ARES_INIT(3)ARES_INIT(3)NAME
ares_init, ares_init_options - Initialize a resolver channel
SYNOPSIS
#include <ares.h>
int ares_init(ares_channel *channelptr)
int ares_init_options(ares_channel *channelptr,
struct ares_options *options, int optmask)
cc file.c -lcares
DESCRIPTION
The ares_init function initializes a communications channel for name
service lookups. If it returns successfully, ares_init will set the
variable pointed to by channelptr to a handle used to identify the name
service channel. The caller should invoke ares_destroy(3) on the han‐
dle when the channel is no longer needed.
The ares_init_options function also initializes a name service channel,
with additional options useful for applications requiring more control
over name service configuration. The optmask parameter specifies which
fields in the structure pointed to by options are set, as follows:
ARES_OPT_FLAGS int flags;
Flags controlling the behavior of the resolver. See
below for a description of possible flag values.
ARES_OPT_TIMEOUT int timeout;
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 compli‐
cated, but scales linearly with the value of time‐
out.) The default is five seconds. This option is
being deprecated by ARES_OPT_TIMEOUTMS starting in c-
ares 1.5.2.
ARES_OPT_TIMEOUTMS
int timeout;
The number of milliseconds each name server is given
to respond to a query on the first try. (After the
first try, the timeout algorithm becomes more compli‐
cated, but scales linearly with the value of time‐
out.) The default is five seconds. Note that this
option is specified with the same struct field as the
former ARES_OPT_TIMEOUT, it is but the option bits
that tell c-ares how to interpret the number. This
option was added in c-ares 1.5.2.
ARES_OPT_TRIES int tries;
The number of tries the resolver will try contacting
each name server before giving up. The default is
four tries.
ARES_OPT_NDOTS int ndots;
The number of dots which must be present in a domain
name for it to be queried for "as is" prior to query‐
ing for it with the default domain extensions
appended. The default value is 1 unless set other‐
wise by resolv.conf or the RES_OPTIONS environment
variable.
ARES_OPT_PORT unsigned short port;
The port to use for queries (both TCP and UDP), in
network byte order. The default value is 53 (in net‐
work byte order), the standard name service port.
ARES_OPT_SERVERS struct in_addr *servers;
int nservers;
The list of IPv4 servers to contact, instead of the
servers specified in resolv.conf or the local named.
In order to allow specification of either IPv4 or
IPv6 name servers, the ares_set_servers(3) function
must be used instead.
ARES_OPT_DOMAINS char **domains;
int ndomains;
The domains to search, instead of the domains speci‐
fied in resolv.conf or the domain derived from the
kernel hostname variable.
ARES_OPT_LOOKUPS char *lookups;
The lookups to perform for host queries. lookups
should be set to a string of the characters "b" or
"f", where "b" indicates a DNS lookup and "f" indi‐
cates a lookup in the hosts file.
ARES_OPT_SOCK_STATE_CB
void (*sock_state_cb)(void *data, int s, int read,
int write);
void *sock_state_cb_data;
A callback function to be invoked when a socket
changes state. s will be passed the socket whose
state has changed; read will be set to true if the
socket should listen for read events, and write will
be set to true if the socket should listen for write
events. The value of sock_state_cb_data will be
passed as the data argument.
The flags field should be the bitwise or of some subset of the follow‐
ing values:
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.
ARES_FLAG_PRIMARY Only query the first server in the list of
servers to query.
ARES_FLAG_IGNTC If a truncated response to a UDP query is
received, do not fall back to TCP; simply con‐
tinue on with the truncated response.
ARES_FLAG_NORECURSE Do not set the "recursion desired" bit on outgo‐
ing queries, so that the name server being con‐
tacted will not try to fetch the answer from
other servers if it doesn't know the answer
locally. Be aware that ares will not do the
recursion for you. Recursion must be handled by
the application calling ares if ARES_FLAG_NORE‐
CURSE is set.
ARES_FLAG_STAYOPEN Do not close communications sockets when the
number of active queries drops to zero.
ARES_FLAG_NOSEARCH Do not use the default search domains; only
query hostnames as-is or as aliases.
ARES_FLAG_NOALIASES Do not honor the HOSTALIASES environment vari‐
able, which normally specifies a file of host‐
name translations.
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.
RETURN VALUES
ares_init or ares_init_options can return any of the following values:
ARES_SUCCESS Initialization succeeded.
ARES_EFILE A configuration file could not be read.
ARES_ENOMEM The process's available memory was exhausted.
ARES_ENOTINITIALIZED
c-ares library initialization not yet performed.
NOTES
When initializing from /etc/resolv.conf, ares_init(3) reads the domain
and search directives to allow lookups of short names relative to the
domains specified. The domain and search directives override one
another. If more that one instance of either domain or search direc‐
tives is specified, the last occurence wins. For more information,
please see the resolv.conf(5) manual page.
SEE ALSOares_destroy(3), ares_dup(3), ares_library_init(3), ares_set_servers(3)AUTHOR
Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.
Copyright (C) 2004-2010 by Daniel Stenberg.
5 March 2010 ARES_INIT(3)