c-ares/docs/ares_getaddrinfo.3

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.TH ARES_GETADDRINFO 3 "4 November 2018"
.SH NAME
ares_getaddrinfo \- Initiate a host query by name and service
.SH SYNOPSIS
.nf
#include <ares.h>

typedef void (*ares_addrinfo_callback)(void *\fIarg\fP, int \fIstatus\fP,
                                       int \fItimeouts\fP,
                                       struct ares_addrinfo *\fIresult\fP)

void ares_getaddrinfo(ares_channel_t *\fIchannel\fP, const char *\fIname\fP,
                      const char* \fIservice\fP,
                      const struct ares_addrinfo_hints *\fIhints\fP,
                      ares_addrinfo_callback \fIcallback\fP, void *\fIarg\fP)
.fi
.SH DESCRIPTION
The \fBares_getaddrinfo(3)\fP function initiates a host query by name on the
name service channel identified by
.IR channel .
The
.I name
and
.I service
parameters give the hostname and service as NULL-terminated C strings.
The
.I hints
parameter is an
.BR ares_addrinfo_hints
structure:
.PP
.RS
.EX
struct ares_addrinfo_hints {
  int ai_flags;
  int ai_family;
  int ai_socktype;
  int ai_protocol;
};
.EE
.RE
.TP
.I ai_family
Specifies desired address family. AF_UNSPEC means return both AF_INET and AF_INET6.
.TP
.I ai_socktype
Specifies desired socket type, for example SOCK_STREAM or SOCK_DGRAM.
Setting this to 0 means any type.
.TP
.I ai_protocol
Setting this to 0 means any protocol.
.TP
.I ai_flags
Specifies additional options, see below.
.PP
.TP 19
.B ARES_AI_NUMERICSERV
If this option is set
.I service
field will be treated as a numeric value.
.TP 19
.B ARES_AI_CANONNAME
The ares_addrinfo structure will return a canonical names list.
.TP 19
.B ARES_AI_NOSORT
Result addresses will not be sorted and no connections to resolved addresses will be attempted.
.TP 19
.B ARES_AI_ENVHOSTS
Read hosts file path from the environment variable
.I CARES_HOSTS .
.PP
When the query is complete or has failed, the ares library will invoke \fIcallback\fP.
Completion or failure of the query may happen immediately, or may happen
during a later call to \fIares_process(3)\fP, \fIares_destroy(3)\fP or
\fIares_cancel(3)\fP.
.PP
If this is called from a thread other than which the main program event loop is
running, care needs to be taken to ensure any file descriptor lists are updated
immediately within the eventloop.  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.
.PP
The callback argument
.I arg
is copied from the \fBares_getaddrinfo(3)\fP argument
.IR arg .
The callback argument
.I status
indicates whether the query succeeded and, if not, how it failed.  It
may have any of the following values:
.TP 19
.B ARES_SUCCESS
The host lookup completed successfully.
.TP 19
.B ARES_ENOTIMP
The ares library does not know how to find addresses of type
.IR family .
.TP 19
.B ARES_ENOTFOUND
The
.I name
was not found.
.TP 19
.B ARES_ENOMEM
Memory was exhausted.
.TP 19
.B ARES_ESERVICE
The textual service name provided could not be dereferenced into a port.
.TP 19
.B ARES_ECANCELLED
The query was cancelled.
.TP 19
.B ARES_EDESTRUCTION
The name service channel
.I channel
is being destroyed; the query will not be completed.
.PP
On successful completion of the query, the callback argument
.I result
points to a
.B 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).
.PP
.RS
.EX
struct ares_addrinfo {
  struct ares_addrinfo_cname *cnames;
  struct ares_addrinfo_node  *nodes;
  char *name;
};
.EE
.RE
.PP
.I ares_addrinfo_node
structure is similar to RFC3493 addrinfo, but without canonname and with extra ttl field.
.RS
.PP
.EX
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;
};
.EE
.RE
.PP
.I ares_addrinfo_cname
structure is a linked list of CNAME records where
.I ttl
is a time to live
.I alias
is a label of the resource record and
.I name
is a value (canonical name) of the resource record.
See RFC2181 10.1.1. CNAME terminology.
.RS
.PP
.EX
struct ares_addrinfo_cname {
  int                         ttl;
  char                       *alias;
  char                       *name;
  struct ares_addrinfo_cname *next;
};
.EE
.RE
.PP
The reserved memory has to be deleted by \fBares_freeaddrinfo(3)\fP.

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.
.SH AVAILABILITY
This function was added in c-ares 1.16.0, released in March 2020.
.SH SEE ALSO
.BR ares_freeaddrinfo (3)
.SH AUTHOR
Christian Ammer
.br
Andrew Selivanov <andrew.selivanov@gmail.com>