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 ARES_OPT_RESOLVCONF
150 char *resolvconf_path; The path to use for reading
151 the resolv.conf file. The resolvconf_path should be
152 set to a path string, and will be honoured on *nix
153 like systems. The default is /etc/resolv.conf
154 The message size to be advertized in EDNS; only takes
155 effect if the ARES_FLAG_EDNS flag is set.
156 The optmask parameter also includes options without a corresponding
158 field in the ares_options structure, as follows:
159 ARES_OPT_ROTATE Perform round-robin selection of the nameservers
161 configured for the channel for each resolution.
162 ARES_OPT_NOROTATE Do not perform round-robin nameserver selection;
164 always use the list of nameservers in the same
165 order.
166 The flags field should be the bitwise or of some subset of the follow‐
168 ing values:
169 ARES_FLAG_USEVC Always use TCP queries (the "virtual circuit")
171 instead of UDP queries. Normally, TCP is only
172 used if a UDP query yields a truncated result.
173 ARES_FLAG_PRIMARY Only query the first server in the list of
175 servers to query.
176 ARES_FLAG_IGNTC If a truncated response to a UDP query is
178 received, do not fall back to TCP; simply con‐
179 tinue on with the truncated response.
180 ARES_FLAG_NORECURSE Do not set the "recursion desired" bit on outgo‐
182 ing queries, so that the name server being con‐
183 tacted will not try to fetch the answer from
184 other servers if it doesn't know the answer
185 locally. Be aware that ares will not do the
186 recursion for you. Recursion must be handled by
187 the application calling ares if ARES_FLAG_NORE‐
188 CURSE is set.
189 ARES_FLAG_STAYOPEN Do not close communications sockets when the
191 number of active queries drops to zero.
192 ARES_FLAG_NOSEARCH Do not use the default search domains; only
194 query hostnames as-is or as aliases.
195 ARES_FLAG_NOALIASES Do not honor the HOSTALIASES environment vari‐
197 able, which normally specifies a file of host‐
198 name translations.
199 ARES_FLAG_NOCHECKRESP Do not discard responses with the SERVFAIL,
201 NOTIMP, or REFUSED response code or responses
202 whose questions don't match the questions in the
203 request. Primarily useful for writing clients
204 which might be used to test or debug name
205 servers.
206 ARES_FLAG_EDNS Include an EDNS pseudo-resource record (RFC
208 2671) in generated requests.
209
211 ares_init_options(3) can return any of the following values:
212 ARES_SUCCESS Initialization succeeded.
214 ARES_EFILE A configuration file could not be read.
216 ARES_ENOMEM The process's available memory was exhausted.
218 ARES_ENOTINITIALIZED
220 c-ares library initialization not yet performed.
221
223 When initializing from /etc/resolv.conf, (or, alternatively when speci‐
224 fied by the resolvconf_path path location) ares_init_options(3) reads
225 the domain and search directives to allow lookups of short names rela‐
226 tive to the domains specified. The domain and search directives over‐
227 ride one another. If more that one instance of either domain or search
228 directives is specified, the last occurrence wins. For more informa‐
229 tion, please see the resolv.conf(5) manual page.
230
232 ares_init(3), ares_destroy(3), ares_dup(3), ares_library_init(3),
233 ares_save_options(3), ares_set_servers(3), ares_set_sortlist(3)
234
236 Greg Hudson, MIT Information Systems
237 Copyright 1998 by the Massachusetts Institute of Technology.
238 Copyright (C) 2004-2010 by Daniel Stenberg.
239 5 March 2010 ARES_INIT(3)