plugin/bind: Bind by interface name (#4522)

* auto make -f Makefile.doc

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

* Bind by interface name

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

* README.md: Interface with multiple address

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

* auto make -f Makefile.doc

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

* auto make -f Makefile.doc

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

* Elaborate more on the behaviour in README.md, revert man/*, fix tests

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

* auto make -f Makefile.doc

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

* --sign-off

Revert man/* to fix DCO check

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

* auto make -f Makefile.doc

* Revert man/* to fix DCO check

Signed-off-by: Mohammad Yosefpor <myusefpur@gmail.com>

Co-authored-by: coredns-auto-go-mod-tidy[bot] <coredns-auto-go-mod-tidy[bot]@users.noreply.github.com>

3 files changed, 43 insertions, 8 deletions

diff --git a/plugin/bind/README.md b/plugin/bind/README.md
index 761aa056a..6ff3fb888 100644
--- a/plugin/bind/README.md
+++ b/plugin/bind/README.md
@@ -11,7 +11,9 @@ another IP instead.
 
 If several addresses are provided, a listener will be open on each of the IP provided.
 
-Each address has to be an IP of one of the interfaces of the host.
+Each address has to be an IP or name of one of the interfaces of the host. Bind by interface name, binds to the IPs on that interface at the time of startup or reload (reload will happen with a SIGHUP or if the config file changes).
+
+If the given argument is an interface name, and that interface has serveral IP addresses, CoreDNS will listen on all of the interface IP addresses (including IPv4 and IPv6).
 
 ## Syntax
 
@@ -50,6 +52,14 @@ The following sample is equivalent to the preceding:
 }
 ~~~
 
+The following server block, binds on localhost with its interface name (both "127.0.0.1" and "::1"):
+
+~~~ corefile
+. {
+    bind lo
+}
+~~~
+
 ## Bugs
 
 When defining more than one server block, take care not to bind more than one server to the same
diff --git a/plugin/bind/setup.go b/plugin/bind/setup.go
index 44c9b8a6f..afca06097 100644
--- a/plugin/bind/setup.go
+++ b/plugin/bind/setup.go
@@ -15,16 +15,40 @@ func setup(c *caddy.Controller) error {
 	// addresses will be consolidated over all BIND directives available in that BlocServer
 	all := []string{}
 	for c.Next() {
-		addrs := c.RemainingArgs()
-		if len(addrs) == 0 {
-			return plugin.Error("bind", fmt.Errorf("at least one address is expected"))
+		args := c.RemainingArgs()
+		if len(args) == 0 {
+			return plugin.Error("bind", fmt.Errorf("at least one address or interface name is expected"))
 		}
-		for _, addr := range addrs {
-			if net.ParseIP(addr) == nil {
-				return plugin.Error("bind", fmt.Errorf("not a valid IP address: %s", addr))
+
+		ifaces, err := net.Interfaces()
+		if err != nil {
+			return plugin.Error("bind", fmt.Errorf("failed to get interfaces list"))
+		}
+
+		var isIface bool
+		for _, arg := range args {
+			isIface = false
+			for _, iface := range ifaces {
+				if arg == iface.Name {
+					isIface = true
+					addrs, err := iface.Addrs()
+					if err != nil {
+						return plugin.Error("bind", fmt.Errorf("failed to get the IP(s) of the interface: %s", arg))
+					}
+					for _, addr := range addrs {
+						if ipnet, ok := addr.(*net.IPNet); ok {
+							all = append(all, ipnet.IP.String())
+						}
+					}
+				}
+			}
+			if !isIface {
+				if net.ParseIP(arg) == nil {
+					return plugin.Error("bind", fmt.Errorf("not a valid IP address: %s", arg))
+				}
+				all = append(all, arg)
 			}
 		}
-		all = append(all, addrs...)
 	}
 	config.ListenHosts = all
 	return nil
diff --git a/plugin/bind/setup_test.go b/plugin/bind/setup_test.go
index 556aa1ea2..b0cf1087b 100644
--- a/plugin/bind/setup_test.go
+++ b/plugin/bind/setup_test.go
@@ -19,6 +19,7 @@ func TestSetup(t *testing.T) {
 		{`bind 1.2.3.4 ::5`, []string{"1.2.3.4", "::5"}, false},
 		{`bind ::1 1.2.3.4 ::5 127.9.9.0`, []string{"::1", "1.2.3.4", "::5", "127.9.9.0"}, false},
 		{`bind ::1 1.2.3.4 ::5 127.9.9.0 noone`, nil, true},
+		{`bind 1.2.3.4 lo`, []string{"1.2.3.4", "127.0.0.1", "::1"}, false},
 	} {
 		c := caddy.NewTestController("dns", test.config)
 		err := setup(c)