mirror of https://github.com/nodejs/node.git
416 lines
15 KiB
Groff
416 lines
15 KiB
Groff
.\"
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.\" Copyright 1998 by the Massachusetts Institute of Technology.
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.\" Copyright (C) 2004-2010 by Daniel Stenberg
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.\"
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.\" SPDX-License-Identifier: MIT
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.\"
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.TH ARES_INIT_OPTIONS 3 "5 March 2010"
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.SH NAME
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ares_init_options, ares_init \- Initialize a resolver channel
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.SH SYNOPSIS
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.nf
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#include <ares.h>
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struct ares_server_failover_options {
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unsigned short retry_chance;
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size_t retry_delay;
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};
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struct ares_options {
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int flags;
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int timeout; /* in seconds or milliseconds, depending on options */
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int tries;
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int ndots;
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unsigned short udp_port;
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unsigned short tcp_port;
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int socket_send_buffer_size;
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int socket_receive_buffer_size;
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struct in_addr *servers;
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int nservers;
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char **domains;
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int ndomains;
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char *lookups;
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ares_sock_state_cb sock_state_cb;
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void *sock_state_cb_data;
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struct apattern *sortlist;
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int nsort;
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int ednspsz;
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char *resolvconf_path;
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char *hosts_path;
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int udp_max_queries;
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int maxtimeout; /* in milliseconds */
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unsigned int qcache_max_ttl; /* in seconds */
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ares_evsys_t evsys;
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struct ares_server_failover_options server_failover_opts;
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};
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int ares_init_options(ares_channel_t **\fIchannelptr\fP,
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const struct ares_options *\fIoptions\fP,
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int \fIoptmask\fP);
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int ares_init(ares_channel_t **\fIchannelptr\fP);
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.fi
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.SH DESCRIPTION
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The \fBares_init(3)\fP function is equivalent to calling
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\fBares_init_options(channelptr, NULL, 0)\fP. It is recommended to use
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\fBares_init_options(3)\fP instead and to set or make configurable the
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appropriate options for your application.
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The \fBares_init_options(3)\fP function initializes a communications channel
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for name service lookups. If it returns successfully,
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\fBares_init_options(3)\fP will set the variable pointed to by
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\fIchannelptr\fP to a handle used to identify the name service channel. The
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caller should invoke \fIares_destroy(3)\fP on the handle when the channel is
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no longer needed.
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It is recommended for an application to have at most one ares channel and use
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this for all DNS queries for the life of the application. When system
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configuration changes, \fIares_reinit(3)\fP can be called to reload the
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configuration if necessary. The recommended concurrent query limit is about
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32k queries, but remembering that when specifying AF_UNSPEC for
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\fBares_getaddrinfo(3)\fP or \fBares_gethostbyname(3)\fP, they may spawn
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2 queries internally. The reason for the limit is c-ares does not allow
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duplicate DNS query ids (which have a maximum of 64k) to be oustanding at a
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given time, and it must randomly search for an available id thus 32k will limit
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the number of searches. This limitation should not be a concern for most
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implementations and c-ares may implement queuing in future releases to lift this
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limitation.
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The \fIoptmask\fP parameter generally specifies which fields in the structure pointed to
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by \fIoptions\fP are set, as follows:
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.TP 18
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.B ARES_OPT_FLAGS
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.B int \fIflags\fP;
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.br
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Flags controlling the behavior of the resolver:
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.RS 4
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.TP 23
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.B ARES_FLAG_USEVC
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Always use TCP queries (the "virtual circuit") instead of UDP
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queries. Normally, TCP is only used if a UDP query yields a truncated
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result.
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.TP 23
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.B ARES_FLAG_PRIMARY
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Only query the first server in the list of servers to query.
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.TP 23
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.B ARES_FLAG_IGNTC
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If a truncated response to a UDP query is received, do not fall back
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to TCP; simply continue on with the truncated response.
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.TP 23
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.B ARES_FLAG_NORECURSE
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Do not set the "recursion desired" bit on outgoing queries, so that the name
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server being contacted will not try to fetch the answer from other servers if
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it doesn't know the answer locally. Be aware that ares will not do the
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recursion for you. Recursion must be handled by the application calling ares
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if \fIARES_FLAG_NORECURSE\fP is set.
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.TP 23
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.B ARES_FLAG_STAYOPEN
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Do not close communications sockets when the number of active queries
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drops to zero.
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.TP 23
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.B ARES_FLAG_NOSEARCH
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Do not use the default search domains; only query hostnames as-is or
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as aliases.
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.TP 23
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.B ARES_FLAG_NOALIASES
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Do not honor the HOSTALIASES environment variable, which normally
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specifies a file of hostname translations.
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.TP 23
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.B ARES_FLAG_NOCHECKRESP
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Do not discard responses with the SERVFAIL, NOTIMP, or REFUSED
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response code or responses whose questions don't match the questions
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in the request. Primarily useful for writing clients which might be
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used to test or debug name servers.
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.TP 23
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.B ARES_FLAG_EDNS
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Include an EDNS pseudo-resource record (RFC 2671) in generated requests. As of
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v1.22, this is on by default if flags are otherwise not set.
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.TP 23
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.B ARES_FLAG_NO_DFLT_SVR
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Do not attempt to add a default local named server if there are no other
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servers available. Instead, fail initialization with \fIARES_ENOSERVER\fP.
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.TP 23
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.B ARES_FLAG_DNS0x20
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Enable support for DNS 0x20 as per https://datatracker.ietf.org/doc/html/draft-vixie-dnsext-dns0x20-00
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which adds additional entropy to the request by randomizing the case of the
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query name. Integrators need to ensure they treat DNS name responses as
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case-insensitive. In rare circumstances this may cause the inability to lookup
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certain domains if the upstream server or the authoritative server for the
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domain is non-compliant.
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.RE
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.TP 18
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.B ARES_OPT_TIMEOUT
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.B int \fItimeout\fP;
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.br
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The number of seconds each name server is given to respond to a query on the
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first try. See \fIARES_OPT_TIMEOUTMS\fP which this value is converted into.
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.TP 18
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.B ARES_OPT_TIMEOUTMS
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.B int \fItimeout\fP;
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.br
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The number of milliseconds each name server is given to respond to a query on
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the first try of any given server. The default is two seconds, however any
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value below 250ms will automatically be set to 250ms (roughly the RTT half-way
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around the world). Note that this option is specified with the same struct field
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as the former \fIARES_OPT_TIMEOUT\fP, it is but the option bits that tell c-ares
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how to interpret the number. This option was added in c-ares 1.5.2.
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As of c-ares 1.32.0, this option is only honored on the first successful query
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to any given server, after that the timeout is automatically calculated based
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on prior query history.
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.TP 18
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.B ARES_OPT_TRIES
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.B int \fItries\fP;
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.br
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The number of tries the resolver will try contacting each name server
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before giving up. The default is three tries.
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.TP 18
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.B ARES_OPT_NDOTS
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.B int \fIndots\fP;
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.br
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The number of dots which must be present in a domain name for it to be
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queried for "as is" prior to querying for it with the default domain
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extensions appended. The default value is 1 unless set otherwise by
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resolv.conf or the RES_OPTIONS environment variable. Valid range is 0-15.
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.TP 18
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.B ARES_OPT_MAXTIMEOUTMS
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.B int \fImaxtimeout\fP;
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.br
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The upper bound for timeout between sequential retry attempts. When retrying
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queries, the timeout is increased from the requested timeout parameter, this
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caps the value.
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.TP 18
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.B ARES_OPT_UDP_PORT
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.B unsigned short \fIudp_port\fP;
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.br
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The port to use for queries over UDP, in host byte order.
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The default value is 53, the standard name service port.
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.TP 18
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.B ARES_OPT_TCP_PORT
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.B unsigned short \fItcp_port\fP;
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.br
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The port to use for queries over TCP, in host byte order.
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The default value is 53, the standard name service port.
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.TP 18
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.B ARES_OPT_SERVERS
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.B struct in_addr *\fIservers\fP;
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.br
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.B int \fInservers\fP;
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.br
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The list of IPv4 servers to contact, instead of the servers specified in
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resolv.conf or the local named. In order to allow specification of either IPv4
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or IPv6 name servers, the \Bares_set_servers(3)\fP function must be used
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instead.
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.TP 18
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.B ARES_OPT_DOMAINS
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.B char **\fIdomains\fP;
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.br
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.B int \fIndomains\fP;
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.br
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The domains to search, instead of the domains specified in resolv.conf
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or the domain derived from the kernel hostname variable.
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.TP 18
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.B ARES_OPT_LOOKUPS
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.B char *\fIlookups\fP;
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.br
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The lookups to perform for host queries.
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.I lookups
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should be set to a string of the characters "b" or "f", where "b"
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indicates a DNS lookup and "f" indicates a lookup in the hosts file.
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.TP 18
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.B ARES_OPT_SOCK_STATE_CB
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.B void (*\fIsock_state_cb\fP)(void *data, ares_socket_t socket_fd, int readable, int writable);
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.br
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.B void *\fIsock_state_cb_data\fP;
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.br
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A callback function to be invoked when a socket changes state.
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.I socket_fd
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will be passed the socket whose state has changed;
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.I readable
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will be set to true if the socket should listen for read events, and
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.I writable
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will be set to true if the socket should listen for write events.
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The value of
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.I sock_state_cb_data
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will be passed as the
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.I data
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argument. The channel lock is held during this callback, if in a multithreaded
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application, care must be taken to ensure lock order is correct to be able to
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handle this and avoid deadlocks.
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Cannot be used with \fBARES_OPT_EVENT_THREAD\fP.
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.TP 18
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.B ARES_OPT_SORTLIST
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.B struct apattern *\fIsortlist\fP;
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.br
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.B int \fInsort\fP;
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.br
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A list of IP address ranges that specifies the order of preference that
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results from \fIares_gethostbyname\fP should be returned in. Note that
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this can only be used with a sortlist retrieved via
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\fBares_save_options(3)\fP (because
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.B struct apattern
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is opaque); to set a fresh sort list, use \fBares_set_sortlist(3)\fP.
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.TP 18
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.B ARES_OPT_SOCK_SNDBUF
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.B int \fIsocket_send_buffer_size\fP;
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.br
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The send buffer size to set for the socket.
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.TP 18
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.B ARES_OPT_SOCK_RCVBUF
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.B int \fIsocket_receive_buffer_size\fP;
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.br
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The receive buffer size to set for the socket.
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.TP 18
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.B ARES_OPT_EDNSPSZ
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.B int \fIednspsz\fP;
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.br
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The message size to be advertised in EDNS; only takes effect if the
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.B ARES_FLAG_EDNS
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flag is set. Defaults to 1232, the recommended size.
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.TP 18
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.B ARES_OPT_RESOLVCONF
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.B char *\fIresolvconf_path\fP;
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.br
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The path to use for reading the resolv.conf file. The
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.I resolvconf_path
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should be set to a path string, and will be honoured on *nix like systems. The
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default is
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.B /etc/resolv.conf
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.br
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.TP 18
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.B ARES_OPT_HOSTS_FILE
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.B char *\fIhosts_path\fP;
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.br
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The path to use for reading the hosts file. The
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.I hosts_path
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should be set to a path string, and will be honoured on *nix like systems. The
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default is
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.B /etc/hosts
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.br
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.TP 18
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.B ARES_OPT_UDP_MAX_QUERIES
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.B int \fIudp_max_queries\fP;
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.br
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The maximum number of udp queries that can be sent on a single ephemeral port
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to a given DNS server before a new ephemeral port is assigned. Any value of 0
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or less will be considered unlimited, and is the default.
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.br
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.TP 18
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.B ARES_OPT_QUERY_CACHE
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.B unsigned int \fIqcache_max_ttl\fP;
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.br
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As of c-ares 1.31.0, the query cache is enabled by default with a TTL of 1hr.
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To disable the query cache, specify this option with a TTL of 0. The query
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cache is based on the returned TTL in the DNS message. Only fully successful
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and NXDOMAIN query results will be cached. Fill in the \fIqcache_max_ttl\fP
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with the maximum number of seconds a query result may be cached which will
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override a larger TTL in the response message. This must be a non-zero value
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otherwise the cache will be disabled. Choose a reasonable value for your
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application such as 300 (5 minutes) or 3600 (1 hour). The query cache is
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automatically flushed if a server configuration change is made.
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.br
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.TP 18
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.B ARES_OPT_EVENT_THREAD
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.B ares_evsys_t \fIevsys\fP;
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.br
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Enable the built-in event thread (Recommended). Introduced in c-ares 1.26.0.
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Set the \fIevsys\fP parameter to \fBARES_EVSYS_DEFAULT\fP (0). Other values are
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reserved for testing and should not be used by integrators.
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This option cannot be used with the \fBARES_OPT_SOCK_STATE_CB\fP option, nor the
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\fIares_set_socket_functions(3)\fP or
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\fIares_set_socket_configure_callback(3)\fP functions.
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When enabled, the integrator is no longer responsible for notifying c-ares of
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any events on the file descriptors, so \fIares_process(3)\fP nor
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\fIares_process_fd(3)\fP should ever be called when this option is enabled.
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As of c-ares 1.29.0, when enabled, it will also automatically re-load the
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system configuration when changes are detected.
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Use \fIares_threadsafety(3)\fP to determine if this option is available to be
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used.
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Returns \fBARES_ENOTIMP\fP if this option is passed but not available, and
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\fBARES_ESERVFAIL\fP if there is a critical failure during initialization of
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the event thread.
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.br
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.TP 18
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.B ARES_OPT_SERVER_FAILOVER
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.B struct ares_server_failover_options \fIserver_failover_opts\fP;
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.br
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Configure server failover retry behavior. When a DNS server fails to
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respond to a query, c-ares will deprioritize the server. On subsequent
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queries, servers with fewer consecutive failures will be selected in
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preference. However, in order to detect when such a server has recovered,
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c-ares will occasionally retry failed servers by probing with a copy of
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the query, without affecting the latency of the actual requested query. The
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\fIares_server_failover_options\fP structure contains options to control this
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behavior.
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The \fIretry_chance\fP field gives the probability (1/N) of retrying a
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failed server on any given query. Setting to a value of 0 disables retries.
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The \fIretry_delay\fP field gives the minimum delay in milliseconds that c-ares
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will wait before retrying a specific failed server.
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If this option is not specificed then c-ares will use a probability of 10%
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and a minimum delay of 5 seconds.
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.br
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.PP
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The \fIoptmask\fP parameter also includes options without a corresponding
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field in the
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.B ares_options
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structure, as follows:
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.TP 23
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.B ARES_OPT_ROTATE
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Perform round-robin selection of the nameservers configured for the channel
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for each resolution.
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.TP 23
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.B ARES_OPT_NOROTATE
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Do not perform round-robin nameserver selection; always use the list of
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nameservers in the same order. The default is not to rotate servers, however
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the system configuration can specify the desire to rotate and this
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configuration value can negate such a system configuration.
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.PP
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.SH RETURN VALUES
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\fBares_init_options(3)\fP and \fBares_init(3)\fP can return any of the
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following values:
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.TP 14
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.B ARES_SUCCESS
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Initialization succeeded.
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.TP 14
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.B ARES_EFILE
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A configuration file could not be read.
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.TP 14
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.B ARES_ENOMEM
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The process's available memory was exhausted.
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.TP 14
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.B ARES_ENOTINITIALIZED
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c-ares library initialization not yet performed.
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.TP 14
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.B ARES_ENOSERVER
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No DNS servers were available to use.
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.SH NOTES
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When initializing from
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.B /etc/resolv.conf,
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(or, alternatively when specified by the
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.I resolvconf_path
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path location)
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\fBares_init_options(3)\fP and \fBares_init(3)\fP reads the \fIdomain\fP and
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\fIsearch\fP directives to allow lookups of short names relative to the domains
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specified. The \fIdomain\fP and \fIsearch\fP directives override one another.
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If more than one instance of either \fIdomain\fP or \fIsearch\fP directives is
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specified, the last occurrence wins. For more information, please see the
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.BR resolv.conf (5)
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manual page.
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.SH SEE ALSO
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.BR ares_reinit (3),
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.BR ares_destroy (3),
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.BR ares_dup (3),
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.BR ares_library_init (3),
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.BR ares_save_options (3),
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.BR ares_set_servers (3),
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.BR ares_set_sortlist (3),
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.BR ares_threadsafety (3)
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