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// 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 darwin freebsd linux netbsd openbsd

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

package net

type dnsConfig struct {
        servers  []string // servers to use
        search   []string // suffixes to append to local name
        ndots    int      // number of dots in name to trigger absolute lookup
        timeout  int      // seconds before giving up on packet
        attempts int      // lost packets before giving up on server
        rotate   bool     // round robin among servers
}

// See resolv.conf(5) on a Linux machine.
// TODO(rsc): Supposed to call uname() and chop the beginning
// of the host name to get the default search domain.
// We assume it's in resolv.conf anyway.
func dnsReadConfig() (*dnsConfig, error) {
        file, err := open("/etc/resolv.conf")
        if err != nil {
                return nil, &DNSConfigError{err}
        }
        conf := new(dnsConfig)
        conf.servers = make([]string, 3)[0:0] // small, but the standard limit
        conf.search = make([]string, 0)
        conf.ndots = 1
        conf.timeout = 5
        conf.attempts = 2
        conf.rotate = false
        for line, ok := file.readLine(); ok; line, ok = file.readLine() {
                f := getFields(line)
                if len(f) < 1 {
                        continue
                }
                switch f[0] {
                case "nameserver": // add one name server
                        a := conf.servers
                        n := len(a)
                        if len(f) > 1 && n < cap(a) {
                                // One more check: make sure server name is
                                // just an IP address.  Otherwise we need DNS
                                // to look it up.
                                name := f[1]
                                switch len(ParseIP(name)) {
                                case 16:
                                        name = "[" + name + "]"
                                        fallthrough
                                case 4:
                                        a = a[0 : n+1]
                                        a[n] = name
                                        conf.servers = a
                                }
                        }

                case "domain": // set search path to just this domain
                        if len(f) > 1 {
                                conf.search = make([]string, 1)
                                conf.search[0] = f[1]
                        } else {
                                conf.search = make([]string, 0)
                        }

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

                case "options": // magic options
                        for i := 1; i < len(f); i++ {
                                s := f[i]
                                switch {
                                case len(s) >= 6 && s[0:6] == "ndots:":
                                        n, _, _ := dtoi(s, 6)
                                        if n < 1 {
                                                n = 1
                                        }
                                        conf.ndots = n
                                case len(s) >= 8 && s[0:8] == "timeout:":
                                        n, _, _ := dtoi(s, 8)
                                        if n < 1 {
                                                n = 1
                                        }
                                        conf.timeout = n
                                case len(s) >= 8 && s[0:9] == "attempts:":
                                        n, _, _ := dtoi(s, 9)
                                        if n < 1 {
                                                n = 1
                                        }
                                        conf.attempts = n
                                case s == "rotate":
                                        conf.rotate = true
                                }
                        }
                }
        }
        file.close()

        return conf, nil
}