add randomd: carries entropy across system restarts

As practiced by Debian (and others) and recommended in the urandom(4) manpage:
https://manpages.debian.org/stretch/manpages/urandom.4.en.html#Configuration

cmd/randomd/randomd.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"io"
+	"io/ioutil"
+	"os"
+	"os/signal"
+	"router7/internal/teelogger"
+	"syscall"
+	"time"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+var log = teelogger.NewConsole()
+
+func seed() error {
+	sf, err := os.OpenFile("/perm/random.seed", os.O_RDWR|os.O_CREATE, 0600)
+	if err != nil {
+		return err
+	}
+	defer sf.Close()
+
+	rf, err := os.OpenFile("/dev/urandom", os.O_RDWR, 0600)
+	if err != nil {
+		return err
+	}
+	defer rf.Close()
+
+	seed := make([]byte, 512)
+	if _, err := sf.Read(seed); err != nil {
+		// fall back to seeding with the current time
+		seed = []byte(time.Now().String())
+	} else {
+		log.Printf("seeding RNG from /perm/random.seed")
+	}
+
+	entropy := struct {
+		entropyCount int64
+		bufSize      int64
+		buf          [512]byte
+	}{
+		entropyCount: int64(len(seed) * 8),
+		bufSize:      int64(len(seed)),
+	}
+	copy(entropy.buf[:], seed)
+	const RNDADDENTROPY = 0x40085203
+	if _, _, errno := unix.Syscall(unix.SYS_IOCTL, rf.Fd(), RNDADDENTROPY, uintptr(unsafe.Pointer(&entropy))); errno != 0 {
+		return errno
+	}
+
+	// Overwrite the seed (re-using a seed compromises security):
+	if _, err := sf.Seek(0, io.SeekStart); err != nil {
+		return err
+	}
+
+	if _, err := io.CopyN(sf, rf, 512); err != nil {
+		return err
+	}
+
+	return sf.Close()
+}
+
+func saveSeed() error {
+	rf, err := os.OpenFile("/dev/urandom", os.O_RDWR, 0600)
+	if err != nil {
+		return err
+	}
+	defer rf.Close()
+
+	seed := make([]byte, 512)
+	if _, err := rf.Read(seed); err != nil {
+		return err
+	}
+	return ioutil.WriteFile("/perm/random.seed", seed, 0600)
+}
+
+func main() {
+	if err := seed(); err != nil {
+		log.Printf("seeding random pool failed: %v", err)
+	}
+
+	// Wait for SIGTERM to store new randomness seed on /perm
+	ch := make(chan os.Signal, 1)
+	signal.Notify(ch, syscall.SIGTERM)
+	<-ch
+
+	if err := saveSeed(); err != nil {
+		log.Printf("persisting random seed failed: %v", err)
+	}
+}