c-ares/docs/ares_getaddrinfo.3

.\"
.\" Copyright 1998 by the Massachusetts Institute of Technology.
.\" SPDX-License-Identifier: MIT
.\"
.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
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 \fIares_getaddrinfo(3)\fP.

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