Implement ECMP for unsafe_routes (#1332)

Author: dioss-Machiel

Diff for: examples/config.yml

@@ -239,7 +239,28 @@ tun:
 
   # Unsafe routes allows you to route traffic over nebula to non-nebula nodes
   # Unsafe routes should be avoided unless you have hosts/services that cannot run nebula
-  # NOTE: The nebula certificate of the "via" node *MUST* have the "route" defined as a subnet in its certificate
+  # Supports weighted ECMP if you define a list of gateways, this can be used for load balancing or redundancy to hosts outside of nebula
+  # NOTES:
+  # * You will only see a single gateway in the routing table if you are not on linux
+  # * If a gateway is not reachable through the overlay another gateway will be selected to send the traffic through, ignoring weights
+  #
+  # unsafe_routes:
+  # # Multiple gateways without defining a weight defaults to a weight of 1, this will balance traffic equally between the three gateways
+  # - route: 192.168.87.0/24
+  #   via:
+  #     - gateway: 10.0.0.1
+  #     - gateway: 10.0.0.2
+  #     - gateway: 10.0.0.3
+  # # Multiple gateways with a weight, this will balance traffic accordingly
+  # - route: 192.168.87.0/24
+  #   via:
+  #     - gateway: 10.0.0.1
+  #       weight: 10
+  #     - gateway: 10.0.0.2
+  #       weight: 5
+  #
+  # NOTE: The nebula certificate of the "via" node(s) *MUST* have the "route" defined as a subnet in its certificate
+  # `via`: single node or list of gateways to use for this route
   # `mtu`: will default to tun mtu if this option is not specified
   # `metric`: will default to 0 if this option is not specified
   # `install`: will default to true, controls whether this route is installed in the systems routing table.

Diff for: inside.go

@@ -8,6 +8,7 @@ import (
 	"github.com/slackhq/nebula/header"
 	"github.com/slackhq/nebula/iputil"
 	"github.com/slackhq/nebula/noiseutil"
+	"github.com/slackhq/nebula/routing"
 )
 
 func (f *Interface) consumeInsidePacket(packet []byte, fwPacket *firewall.Packet, nb, out []byte, q int, localCache firewall.ConntrackCache) {
@@ -49,7 +50,7 @@ func (f *Interface) consumeInsidePacket(packet []byte, fwPacket *firewall.Packet
 		return
 	}
 
-	hostinfo, ready := f.getOrHandshake(fwPacket.RemoteAddr, func(hh *HandshakeHostInfo) {
+	hostinfo, ready := f.getOrHandshakeConsiderRouting(fwPacket, func(hh *HandshakeHostInfo) {
 		hh.cachePacket(f.l, header.Message, 0, packet, f.sendMessageNow, f.cachedPacketMetrics)
 	})
 
@@ -121,22 +122,94 @@ func (f *Interface) rejectOutside(packet []byte, ci *ConnectionState, hostinfo *
 	f.sendNoMetrics(header.Message, 0, ci, hostinfo, netip.AddrPort{}, out, nb, packet, q)
 }
 
+// Handshake will attempt to initiate a tunnel with the provided vpn address if it is within our vpn networks. This is a no-op if the tunnel is already established or being established
 func (f *Interface) Handshake(vpnAddr netip.Addr) {
-	f.getOrHandshake(vpnAddr, nil)
+	f.getOrHandshakeNoRouting(vpnAddr, nil)
 }
 
-// getOrHandshake returns nil if the vpnAddr is not routable.
+// getOrHandshakeNoRouting returns nil if the vpnAddr is not routable.
 // If the 2nd return var is false then the hostinfo is not ready to be used in a tunnel
-func (f *Interface) getOrHandshake(vpnAddr netip.Addr, cacheCallback func(*HandshakeHostInfo)) (*HostInfo, bool) {
+func (f *Interface) getOrHandshakeNoRouting(vpnAddr netip.Addr, cacheCallback func(*HandshakeHostInfo)) (*HostInfo, bool) {
 	_, found := f.myVpnNetworksTable.Lookup(vpnAddr)
-	if !found {
-		vpnAddr = f.inside.RouteFor(vpnAddr)
-		if !vpnAddr.IsValid() {
-			return nil, false
+	if found {
+		return f.handshakeManager.GetOrHandshake(vpnAddr, cacheCallback)
+	}
+
+	return nil, false
+}
+
+// getOrHandshakeConsiderRouting will try to find the HostInfo to handle this packet, starting a handshake if necessary.
+// If the 2nd return var is false then the hostinfo is not ready to be used in a tunnel.
+func (f *Interface) getOrHandshakeConsiderRouting(fwPacket *firewall.Packet, cacheCallback func(*HandshakeHostInfo)) (*HostInfo, bool) {
+
+	destinationAddr := fwPacket.RemoteAddr
+
+	hostinfo, ready := f.getOrHandshakeNoRouting(destinationAddr, cacheCallback)
+
+	// Host is inside the mesh, no routing required
+	if hostinfo != nil {
+		return hostinfo, ready
+	}
+
+	gateways := f.inside.RoutesFor(destinationAddr)
+
+	switch len(gateways) {
+	case 0:
+		return nil, false
+	case 1:
+		// Single gateway route
+		return f.handshakeManager.GetOrHandshake(gateways[0].Addr(), cacheCallback)
+	default:
+		// Multi gateway route, perform ECMP categorization
+		gatewayAddr, balancingOk := routing.BalancePacket(fwPacket, gateways)
+
+		if !balancingOk {
+			// This happens if the gateway buckets were not calculated, this _should_ never happen
+			f.l.Error("Gateway buckets not calculated, fallback from ECMP to random routing. Please report this bug.")
 		}
+
+		var handshakeInfoForChosenGateway *HandshakeHostInfo
+		var hhReceiver = func(hh *HandshakeHostInfo) {
+			handshakeInfoForChosenGateway = hh
+		}
+
+		// Store the handshakeHostInfo for later.
+		// If this node is not reachable we will attempt other nodes, if none are reachable we will
+		// cache the packet for this gateway.
+		if hostinfo, ready = f.handshakeManager.GetOrHandshake(gatewayAddr, hhReceiver); ready {
+			return hostinfo, true
+		}
+
+		// It appears the selected gateway cannot be reached, find another gateway to fallback on.
+		// The current implementation breaks ECMP but that seems better than no connectivity.
+		// If ECMP is also required when a gateway is down then connectivity status
+		// for each gateway needs to be kept and the weights recalculated when they go up or down.
+		// This would also need to interact with unsafe_route updates through reloading the config or
+		// use of the use_system_route_table option
+
+		if f.l.Level >= logrus.DebugLevel {
+			f.l.WithField("destination", destinationAddr).
+				WithField("originalGateway", gatewayAddr).
+				Debugln("Calculated gateway for ECMP not available, attempting other gateways")
+		}
+
+		for i := range gateways {
+			// Skip the gateway that failed previously
+			if gateways[i].Addr() == gatewayAddr {
+				continue
+			}
+
+			// We do not need the HandshakeHostInfo since we cache the packet in the originally chosen gateway
+			if hostinfo, ready = f.handshakeManager.GetOrHandshake(gateways[i].Addr(), nil); ready {
+				return hostinfo, true
+			}
+		}
+
+		// No gateways reachable, cache the packet in the originally chosen gateway
+		cacheCallback(handshakeInfoForChosenGateway)
+		return hostinfo, false
 	}
 
-	return f.handshakeManager.GetOrHandshake(vpnAddr, cacheCallback)
 }
 
 func (f *Interface) sendMessageNow(t header.MessageType, st header.MessageSubType, hostinfo *HostInfo, p, nb, out []byte) {
@@ -163,7 +236,7 @@ func (f *Interface) sendMessageNow(t header.MessageType, st header.MessageSubTyp
 
 // SendMessageToVpnAddr handles real addr:port lookup and sends to the current best known address for vpnAddr
 func (f *Interface) SendMessageToVpnAddr(t header.MessageType, st header.MessageSubType, vpnAddr netip.Addr, p, nb, out []byte) {
-	hostInfo, ready := f.getOrHandshake(vpnAddr, func(hh *HandshakeHostInfo) {
+	hostInfo, ready := f.getOrHandshakeNoRouting(vpnAddr, func(hh *HandshakeHostInfo) {
 		hh.cachePacket(f.l, t, st, p, f.SendMessageToHostInfo, f.cachedPacketMetrics)
 	})
 

Diff for: overlay/device.go

@@ -3,13 +3,15 @@ package overlay
 import (
 	"io"
 	"net/netip"
+
+	"github.com/slackhq/nebula/routing"
 )
 
 type Device interface {
 	io.ReadWriteCloser
 	Activate() error
 	Networks() []netip.Prefix
 	Name() string
-	RouteFor(netip.Addr) netip.Addr
+	RoutesFor(netip.Addr) routing.Gateways
 	NewMultiQueueReader() (io.ReadWriteCloser, error)
 }

Diff for: overlay/route.go

@@ -11,13 +11,14 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/routing"
 )
 
 type Route struct {
 	MTU     int
 	Metric  int
 	Cidr    netip.Prefix
-	Via     netip.Addr
+	Via     routing.Gateways
 	Install bool
 }
 
@@ -47,15 +48,17 @@ func (r Route) String() string {
 	return s
 }
 
-func makeRouteTree(l *logrus.Logger, routes []Route, allowMTU bool) (*bart.Table[netip.Addr], error) {
-	routeTree := new(bart.Table[netip.Addr])
+func makeRouteTree(l *logrus.Logger, routes []Route, allowMTU bool) (*bart.Table[routing.Gateways], error) {
+	routeTree := new(bart.Table[routing.Gateways])
 	for _, r := range routes {
 		if !allowMTU && r.MTU > 0 {
 			l.WithField("route", r).Warnf("route MTU is not supported in %s", runtime.GOOS)
 		}
 
-		if r.Via.IsValid() {
-			routeTree.Insert(r.Cidr, r.Via)
+		gateways := r.Via
+		if len(gateways) > 0 {
+			routing.CalculateBucketsForGateways(gateways)
+			routeTree.Insert(r.Cidr, gateways)
 		}
 	}
 	return routeTree, nil
@@ -201,14 +204,63 @@ func parseUnsafeRoutes(c *config.C, networks []netip.Prefix) ([]Route, error) {
 			return nil, fmt.Errorf("entry %v.via in tun.unsafe_routes is not present", i+1)
 		}
 
-		via, ok := rVia.(string)
-		if !ok {
-			return nil, fmt.Errorf("entry %v.via in tun.unsafe_routes is not a string: found %T", i+1, rVia)
-		}
+		var gateways routing.Gateways
 
-		viaVpnIp, err := netip.ParseAddr(via)
-		if err != nil {
-			return nil, fmt.Errorf("entry %v.via in tun.unsafe_routes failed to parse address: %v", i+1, err)
+		switch via := rVia.(type) {
+		case string:
+			viaIp, err := netip.ParseAddr(via)
+			if err != nil {
+				return nil, fmt.Errorf("entry %v.via in tun.unsafe_routes failed to parse address: %v", i+1, err)
+			}
+
+			gateways = routing.Gateways{routing.NewGateway(viaIp, 1)}
+
+		case []interface{}:
+			gateways = make(routing.Gateways, len(via))
+			for ig, v := range via {
+				gatewayMap, ok := v.(map[interface{}]interface{})
+				if !ok {
+					return nil, fmt.Errorf("entry %v in tun.unsafe_routes[%v].via is invalid", i+1, ig+1)
+				}
+
+				rGateway, ok := gatewayMap["gateway"]
+				if !ok {
+					return nil, fmt.Errorf("entry .gateway in tun.unsafe_routes[%v].via[%v] is not present", i+1, ig+1)
+				}
+
+				parsedGateway, ok := rGateway.(string)
+				if !ok {
+					return nil, fmt.Errorf("entry .gateway in tun.unsafe_routes[%v].via[%v] is not a string", i+1, ig+1)
+				}
+
+				gatewayIp, err := netip.ParseAddr(parsedGateway)
+				if err != nil {
+					return nil, fmt.Errorf("entry .gateway in tun.unsafe_routes[%v].via[%v] failed to parse address: %v", i+1, ig+1, err)
+				}
+
+				rGatewayWeight, ok := gatewayMap["weight"]
+				if !ok {
+					rGatewayWeight = 1
+				}
+
+				gatewayWeight, ok := rGatewayWeight.(int)
+				if !ok {
+					_, err = strconv.ParseInt(rGatewayWeight.(string), 10, 32)
+					if err != nil {
+						return nil, fmt.Errorf("entry .weight in tun.unsafe_routes[%v].via[%v] is not an integer", i+1, ig+1)
+					}
+				}
+
+				if gatewayWeight < 1 || gatewayWeight > math.MaxInt32 {
+					return nil, fmt.Errorf("entry .weight in tun.unsafe_routes[%v].via[%v] is not in range (1-%d) : %v", i+1, ig+1, math.MaxInt32, gatewayWeight)
+				}
+
+				gateways[ig] = routing.NewGateway(gatewayIp, gatewayWeight)
+
+			}
+
+		default:
+			return nil, fmt.Errorf("entry %v.via in tun.unsafe_routes is not a string or list of gateways: found %T", i+1, rVia)
 		}
 
 		rRoute, ok := m["route"]
@@ -226,7 +278,7 @@ func parseUnsafeRoutes(c *config.C, networks []netip.Prefix) ([]Route, error) {
 		}
 
 		r := Route{
-			Via:     viaVpnIp,
+			Via:     gateways,
 			MTU:     mtu,
 			Metric:  metric,
 			Install: install,