New ECS modifier option 'add-if-missing' (#389)

cmd/routedns/main.go

@@ -498,6 +498,8 @@ func instantiateGroup(id string, g group, resolvers map[string]rdns.Resolver) er
 		switch g.ECSOp {
 		case "add":
 			f = rdns.ECSModifierAdd(g.ECSAddress, g.ECSPrefix4, g.ECSPrefix6)
+		case "add-if-missing":
+			f = rdns.ECSModifierAddIfMissing(g.ECSAddress, g.ECSPrefix4, g.ECSPrefix6)
 		case "delete":
 			f = rdns.ECSModifierDelete
 		case "privacy":

doc/configuration.md

@@ -855,6 +855,7 @@ Example config files: [client-blocklist.toml](../cmd/routedns/example-config/cli
 A client subnet modifier is used to either remove ECS options from a query, replace/add one, or improve privacy by hiding more bits of the address. The following operation are supported by the subnet modifier:
 
 - `add` - Add an ECS option to a query. If there is one already it is replaced. If no `ecs-address` is provided, the address of the client is used (with `ecs-prefix4` or `ecs-prefix6` applied).
+- `add-if-missing` - Add an ECS option to a query if none was provided by the client. If no `ecs-address` is provided, the address of the client is used (with `ecs-prefix4` or `ecs-prefix6` applied).
 - `delete` - Remove the ECS option completely from the EDNS0 record.
 - `privacy` - Restrict the number of bits in the address to the number in `ecs-prefix4`/`ecs-prefix6`.
 
@@ -865,7 +866,7 @@ Client Subnet modifiers are instantiated with `type = "ecs-modifier"` in the gro
 Options:
 
 - `resolvers` - Array of upstream resolvers, only one is supported.
-- `ecs-op` - Operation to be performed on query options. Either `add`, `delete`, or `privacy`. Does nothing if not specified.
+- `ecs-op` - Operation to be performed on query options. Either `add`, `add-if-missing`, `delete`, or `privacy`. Does nothing if not specified.
 - `ecs-address` - The address to use in the option. Only used for add operations. If given, will set the address to a fixed value. If missing, the address of the client is used (with the appropriate `ecs-prefix` applied).
 - `ecs-prefix4` and `ecs-prefix6` - Source prefix length. Mask for the address. Only used for add and privacy operations.
 

ecs-modifier.go

@@ -119,6 +119,31 @@ func ECSModifierAdd(addr net.IP, prefix4, prefix6 uint8) ECSModifierFunc {
 	}
 }
 
+func ECSModifierAddIfMissing(addr net.IP, prefix4, prefix6 uint8) ECSModifierFunc {
+	addFunc := ECSModifierAdd(addr, prefix4, prefix6)
+
+	return func(id string, q *dns.Msg, ci ClientInfo) {
+		// See if we have an ECS option already
+		edns0 := q.IsEdns0()
+		if edns0 != nil {
+			// Find the ECS option
+			for _, opt := range edns0.Option {
+				ecs, ok := opt.(*dns.EDNS0_SUBNET)
+				if ok {
+					logger(id, q, ci).WithFields(logrus.Fields{
+						"addr": ecs.Address,
+						"mask": ecs.SourceNetmask,
+					}).Debug("ecs option already present")
+					return // There's an ECS option already, don't touch it
+				}
+			}
+		}
+
+		// No ECS option found, add it
+		addFunc(id, q, ci)
+	}
+}
+
 func ECSModifierPrivacy(prefix4, prefix6 uint8) ECSModifierFunc {
 	return func(id string, q *dns.Msg, ci ClientInfo) {
 		edns0 := q.IsEdns0()