1ARES_INIT(3) Library Functions Manual ARES_INIT(3)
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6 ares_init, ares_init_options - Initialize a resolver channel
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9 #include <ares.h>
10 int ares_init(ares_channel *channelptr)
12 int ares_init_options(ares_channel *channelptr,
13 struct ares_options *options, int optmask)
14 cc file.c -lcares
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18 The ares_init function initializes a communications channel for name
19 service lookups. If it returns successfully, ares_init will set the
20 variable pointed to by channelptr to a handle used to identify the name
21 service channel. The caller should invoke ares_destroy(3) on the han‐
22 dle when the channel is no longer needed.
23 The ares_init_options function also initializes a name service channel,
25 with additional options useful for applications requiring more control
26 over name service configuration. The optmask parameter specifies which
27 fields in the structure pointed to by options are set, as follows:
28 ARES_OPT_FLAGS int flags;
30 Flags controlling the behavior of the resolver. See
31 below for a description of possible flag values.
32 ARES_OPT_TIMEOUT int timeout;
34 The number of seconds each name server is given to
35 respond to a query on the first try. (After the
36 first try, the timeout algorithm becomes more compli‐
37 cated, but scales linearly with the value of time‐
38 out.) The default is five seconds. This option is
39 being deprecated by ARES_OPT_TIMEOUTMS starting in c-
40 ares 1.5.2.
41 ARES_OPT_TIMEOUTMS
43 int timeout;
44 The number of milliseconds each name server is given
45 to respond to a query on the first try. (After the
46 first try, the timeout algorithm becomes more compli‐
47 cated, but scales linearly with the value of time‐
48 out.) The default is five seconds. Note that this
49 option is specified with the same struct field as the
50 former ARES_OPT_TIMEOUT, it is but the option bits
51 that tell c-ares how to interpret the number. This
52 option was added in c-ares 1.5.2.
53 ARES_OPT_TRIES int tries;
55 The number of tries the resolver will try contacting
56 each name server before giving up. The default is
57 four tries.
58 ARES_OPT_NDOTS int ndots;
60 The number of dots which must be present in a domain
61 name for it to be queried for "as is" prior to query‐
62 ing for it with the default domain extensions
63 appended. The default value is 1 unless set other‐
64 wise by resolv.conf or the RES_OPTIONS environment
65 variable.
66 ARES_OPT_PORT unsigned short port;
68 The port to use for queries (both TCP and UDP), in
69 network byte order. The default value is 53 (in net‐
70 work byte order), the standard name service port.
71 ARES_OPT_SERVERS struct in_addr *servers;
73 int nservers;
74 The list of IPv4 servers to contact, instead of the
75 servers specified in resolv.conf or the local named.
76 In order to allow specification of either IPv4 or
77 IPv6 name servers, the ares_set_servers(3) function
78 must be used instead.
79 ARES_OPT_DOMAINS char **domains;
81 int ndomains;
82 The domains to search, instead of the domains speci‐
83 fied in resolv.conf or the domain derived from the
84 kernel hostname variable.
85 ARES_OPT_LOOKUPS char *lookups;
87 The lookups to perform for host queries. lookups
88 should be set to a string of the characters "b" or
89 "f", where "b" indicates a DNS lookup and "f" indi‐
90 cates a lookup in the hosts file.
91 ARES_OPT_SOCK_STATE_CB
93 void (*sock_state_cb)(void *data, int s, int read,
94 int write);
95 void *sock_state_cb_data;
96 A callback function to be invoked when a socket
97 changes state. s will be passed the socket whose
98 state has changed; read will be set to true if the
99 socket should listen for read events, and write will
100 be set to true if the socket should listen for write
101 events. The value of sock_state_cb_data will be
102 passed as the data argument.
103 The flags field should be the bitwise or of some subset of the follow‐
105 ing values:
106 ARES_FLAG_USEVC Always use TCP queries (the "virtual circuit")
108 instead of UDP queries. Normally, TCP is only
109 used if a UDP query yields a truncated result.
110 ARES_FLAG_PRIMARY Only query the first server in the list of
112 servers to query.
113 ARES_FLAG_IGNTC If a truncated response to a UDP query is
115 received, do not fall back to TCP; simply con‐
116 tinue on with the truncated response.
117 ARES_FLAG_NORECURSE Do not set the "recursion desired" bit on outgo‐
119 ing queries, so that the name server being con‐
120 tacted will not try to fetch the answer from
121 other servers if it doesn't know the answer
122 locally. Be aware that ares will not do the
123 recursion for you. Recursion must be handled by
124 the application calling ares if ARES_FLAG_NORE‐
125 CURSE is set.
126 ARES_FLAG_STAYOPEN Do not close communications sockets when the
128 number of active queries drops to zero.
129 ARES_FLAG_NOSEARCH Do not use the default search domains; only
131 query hostnames as-is or as aliases.
132 ARES_FLAG_NOALIASES Do not honor the HOSTALIASES environment vari‐
134 able, which normally specifies a file of host‐
135 name translations.
136 ARES_FLAG_NOCHECKRESP Do not discard responses with the SERVFAIL,
138 NOTIMP, or REFUSED response code or responses
139 whose questions don't match the questions in the
140 request. Primarily useful for writing clients
141 which might be used to test or debug name
142 servers.
143
145 ares_init or ares_init_options can return any of the following values:
146 ARES_SUCCESS Initialization succeeded.
148 ARES_EFILE A configuration file could not be read.
150 ARES_ENOMEM The process's available memory was exhausted.
152 ARES_ENOTINITIALIZED
154 c-ares library initialization not yet performed.
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157 When initializing from /etc/resolv.conf, ares_init(3) reads the domain
158 and search directives to allow lookups of short names relative to the
159 domains specified. The domain and search directives override one
160 another. If more that one instance of either domain or search direc‐
161 tives is specified, the last occurence wins. For more information,
162 please see the resolv.conf(5) manual page.
163
165 ares_destroy(3), ares_dup(3), ares_library_init(3), ares_set_servers(3)
166
168 Greg Hudson, MIT Information Systems
169 Copyright 1998 by the Massachusetts Institute of Technology.
170 Copyright (C) 2004-2010 by Daniel Stenberg.
171 5 March 2010 ARES_INIT(3)