resolver: dn_comp(), dn_expand(), get_resfield(), herror(), res_init(),
res_mkquery(), res_query(), res_search(), res_send(), set_resfield()-
These routines are used for making, sending, and interpreting query and
reply messages with Internet domain name servers.
Global configuration and state information used by the resolver rou‐
tines are kept in the structure and are defined in Most of the fields
have reasonable defaults and can be ignored. The resolver options are
stored in the field and are listed below. The options are stored as a
simple bit mask containing the bitwise OR of the options enabled.
In a multithreaded environment, a thread specific structure is allo‐
cated for each thread.
True if the initial name server address and default domain name
initialized (i.e., has been called).
Print debugging messages.
Accept authoritative answers only.
With this option, should continue until it
finds an authoritative answer or finds an
error. Currently this is not implemented.
Query the primary server only.
Currently this is not implemented.
Use TCP connections for queries instead of UDP
Used with 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.
The name server will set the truncation bit
if all of the data does not fit into the
response datagram packet. If is set, will
not retry the query with TCP (i.e., ignore
Set the recursion-desired bit in queries.
This is the default. (does not do iterative
queries and expects the name server to han‐
If set, appends the default domain name to single-
component names (those that do not contain a
dot). This option is enabled by default.
If this option is set,
searches for host names in the current
domain and in parent domains; see host‐
name(5). This is used by the standard host
lookup routine (see gethostent(3N)). This
option is enabled by default.
Initialization of the resolver structure normally occurs on the first
call to one of the resolver routines below. If there are errors in the
configuration file, they are silently ignored.
The values for retransmission timeout and number of retries to be
attempted can be configured. These correspond to the retrans and retry
fields in the structure. The following three options, listed in the
order of precedence, have been provided for configuring the retransmis‐
sion timeout and retry values.
1. Environment Variables,
2. Configuration file
3. Through calls to API
retrans and retry can be configured through the Environmental Variables
and as follows:
Alternatively in the following name-value pairs can be added :
While the Environmental Variables and entries in the file are inter‐
preted when the API is called, the API has to be explicitly called from
within the code. Setting the retrans and retry values through a lower
precedence option will be ignored if these values have been configured
through higher precedence option. A message is flagged in when an
invalid value is specified in either or Environmental variables. The
retrans is to be specified in milliseconds, and its default value is
5000 milliseconds. The default value for retry is 4.
Reads the configuration file,
to get the default domain name, search list, the
Internet address of the local name server(s), the
values for retrans and retry. If no server is con‐
figured, 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 This
environment variable may contain several blank sep‐
arated tokens and overrides the search list on a
per process basis. This is similar to the command
in the configuration file. Another environment
variable such as, can be set to override certain
internal resolver options which are set by calling
some of the configuration routines above, or by
using the configuration file's command. The syntax
of the environment variable is explained in
resolver(4). The entries for retrans and retry can
be overridden by and Environmental Variables
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 informa‐
tion of the specified type and class for the speci‐
fied fully-qualified domain name dname. The reply
message is left in the answer buffer with length
anslen supplied by the caller.
Makes a query and awaits a response much like
but in addition, it implements the default and
search rules controlled by the and options. It
returns the first successful reply.
Sets the value for the
retry and retrans fields in the structure. The
value for the retrans option must be specified in
milliseconds. This routine also validates the val‐
ues which the user tries to set for the retry and
retrans options. returns on success, and on fail‐
ure. Calls to fails when values for the field
passed as argument is already set by any higher
precedence option like entering name-value pairs in
the file or setting the Environmental Variables and
Get the value for the
retry and retrans fields in the structure. returns
the value of the field requested on success and if
on failure. It fails when the arguments do not
refer to retrans or retry.
Routines described here are lower-level routines used by
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 but can be any of the query types
defined in The domain name for the query is given
by dname. class can be any of the query classes
defined in type can be any of the query types
defined in data is the data for an inverse query
newrr is currently unused but is intended for mak‐
ing update messages.
Sends a pre-formatted query and returns an answer.
It calls if is not set, sends the query to the
local name server, and handles timeouts and
retries. returns the length of the reply message,
or −1 if there were errors.
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. length is the size of the array pointed to
by comp_dn. The compression uses an array of
pointers dnptrs to previously compressed names in
the current message. The first pointer points to
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 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.
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
supports existing applications to print an error message
a failure. ANSI applications must specify
the following definition for
The argument string s is printed first, fol‐
lowed by a colon, a blank, the message, and
a new-line. may be removed in future
Error return status from is indicated by a return value of −1. The
external integer can then be checked to see whether this is a temporary
failure or an invalid or unknown host.
In a multithreaded application using kernel thread, a thread specific
is allocated for each thread.
can have the following values:
No such host is known.
This is usually a temporary error and means that the local
did not receive a response from an authori‐
tative server. A retry at some later time
Some unexpected server failure was encountered.
This is a non-recoverable error.
The name is known to the name server,
but there is no data of the requested type
associated with this name; this is not a
temporary error. Another type of request to
the name server using this domain name will
result in an answer.
The field should be modified only by using call, and should not be
manipulated directly. is referenced as an for non-threaded applica‐
tions and is defined as function call macro for multithreaded applica‐
tions in file Applications which reference should include
is referenced as an for non-threaded applications and is defined as
function call macro for multithreaded application in file Applications
which reference should include
These resolver routines were developed by the University of California,
Resolver configuration file.
SEE ALSOnamed(1M), gethostent(3N), resolver(4), hostname(5), thread_safety(5),
RFC1034, RFC1035, RFC1535.