ares_init, ares_init_options - Initialize a resolver channel
int ares_init(ares_channel *channel)
int ares_init_options(ares_channel *channel,
struct ares_options *options, int optmask)
cc file.c -lares
The ares_init function initializes a communications channel for name
service lookups. If it returns successfully, ares_init will set the
variable pointed to by channel 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.
The ares_init_options function also initializes a name service channel,
with additional options useful for applications requiring more control
over name service configuration. The optmask parameter 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. See
below for a description of possible flag values.
ARES_OPT_TIMEOUT int timeout;
The number of seconds each name server is given to
respond to a query on the first try. (After the
first try, the timeout algorithm becomes more
complicated, but scales linearly with the value of
timeout.) The default is five seconds.
ARES_OPT_TRIES int tries;
The number of tries the resolver will try contacting
each name server before giving up. The default is
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
ARES_OPT_PORT unsigned short port;
The port to use for queries (both TCP and UDP), in
network byte order. The default value is 53 (in
network byte order), the standard name service port.
ARES_OPT_SERVERS struct in_addr *servers;
The list of servers to contact, instead of the
servers specified in resolv.conf or the local named.
ARES_OPT_DOMAINS char **domains;
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.
The flags field should be the bitwise or of some subset of the
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
ARES_FLAG_STAYOPEN Do not close communciations 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
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
ares_init or ares_init_options 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.
Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.
21 July 1998