.\" .\" Copyright 1998 by the Massachusetts Institute of Technology. .\" Copyright (C) 2004-2010 by Daniel Stenberg .\" .\" Permission to use, copy, modify, and distribute this .\" software and its documentation for any purpose and without .\" fee is hereby granted, provided that the above copyright .\" notice appear in all copies and that both that copyright .\" notice and this permission notice appear in supporting .\" documentation, and that the name of M.I.T. not be used in .\" advertising or publicity pertaining to distribution of the .\" software without specific, written prior permission. .\" M.I.T. makes no representations about the suitability of .\" this software for any purpose. It is provided "as is" .\" without express or implied warranty. .\" .\" SPDX-License-Identifier: MIT .\" .TH ARES_INIT 3 "5 March 2010" .SH NAME ares_init_options \- Initialize a resolver channel .SH SYNOPSIS .nf #include 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 ares_init_options(ares_channel *\fIchannelptr\fP, struct ares_options *\fIoptions\fP, int \fIoptmask\fP) .fi .SH DESCRIPTION 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. 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. See below for a description of possible flag values. .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. (After the first try, the timeout algorithm becomes more complicated, but scales linearly with the value of \fItimeout\fP.) The default is two seconds. This option is being deprecated by \fIARES_OPT_TIMEOUTMS\fP starting in c-ares 1.5.2. .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. (After the first try, the timeout algorithm becomes more complicated, but scales linearly with the value of \fItimeout\fP.) The default is two seconds. 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. .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. .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 .BR ares_set_servers(3) 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. .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 advertized in EDNS; only takes effect if the .B ARES_FLAG_EDNS flag is set. .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 .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. .PP The .I flags field should be the bitwise or of some subset of the following values: .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. .SH RETURN VALUES \fBares_init_options(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. .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 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_init(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) .SH AUTHOR Greg Hudson, MIT Information Systems .br Copyright 1998 by the Massachusetts Institute of Technology. .br Copyright (C) 2004-2010 by Daniel Stenberg.