diff --git a/src/yggdrasil/ckr.go b/src/yggdrasil/ckr.go
index 2b6c0d1..96fde27 100644
--- a/src/yggdrasil/ckr.go
+++ b/src/yggdrasil/ckr.go
@@ -12,12 +12,14 @@ import (
 // allow traffic for non-Yggdrasil ranges to be routed over Yggdrasil.
 
 type cryptokey struct {
-	core       *Core
-	enabled    bool
-	ipv4routes []cryptokey_route
-	ipv6routes []cryptokey_route
-	ipv4cache  map[address]cryptokey_route
-	ipv6cache  map[address]cryptokey_route
+	core        *Core
+	enabled     bool
+	ipv4routes  []cryptokey_route
+	ipv6routes  []cryptokey_route
+	ipv4cache   map[address]cryptokey_route
+	ipv6cache   map[address]cryptokey_route
+	ipv4sources []net.IPNet
+	ipv6sources []net.IPNet
 }
 
 type cryptokey_route struct {
@@ -31,12 +33,60 @@ func (c *cryptokey) init(core *Core) {
 	c.ipv6routes = make([]cryptokey_route, 0)
 	c.ipv4cache = make(map[address]cryptokey_route, 0)
 	c.ipv6cache = make(map[address]cryptokey_route, 0)
+	c.ipv4sources = make([]net.IPNet, 0)
+	c.ipv6sources = make([]net.IPNet, 0)
 }
 
 func (c *cryptokey) isEnabled() bool {
 	return c.enabled
 }
 
+func (c *cryptokey) isValidSource(addr address) bool {
+	ip := net.IP(addr[:])
+
+	// Does this match our node's address?
+	if addr == c.core.router.addr {
+		return true
+	}
+
+	// Does this match our node's subnet?
+	var subnet net.IPNet
+	copy(subnet.IP, c.core.router.subnet[:])
+	copy(subnet.Mask, net.CIDRMask(64, 128))
+	if subnet.Contains(ip) {
+		return true
+	}
+
+	// Does it match a configured CKR source?
+	for _, subnet := range c.ipv6sources {
+		if subnet.Contains(ip) {
+			return true
+		}
+	}
+
+	// Doesn't match any of the above
+	return false
+}
+
+func (c *cryptokey) addSourceSubnet(cidr string) error {
+	// Is the CIDR we've been given valid?
+	_, ipnet, err := net.ParseCIDR(cidr)
+	if err != nil {
+		return err
+	}
+
+	// Check if we already have this CIDR
+	for _, subnet := range c.ipv6sources {
+		if subnet.String() == ipnet.String() {
+			return errors.New("Source subnet already configured")
+		}
+	}
+
+	// Add the source subnet
+	c.ipv6sources = append(c.ipv6sources, *ipnet)
+	return nil
+}
+
 func (c *cryptokey) addRoute(cidr string, dest string) error {
 	// Is the CIDR we've been given valid?
 	ipaddr, ipnet, err := net.ParseCIDR(cidr)
diff --git a/src/yggdrasil/config/config.go b/src/yggdrasil/config/config.go
index 6f1871f..9594745 100644
--- a/src/yggdrasil/config/config.go
+++ b/src/yggdrasil/config/config.go
@@ -39,6 +39,7 @@ type SessionFirewall struct {
 
 // TunnelRouting contains the crypto-key routing tables for tunneling
 type TunnelRouting struct {
-	Enable     bool              `comment:"Enable or disable tunneling."`
-	IPv6Routes map[string]string `comment:"IPv6 subnets, mapped to the public keys to which they should be routed."`
+	Enable      bool              `comment:"Enable or disable tunneling."`
+	IPv6Routes  map[string]string `comment:"IPv6 subnets, mapped to the public keys to which they should be routed."`
+	IPv6Sources []string          `comment:"Allow source addresses in these subnets."`
 }
diff --git a/src/yggdrasil/router.go b/src/yggdrasil/router.go
index dcce727..e363464 100644
--- a/src/yggdrasil/router.go
+++ b/src/yggdrasil/router.go
@@ -34,6 +34,7 @@ import (
 type router struct {
 	core      *Core
 	addr      address
+	subnet    subnet
 	in        <-chan []byte // packets we received from the network, link to peer's "out"
 	out       func([]byte)  // packets we're sending to the network, link to peer's "in"
 	recv      chan<- []byte // place where the tun pulls received packets from
@@ -47,6 +48,7 @@ type router struct {
 func (r *router) init(core *Core) {
 	r.core = core
 	r.addr = *address_addrForNodeID(&r.core.dht.nodeID)
+	r.subnet = *address_subnetForNodeID(&r.core.dht.nodeID)
 	in := make(chan []byte, 32) // TODO something better than this...
 	p := r.core.peers.newPeer(&r.core.boxPub, &r.core.sigPub, &boxSharedKey{}, "(self)")
 	p.out = func(packet []byte) {
@@ -128,6 +130,9 @@ func (r *router) sendPacket(bs []byte) {
 	var snet subnet
 	copy(sourceAddr[:], bs[8:])
 	copy(sourceSubnet[:], bs[8:])
+	if !r.cryptokey.isValidSource(sourceAddr) {
+		return
+	}
 	copy(dest[:], bs[24:])
 	copy(snet[:], bs[24:])
 	if !dest.isValid() && !snet.isValid() {
@@ -141,10 +146,6 @@ func (r *router) sendPacket(bs []byte) {
 		} else {
 			return
 		}
-	} else {
-		if !sourceAddr.isValid() && !sourceSubnet.isValid() {
-			return
-		}
 	}
 	doSearch := func(packet []byte) {
 		var nodeID, mask *NodeID