ares_init, ares_init_options - Initialize a resolver channel
int ares_init(ares_channel *channelptr)
int ares_init_options(ares_channel *channelptr,
struct ares_options *options, int optmask)
cc file.c -lcares
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 handle 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
complicated, but scales linearly with the value of timeout.) The
default is five seconds. This option is being deprecated by
ARES_OPT_TIMEOUTMS starting in c-ares 1.5.2.
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 complicated, 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 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.
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 network byte order), the standard name ser-
ARES_OPT_SERVERS struct in_addr *servers;
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;
The domains to search, instead of the domains specified 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" indicates a lookup in the hosts file.
void (*sock_state_cb)(void *data, int s, int read, int write);
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 following 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 trun-
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 continue on with the truncated response.
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. Be aware
that ares will not do the recursion for you. Recursion must be
handled by the application calling ares if ARES_FLAG_NORECURSE is
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 variable, which normally
specifies a file of hostname translations.
ARES_FLAG_NOCHECKRESP Do not discard responses with the SERVFAIL, NOTIMP, or REFUSED
response code or responses whose questions don't match the ques-
tions in the request. Primarily useful for writing clients which
might be used to test or debug name servers.
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.
c-ares library initialization not yet performed.
When initializing from /etc/resolv.conf, ares_init(3) reads the domain and search direc-
tives 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 directives is specified, the last occurence wins. For more information, please see
the resolv.conf(5) manual page.
ares_destroy(3), ares_dup(3), ares_library_init(3), ares_set_servers(3)
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)