ARES_INIT(3) FreeBSD Library Functions Manual 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;
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 _destroy§ion=3">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)
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