Ubuntu Manpage: ares_init_options, ares_init - Initialize a resolver channel

Provided by: libc-ares-dev_1.27.0-1.0ubuntu1_amd64

NAME

       ares_init_options, ares_init - Initialize a resolver channel

SYNOPSIS

       #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 */
         ares_evsys_t evsys;
       };

       int ares_init_options(ares_channel_t **channelptr,
                             const struct ares_options *options,
                             int optmask);

       int ares_init(ares_channel_t **channelptr);

DESCRIPTION

The ares_init(3) function is equivalent to calling ares_init_options(channelptr, 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_FLAG_NO_DFLT_SVR Do not attempt to add a default local named server if there are
no other servers available. Instead, fail initialization with
ARES_ENOSERVER.

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.

Cannot be used with ARES_OPT_EVENT_THREAD.

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 1232, 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).

ARES_OPT_EVENT_THREAD
ares_evsys_t evsys;
Enable the built-in event thread (Recommended). Introduced in c-ares
1.26.0. Set the evsys parameter to ARES_EVSYS_DEFAULT (0). Other
values are reserved for testing and should not be used by integrators.

This option cannot be used with the ARES_OPT_SOCK_STATE_CB option, nor
the ares_set_socket_functions(3) or
ares_set_socket_configure_callback(3) functions.

When enabled, the integrator is no longer responsible for notifying c-
ares of any events on the file descriptors, so ares_process(3) nor
ares_process_fd(3) should ever be called when this option is enabled.

Use ares_threadsafety(3) to determine if this option is available to be
used.

Returns ARES_ENOTIMP if this option is passed but not available, and
ARES_ESERVFAIL if there is a critical failure during initialization of
the event thread.

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.

RETURN VALUES

       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.

       ARES_ENOSERVER
                     No DNS servers were available to use.

NOTES

       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.

AUTHOR