curl/ares/ares_init.3

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.\" Copyright 1998 by the Massachusetts Institute of Technology.
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.TH ARES_INIT 3 "21 July 1998"
.SH NAME
ares_init, ares_init_options \- Initialize a resolver channel
.SH SYNOPSIS
.nf
.B #include <ares.h>
.PP
.B int ares_init(ares_channel *\fIchannel\fP)
.B int ares_init_options(ares_channel *\fIchannel\fP,
.B 	struct ares_options *\fIoptions\fP, int \fIoptmask\fP)
.PP
.B cc file.c -lares
.fi
.SH DESCRIPTION
The
.B ares_init
function initializes a communications channel for name service
lookups.  If it returns successfully,
.B ares_init
will set the variable pointed to by
.I channel
to a handle used to identify the name service channel.  The caller
should invoke
.BR ares_destroy (3)
on the handle when the channel is no longer needed.
.PP
The
.B ares_init_options
function also initializes a name service channel, with additional
options useful for applications requiring more control over name
service configuration.  The
.I optmask
parameter specifies which fields in the structure pointed to by
.I options
are set, as follows:
.PP
.TP 18
.B ARES_OPT_FLAGS
.B int \fIflags\fP;
.br
Flags controlling the behavior of the resolver.  See below for a
description of possible flag values.
.TP 18
.B ARES_OPT_TIMEOUT
.B int \fItimeout\fP;
.br
The number of seconds each name server is given to respond to a query
on the first try.  (After the first try, the timeout algorithm becomes
more complicated, but scales linearly with the value of
\fItimeout\fP.)  The default is five seconds.
.TP 18
.B ARES_OPT_TRIES
.B int \fItries\fP;
.br
The number of tries the resolver will try contacting each name server
before giving up.  The default is four tries.
.TP 18
.B ARES_OPT_NDOTS
.B int \fIndots\fP;
.br
The number of dots which must be present in a domain name for it to be
queried for "as is" prior to querying for it with the default domain
extensions appended.  The default value is 1 unless set otherwise by
resolv.conf or the RES_OPTIONS environment variable.
.TP 18
.B ARES_OPT_PORT
.B unsigned short \fIport\fP;
.br
The port to use for queries (both TCP and UDP), in network byte order.
The default value is 53 (in network byte order), the standard name
service port.
.TP 18
.B ARES_OPT_SERVERS
.B struct in_addr *\fIservers\fP;
.br
.B int \fInservers\fP;
.br
The list of servers to contact, instead of the servers specified in
resolv.conf or the local named.
.TP 18
.B ARES_OPT_DOMAINS
.B char **\fIdomains\fP;
.br
.B int \fIndomains\fP;
.br
The domains to search, instead of the domains specified in resolv.conf
or the domain derived from the kernel hostname variable.
.TP 18
.B ARES_OPT_LOOKUPS
.B char *\fIlookups\fP;
.br
The lookups to perform for host queries.  
.I lookups
should be set to a string of the characters "b" or "f", where "b"
indicates a DNS lookup and "f" indicates a lookup in the hosts file.
.PP
The
.I flags
field should be the bitwise or of some subset of the following values:
.TP 23
.B ARES_FLAG_USEVC
Always use TCP queries (the "virtual circuit") instead of UDP
queries.  Normally, TCP is only used if a UDP query yields a truncated
result.
.TP 23
.B ARES_FLAG_PRIMARY
Only query the first server in the list of servers to query.
.TP 23
.B ARES_FLAG_IGNTC
If a truncated response to a UDP query is received, do not fall back
to TCP; simply continue on with the truncated response.
.TP 23
.B ARES_FLAG_NORECURSE
Do not set the "recursion desired" bit on outgoing queries, so that
the name server being contacted will not try to fetch the answer from
other servers if it doesn't know the answer locally.
.TP 23
.B ARES_FLAG_STAYOPEN
Do not close communciations sockets when the number of active queries
drops to zero.
.TP 23
.B ARES_FLAG_NOSEARCH
Do not use the default search domains; only query hostnames as-is or
as aliases.
.TP 23
.B ARES_FLAG_NOALIASES
Do not honor the HOSTALIASES environment variable, which normally
specifies a file of hostname translations.
.TP 23
.B ARES_FLAG_NOCHECKRESP
Do not discard responses with the SERVFAIL, NOTIMP, or REFUSED
response code or responses whose questions don't match the questions
in the request.  Primarily useful for writing clients which might be
used to test or debug name servers.
.SH RETURN VALUES
.I ares_init
or
.I ares_init_options
can return any of the following values:
.TP 14
.B ARES_SUCCESS
Initialization succeeded.
.TP 14
.B ARES_EFILE
A configuration file could not be read.
.TP 14
.B ARES_ENOMEM
The process's available memory was exhausted.
.SH SEE ALSO
.BR ares_destroy (3)
.SH AUTHOR
Greg Hudson, MIT Information Systems
.br
Copyright 1998 by the Massachusetts Institute of Technology.