fix: update DHCP client to use Request-Ack sequence after an Offer

The problem was that our DHCP client was always doing
Discover-Offer-Request-Ack sequence as if it doesn't have any IP lease
which was breaking some DHCP servers leading to an error which in turn
makes Talos retry harder hitting the same error over and over again.

Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>

internal/app/networkd/pkg/address/dhcp.go

@@ -25,6 +25,7 @@ const dhcpReceivedRouteMetric uint32 = 1024
 
 // DHCP implements the Addressing interface.
 type DHCP struct {
+	Offer       *dhcpv4.DHCPv4
 	Ack         *dhcpv4.DHCPv4
 	NetIf       *net.Interface
 	DHCPOptions config.DHCPOptions
@@ -46,11 +47,7 @@ func (d *DHCP) Link() *net.Interface {
 // Discover handles the DHCP client exchange stores the DHCP Ack.
 func (d *DHCP) Discover(ctx context.Context, link *net.Interface) error {
 	d.NetIf = link
-	ack, err := d.discover(ctx)
-
-	if ack != nil {
-		d.Ack = ack
-	}
+	err := d.discover(ctx)
 
 	return err
 }
@@ -193,7 +190,7 @@ func (d *DHCP) Hostname() (hostname string) {
 }
 
 // discover handles the actual DHCP conversation.
-func (d *DHCP) discover(ctx context.Context) (*dhcpv4.DHCPv4, error) {
+func (d *DHCP) discover(ctx context.Context) error {
 	opts := []dhcpv4.OptionCode{
 		dhcpv4.OptionClasslessStaticRoute,
 		dhcpv4.OptionDomainNameServer,
@@ -219,23 +216,36 @@ func (d *DHCP) discover(ctx context.Context) (*dhcpv4.DHCPv4, error) {
 	// debug logging option
 	cli, err := nclient4.New(d.NetIf.Name)
 	if err != nil {
-		return nil, err
+		return err
 	}
 
 	// nolint: errcheck
 	defer cli.Close()
 
-	lease, err := cli.Request(ctx, mods...)
+	var lease *nclient4.Lease
+
+	if d.Offer != nil {
+		lease, err = cli.RequestFromOffer(ctx, d.Offer, mods...)
+	} else {
+		lease, err = cli.Request(ctx, mods...)
+	}
+
 	if err != nil {
 		// TODO: Make this a well defined error so we can make it not fatal
 		log.Printf("failed dhcp request for %q: %v", d.NetIf.Name, err)
 
-		return nil, err
+		// clear offer if request fails to start with discover sequence next time
+		d.Offer = nil
+
+		return err
 	}
 
 	log.Printf("DHCP ACK on %q: %s", d.NetIf.Name, collapseSummary(lease.ACK.Summary()))
 
-	return lease.ACK, err
+	d.Ack = lease.ACK
+	d.Offer = lease.Offer
+
+	return err
 }
 
 func collapseSummary(summary string) string {