From 719ef16b93e149d18558401aa87a7544b2b1cd31 Mon Sep 17 00:00:00 2001
From: Simon Zolin <s.zolin@adguard.com>
Date: Mon, 24 Aug 2020 13:01:55 +0300
Subject: [PATCH] * DHCP: don't replace the host name from static lease

When a static lease contains a host name
 and client sends its own host name:

1. don't replace the host name from static lease with it
2. send option FQDN to the client in Ack response packet
---
 dhcpd/dhcpd.go |  1 -
 dhcpd/v4.go    | 51 +++++++++++++++++++++++++++++++++++++++-----------
 2 files changed, 40 insertions(+), 12 deletions(-)

diff --git a/dhcpd/dhcpd.go b/dhcpd/dhcpd.go
index f6e19d5a..ad109b2f 100644
--- a/dhcpd/dhcpd.go
+++ b/dhcpd/dhcpd.go
@@ -51,7 +51,6 @@ const (
 	LeaseChangedAdded = iota
 	LeaseChangedAddedStatic
 	LeaseChangedRemovedStatic
-	LeaseChangedBlacklisted
 
 	LeaseChangedDBStore
 )
diff --git a/dhcpd/v4.go b/dhcpd/v4.go
index ac0f9808..b283079a 100644
--- a/dhcpd/v4.go
+++ b/dhcpd/v4.go
@@ -365,8 +365,6 @@ func (s *v4Server) processDiscover(req *dhcpv4.DHCPv4, resp *dhcpv4.DHCPv4) *Lea
 
 		s.conf.notify(LeaseChangedDBStore)
 
-		// s.conf.notify(LeaseChangedBlacklisted)
-
 	} else {
 		reqIP := req.Options.Get(dhcpv4.OptionRequestedIPAddress)
 		if len(reqIP) != 0 &&
@@ -375,13 +373,40 @@ func (s *v4Server) processDiscover(req *dhcpv4.DHCPv4, resp *dhcpv4.DHCPv4) *Lea
 		}
 	}
 
-	hostname := req.Options.Get(dhcpv4.OptionHostName)
-	lease.Hostname = string(hostname)
-
 	resp.UpdateOption(dhcpv4.OptMessageType(dhcpv4.MessageTypeOffer))
 	return lease
 }
 
+type optFQDN struct {
+	name string
+}
+
+func (o *optFQDN) String() string {
+	return "optFQDN"
+}
+
+// flags[1]
+// A-RR[1]
+// PTR-RR[1]
+// name[]
+func (o *optFQDN) ToBytes() []byte {
+	b := make([]byte, 3+len(o.name))
+	i := 0
+
+	b[i] = 0x03 // f_server_overrides | f_server
+	i++
+
+	b[i] = 255 // A-RR
+	i++
+
+	b[i] = 255 // PTR-RR
+	i++
+
+	copy(b[i:], []byte(o.name))
+	return b
+
+}
+
 // Process Request request and return lease
 // Return false if we don't need to reply
 func (s *v4Server) processRequest(req *dhcpv4.DHCPv4, resp *dhcpv4.DHCPv4) (*Lease, bool) {
@@ -414,12 +439,6 @@ func (s *v4Server) processRequest(req *dhcpv4.DHCPv4, resp *dhcpv4.DHCPv4) (*Lea
 				return nil, true
 			}
 
-			if !bytes.Equal([]byte(l.Hostname), hostname) {
-				s.leasesLock.Unlock()
-				log.Debug("DHCPv4: Mismatched OptionHostName in Request message for %s", mac)
-				return nil, true
-			}
-
 			lease = l
 			break
 		}
@@ -432,7 +451,17 @@ func (s *v4Server) processRequest(req *dhcpv4.DHCPv4, resp *dhcpv4.DHCPv4) (*Lea
 	}
 
 	if lease.Expiry.Unix() != leaseExpireStatic {
+		lease.Hostname = string(hostname)
 		s.commitLease(lease)
+	} else if len(lease.Hostname) != 0 {
+		o := &optFQDN{
+			name: string(lease.Hostname),
+		}
+		fqdn := dhcpv4.Option{
+			Code:  dhcpv4.OptionFQDN,
+			Value: o,
+		}
+		resp.UpdateOption(fqdn)
 	}
 
 	resp.UpdateOption(dhcpv4.OptMessageType(dhcpv4.MessageTypeAck))