ares_init_options(3)
ARES_INIT(3) C LIBRARY FUNCTIONS ARES_INIT(3)
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;
};
int ares_init_options(ares_channel *channelptr,
struct ares_options *options,
int optmask)
DESCRIPTION
The ares_init_options(3) function initializes a communica-
tions channel for name service lookups. If it returns suc-
cessfully, 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
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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 compli-
cated, 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
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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
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.
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 apat-
tern 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
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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.
The optmask parameter also includes options without a
corresponding field in the ares_options structure, as fol-
lows:
ARES_OPT_ROTATE Perform round-robin selection of the
nameservers configured for the chan-
nel 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
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or as aliases.
ARES_FLAG_NOALIASES Do not honor the HOSTALIASES environ-
ment variable, which normally speci-
fies 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 writ-
ing 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 per-
formed.
NOTES
When initializing from /etc/resolv.conf,
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.
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