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_mkquery(
int op
const char *dname
int class
int type
const char *data
int datalen
struct rrec *newrr
char *buf
int buflen
)
res_send(
const u_char *msg
int msglen
u_char *answer
int anslen
)
res_init(
)
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
u_char *exp_dn
int length
)
Global configuration and state information that is used by the
resolver routines is kept in the structure
_res.
Most of the values have reasonable 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.
RES_INIT
)
has been called).
RES_DEBUG
RES_AAONLY
)
should continue until it finds an authoritative answer or finds an error.
Currently this is not implemented.
RES_USEVC
RES_STAYOPEN
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
RES_RECURSE
)
does not do iterative queries and expects the name server
to handle recursion.)
RES_DEFNAMES
)
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
)
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
RES_USE_EDNS0
The
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
_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 following routines.
The
res_query()
function provides an interface to the server query mechanism.
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-qualified domain name
dname
.
The reply message is left in the
answer
buffer with length
anslen
supplied by the caller.
The
res_search()
routine makes a query and awaits a response like
res_query(
),
but in addition, it implements the default and search rules
controlled by the
RES_DEFNAMES
and
RES_DNSRCH
options.
It returns the first successful reply.
The remaining routines are lower-level routines used by
res_query().
The
res_mkquery(
)
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
<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_send()
routine
sends a pre-formatted query and returns an answer.
It will call
res_init(
)
if
RES_INIT
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
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.
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.
/etc/resolv.conf