dhcpv4: add a client method to facilitate lease renewals

Signed-off-by: Dennis Marttinen <twelho@welho.tech>

5 files changed, 73 insertions, 1 deletions

diff --git a/dhcpv4/dhcpv4.go b/dhcpv4/dhcpv4.go
index 900bb22..e2fd190 100644
--- a/dhcpv4/dhcpv4.go
+++ b/dhcpv4/dhcpv4.go
@@ -231,6 +231,7 @@ func NewInform(hwaddr net.HardwareAddr, localIP net.IP, modifiers ...Modifier) (
 }
 
 // NewRequestFromOffer builds a DHCPv4 request from an offer.
+// It assumes the SELECTING state by default, see Section 4.3.2 in RFC 2131 for more details.
 func NewRequestFromOffer(offer *DHCPv4, modifiers ...Modifier) (*DHCPv4, error) {
 	return New(PrependModifiers(modifiers,
 		WithReply(offer),
@@ -248,6 +249,21 @@ func NewRequestFromOffer(offer *DHCPv4, modifiers ...Modifier) (*DHCPv4, error)
 	)...)
 }
 
+// NewRenewFromOffer builds a DHCPv4 RENEW-style request from an offer. RENEW requests have minor
+// changes to their options compared to SELECT requests as specified by RFC 2131, section 4.3.2.
+func NewRenewFromOffer(offer *DHCPv4, modifiers ...Modifier) (*DHCPv4, error) {
+	return NewRequestFromOffer(offer, PrependModifiers(modifiers,
+		// The server identifier option must not be filled in
+		WithoutOption(OptionServerIdentifier),
+		// The requested IP address must not be filled in
+		WithoutOption(OptionRequestedIPAddress),
+		// The client IP must be filled in with the IP offered to the client
+		WithClientIP(offer.YourIPAddr),
+		// The renewal request must use unicast
+		WithBroadcast(false),
+	)...)
+}
+
 // NewReplyFromRequest builds a DHCPv4 reply from a request.
 func NewReplyFromRequest(request *DHCPv4, modifiers ...Modifier) (*DHCPv4, error) {
 	return New(PrependModifiers(modifiers,
diff --git a/dhcpv4/nclient4/client.go b/dhcpv4/nclient4/client.go
index 02bd1f6..d40e1a9 100644
--- a/dhcpv4/nclient4/client.go
+++ b/dhcpv4/nclient4/client.go
@@ -493,6 +493,7 @@ func (e *ErrNak) Error() string {
 }
 
 // RequestFromOffer sends a Request message and waits for an response.
+// It assumes the SELECTING state by default, see Section 4.3.2 in RFC 2131 for more details.
 func (c *Client) RequestFromOffer(ctx context.Context, offer *dhcpv4.DHCPv4, modifiers ...dhcpv4.Modifier) (*Lease, error) {
 	// TODO(chrisko): should this be unicast to the server?
 	request, err := dhcpv4.NewRequestFromOffer(offer, dhcpv4.PrependModifiers(modifiers,
diff --git a/dhcpv4/nclient4/lease.go b/dhcpv4/nclient4/lease.go
index 184fae2..1895dd0 100644
--- a/dhcpv4/nclient4/lease.go
+++ b/dhcpv4/nclient4/lease.go
@@ -3,6 +3,7 @@
 package nclient4
 
 import (
+	"context"
 	"fmt"
 	"net"
 	"time"
@@ -36,3 +37,41 @@ func (c *Client) Release(lease *Lease, modifiers ...dhcpv4.Modifier) error {
 	}
 	return err
 }
+
+// Renew sends a DHCPv4 request to the server to renew the given lease. The renewal information is
+// sourced from the initial offer in the lease, and the ACK of the lease is updated to the ACK of
+// the latest renewal. This avoids issues with DHCP servers that omit information needed to build a
+// completely new lease from their renewal ACK (such as the Windows DHCP Server).
+func (c *Client) Renew(ctx context.Context, lease *Lease, modifiers ...dhcpv4.Modifier) error {
+	if lease == nil {
+		return fmt.Errorf("lease is nil")
+	}
+
+	request, err := dhcpv4.NewRenewFromOffer(lease.Offer, dhcpv4.PrependModifiers(modifiers,
+		dhcpv4.WithOption(dhcpv4.OptMaxMessageSize(MaxMessageSize)))...)
+	if err != nil {
+		return fmt.Errorf("unable to create a request: %w", err)
+	}
+
+	// Servers are supposed to only respond to Requests containing their server identifier,
+	// but sometimes non-compliant servers respond anyway.
+	// Clients are not required to validate this field, but servers are required to
+	// include the server identifier in their Offer per RFC 2131 Section 4.3.1 Table 3.
+	response, err := c.SendAndRead(ctx, c.serverAddr, request, IsAll(
+		IsCorrectServer(lease.Offer.ServerIdentifier()),
+		IsMessageType(dhcpv4.MessageTypeAck, dhcpv4.MessageTypeNak)))
+	if err != nil {
+		return fmt.Errorf("got an error while processing the request: %w", err)
+	}
+	if response.MessageType() == dhcpv4.MessageTypeNak {
+		return &ErrNak{
+			Offer: lease.Offer,
+			Nak:   response,
+		}
+	}
+
+	// Update the ACK of the lease with the ACK of the latest renewal
+	lease.ACK = response
+
+	return nil
+}
diff --git a/dhcpv4/nclient4/lease_test.go b/dhcpv4/nclient4/lease_test.go
index 361847f..d27eeca 100644
--- a/dhcpv4/nclient4/lease_test.go
+++ b/dhcpv4/nclient4/lease_test.go
@@ -1,4 +1,4 @@
-// this tests nclient4 with lease and release
+// this tests nclient4 with lease, renew and release
 
 package nclient4
 
@@ -236,6 +236,14 @@ func (sll *testServerLeaseList) runTest(t *testing.T) {
 		sll.lastTestSvrErrLock.RLock()
 		keepgoing := chkerr(err, sll.lastTestSvrErr, l.ShouldFail, t)
 		sll.lastTestSvrErrLock.RUnlock()
+
+		if keepgoing {
+			err = clnt.Renew(context.Background(), lease)
+			sll.lastTestSvrErrLock.RLock()
+			keepgoing = chkerr(err, sll.lastTestSvrErr, l.ShouldFail, t)
+			sll.lastTestSvrErrLock.RUnlock()
+		}
+
 		if keepgoing {
 			err = clnt.Release(lease)
 			//this sleep is to make sure release is handled by server
diff --git a/dhcpv4/server4/server_test.go b/dhcpv4/server4/server_test.go
index b3f6d9d..7be868c 100644
--- a/dhcpv4/server4/server_test.go
+++ b/dhcpv4/server4/server_test.go
@@ -115,6 +115,14 @@ func TestServer(t *testing.T) {
 		require.Equal(t, xid, p.TransactionID)
 		require.Equal(t, ifaces[0].HardwareAddr, p.ClientHWAddr)
 	}
+
+	err = c.Renew(context.Background(), lease, modifiers...)
+	require.NoError(t, err)
+	require.NotNil(t, lease.Offer, lease.ACK)
+	for _, p := range []*dhcpv4.DHCPv4{lease.Offer, lease.ACK} {
+		require.Equal(t, xid, p.TransactionID)
+		require.Equal(t, ifaces[0].HardwareAddr, p.ClientHWAddr)
+	}
 }
 
 func TestBadAddrFamily(t *testing.T) {