ares_init_options(3) Library Functions Manual 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(3) 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 resolvconfares_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.
SEE ALSO
ares_reinit(3), ares_destroy(3), ares_dup(3), ares_library_init(3),
ares_save_options(3), ares_set_servers(3), ares_set_sortlist(3),
ares_threadsafety(3)
5 March 2010 ares_init_options(3)
c-ares 1.34.1 - Generated Fri Oct 11 05:28:39 CDT 2024