Provided by: libc-ares-dev_1.18.1-1build1_amd64 
NAME
ares_init_options - Initialize a resolver channel
SYNOPSIS
#include <ares.h>
struct ares_options {
int flags;
int timeout; /* in seconds or milliseconds, depending on options */
int tries;
int ndots;
unsigned short udp_port;
unsigned short tcp_port;
int socket_send_buffer_size;
int socket_receive_buffer_size;
struct in_addr *servers;
int nservers;
char **domains;
int ndomains;
char *lookups;
ares_sock_state_cb sock_state_cb;
void *sock_state_cb_data;
struct apattern *sortlist;
int nsort;
int ednspsz;
char *resolvconf_path;
};
int ares_init_options(ares_channel *channelptr,
struct ares_options *options,
int optmask)
DESCRIPTION
The ares_init_options(3) function initializes a communications channel for name service
lookups. If it returns successfully, ares_init_options(3) 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 optmask parameter generally 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_UDP_PORT unsigned short udp_port;
The port to use for queries over UDP, in network byte order. The
default value is 53 (in network byte order), the standard name service
port.
ARES_OPT_TCP_PORT unsigned short tcp_port;
The port to use for queries over TCP, 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, ares_socket_t socket_fd, int readable,
int writable);
void *sock_state_cb_data;
A callback function to be invoked when a socket changes state.
socket_fd will be passed the socket whose state has changed; readable
will be set to true if the socket should listen for read events, and
writable 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.
ARES_OPT_SORTLIST struct apattern *sortlist;
int nsort;
A list of IP address ranges that specifies the order of preference that
results from ares_gethostbyname should be returned in. Note that this
can only be used with a sortlist retrieved via ares_save_options(3)
(because struct apattern is opaque); to set a fresh sort list, use
ares_set_sortlist(3).
ARES_OPT_SOCK_SNDBUF
int socket_send_buffer_size;
The send buffer size to set for the socket.
ARES_OPT_SOCK_RCVBUF
int socket_receive_buffer_size;
The receive buffer size to set for the socket.
ARES_OPT_EDNSPSZ int ednspsz;
The message size to be advertized in EDNS; only takes effect if the
ARES_FLAG_EDNS flag is set.
ARES_OPT_RESOLVCONF
char *resolvconf_path;
The path to use for reading the resolv.conf file. The resolvconf_path
should be set to a path string, and will be honoured on *nix like
systems. The default is /etc/resolv.conf
The optmask parameter also includes options without a corresponding field in the
ares_options structure, as follows:
ARES_OPT_ROTATE Perform round-robin selection of the nameservers configured for the
channel for each resolution.
ARES_OPT_NOROTATE Do not perform round-robin nameserver selection; always use the
list of nameservers in the same order.
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.
ARES_FLAG_EDNS Include an EDNS pseudo-resource record (RFC 2671) in generated
requests.
RETURN VALUES
ares_init_options(3) 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, (or, alternatively when specified by the
resolvconf_path path location) ares_init_options(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 occurrence wins. For more information, please see the
resolv.conf(5) manual page.
SEE ALSO
ares_init(3), ares_destroy(3), ares_dup(3), ares_library_init(3), ares_save_options(3),
ares_set_servers(3), ares_set_sortlist(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)