1 files changed, 157 insertions, 0 deletions
diff --git a/src/net/dnsconfig_unix.go b/src/net/dnsconfig_unix.go
new file mode 100644
index 0000000..94cd09e
--- /dev/null
+++ b/src/net/dnsconfig_unix.go
@@ -0,0 +1,157 @@
+// 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.
+
+//go:build !js && !windows
+
+// Read system DNS config from /etc/resolv.conf
+
+package net
+
+import (
+	"internal/bytealg"
+	"time"
+)
+
+// See resolv.conf(5) on a Linux machine.
+func dnsReadConfig(filename string) *dnsConfig {
+	conf := &dnsConfig{
+		ndots:    1,
+		timeout:  5 * time.Second,
+		attempts: 2,
+	}
+	file, err := open(filename)
+	if err != nil {
+		conf.servers = defaultNS
+		conf.search = dnsDefaultSearch()
+		conf.err = err
+		return conf
+	}
+	defer file.close()
+	if fi, err := file.file.Stat(); err == nil {
+		conf.mtime = fi.ModTime()
+	} else {
+		conf.servers = defaultNS
+		conf.search = dnsDefaultSearch()
+		conf.err = err
+		return conf
+	}
+	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
+		if len(line) > 0 && (line[0] == ';' || line[0] == '#') {
+			// comment.
+			continue
+		}
+		f := getFields(line)
+		if len(f) < 1 {
+			continue
+		}
+		switch f[0] {
+		case "nameserver": // add one name server
+			if len(f) > 1 && len(conf.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(f[1]) != nil {
+					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
+				} else if ip, _ := parseIPv6Zone(f[1]); ip != nil {
+					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
+				}
+			}
+
+		case "domain": // set search path to just this domain
+			if len(f) > 1 {
+				conf.search = []string{ensureRooted(f[1])}
+			}
+
+		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] = ensureRooted(f[i+1])
+			}
+
+		case "options": // magic options
+			for _, s := range f[1:] {
+				switch {
+				case hasPrefix(s, "ndots:"):
+					n, _, _ := dtoi(s[6:])
+					if n < 0 {
+						n = 0
+					} else if n > 15 {
+						n = 15
+					}
+					conf.ndots = n
+				case hasPrefix(s, "timeout:"):
+					n, _, _ := dtoi(s[8:])
+					if n < 1 {
+						n = 1
+					}
+					conf.timeout = time.Duration(n) * time.Second
+				case hasPrefix(s, "attempts:"):
+					n, _, _ := dtoi(s[9:])
+					if n < 1 {
+						n = 1
+					}
+					conf.attempts = n
+				case s == "rotate":
+					conf.rotate = true
+				case s == "single-request" || s == "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."
+					conf.singleRequest = true
+				case s == "use-vc" || s == "usevc" || s == "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
+					conf.useTCP = true
+				default:
+					conf.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"
+			conf.lookup = f[1:]
+
+		default:
+			conf.unknownOpt = true
+		}
+	}
+	if len(conf.servers) == 0 {
+		conf.servers = defaultNS
+	}
+	if len(conf.search) == 0 {
+		conf.search = dnsDefaultSearch()
+	}
+	return conf
+}
+
+func dnsDefaultSearch() []string {
+	hn, err := getHostname()
+	if err != nil {
+		// best effort
+		return nil
+	}
+	if i := bytealg.IndexByteString(hn, '.'); i >= 0 && i < len(hn)-1 {
+		return []string{ensureRooted(hn[i+1:])}
+	}
+	return nil
+}
+
+func hasPrefix(s, prefix string) bool {
+	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
+}
+
+func ensureRooted(s string) string {
+	if len(s) > 0 && s[len(s)-1] == '.' {
+		return s
+	}
+	return s + "."
+}