docs/ares_init_options.3

.\"
.\" Copyright 1998 by the Massachusetts Institute of Technology.
.\" Copyright (C) 2004-2010 by Daniel Stenberg
.\"
.\" SPDX-License-Identifier: MIT
.\"
.TH ARES_INIT_OPTIONS 3 "5 March 2010"
.SH NAME
ares_init_options, ares_init \- Initialize a resolver channel
.SH SYNOPSIS
.nf
#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 **\fIchannelptr\fP,
                      const struct ares_options *\fIoptions\fP,
                      int \fIoptmask\fP);

int ares_init(ares_channel_t **\fIchannelptr\fP);

.fi
.SH DESCRIPTION
The \fBares_init(3)\fP function is equivalent to calling
\fBares_init_options(channelptr, NULL, 0)\fP.  It is recommended to use
\fBares_init_options(3)\fP instead and to set or make configurable the
appropriate options for your application.

The \fBares_init_options(3)\fP function initializes a communications channel
for name service lookups.  If it returns successfully,
\fBares_init_options(3)\fP will set the variable pointed to by
\fIchannelptr\fP to a handle used to identify the name service channel.  The
caller should invoke \fIares_destroy(3)\fP 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, \fIares_reinit(3)\fP 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
\fBares_getaddrinfo(3)\fP or \fBares_gethostbyname(3)\fP, 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 \fIoptmask\fP parameter generally specifies which fields in the structure pointed to
by \fIoptions\fP are set, as follows:
.TP 18
.B ARES_OPT_FLAGS
.B int \fIflags\fP;
.br
Flags controlling the behavior of the resolver:
.RS 4
.TP 23
.B 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.
.TP 23
.B ARES_FLAG_PRIMARY
Only query the first server in the list of servers to query.
.TP 23
.B 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.
.TP 23
.B 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 \fIARES_FLAG_NORECURSE\fP is set.
.TP 23
.B ARES_FLAG_STAYOPEN
Do not close communications sockets when the number of active queries
drops to zero.
.TP 23
.B ARES_FLAG_NOSEARCH
Do not use the default search domains; only query hostnames as-is or
as aliases.
.TP 23
.B ARES_FLAG_NOALIASES
Do not honor the HOSTALIASES environment variable, which normally
specifies a file of hostname translations.
.TP 23
.B 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.
.TP 23
.B 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.
.TP 23
.B 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 \fIARES_ENOSERVER\fP.
.TP 23
.B 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.
.RE
.TP 18
.B ARES_OPT_TIMEOUT
.B int \fItimeout\fP;
.br
The number of seconds each name server is given to respond to a query on the
first try.  See \fIARES_OPT_TIMEOUTMS\fP which this value is converted into.
.TP 18
.B ARES_OPT_TIMEOUTMS
.B int \fItimeout\fP;
.br
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 \fIARES_OPT_TIMEOUT\fP, 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.
.TP 18
.B ARES_OPT_TRIES
.B int \fItries\fP;
.br
The number of tries the resolver will try contacting each name server
before giving up.  The default is three tries.
.TP 18
.B ARES_OPT_NDOTS
.B int \fIndots\fP;
.br
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.
.TP 18
.B ARES_OPT_MAXTIMEOUTMS
.B int \fImaxtimeout\fP;
.br
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.
.TP 18
.B ARES_OPT_UDP_PORT
.B unsigned short \fIudp_port\fP;
.br
The port to use for queries over UDP, in host byte order.
The default value is 53, the standard name service port.
.TP 18
.B ARES_OPT_TCP_PORT
.B unsigned short \fItcp_port\fP;
.br
The port to use for queries over TCP, in host byte order.
The default value is 53, the standard name service port.
.TP 18
.B ARES_OPT_SERVERS
.B struct in_addr *\fIservers\fP;
.br
.B int \fInservers\fP;
.br
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 \Bares_set_servers(3)\fP function must be used
instead.
.TP 18
.B ARES_OPT_DOMAINS
.B char **\fIdomains\fP;
.br
.B int \fIndomains\fP;
.br
The domains to search, instead of the domains specified in resolv.conf
or the domain derived from the kernel hostname variable.
.TP 18
.B ARES_OPT_LOOKUPS
.B char *\fIlookups\fP;
.br
The lookups to perform for host queries.
.I 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.
.TP 18
.B ARES_OPT_SOCK_STATE_CB
.B void (*\fIsock_state_cb\fP)(void *data, ares_socket_t socket_fd, int readable, int writable);
.br
.B void *\fIsock_state_cb_data\fP;
.br
A callback function to be invoked when a socket changes state.
.I socket_fd
will be passed the socket whose state has changed;
.I readable
will be set to true if the socket should listen for read events, and
.I writable
will be set to true if the socket should listen for write events.
The value of
.I sock_state_cb_data
will be passed as the
.I 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 \fBARES_OPT_EVENT_THREAD\fP.
.TP 18
.B ARES_OPT_SORTLIST
.B struct apattern *\fIsortlist\fP;
.br
.B int \fInsort\fP;
.br
A list of IP address ranges that specifies the order of preference that
results from \fIares_gethostbyname\fP should be returned in.  Note that
this can only be used with a sortlist retrieved via
\fBares_save_options(3)\fP (because
.B struct apattern
is opaque); to set a fresh sort list, use \fBares_set_sortlist(3)\fP.
.TP 18
.B ARES_OPT_SOCK_SNDBUF
.B int \fIsocket_send_buffer_size\fP;
.br
The send buffer size to set for the socket.
.TP 18
.B ARES_OPT_SOCK_RCVBUF
.B int \fIsocket_receive_buffer_size\fP;
.br
The receive buffer size to set for the socket.
.TP 18
.B ARES_OPT_EDNSPSZ
.B int \fIednspsz\fP;
.br
The message size to be advertised in EDNS; only takes effect if the
.B ARES_FLAG_EDNS
flag is set.  Defaults to 1232, the recommended size.
.TP 18
.B ARES_OPT_RESOLVCONF
.B char *\fIresolvconf_path\fP;
.br
The path to use for reading the resolv.conf file. The
.I resolvconf_path
should be set to a path string, and will be honoured on *nix like systems. The
default is
.B /etc/resolv.conf
.br
.TP 18
.B ARES_OPT_HOSTS_FILE
.B char *\fIhosts_path\fP;
.br
The path to use for reading the hosts file. The
.I hosts_path
should be set to a path string, and will be honoured on *nix like systems. The
default is
.B /etc/hosts
.br
.TP 18
.B ARES_OPT_UDP_MAX_QUERIES
.B int \fIudp_max_queries\fP;
.br
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.
.br
.TP 18
.B ARES_OPT_QUERY_CACHE
.B unsigned int \fIqcache_max_ttl\fP;
.br
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 \fIqcache_max_ttl\fP
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.
.br
.TP 18
.B ARES_OPT_EVENT_THREAD
.B ares_evsys_t \fIevsys\fP;
.br
Enable the built-in event thread (Recommended). Introduced in c-ares 1.26.0.
Set the \fIevsys\fP parameter to \fBARES_EVSYS_DEFAULT\fP (0).  Other values are
reserved for testing and should not be used by integrators.

This option cannot be used with the \fBARES_OPT_SOCK_STATE_CB\fP option, nor the
\fIares_set_socket_functions(3)\fP or
\fIares_set_socket_configure_callback(3)\fP functions.

When enabled, the integrator is no longer responsible for notifying c-ares of
any events on the file descriptors, so \fIares_process(3)\fP nor
\fIares_process_fd(3)\fP 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 \fIares_threadsafety(3)\fP to determine if this option is available to be
used.

Returns \fBARES_ENOTIMP\fP if this option is passed but not available, and
\fBARES_ESERVFAIL\fP if there is a critical failure during initialization of
the event thread.
.br
.TP 18
.B ARES_OPT_SERVER_FAILOVER
.B struct ares_server_failover_options \fIserver_failover_opts\fP;
.br
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
\fIares_server_failover_options\fP structure contains options to control this
behavior.
The \fIretry_chance\fP field gives the probability (1/N) of retrying a
failed server on any given query.  Setting to a value of 0 disables retries.
The \fIretry_delay\fP 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.
.br
.PP
The \fIoptmask\fP parameter also includes options without a corresponding
field in the
.B ares_options
structure, as follows:
.TP 23
.B ARES_OPT_ROTATE
Perform round-robin selection of the nameservers configured for the channel
for each resolution.
.TP 23
.B 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.
.PP

.SH RETURN VALUES
\fBares_init_options(3)\fP and \fBares_init(3)\fP can return any of the
following values:
.TP 14
.B ARES_SUCCESS
Initialization succeeded.
.TP 14
.B ARES_EFILE
A configuration file could not be read.
.TP 14
.B ARES_ENOMEM
The process's available memory was exhausted.
.TP 14
.B ARES_ENOTINITIALIZED
c-ares library initialization not yet performed.
.TP 14
.B ARES_ENOSERVER
No DNS servers were available to use.
.SH NOTES
When initializing from
.B /etc/resolv.conf,
(or, alternatively when specified by the
.I resolvconf_path
path location)
\fBares_init_options(3)\fP and \fBares_init(3)\fP reads the \fIdomain\fP and
\fIsearch\fP directives to allow lookups of short names relative to the domains
specified. The \fIdomain\fP and \fIsearch\fP directives override one another.
If more than one instance of either \fIdomain\fP or \fIsearch\fP directives is
specified, the last occurrence wins. For more information, please see the
.BR resolv.conf (5)
manual page.
.SH SEE ALSO
.BR ares_reinit (3),
.BR ares_destroy (3),
.BR ares_dup (3),
.BR ares_library_init (3),
.BR ares_save_options (3),
.BR ares_set_servers (3),
.BR ares_set_sortlist (3),
.BR ares_threadsafety (3)