Tribblix: manual page: ares_init

ARES_INIT_OPTIONS(3) Introduction to Library Functions ARES_INIT_OPTIONS(3)

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.

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUES

NOTES

SEE ALSO