 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 master servers.
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 */