diff --git a/dhcpv4/dhcpv4.go b/dhcpv4/dhcpv4.go
index 1d8ddef1..0bfea480 100644
--- a/dhcpv4/dhcpv4.go
+++ b/dhcpv4/dhcpv4.go
@@ -262,7 +262,11 @@ func NewReplyFromRequest(request *DHCPv4, modifiers ...Modifier) (*DHCPv4, error
 	return New(PrependModifiers(modifiers,
 		WithReply(request),
 		WithGatewayIP(request.GatewayIPAddr),
-		WithRelayAgentInfo(request),
+		WithOptionCopied(request, OptionRelayAgentInformation),
+
+		// RFC 6842 states the Client Identifier option must be copied
+		// from the request if a client specified it.
+		WithOptionCopied(request, OptionClientIdentifier),
 	)...)
 }
 
diff --git a/dhcpv4/modifiers.go b/dhcpv4/modifiers.go
index 2bf79b83..0ab35bc5 100644
--- a/dhcpv4/modifiers.go
+++ b/dhcpv4/modifiers.go
@@ -43,18 +43,16 @@ func WithGatewayIP(ip net.IP) Modifier {
 	}
 }
 
-// WithRelayAgentInfo copies the relay options from the request to the reply.
-func WithRelayAgentInfo(request *DHCPv4) Modifier {
+// WithOptionCopied copies the value of option opt from request.
+func WithOptionCopied(request *DHCPv4, opt OptionCode) Modifier {
 	return func(d *DHCPv4) {
-		// If request has Relay Agent Info copy it to the reply
-		if relayOpt := request.RelayAgentInfo(); relayOpt != nil {
-			d.UpdateOption(Option{Code: OptionRelayAgentInformation, Value: relayOpt})
+		if val := request.Options.Get(opt); val != nil {
+			d.UpdateOption(OptGeneric(opt, val))
 		}
 	}
 }
 
-// WithReply fills in opcode, hwtype, xid, clienthwaddr, flags, and gateway ip
-// addr from the given packet.
+// WithReply fills in opcode, hwtype, xid, clienthwaddr, and flags from the given packet.
 func WithReply(request *DHCPv4) Modifier {
 	return func(d *DHCPv4) {
 		if request.OpCode == OpcodeBootRequest {
diff --git a/dhcpv4/option_misc.go b/dhcpv4/option_misc.go
index 01de95f7..e91b34d8 100644
--- a/dhcpv4/option_misc.go
+++ b/dhcpv4/option_misc.go
@@ -16,3 +16,8 @@ func OptDomainSearch(labels *rfc1035label.Labels) Option {
 func OptClientArch(archs ...iana.Arch) Option {
 	return Option{Code: OptionClientSystemArchitectureType, Value: iana.Archs(archs)}
 }
+
+// OptClientIdentifier returns a new Client Identifier option.
+func OptClientIdentifier(ident []byte) Option {
+	return OptGeneric(OptionClientIdentifier, ident)
+}