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 };
32 int ares_init_options(ares_channel *channelptr,
34 struct ares_options *options,
35 int optmask)
36
38 The ares_init_options(3) function initializes a communications channel
39 for name service lookups. If it returns successfully,
40 ares_init_options(3) will set the variable pointed to by channelptr to
41 a handle used to identify the name service channel. The caller should
42 invoke ares_destroy(3) on the handle when the channel is no longer
43 needed.
44 The optmask parameter generally specifies which fields in the structure
46 pointed to by options are set, as follows:
47 ARES_OPT_FLAGS int flags;
49 Flags controlling the behavior of the resolver. See
50 below for a description of possible flag values.
51 ARES_OPT_TIMEOUT int timeout;
53 The number of seconds each name server is given to
54 respond to a query on the first try. (After the
55 first try, the timeout algorithm becomes more compli‐
56 cated, but scales linearly with the value of time‐
57 out.) The default is five seconds. This option is
58 being deprecated by ARES_OPT_TIMEOUTMS starting in c-
59 ares 1.5.2.
60 ARES_OPT_TIMEOUTMS
62 int timeout;
63 The number of milliseconds each name server is given
64 to respond to a query on the first try. (After the
65 first try, the timeout algorithm becomes more compli‐
66 cated, but scales linearly with the value of time‐
67 out.) The default is five seconds. Note that this
68 option is specified with the same struct field as the
69 former ARES_OPT_TIMEOUT, it is but the option bits
70 that tell c-ares how to interpret the number. This
71 option was added in c-ares 1.5.2.
72 ARES_OPT_TRIES int tries;
74 The number of tries the resolver will try contacting
75 each name server before giving up. The default is
76 four tries.
77 ARES_OPT_NDOTS int ndots;
79 The number of dots which must be present in a domain
80 name for it to be queried for "as is" prior to query‐
81 ing for it with the default domain extensions
82 appended. The default value is 1 unless set other‐
83 wise by resolv.conf or the RES_OPTIONS environment
84 variable.
85 ARES_OPT_UDP_PORT unsigned short udp_port;
87 The port to use for queries over UDP, in network byte
88 order. The default value is 53 (in network byte
89 order), the standard name service port.
90 ARES_OPT_TCP_PORT unsigned short tcp_port;
92 The port to use for queries over TCP, in network byte
93 order. The default value is 53 (in network byte
94 order), the standard name service port.
95 ARES_OPT_SERVERS struct in_addr *servers;
97 int nservers;
98 The list of IPv4 servers to contact, instead of the
99 servers specified in resolv.conf or the local named.
100 In order to allow specification of either IPv4 or
101 IPv6 name servers, the ares_set_servers(3) function
102 must be used 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
135 ares_save_options(3) (because struct apattern is
136 opaque); to set a fresh sort list, use
137 ares_set_sortlist(3).
138 ARES_OPT_SOCK_SNDBUF
140 int socket_send_buffer_size;
141 The send buffer size to set for the socket.
142 ARES_OPT_SOCK_RCVBUF
144 int socket_receive_buffer_size;
145 The receive buffer size to set for the socket.
146 ARES_OPT_EDNSPSZ int ednspsz;
148 The message size to be advertized in EDNS; only takes
149 effect if the ARES_FLAG_EDNS flag is set.
150 ARES_OPT_RESOLVCONF
152 char *resolvconf_path;
153 The path to use for reading the resolv.conf file. The
154 resolvconf_path should be set to a path string, and
155 will be honoured on *nix like systems. The default is
156 /etc/resolv.conf
157 The optmask parameter also includes options without a corresponding
159 field in the ares_options structure, as follows:
160 ARES_OPT_ROTATE Perform round-robin selection of the nameservers
162 configured for the channel for each resolution.
163 ARES_OPT_NOROTATE Do not perform round-robin nameserver selection;
165 always use the list of nameservers in the same
166 order.
167 The flags field should be the bitwise or of some subset of the follow‐
169 ing values:
170 ARES_FLAG_USEVC Always use TCP queries (the "virtual circuit")
172 instead of UDP queries. Normally, TCP is only
173 used if a UDP query yields a truncated result.
174 ARES_FLAG_PRIMARY Only query the first server in the list of
176 servers to query.
177 ARES_FLAG_IGNTC If a truncated response to a UDP query is
179 received, do not fall back to TCP; simply con‐
180 tinue on with the truncated response.
181 ARES_FLAG_NORECURSE Do not set the "recursion desired" bit on outgo‐
183 ing queries, so that the name server being con‐
184 tacted will not try to fetch the answer from
185 other servers if it doesn't know the answer
186 locally. Be aware that ares will not do the
187 recursion for you. Recursion must be handled by
188 the application calling ares if ARES_FLAG_NORE‐
189 CURSE is set.
190 ARES_FLAG_STAYOPEN Do not close communications sockets when the
192 number of active queries drops to zero.
193 ARES_FLAG_NOSEARCH Do not use the default search domains; only
195 query hostnames as-is or as aliases.
196 ARES_FLAG_NOALIASES Do not honor the HOSTALIASES environment vari‐
198 able, which normally specifies a file of host‐
199 name translations.
200 ARES_FLAG_NOCHECKRESP Do not discard responses with the SERVFAIL,
202 NOTIMP, or REFUSED response code or responses
203 whose questions don't match the questions in the
204 request. Primarily useful for writing clients
205 which might be used to test or debug name
206 servers.
207 ARES_FLAG_EDNS Include an EDNS pseudo-resource record (RFC
209 2671) in generated requests.
210
212 ares_init_options(3) can return any of the following values:
213 ARES_SUCCESS Initialization succeeded.
215 ARES_EFILE A configuration file could not be read.
217 ARES_ENOMEM The process's available memory was exhausted.
219 ARES_ENOTINITIALIZED
221 c-ares library initialization not yet performed.
222
224 When initializing from /etc/resolv.conf, (or, alternatively when speci‐
225 fied by the resolvconf_path path location) ares_init_options(3) reads
226 the domain and search directives to allow lookups of short names rela‐
227 tive to the domains specified. The domain and search directives over‐
228 ride one another. If more that one instance of either domain or search
229 directives is specified, the last occurrence wins. For more informa‐
230 tion, please see the resolv.conf(5) manual page.
231
233 ares_init(3), ares_destroy(3), ares_dup(3), ares_library_init(3),
234 ares_save_options(3), ares_set_servers(3), ares_set_sortlist(3)
235
237 Greg Hudson, MIT Information Systems
238 Copyright 1998 by the Massachusetts Institute of Technology.
239 Copyright (C) 2004-2010 by Daniel Stenberg.
240 5 March 2010 ARES_INIT(3)