f-46: dhcpd: build dhcp replies over insomniacslk/dhcp #46

internal/dhcpd décide quelles options partent vers quelle MAC, sans I/O ni
état. L'encodage RFC 3442 de l'option 121 est délégué à la bibliothèque, qui le
teste elle-même.

L'option 3 est omise plutôt qu'envoyée vide : la dhcp-option=3 nue n'existait
que pour empêcher dnsmasq d'annoncer sa propre adresse. La route par défaut est
conditionnée au subnet (DefaultGateway) et à l'interface (Host.DefaultRoute),
ce qui remplace le mécanisme de tags.

24 tests plus un fuzz sur BuildReply. Les assertions d'encodage sont validées
par mutation.

Signed-off-by: GnomeZworc <nicolas.boufidjeline@g3e.fr>

internal/dhcpd/reply.go

@@ -0,0 +1,97 @@
+package dhcpd
+
+import (
+	"errors"
+	"fmt"
+	"net"
+
+	"git.g3e.fr/syonad/two/internal/metadata"
+
+	"github.com/insomniacslk/dhcp/dhcpv4"
+)
+
+var (
+	ErrNoInterfaceIP = errors.New("interface ip is required: guests would have no route to the metadata server")
+	ErrNoNetwork     = errors.New("subnet network is required")
+	ErrNoHostIP      = errors.New("host ip is required")
+	ErrNoRequest     = errors.New("request is nil")
+)
+
+func metadataRoute() *net.IPNet {
+	return &net.IPNet{
+		IP:   net.ParseIP(metadata.ServiceIP).To4(),
+		Mask: net.CIDRMask(32, 32),
+	}
+}
+
+func defaultRoute() *net.IPNet {
+	return &net.IPNet{
+		IP:   net.IPv4zero.To4(),
+		Mask: net.CIDRMask(0, 32),
+	}
+}
+
+func Routes(c SubnetConfig, h Host) (dhcpv4.Routes, error) {
+	if c.InterfaceIP == nil {
+		return nil, ErrNoInterfaceIP
+	}
+
+	routes := dhcpv4.Routes{{Dest: metadataRoute(), Router: c.InterfaceIP}}
+
+	if c.VPCRoute != nil {
+		routes = append(routes, &dhcpv4.Route{Dest: c.VPCRoute, Router: c.InterfaceIP})
+	}
+	if h.DefaultRoute && c.DefaultGateway != nil {
+		routes = append(routes, &dhcpv4.Route{Dest: defaultRoute(), Router: c.DefaultGateway})
+	}
+	return routes, nil
+}
+
+func replyType(req *dhcpv4.DHCPv4) (dhcpv4.MessageType, error) {
+	switch req.MessageType() {
+	case dhcpv4.MessageTypeDiscover:
+		return dhcpv4.MessageTypeOffer, nil
+	case dhcpv4.MessageTypeRequest:
+		return dhcpv4.MessageTypeAck, nil
+	default:
+		return 0, fmt.Errorf("no reply built for message type %s", req.MessageType())
+	}
+}
+
+func BuildReply(c SubnetConfig, h Host, req *dhcpv4.DHCPv4) (*dhcpv4.DHCPv4, error) {
+	if req == nil {
+		return nil, ErrNoRequest
+	}
+	if c.Network == nil {
+		return nil, ErrNoNetwork
+	}
+	if h.IP == nil {
+		return nil, ErrNoHostIP
+	}
+
+	kind, err := replyType(req)
+	if err != nil {
+		return nil, err
+	}
+
+	routes, err := Routes(c, h)
+	if err != nil {
+		return nil, err
+	}
+
+	mods := []dhcpv4.Modifier{
+		dhcpv4.WithMessageType(kind),
+		dhcpv4.WithServerIP(c.InterfaceIP),
+		dhcpv4.WithYourIP(h.IP),
+		dhcpv4.WithNetmask(c.Network.Mask),
+		dhcpv4.WithLeaseTime(uint32(LeaseTime.Seconds())),
+		dhcpv4.WithOption(dhcpv4.OptServerIdentifier(c.InterfaceIP)),
+		dhcpv4.WithOption(dhcpv4.OptDNS(DNSServers()...)),
+		dhcpv4.WithOption(dhcpv4.OptClasslessStaticRoute(routes...)),
+	}
+	if h.DefaultRoute && c.DefaultGateway != nil {
+		mods = append(mods, dhcpv4.WithOption(dhcpv4.OptRouter(c.DefaultGateway)))
+	}
+
+	return dhcpv4.NewReplyFromRequest(req, mods...)
+}