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Diffstat (limited to 'src/net/dnsconfig_unix.go')
| -rw-r--r-- | src/net/dnsconfig_unix.go | 191 |
1 files changed, 191 insertions, 0 deletions
diff --git a/src/net/dnsconfig_unix.go b/src/net/dnsconfig_unix.go new file mode 100644 index 0000000..877e77c --- /dev/null +++ b/src/net/dnsconfig_unix.go @@ -0,0 +1,191 @@ +// 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 ( + "internal/bytealg" + "os" + "sync/atomic" + "time" +) + +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(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 +} + +// 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 (c *dnsConfig) serverOffset() uint32 { + if c.rotate { + return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start + } + return 0 +} + +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 + "." +} |