update listeners when netlink reports an address change

related to issue #21

gokrazy.go

@@ -17,6 +17,7 @@ import (
 	"os/exec"
 	"os/signal"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/gokrazy/gokrazy/internal/iface"
@@ -145,6 +146,30 @@ func Supervise(commands []*exec.Cmd) error {
 		return fmt.Errorf("updating listeners: %v", err)
 	}
 
+	if nl, err := listenNetlink(); err == nil {
+		go func() {
+			for {
+				msgs, err := nl.ReadMsgs()
+				if err != nil {
+					log.Printf("netlink.ReadMsgs: %v", err)
+					return
+				}
+
+				for _, m := range msgs {
+					if m.Header.Type != syscall.RTM_NEWADDR &&
+						m.Header.Type != syscall.RTM_DELADDR {
+						continue
+					}
+					if err := updateListeners("80"); err != nil {
+						log.Printf("updating listeners: %v", err)
+					}
+				}
+			}
+		}()
+	} else {
+		log.Printf("cannot listen for new IP addresses: %v", err)
+	}
+
 	go func() {
 		c := make(chan os.Signal, 1)
 		signal.Notify(c, unix.SIGHUP)

netlink.go

@@ -0,0 +1,49 @@
+package gokrazy
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+type netlinkListener struct {
+	fd  int
+	buf []byte
+}
+
+func listenNetlink() (*netlinkListener, error) {
+	fd, err := syscall.Socket(syscall.AF_NETLINK, syscall.SOCK_DGRAM, syscall.NETLINK_ROUTE)
+	if err != nil {
+		return nil, fmt.Errorf("socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE): %v", err)
+	}
+
+	saddr := &syscall.SockaddrNetlink{
+		Family: syscall.AF_NETLINK,
+		Groups: (1 << (syscall.RTNLGRP_IPV4_IFADDR - 1)) |
+			(1 << (syscall.RTNLGRP_IPV6_IFADDR - 1)),
+	}
+
+	if err := syscall.Bind(fd, saddr); err != nil {
+		return nil, fmt.Errorf("bind: %v", err)
+	}
+
+	return &netlinkListener{
+		fd: fd,
+		// use the page size as buffer size, like libnl
+		buf: make([]byte, os.Getpagesize()),
+	}, nil
+}
+
+func (l *netlinkListener) ReadMsgs() ([]syscall.NetlinkMessage, error) {
+	n, err := syscall.Read(l.fd, l.buf)
+	if err != nil {
+		return nil, fmt.Errorf("Read: %v", err)
+	}
+
+	msgs, err := syscall.ParseNetlinkMessage(l.buf[:n])
+	if err != nil {
+		return nil, fmt.Errorf("ParseNetlinkMessage: %v", err)
+	}
+
+	return msgs, nil
+}