libnetwork/iptables: move IPTable.LoopbackByVersion() to a utility

Not critical, but when used from ChainInfo, we had to construct an IPTable
based on the version of the ChainInfo, which then only used the version
we passed to get the right loopback.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/iptables/iptables.go

@@ -94,6 +94,19 @@ func (e ChainError) Error() string {
 	return fmt.Sprintf("error iptables %s: %s", e.Chain, string(e.Output))
 }
 
+// loopbackAddress returns the loopback address for the given IP version.
+func loopbackAddress(version IPVersion) string {
+	switch version {
+	case IPv4, "":
+		// IPv4 (default for backward-compatibility)
+		return "127.0.0.0/8"
+	case IPv6:
+		return "::1/128"
+	default:
+		panic("unknown IP version: " + version)
+	}
+}
+
 func detectIptables() {
 	path, err := exec.LookPath("iptables")
 	if err != nil {
@@ -183,14 +196,6 @@ func (iptable IPTable) NewChain(name string, table Table, hairpinMode bool) (*Ch
 	}, nil
 }
 
-// LoopbackByVersion returns loopback address by version
-func (iptable IPTable) LoopbackByVersion() string {
-	if iptable.ipVersion == IPv6 {
-		return "::1/128"
-	}
-	return "127.0.0.0/8"
-}
-
 // ProgramChain is used to add rules to a chain
 func (iptable IPTable) ProgramChain(c *ChainInfo, bridgeName string, hairpinMode, enable bool) error {
 	if c.Name == "" {
@@ -232,7 +237,7 @@ func (iptable IPTable) ProgramChain(c *ChainInfo, bridgeName string, hairpinMode
 			"-j", c.Name,
 		}
 		if !hairpinMode {
-			output = append(output, "!", "--dst", iptable.LoopbackByVersion())
+			output = append(output, "!", "--dst", loopbackAddress(iptable.ipVersion))
 		}
 		if !iptable.Exists(Nat, "OUTPUT", output...) && enable {
 			if err := c.Output(Append, output...); err != nil {
@@ -443,15 +448,15 @@ func (c *ChainInfo) Output(action Action, args ...string) error {
 
 // Remove removes the chain.
 func (c *ChainInfo) Remove() error {
-	iptable := GetIptable(c.IPVersion)
 	// Ignore errors - This could mean the chains were never set up
 	if c.Table == Nat {
 		_ = c.Prerouting(Delete, "-m", "addrtype", "--dst-type", "LOCAL", "-j", c.Name)
-		_ = c.Output(Delete, "-m", "addrtype", "--dst-type", "LOCAL", "!", "--dst", iptable.LoopbackByVersion(), "-j", c.Name)
+		_ = c.Output(Delete, "-m", "addrtype", "--dst-type", "LOCAL", "!", "--dst", loopbackAddress(c.IPVersion), "-j", c.Name)
 		_ = c.Output(Delete, "-m", "addrtype", "--dst-type", "LOCAL", "-j", c.Name) // Created in versions <= 0.1.6
 		_ = c.Prerouting(Delete)
 		_ = c.Output(Delete)
 	}
+	iptable := GetIptable(c.IPVersion)
 	_, _ = iptable.Raw("-t", string(c.Table), "-F", c.Name)
 	_, _ = iptable.Raw("-t", string(c.Table), "-X", c.Name)
 	return nil