noble (3) ares_getaddrinfo.3.gz

NAME

       ares_getaddrinfo - Initiate a host query by name and service

SYNOPSIS

       #include <ares.h>

       typedef void (*ares_addrinfo_callback)(void *arg, int status,
                                              int timeouts,
                                              struct ares_addrinfo *result)

       void ares_getaddrinfo(ares_channel_t *channel, const char *name,
                             const char* service,
                             const struct ares_addrinfo_hints *hints,
                             ares_addrinfo_callback callback, void *arg)

DESCRIPTION

       The ares_getaddrinfo(3) function initiates a host query by name on the name service channel identified by
       channel.  The name and service parameters give the hostname and service  as  NULL-terminated  C  strings.
       The hints parameter is an ares_addrinfo_hints structure:

              struct ares_addrinfo_hints {
                int ai_flags;
                int ai_family;
                int ai_socktype;
                int ai_protocol;
              };

       ai_family
              Specifies desired address family. AF_UNSPEC means return both AF_INET and AF_INET6.

       ai_socktype
              Specifies desired socket type, for example SOCK_STREAM or SOCK_DGRAM.  Setting this to 0 means any
              type.

       ai_protocol
              Setting this to 0 means any protocol.

       ai_flags
              Specifies additional options, see below.

       ARES_AI_NUMERICSERV
                          If this option is set service field will be treated as a numeric value.

       ARES_AI_CANONNAME  The ares_addrinfo structure will return a canonical names list.

       ARES_AI_NOSORT     Result addresses will not be sorted and no connections to resolved addresses  will  be
                          attempted.

       ARES_AI_ENVHOSTS   Read hosts file path from the environment variable CARES_HOSTS .

       When  the  query is complete or has failed, the ares library will invoke callback.  Completion or failure
       of  the  query  may  happen  immediately,  or  may  happen  during  a  later  call  to   ares_process(3),
       ares_destroy(3) or ares_cancel(3).

       When  the associated callback is called, it is called with a channel lock so care must be taken to ensure
       any processing is minimal to prevent DNS channel stalls.

       The callback may be triggered from a different thread than the one which called ares_getaddrinfo(3).

       For integrators running their own event loops and not using ARES_OPT_EVENT_THREAD, care needs to be taken
       to ensure any file descriptor lists are updated immediately within the eventloop when notified.

       The  callback  argument  arg  is copied from the ares_getaddrinfo(3) argument arg.  The callback argument
       status indicates whether the query succeeded and, if not,  how  it  failed.   It  may  have  any  of  the
       following values:

       ARES_SUCCESS       The host lookup completed successfully.

       ARES_ENOTIMP       The ares library does not know how to find addresses of type family.

       ARES_ENOTFOUND     The name was not found.

       ARES_ENOMEM        Memory was exhausted.

       ARES_ESERVICE      The textual service name provided could not be dereferenced into a port.

       ARES_ECANCELLED    The query was cancelled.

       ARES_EDESTRUCTION  The name service channel channel is being destroyed; the query will not be completed.

       On  successful  completion  of  the  query, the callback argument result points to a struct ares_addrinfo
       which contains two linked lists, one with resolved addresses and  another  with  canonical  names.   Also
       included is the official name of the host (analogous to gethostbyname() h_name).

              struct ares_addrinfo {
                struct ares_addrinfo_cname *cnames;
                struct ares_addrinfo_node  *nodes;
                char *name;
              };

       ares_addrinfo_node  structure  is  similar  to RFC3493 addrinfo, but without canonname and with extra ttl
       field.

              struct ares_addrinfo_node {
                int                        ai_ttl;
                int                        ai_flags;
                int                        ai_family;
                int                        ai_socktype;
                int                        ai_protocol;
                ares_socklen_t             ai_addrlen;
                struct sockaddr           *ai_addr;
                struct ares_addrinfo_node *ai_next;
              };

       ares_addrinfo_cname structure is a linked list of CNAME records where ttl is a time to live  alias  is  a
       label  of  the  resource record and name is a value (canonical name) of the resource record.  See RFC2181
       10.1.1. CNAME terminology.

              struct ares_addrinfo_cname {
                int                         ttl;
                char                       *alias;
                char                       *name;
                struct ares_addrinfo_cname *next;
              };

       The reserved memory has to be deleted by ares_freeaddrinfo(3).

       The result is sorted according to RFC6724 except:
        - Rule 3 (Avoid deprecated addresses)
        - Rule 4 (Prefer home addresses)
        - Rule 7 (Prefer native transport)

       Please note that the function will attempt a connection on each of the resolved addresses as per RFC6724.

AVAILABILITY

       This function was added in c-ares 1.16.0, released in March 2020.

SEE ALSO

       ares_freeaddrinfo(3)

AUTHOR

       Christian Ammer
       Andrew Selivanov <andrew.selivanov@gmail.com>

                                                 4 November 2018                             ARES_GETADDRINFO(3)