Re-Add DHCP discover fallback for ISPs that don't advertise DHCP

Add the domain as it is needed (multiple domains on a home net is niche)
Only replace the record if a local one was found
Use proper slice updating

internal/dhcp4/dhcp4.go

@@ -52,6 +52,8 @@ type Client struct {
 	timeNow      func() time.Time
 	generateXID  func() uint32
 
+	timeoutCount int
+
 	// last DHCPACK packet for renewal/release
 	Ack *layers.DHCPv4
 }
@@ -129,7 +131,18 @@ func (c *Client) ObtainOrRenew() bool {
 	ack, err := c.dhcpRequest()
 	if err != nil {
 		if errno, ok := err.(syscall.Errno); ok && errno == syscall.EAGAIN {
-			c.err = fmt.Errorf("DHCP: timeout (server(s) unreachable)")
+			var serverip net.IP
+			for _, opt := range c.Ack.Options {
+				if opt.Type == layers.DHCPOptServerID {
+					serverip = opt.Data
+				}
+			}
+			c.err = fmt.Errorf("DHCP: timeout (server(s) unreachable: %v)", serverip)
+			c.timeoutCount++
+			if c.timeoutCount > 3 {
+				c.timeoutCount = 0
+				c.Ack = nil // start over at DHCPDISCOVER it has failed 3 times
+			}
 			return true // temporary error
 		}
 		if err == errNAK {
@@ -154,6 +167,7 @@ func (c *Client) ObtainOrRenew() bool {
 		}
 	}
 	c.cfg.RenewAfter = c.timeNow().Add(lease.RenewalTime)
+	c.timeoutCount = 0
 	return true
 }
 

internal/dns/dns.go

@@ -546,15 +546,15 @@ func (s *Server) handleRequest(w dns.ResponseWriter, r *dns.Msg) {
 		}
 		if len(in.Answer) > 1 {
 			if in.Answer[0].Header().Rrtype == dns.TypeCNAME {
-				for _, rr := range in.Answer {
-					if rr.Header().Rrtype == dns.TypeA {
-						if newRR, err := s.resolveSubname("", dns.Question{strings.ToLower(rr.Header().Name), dns.TypeA, dns.ClassINET}); err == nil {
-							in.Answer[len(in.Answer)-1] = newRR
+				for i, rr := range in.Answer {
+					if rr != nil && rr.Header() != nil && rr.Header().Rrtype == dns.TypeA {
+						newRR, err := s.resolveSubname(string(s.domain), dns.Question{strings.ToLower(rr.Header().Name), dns.TypeA, dns.ClassINET})
+						if err == nil && newRR != nil {
+							in.Answer[i] = newRR
 						}
 					}
 				}
 			}
-
 		}
 		w.WriteMsg(in)
 		if idx > 0 {