From 2ef8bf6ea0ad1673f6ba7383e9dc7c5acb2a09ce Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <stapelberg@google.com>
Date: Mon, 4 Jun 2018 08:02:28 +0200
Subject: [PATCH] dnsd/dhcp4d: listen on configured address (not hard-coded)

---
 cmd/dhcp4d/dhcp4d.go            |  5 ++++-
 cmd/dnsd/dnsd.go                | 10 +++++++---
 internal/dhcp4d/dhcp4d.go       | 19 ++++++++++++++-----
 internal/dhcp4d/dhcp4d_test.go  | 32 +++++++++++++++++++++++++++++++-
 internal/netconfig/netconfig.go | 22 ++++++++++++++++++++++
 5 files changed, 78 insertions(+), 10 deletions(-)

diff --git a/cmd/dhcp4d/dhcp4d.go b/cmd/dhcp4d/dhcp4d.go
index 2a1388e..6936196 100644
--- a/cmd/dhcp4d/dhcp4d.go
+++ b/cmd/dhcp4d/dhcp4d.go
@@ -22,7 +22,10 @@ func logic() error {
 		return err
 	}
 	errs := make(chan error)
-	handler := dhcp4d.NewHandler()
+	handler, err := dhcp4d.NewHandler("/perm")
+	if err != nil {
+		return err
+	}
 	handler.Leases = func(leases []*dhcp4d.Lease) {
 		b, err := json.Marshal(leases)
 		if err != nil {
diff --git a/cmd/dnsd/dnsd.go b/cmd/dnsd/dnsd.go
index 82a44fe..ce0a50a 100644
--- a/cmd/dnsd/dnsd.go
+++ b/cmd/dnsd/dnsd.go
@@ -12,12 +12,16 @@ import (
 
 	"router7/internal/dhcp4d"
 	"router7/internal/dns"
+	"router7/internal/netconfig"
 )
 
 func logic() error {
-	// TODO: serve on correct IP address
 	// TODO: set correct upstream DNS resolver(s)
-	srv := dns.NewServer("192.168.42.1:53", "lan")
+	ip, err := netconfig.LinkAddress("/perm", "lan0")
+	if err != nil {
+		return err
+	}
+	srv := dns.NewServer(ip.String()+":53", "lan")
 	readLeases := func() error {
 		b, err := ioutil.ReadFile("/perm/dhcp4d/leases.json")
 		if err != nil {
@@ -31,7 +35,7 @@ func logic() error {
 		return nil
 	}
 	if err := readLeases(); err != nil {
-		log.Printf("readLeases: %v", err)
+		log.Printf("cannot resolve DHCP hostnames: %v", err)
 	}
 	ch := make(chan os.Signal, 1)
 	signal.Notify(ch, syscall.SIGUSR1)
diff --git a/internal/dhcp4d/dhcp4d.go b/internal/dhcp4d/dhcp4d.go
index 6cc4199..0fb8fe9 100644
--- a/internal/dhcp4d/dhcp4d.go
+++ b/internal/dhcp4d/dhcp4d.go
@@ -7,6 +7,8 @@ import (
 	"net"
 	"time"
 
+	"router7/internal/netconfig"
+
 	"github.com/krolaw/dhcp4"
 )
 
@@ -31,14 +33,21 @@ type Handler struct {
 }
 
 // TODO: restore leases from permanent storage
-func NewHandler() *Handler {
-	serverIP := net.IP{192, 168, 42, 1} // TODO: customizeable
+func NewHandler(dir string) (*Handler, error) {
+	serverIP, err := netconfig.LinkAddress(dir, "lan0")
+	if err != nil {
+		return nil, err
+	}
+	serverIP = serverIP.To4()
+	start := make(net.IP, len(serverIP))
+	copy(start, serverIP)
+	start[len(start)-1] += 1
 	return &Handler{
 		leasesHW:    make(map[string]*Lease),
 		leasesIP:    make(map[int]*Lease),
 		serverIP:    serverIP,
-		start:       net.IP{192, 168, 42, 2},
-		leaseRange:  50,
+		start:       start,
+		leaseRange:  200,
 		leasePeriod: 2 * time.Hour,
 		options: dhcp4.Options{
 			dhcp4.OptionSubnetMask:       []byte{255, 255, 255, 0},
@@ -47,7 +56,7 @@ func NewHandler() *Handler {
 			dhcp4.OptionDomainName:       []byte("lan"),
 			dhcp4.OptionDomainSearch:     []byte{0x03, 'l', 'a', 'n', 0x00},
 		},
-	}
+	}, nil
 }
 
 func (h *Handler) findLease() int {
diff --git a/internal/dhcp4d/dhcp4d_test.go b/internal/dhcp4d/dhcp4d_test.go
index bfc935f..3e42a1f 100644
--- a/internal/dhcp4d/dhcp4d_test.go
+++ b/internal/dhcp4d/dhcp4d_test.go
@@ -2,19 +2,49 @@ package dhcp4d
 
 import (
 	"bytes"
+	"io/ioutil"
 	"net"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/krolaw/dhcp4"
 )
 
+const goldenInterfaces = `
+{
+  "interfaces":[
+    {
+      "hardware_addr": "02:73:53:00:ca:fe",
+      "name": "uplink0"
+    },
+    {
+      "hardware_addr": "02:73:53:00:b0:0c",
+      "name": "lan0",
+      "addr": "192.168.42.1/24"
+    }
+  ]
+}
+`
+
 func TestLease(t *testing.T) {
+	tmpdir, err := ioutil.TempDir("", "dhcp4dtest")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+	if err := ioutil.WriteFile(filepath.Join(tmpdir, "interfaces.json"), []byte(goldenInterfaces), 0644); err != nil {
+		t.Fatal(err)
+	}
 	var (
 		addr         = net.IP{192, 168, 42, 23}
 		hardwareAddr = net.HardwareAddr{0x11, 0x22, 0x33, 0x44, 0x55, 0x66}
 		hostname     = "xps"
 	)
-	handler := NewHandler()
+	handler, err := NewHandler(tmpdir)
+	if err != nil {
+		t.Fatal(err)
+	}
 	leasedCalled := false
 	handler.Leases = func(leases []*Lease) {
 		if got, want := len(leases), 1; got != want {
diff --git a/internal/netconfig/netconfig.go b/internal/netconfig/netconfig.go
index f6c0109..9fc32c3 100644
--- a/internal/netconfig/netconfig.go
+++ b/internal/netconfig/netconfig.go
@@ -156,6 +156,28 @@ type InterfaceConfig struct {
 	Interfaces []InterfaceDetails `json:"interfaces"`
 }
 
+// LinkAddress returns the IP address configured for the interface ifname in
+// interfaces.json.
+func LinkAddress(dir, ifname string) (net.IP, error) {
+	fn := filepath.Join(dir, "interfaces.json")
+	b, err := ioutil.ReadFile(fn)
+	if err != nil {
+		return nil, err
+	}
+	var cfg InterfaceConfig
+	if err := json.Unmarshal(b, &cfg); err != nil {
+		return nil, err
+	}
+	for _, details := range cfg.Interfaces {
+		if details.Name != ifname {
+			continue
+		}
+		ip, _, err := net.ParseCIDR(details.Addr)
+		return ip, err
+	}
+	return nil, fmt.Errorf("%s does not configure interface %q", fn, ifname)
+}
+
 func applyInterfaces(dir, root string) error {
 	b, err := ioutil.ReadFile(filepath.Join(dir, "interfaces.json"))
 	if err != nil {