ares_init_options man page

ares_init_options — Initialize a resolver channel


#include <ares.h>

int ares_init_options(ares_channel *channelptr,
                      struct ares_options *options,
                      int optmask)


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:

int flags;
Flags controlling the behavior of the resolver. See below for a description of possible flag values.
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.
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.
int tries;
The number of tries the resolver will try contacting each name server before giving up. The default is four tries.
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.
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.
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.
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.
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.
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);
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.
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).
int socket_send_buffer_size;
The send buffer size to set for the socket.
int socket_receive_buffer_size;
The receive buffer size to set for the socket.
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 follows:

Perform round-robin selection of the nameservers configured for the channel for each resolution.
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:

Always use TCP queries (the "virtual circuit") instead of UDP queries. Normally, TCP is only used if a UDP query yields a truncated result.
Only query the first server in the list of servers to query.
If a truncated response to a UDP query is received, do not fall back to TCP; simply continue on with the truncated response.
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.
Do not close communications sockets when the number of active queries drops to zero.
Do not use the default search domains; only query hostnames as-is or as aliases.
Do not honor the HOSTALIASES environment variable, which normally specifies a file of hostname translations.
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.
Include an EDNS pseudo-resource record (RFC 2671) in generated requests.

Return Values

ares_init_options(3) can return any of the following values:

Initialization succeeded.
A configuration file could not be read.
The process's available memory was exhausted.
c-ares library initialization not yet performed.


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)


Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.
Copyright (C) 2004-2010 by Daniel Stenberg.

Referenced By

ares_destroy_options(3), ares_gethostbyname_file(3), ares_get_servers(3), ares_init(3), ares_save_options(3), ares_set_servers(3), ares_set_socket_callback(3), ares_set_socket_configure_callback(3), ares_set_sortlist(3).

Explore man page connections for ares_init_options(3).

5 March 2010