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 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.
- 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
complicated, but scales linearly with the value of timeout.) 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 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 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.
- 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 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 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 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 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 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 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 directives is specified, the last occurence
wins. For more information, please see the
resolv.conf(5) manual page.
SEE ALSO¶
ares_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.