remote_allow_ranges: allow inside CIDR specific remote_allow_lists (#540)

This allows you to configure remote allow lists specific to different
subnets of the inside CIDR. Example:

    remote_allow_ranges:
      10.42.42.0/24:
        192.168.0.0/16: true

This would only allow hosts with a VPN IP in the 10.42.42.0/24 range to
have private IPs (and thus don't connect over public IPs).

The PR also refactors AllowList into RemoteAllowList and LocalAllowList to make it clearer which methods are allowed on which allow list.

allow_list.go

@@ -9,6 +9,18 @@ import (
 type AllowList struct {
 	// The values of this cidrTree are `bool`, signifying allow/deny
 	cidrTree *CIDR6Tree
+}
+
+type RemoteAllowList struct {
+	AllowList *AllowList
+
+	// Inside Range Specific, keys of this tree are inside CIDRs and values
+	// are *AllowList
+	insideAllowLists *CIDR6Tree
+}
+
+type LocalAllowList struct {
+	AllowList *AllowList
 
 	// To avoid ambiguity, all rules must be true, or all rules must be false.
 	nameRules []AllowListNameRule
@@ -61,7 +73,14 @@ func (al *AllowList) AllowIpV6(hi, lo uint64) bool {
 	}
 }
 
-func (al *AllowList) AllowName(name string) bool {
+func (al *LocalAllowList) Allow(ip net.IP) bool {
+	if al == nil {
+		return true
+	}
+	return al.AllowList.Allow(ip)
+}
+
+func (al *LocalAllowList) AllowName(name string) bool {
 	if al == nil || len(al.nameRules) == 0 {
 		return true
 	}
@@ -75,3 +94,47 @@ func (al *AllowList) AllowName(name string) bool {
 	// If no rules match, return the default, which is the inverse of the rules
 	return !al.nameRules[0].Allow
 }
+
+func (al *RemoteAllowList) AllowUnknownVpnIp(ip net.IP) bool {
+	if al == nil {
+		return true
+	}
+	return al.AllowList.Allow(ip)
+}
+
+func (al *RemoteAllowList) Allow(vpnIp uint32, ip net.IP) bool {
+	if !al.getInsideAllowList(vpnIp).Allow(ip) {
+		return false
+	}
+	return al.AllowList.Allow(ip)
+}
+
+func (al *RemoteAllowList) AllowIpV4(vpnIp uint32, ip uint32) bool {
+	if al == nil {
+		return true
+	}
+	if !al.getInsideAllowList(vpnIp).AllowIpV4(ip) {
+		return false
+	}
+	return al.AllowList.AllowIpV4(ip)
+}
+
+func (al *RemoteAllowList) AllowIpV6(vpnIp uint32, hi, lo uint64) bool {
+	if al == nil {
+		return true
+	}
+	if !al.getInsideAllowList(vpnIp).AllowIpV6(hi, lo) {
+		return false
+	}
+	return al.AllowList.AllowIpV6(hi, lo)
+}
+
+func (al *RemoteAllowList) getInsideAllowList(vpnIp uint32) *AllowList {
+	if al.insideAllowLists != nil {
+		inside := al.insideAllowLists.MostSpecificContainsIpV4(vpnIp)
+		if inside != nil {
+			return inside.(*AllowList)
+		}
+	}
+	return nil
+}