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 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
True if the initial name server address and default domain name are
initialized (i.e.,
res_init();
has been called).
RES_DEBUG
Print debugging messages.
RES_AAONLY
Accept authoritative answers only.
With this option,
res_send();
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
Unused currently (ignore truncation errors, i.e., don't retry with
TCP).
RES_RECURSE
Set the recursion-desired bit in queries.
This is the default.
.Pf ( Fn res_send
does not do iterative queries and expects the name server
to handle recursion.)
RES_DEFNAMES
If set,
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_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 not meaningful on
OpenBSD.
RES_USE_EDNS0
Enables support for OPT pseudo-RR for EDNS0 extension.
With this 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.
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
searchlist
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.
Values for the
class
and
type
fields
are defined in
‹arpa/nameser.h›.
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
dnptrs
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.