useTCP: OpenBSD & FreeBSD support

src/net/dnsconfig_unix.go

@@ -124,11 +124,13 @@ func dnsReadConfig(filename string) *dnsConfig {
 					//  This option disables the behavior and makes glibc
 					//  perform the IPv6 and IPv4 requests sequentially."
 					conf.singleRequest = true
-				case s == "use-vc":
-					// Linux glibc option:
+				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

src/net/dnsconfig_unix_test.go

@@ -125,7 +125,29 @@ var dnsReadConfigTests = []struct {
 		},
 	},
 	{
-		name: "testdata/use-vc-resolv.conf",
+		name: "testdata/linux-use-vc-resolv.conf",
+		want: &dnsConfig{
+			servers:  defaultNS,
+			ndots:    1,
+			useTCP:   true,
+			timeout:  5 * time.Second,
+			attempts: 2,
+			search:   []string{"domain.local."},
+		},
+	},
+	{
+		name: "testdata/freebsd-usevc-resolv.conf",
+		want: &dnsConfig{
+			servers:  defaultNS,
+			ndots:    1,
+			useTCP:   true,
+			timeout:  5 * time.Second,
+			attempts: 2,
+			search:   []string{"domain.local."},
+		},
+	},
+	{
+		name: "testdata/openbsd-tcp-resolv.conf",
 		want: &dnsConfig{
 			servers:  defaultNS,
 			ndots:    1,

src/net/testdata/freebsd-usevc-resolv.conf

@@ -0,0 +1 @@
+options usevc

src/net/testdata/openbsd-tcp-resolv.conf

@@ -0,0 +1 @@
+options tcp