ares_init_options, ares_init − Initialize a resolver channel
#include <ares.h>
struct
ares_options {
int flags;
int timeout; /* in seconds or milliseconds, depending on
options */
int tries;
int ndots;
unsigned short udp_port;
unsigned short tcp_port;
int socket_send_buffer_size;
int socket_receive_buffer_size;
struct in_addr *servers;
int nservers;
char **domains;
int ndomains;
char *lookups;
ares_sock_state_cb sock_state_cb;
void *sock_state_cb_data;
struct apattern *sortlist;
int nsort;
int ednspsz;
char *resolvconf_path;
char *hosts_path;
int udp_max_queries;
int maxtimeout; /* in milliseconds */
unsigned int qcache_max_ttl; /* in seconds */
};
int
ares_init_options(ares_channel_t **channelptr,
const struct ares_options *options,
int optmask);
int ares_init(ares_channel_t **channelptr);
The ares_init(3) function is equivalent to calling ares_init_options(NULL, 0). It is recommended to use ares_init_options(3) instead and to set or make configurable the appropriate options for your application.
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.
It is recommended for an application to have at most one ares channel and use this for all DNS queries for the life of the application. When system configuration changes, ares_reinit(3) can be called to reload the configuration if necessary. The recommended concurrent query limit is about 32k queries, but remembering that when specifying AF_UNSPEC for ares_getaddrinfo(3) or ares_gethostbyname(3), they may spawn 2 queries internally. The reason for the limit is c-ares does not allow duplicate DNS query ids (which have a maximum of 64k) to be oustanding at a given time, and it must randomly search for an available id thus 32k will limit the number of searches. This limitation should not be a concern for most implementations and c-ares may implement queuing in future releases to lift this limitation.
The optmask parameter generally specifies which fields in the structure pointed to by options are set, as follows:
ARES_OPT_FLAGS |
int flags; |
Flags controlling the behavior of the resolver:
ARES_FLAG_USEVC |
Always use TCP queries (the "virtual circuit") instead of UDP queries. Normally, TCP is only used if a UDP query yields a truncated result. | ||
ARES_FLAG_PRIMARY |
Only query the first server in the list of servers to query. | ||
ARES_FLAG_IGNTC |
If a truncated response to a UDP query is received, do not fall back to TCP; simply continue on with the truncated response. | ||
ARES_FLAG_NORECURSE |
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. | ||
ARES_FLAG_STAYOPEN |
Do not close communications sockets when the number of active queries drops to zero. | ||
ARES_FLAG_NOSEARCH |
Do not use the default search domains; only query hostnames as-is or as aliases. | ||
ARES_FLAG_NOALIASES |
Do not honor the HOSTALIASES environment variable, which normally specifies a file of hostname translations. | ||
ARES_FLAG_NOCHECKRESP |
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. | ||
ARES_FLAG_EDNS |
Include an EDNS pseudo-resource record (RFC 2671) in generated requests. As of v1.22, this is on by default if flags are otherwise not set. | ||
ARES_OPT_TIMEOUT |
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
two seconds. This option is being deprecated by
ARES_OPT_TIMEOUTMS starting in c-ares 1.5.2.
ARES_OPT_TIMEOUTMS
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
two 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.
ARES_OPT_TRIES |
int tries; |
The number of tries the resolver will try contacting each name server before giving up. The default is three tries.
ARES_OPT_NDOTS |
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.
ARES_OPT_MAXTIMEOUTMS
int
maxtimeout;
The upper bound for timeout between sequential retry
attempts. When retrying queries, the timeout is increased
from the requested timeout parameter, this caps the
value.
ARES_OPT_UDP_PORT |
unsigned short udp_port; |
The port to use for queries over UDP, in host byte order. The default value is 53, the standard name service port.
ARES_OPT_TCP_PORT |
unsigned short tcp_port; |
The port to use for queries over TCP, in host byte order. The default value is 53, the standard name service port.
ARES_OPT_SERVERS |
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
0 instead.
ARES_OPT_DOMAINS |
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.
ARES_OPT_LOOKUPS |
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.
ARES_OPT_SOCK_STATE_CB
void
(*sock_state_cb)(void *data, ares_socket_t
socket_fd, int readable, int writable);
void *sock_state_cb_data;
A callback function to be invoked when a socket changes
state. socket_fd will be passed the socket whose
state has changed; readable will be set to true if
the socket should listen for read events, and
writable 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.
ARES_OPT_SORTLIST |
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).
ARES_OPT_SOCK_SNDBUF
int
socket_send_buffer_size;
The send buffer size to set for the socket.
ARES_OPT_SOCK_RCVBUF
int
socket_receive_buffer_size;
The receive buffer size to set for the socket.
ARES_OPT_EDNSPSZ |
int ednspsz; |
The message size to be advertised in EDNS; only takes effect if the ARES_FLAG_EDNS flag is set. Defaults to 1280, the recommended size.
ARES_OPT_RESOLVCONF
char
*resolvconf_path;
The path to use for reading the resolv.conf file. The
resolvconf_path should be set to a path string, and
will be honoured on *nix like systems. The default is
/etc/resolv.conf
ARES_OPT_HOSTS_FILE
char
*hosts_path;
The path to use for reading the hosts file. The
hosts_path should be set to a path string, and will
be honoured on *nix like systems. The default is
/etc/hosts
ARES_OPT_UDP_MAX_QUERIES
int
udp_max_queries;
The maximum number of udp queries that can be sent on a
single ephemeral port to a given DNS server before a new
ephemeral port is assigned. Any value of 0 or less will be
considered unlimited, and is the default.
ARES_OPT_QUERY_CACHE
unsigned int
qcache_max_ttl;
Enable the built-in query cache. Will cache queries based on
the returned TTL in the DNS message. Only fully successful
and NXDOMAIN query results will be cached. Fill in the
qcache_max_ttl with the maximum number of seconds a
query result may be cached which will override a larger TTL
in the response message. This must be a non-zero value
otherwise the cache will be disabled. Choose a reasonable
value for your application such as 300 (5 minutes) or 3600
(1 hour).
The optmask parameter also includes options without a corresponding field in the ares_options structure, as follows:
ARES_OPT_ROTATE |
Perform round-robin selection of the nameservers configured for the channel for each resolution. | ||
ARES_OPT_NOROTATE |
Do not perform round-robin nameserver selection; always use the list of nameservers in the same order. |
ares_init_options(3) and ares_init(3) can return any of the following values:
ARES_SUCCESS |
Initialization succeeded. |
|||
ARES_EFILE |
A configuration file could not be read. |
|||
ARES_ENOMEM |
The process’s available memory was exhausted. |
ARES_ENOTINITIALIZED
c-ares library initialization not yet performed.
When initializing from /etc/resolv.conf, (or, alternatively when specified by the resolvconf_path path location) ares_init_options(3) and ares_init(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 than 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.
ares_reinit(3), ares_destroy(3), ares_dup(3), ares_library_init(3), ares_save_options(3), ares_set_servers(3), ares_set_sortlist(3), ares_threadsafety(3)
Greg Hudson, MIT
Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.
Copyright (C) 2004-2010 by Daniel Stenberg.