RESOLVER(3) BSD Library Functions Manual RESOLVER(3)
NAME
resquery, ressearch, resmkquery, ressend, resinit, dncomp,
dnexpand, dnskipname, nsget16, nsget32, nsput16, nsput32 --
resolver routines
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
##include <>
##include <>
##include <>
##include <>
int
resquery(const char *dname, int class, int type, uchar *answer,
int anslen);
int
ressearch(const char *dname, int class, int type, uchar *answer,
int anslen);
int
resmkquery(int op, const char *dname, int class, int type,
const uchar *data, int datalen, const uchar *newrrin, uchar *buf,
int buflen);
int
ressend(const uchar *msg, int msglen, uchar *answer, int anslen);
int
resinit();
int
dncomp(const char *expdn, uchar *compdn, int length, uchar **dnptrs,
uchar **lastdnptr);
int
dnexpand(const uchar *msg, const uchar *eomorig,
const uchar *compdn, char *expdn, int length);
int
dnskipname(const uchar *compdn, const uchar *eom);
uint
nsget16(const uchar *src);
ulong
nsget32(const uchar *src);
void
nsput16(uint src, uchar *dst);
void
nsput32(ulong src, uchar *dst);
DESCRIPTION
These routines are used for making, sending and interpreting query and
reply messages with Internet domain name servers.
Global configuration and state information that is used by the resolver
routines is kept in the structure res. Most of the values have reason-
able defaults and can be ignored. Options stored in 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.
RESINIT True if the initial name server address and default domain
name are initialized (i.e., resinit() has been called).
RESDEBUG Print debugging messages.
RESAONLY Accept authoritative answers only. With this option,
ressend() should continue until it finds an authoritative
answer or finds an error. Currently this is not imple-
mented.
RESUSEVC Use TCP connections for queries instead of UDP datagrams.
RESTAYOPEN Used with RESUSEVC 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.
RESIGNTC Unused currently (ignore truncation errors, i.e., don't
retry with TCP).
RESRECURSE Set the recursion-desired bit in queries. This is the
default. (ressend() does not do iterative queries and
expects the name server to handle recursion.)
RESDEFNAMES If set, ressearch() will append the default domain name
to single-component names (those that do not contain a
dot). This option is enabled by default.
RESDNSRCH If this option is set, ressearch() 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.
RESNOALIASES This option turns off the user level aliasing feature con-
trolled by the ``HOSTALIASES'' environment variable. Net-
work daemons should set this option.
RESUSEINET6 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.
RESUSEDNS0 Enables support for OPT pseudo-R for EDNS0 extension.
With the option, resolver code will attach OPT pseudo-R
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.
The resinit() routine reads the configuration file (if any; see
resolver(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 over-
ride the search list on a per-process basis. This is similar to the
search command in the configuration file. Another environment variable
``RESOPTIONS'' can be set to override certain internal resolver options
which are otherwise set by changing fields in the res structure or are
inherited from the configuration file's options command. The syntax of
the ``RESOPTIONS'' environment variable is explained in resolver(5).
Initialization normally occurs on the first call to one of the following
routines.
The resquery() function provides an interface to the server query mecha-
nism. It 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-qual-
ified domain name dname. The reply message is left in the answer buffer
with length anslen supplied by the caller.
The ressearch() routine makes a query and awaits a response like
resquery(), but in addition, it implements the default and search rules
controlled by the RESDEFNAMES and RESDNSRCH options. It returns the
first successful reply.
The remaining routines are lower-level routines used by resquery(). The
resmkquery() function 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 . The domain name for the query
is given by dname. Newrr is currently unused but is intended for making
update messages.
The ressend() routine sends a pre-formatted query and returns an answer.
It will call resinit() if RESINIT is not set, send the query to the
local name server, and handle timeouts and retries. The length of the
reply message is returned, or -1 if there were errors.
The dncomp() function compresses the domain name expdn and stores it in
compdn. The size of the compressed name is returned or -1 if there were
errors. The size of the array pointed to by compdn 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 NUL. The limit to the array is
specified by lastdnptr. A side effect of dncomp() is to update the list
of pointers for labels inserted into the message as the name is com-
pressed. If dnptr is NUL, names are not compressed. If lastdnptr is
NUL, the list of labels is not updated.
The dnexpand() entry expands the compressed domain name compdn 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. The uncom-
pressed name is placed in the buffer indicated by expdn which is of size
length. The size of compressed name is returned or -1 if there was an
error.
The dnskipname() function skips over a compressed domain name, which
starts at a location pointed to by compdn. The compressed name is con-
tained in a query or reply message; eom is a pointer to the end of the
message. The size of compressed name is returned or -1 if there was an
error.
The nsget16() function gets a 16-bit quantity from a buffer pointed to
by src.
The nsget32() function gets a 32-bit quantity from a buffer pointed to
by src.
The nsput16() function puts a 16-bit quantity src to a buffer pointed to
by dst.
The nsput32() function puts a 32-bit quantity src to a buffer pointed to
by dst.
FILES
/etc/resolv.conf The configuration file, see resolver(5).
SEE ALSO
gethostbyname(3), resolver(5), hostname(7), named(8)
RFC1032, RFC1033, RFC1034, RFC1035, RFC974
Name Server Operations Guide for BIND.
HISTORY
The resquery function appeared in 4.3BSD.
BSD June 4, 1993 BSD
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