1ARES_INIT(3) Library Functions Manual ARES_INIT(3)
2
6 ares_init_options - Initialize a resolver channel
7
9 #include <ares.h>
10 struct ares_options {
12 int flags;
13 int timeout; /* in seconds or milliseconds, depending on options */
14 int tries;
15 int ndots;
16 unsigned short udp_port;
17 unsigned short tcp_port;
18 int socket_send_buffer_size;
19 int socket_receive_buffer_size;
20 struct in_addr *servers;
21 int nservers;
22 char **domains;
23 int ndomains;
24 char *lookups;
25 ares_sock_state_cb sock_state_cb;
26 void *sock_state_cb_data;
27 struct apattern *sortlist;
28 int nsort;
29 int ednspsz;
30 char *resolvconf_path;
31 char *hosts_path;
32 int udp_max_queries;
33 };
34 int ares_init_options(ares_channel *channelptr,
36 struct ares_options *options,
37 int optmask)
38
40 The ares_init_options(3) function initializes a communications channel
41 for name service lookups. If it returns successfully, ares_init_op‐
42 tions(3) will set the variable pointed to by channelptr to a handle
43 used to identify the name service channel. The caller should invoke
44 ares_destroy(3) on the handle when the channel is no longer needed.
45 The optmask parameter generally specifies which fields in the structure
47 pointed to by options are set, as follows:
48 ARES_OPT_FLAGS int flags;
50 Flags controlling the behavior of the resolver. See
51 below for a description of possible flag values.
52 ARES_OPT_TIMEOUT int timeout;
54 The number of seconds each name server is given to
55 respond to a query on the first try. (After the
56 first try, the timeout algorithm becomes more compli‐
57 cated, but scales linearly with the value of time‐
58 out.) The default is two seconds. This option is be‐
59 ing deprecated by ARES_OPT_TIMEOUTMS starting in c-
60 ares 1.5.2.
61 ARES_OPT_TIMEOUTMS
63 int timeout;
64 The number of milliseconds each name server is given
65 to respond to a query on the first try. (After the
66 first try, the timeout algorithm becomes more compli‐
67 cated, but scales linearly with the value of time‐
68 out.) The default is two seconds. Note that this op‐
69 tion is specified with the same struct field as the
70 former ARES_OPT_TIMEOUT, it is but the option bits
71 that tell c-ares how to interpret the number. This
72 option was added in c-ares 1.5.2.
73 ARES_OPT_TRIES int tries;
75 The number of tries the resolver will try contacting
76 each name server before giving up. The default is
77 three tries.
78 ARES_OPT_NDOTS int ndots;
80 The number of dots which must be present in a domain
81 name for it to be queried for "as is" prior to query‐
82 ing for it with the default domain extensions ap‐
83 pended. The default value is 1 unless set otherwise
84 by resolv.conf or the RES_OPTIONS environment vari‐
85 able.
86 ARES_OPT_UDP_PORT unsigned short udp_port;
88 The port to use for queries over UDP, in host byte
89 order. The default value is 53, the standard name
90 service port.
91 ARES_OPT_TCP_PORT unsigned short tcp_port;
93 The port to use for queries over TCP, in host byte
94 order. The default value is 53, the standard name
95 service port.
96 ARES_OPT_SERVERS struct in_addr *servers;
98 int nservers;
99 The list of IPv4 servers to contact, instead of the
100 servers specified in resolv.conf or the local named.
101 In order to allow specification of either IPv4 or
102 IPv6 name servers, the 0 instead.
103 ARES_OPT_DOMAINS char **domains;
105 int ndomains;
106 The domains to search, instead of the domains speci‐
107 fied in resolv.conf or the domain derived from the
108 kernel hostname variable.
109 ARES_OPT_LOOKUPS char *lookups;
111 The lookups to perform for host queries. lookups
112 should be set to a string of the characters "b" or
113 "f", where "b" indicates a DNS lookup and "f" indi‐
114 cates a lookup in the hosts file.
115 ARES_OPT_SOCK_STATE_CB
117 void (*sock_state_cb)(void *data, ares_socket_t
118 socket_fd, int readable, int writable);
119 void *sock_state_cb_data;
120 A callback function to be invoked when a socket
121 changes state. socket_fd will be passed the socket
122 whose state has changed; readable will be set to true
123 if the socket should listen for read events, and
124 writable will be set to true if the socket should
125 listen for write events. The value of
126 sock_state_cb_data will be passed as the data argu‐
127 ment.
128 ARES_OPT_SORTLIST struct apattern *sortlist;
130 int nsort;
131 A list of IP address ranges that specifies the order
132 of preference that results from ares_gethostbyname
133 should be returned in. Note that this can only be
134 used with a sortlist retrieved via ares_save_op‐
135 tions(3) (because struct apattern is opaque); to set
136 a fresh sort list, use ares_set_sortlist(3).
137 ARES_OPT_SOCK_SNDBUF
139 int socket_send_buffer_size;
140 The send buffer size to set for the socket.
141 ARES_OPT_SOCK_RCVBUF
143 int socket_receive_buffer_size;
144 The receive buffer size to set for the socket.
145 ARES_OPT_EDNSPSZ int ednspsz;
147 The message size to be advertized in EDNS; only takes
148 effect if the ARES_FLAG_EDNS flag is set.
149 ARES_OPT_RESOLVCONF
151 char *resolvconf_path;
152 The path to use for reading the resolv.conf file. The
153 resolvconf_path should be set to a path string, and
154 will be honoured on *nix like systems. The default is
155 /etc/resolv.conf
156 ARES_OPT_HOSTS_FILE
158 char *hosts_path;
159 The path to use for reading the hosts file. The
160 hosts_path should be set to a path string, and will
161 be honoured on *nix like systems. The default is
162 /etc/hosts
163 ARES_OPT_UDP_MAX_QUERIES
165 int udp_max_queries;
166 The maximum number of udp queries that can be sent on
167 a single ephemeral port to a given DNS server before
168 a new ephemeral port is assigned. Any value of 0 or
169 less will be considered unlimited, and is the de‐
170 fault.
171 The optmask parameter also includes options without a corresponding
173 field in the ares_options structure, as follows:
174 ARES_OPT_ROTATE Perform round-robin selection of the nameservers
176 configured for the channel for each resolution.
177 ARES_OPT_NOROTATE Do not perform round-robin nameserver selection;
179 always use the list of nameservers in the same
180 order.
181 The flags field should be the bitwise or of some subset of the follow‐
183 ing values:
184 ARES_FLAG_USEVC Always use TCP queries (the "virtual circuit")
186 instead of UDP queries. Normally, TCP is only
187 used if a UDP query yields a truncated result.
188 ARES_FLAG_PRIMARY Only query the first server in the list of
190 servers to query.
191 ARES_FLAG_IGNTC If a truncated response to a UDP query is re‐
193 ceived, do not fall back to TCP; simply continue
194 on with the truncated response.
195 ARES_FLAG_NORECURSE Do not set the "recursion desired" bit on outgo‐
197 ing queries, so that the name server being con‐
198 tacted will not try to fetch the answer from
199 other servers if it doesn't know the answer lo‐
200 cally. Be aware that ares will not do the recur‐
201 sion for you. Recursion must be handled by the
202 application calling ares if ARES_FLAG_NORECURSE
203 is set.
204 ARES_FLAG_STAYOPEN Do not close communications sockets when the
206 number of active queries drops to zero.
207 ARES_FLAG_NOSEARCH Do not use the default search domains; only
209 query hostnames as-is or as aliases.
210 ARES_FLAG_NOALIASES Do not honor the HOSTALIASES environment vari‐
212 able, which normally specifies a file of host‐
213 name translations.
214 ARES_FLAG_NOCHECKRESP Do not discard responses with the SERVFAIL,
216 NOTIMP, or REFUSED response code or responses
217 whose questions don't match the questions in the
218 request. Primarily useful for writing clients
219 which might be used to test or debug name
220 servers.
221 ARES_FLAG_EDNS Include an EDNS pseudo-resource record (RFC
223 2671) in generated requests.
224
226 ares_init_options(3) can return any of the following values:
227 ARES_SUCCESS Initialization succeeded.
229 ARES_EFILE A configuration file could not be read.
231 ARES_ENOMEM The process's available memory was exhausted.
233 ARES_ENOTINITIALIZED
235 c-ares library initialization not yet performed.
236
238 When initializing from /etc/resolv.conf, (or, alternatively when speci‐
239 fied by the resolvconf_path path location) ares_init_options(3) reads
240 the domain and search directives to allow lookups of short names rela‐
241 tive to the domains specified. The domain and search directives over‐
242 ride one another. If more than one instance of either domain or search
243 directives is specified, the last occurrence wins. For more informa‐
244 tion, please see the resolv.conf(5) manual page.
245
247 ares_init(3), ares_destroy(3), ares_dup(3), ares_library_init(3),
248 ares_save_options(3), ares_set_servers(3), ares_set_sortlist(3)
249
251 Greg Hudson, MIT Information Systems
252 Copyright 1998 by the Massachusetts Institute of Technology.
253 Copyright (C) 2004-2010 by Daniel Stenberg.
254 5 March 2010 ARES_INIT(3)