plucky (3) ares_init_options.3.gz

Provided by: libc-ares-dev_1.34.4-2.1_amd64 bug

NAME

       ares_init_options, ares_init - Initialize a resolver channel

SYNOPSIS

       #include <ares.h>

       struct ares_server_failover_options {
         unsigned short retry_chance;
         size_t retry_delay;
       };

       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;
         struct ares_server_failover_options server_failover_opts;
       };

       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_FLAG_DNS0x20      Enable        support        for        DNS        0x20         as         per
                                  https://datatracker.ietf.org/doc/html/draft-vixie-dnsext-dns0x20-00 which adds
                                  additional entropy to the request by randomizing the case of the  query  name.
                                  Integrators  need to ensure they treat DNS name responses as case-insensitive.
                                  In rare circumstances this may cause the inability to lookup  certain  domains
                                  if  the  upstream  server  or  the authoritative server for the domain is non-
                                  compliant.

       ARES_OPT_TIMEOUT  int timeout;
                         The number of seconds each name server is given to respond to a query on the first try.
                         See ARES_OPT_TIMEOUTMS which this value is converted into.

       ARES_OPT_TIMEOUTMS
                         int timeout;
                         The number of milliseconds each name server is given to respond to a query on the first
                         try of any given server. The default is two seconds, however any value below 250ms will
                         automatically  be  set  to 250ms (roughly the RTT half-way around the world). 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.

                         As of c-ares 1.32.0, this option is only honored on the first successful query  to  any
                         given  server,  after that the timeout is automatically calculated based on prior query
                         history.

       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.
                         Valid range is 0-15.

       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.  The channel lock is  held  during  this  callback,  if  in  a  multithreaded
                         application,  care  must  be taken to ensure lock order is correct to be able to handle
                         this and avoid deadlocks.

                         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;
                         As  of  c-ares  1.31.0,  the  query  cache is enabled by default with a TTL of 1hr.  To
                         disable the query cache, specify this option with a TTL of 0.  The query cache is 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 query cache
                         is automatically flushed if a server configuration change is made.

       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.

                         As  of  c-ares  1.29.0,  when  enabled,  it  will also automatically re-load the system
                         configuration when changes are detected.

                         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.

       ARES_OPT_SERVER_FAILOVER
                         struct ares_server_failover_options server_failover_opts;
                         Configure  server  failover  retry  behavior.   When a DNS server fails to respond to a
                         query, c-ares will deprioritize the server.  On subsequent queries, servers with  fewer
                         consecutive  failures will be selected in preference.  However, in order to detect when
                         such a server has recovered, c-ares will occasionally retry failed servers  by  probing
                         with  a copy of the query, without affecting the latency of the actual requested query.
                         The ares_server_failover_options structure contains options to control  this  behavior.
                         The  retry_chance  field gives the probability (1/N) of retrying a failed server on any
                         given query.  Setting to a value of 0 disables retries.  The  retry_delay  field  gives
                         the  minimum  delay  in  milliseconds  that c-ares will wait before retrying a specific
                         failed server.  If this option is not specificed then c-ares will use a probability  of
                         10% and a minimum delay of 5 seconds.

       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.  The default is not to rotate servers, however  the
                              system configuration can specify the desire to rotate and this configuration value
                              can negate such a system configuration.

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.

SEE ALSO

       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)

                                                  5 March 2010                              ARES_INIT_OPTIONS(3)