// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris

// Read system DNS config from /etc/resolv.conf

package net

import (
	
	
	
	
)

var (
	defaultNS   = []string{"127.0.0.1:53", "[::1]:53"}
	getHostname = os.Hostname // variable for testing
)

type dnsConfig struct {
	servers       []string      // server addresses (in host:port form) to use
	search        []string      // rooted suffixes to append to local name
	ndots         int           // number of dots in name to trigger absolute lookup
	timeout       time.Duration // wait before giving up on a query, including retries
	attempts      int           // lost packets before giving up on server
	rotate        bool          // round robin among servers
	unknownOpt    bool          // anything unknown was encountered
	lookup        []string      // OpenBSD top-level database "lookup" order
	err           error         // any error that occurs during open of resolv.conf
	mtime         time.Time     // time of resolv.conf modification
	soffset       uint32        // used by serverOffset
	singleRequest bool          // use sequential A and AAAA queries instead of parallel queries
	useTCP        bool          // force usage of TCP for DNS resolutions
}

// See resolv.conf(5) on a Linux machine.
func dnsReadConfig( string) *dnsConfig {
	 := &dnsConfig{
		ndots:    1,
		timeout:  5 * time.Second,
		attempts: 2,
	}
	,  := open()
	if  != nil {
		.servers = defaultNS
		.search = dnsDefaultSearch()
		.err = 
		return 
	}
	defer .close()
	if ,  := .file.Stat();  == nil {
		.mtime = .ModTime()
	} else {
		.servers = defaultNS
		.search = dnsDefaultSearch()
		.err = 
		return 
	}
	for ,  := .readLine(); ; ,  = .readLine() {
		if len() > 0 && ([0] == ';' || [0] == '#') {
			// comment.
			continue
		}
		 := getFields()
		if len() < 1 {
			continue
		}
		switch [0] {
		case "nameserver": // add one name server
			if len() > 1 && len(.servers) < 3 { // small, but the standard limit
				// One more check: make sure server name is
				// just an IP address. Otherwise we need DNS
				// to look it up.
				if parseIPv4([1]) != nil {
					.servers = append(.servers, JoinHostPort([1], "53"))
				} else if ,  := parseIPv6Zone([1]);  != nil {
					.servers = append(.servers, JoinHostPort([1], "53"))
				}
			}

		case "domain": // set search path to just this domain
			if len() > 1 {
				.search = []string{ensureRooted([1])}
			}

		case "search": // set search path to given servers
			.search = make([]string, len()-1)
			for  := 0;  < len(.search); ++ {
				.search[] = ensureRooted([+1])
			}

		case "options": // magic options
			for ,  := range [1:] {
				switch {
				case hasPrefix(, "ndots:"):
					, ,  := dtoi([6:])
					if  < 0 {
						 = 0
					} else if  > 15 {
						 = 15
					}
					.ndots = 
				case hasPrefix(, "timeout:"):
					, ,  := dtoi([8:])
					if  < 1 {
						 = 1
					}
					.timeout = time.Duration() * time.Second
				case hasPrefix(, "attempts:"):
					, ,  := dtoi([9:])
					if  < 1 {
						 = 1
					}
					.attempts = 
				case  == "rotate":
					.rotate = true
				case  == "single-request" ||  == "single-request-reopen":
					// Linux option:
					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
					// "By default, glibc performs IPv4 and IPv6 lookups in parallel [...]
					//  This option disables the behavior and makes glibc
					//  perform the IPv6 and IPv4 requests sequentially."
					.singleRequest = true
				case  == "use-vc" ||  == "usevc" ||  == "tcp":
					// Linux (use-vc), FreeBSD (usevc) and OpenBSD (tcp) option:
					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
					// "Sets RES_USEVC in _res.options.
					//  This option forces the use of TCP for DNS resolutions."
					// https://www.freebsd.org/cgi/man.cgi?query=resolv.conf&sektion=5&manpath=freebsd-release-ports
					// https://man.openbsd.org/resolv.conf.5
					.useTCP = true
				default:
					.unknownOpt = true
				}
			}

		case "lookup":
			// OpenBSD option:
			// https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5
			// "the legal space-separated values are: bind, file, yp"
			.lookup = [1:]

		default:
			.unknownOpt = true
		}
	}
	if len(.servers) == 0 {
		.servers = defaultNS
	}
	if len(.search) == 0 {
		.search = dnsDefaultSearch()
	}
	return 
}

// serverOffset returns an offset that can be used to determine
// indices of servers in c.servers when making queries.
// When the rotate option is enabled, this offset increases.
// Otherwise it is always 0.
func ( *dnsConfig) () uint32 {
	if .rotate {
		return atomic.AddUint32(&.soffset, 1) - 1 // return 0 to start
	}
	return 0
}

func dnsDefaultSearch() []string {
	,  := getHostname()
	if  != nil {
		// best effort
		return nil
	}
	if  := bytealg.IndexByteString(, '.');  >= 0 &&  < len()-1 {
		return []string{ensureRooted([+1:])}
	}
	return nil
}

func hasPrefix(,  string) bool {
	return len() >= len() && [:len()] == 
}

func ensureRooted( string) string {
	if len() > 0 && [len()-1] == '.' {
		return 
	}
	return  + "."
}