proxyd: re-write block tags to enforce consensus (#5586)

proxyd/proxyd/backend.go

@@ -90,6 +90,11 @@ var (
 		Message:       "backend is currently not healthy to serve traffic",
 		HTTPErrorCode: 503,
 	}
+	ErrBlockOutOfRange = &RPCErr{
+		Code:          JSONRPCErrorInternal - 19,
+		Message:       "block is out of range",
+		HTTPErrorCode: 400,
+	}
 
 	ErrBackendUnexpectedJSONRPC = errors.New("backend returned an unexpected JSON-RPC response")
 )
@@ -214,6 +219,12 @@ func WithMaxErrorRateThreshold(maxErrorRateThreshold float64) BackendOpt {
 	}
 }
 
+type indexedReqRes struct {
+	index int
+	req   *RPCReq
+	res   *RPCRes
+}
+
 func NewBackend(
 	name string,
 	rpcURL string,
@@ -593,47 +604,96 @@ func (b *BackendGroup) Forward(ctx context.Context, rpcReqs []*RPCReq, isBatch b
 
 	backends := b.Backends
 
-	// When `consensus_aware` is set to `true`, the backend group acts as a load balancer
-	// serving traffic from any backend that agrees in the consensus group
+	overriddenResponses := make([]*indexedReqRes, 0)
+	rewrittenReqs := make([]*RPCReq, 0, len(rpcReqs))
+
 	if b.Consensus != nil {
+		// When `consensus_aware` is set to `true`, the backend group acts as a load balancer
+		// serving traffic from any backend that agrees in the consensus group
 		backends = b.loadBalancedConsensusGroup()
+
+		// We also rewrite block tags to enforce compliance with consensus
+		rctx := RewriteContext{latest: b.Consensus.GetConsensusBlockNumber()}
+
+		for i, req := range rpcReqs {
+			res := RPCRes{JSONRPC: JSONRPCVersion, ID: req.ID}
+			result, err := RewriteTags(rctx, req, &res)
+			switch result {
+			case RewriteOverrideError:
+				overriddenResponses = append(overriddenResponses, &indexedReqRes{
+					index: i,
+					req:   req,
+					res:   &res,
+				})
+				if errors.Is(err, ErrRewriteBlockOutOfRange) {
+					res.Error = ErrBlockOutOfRange
+				} else {
+					res.Error = ErrParseErr
+				}
+			case RewriteOverrideResponse:
+				overriddenResponses = append(overriddenResponses, &indexedReqRes{
+					index: i,
+					req:   req,
+					res:   &res,
+				})
+			case RewriteOverrideRequest, RewriteNone:
+				rewrittenReqs = append(rewrittenReqs, req)
+			}
+		}
+		rpcReqs = rewrittenReqs
 	}
 
 	rpcRequestsTotal.Inc()
 
 	for _, back := range backends {
-		res, err := back.Forward(ctx, rpcReqs, isBatch)
-		if errors.Is(err, ErrMethodNotWhitelisted) {
-			return nil, err
+		res := make([]*RPCRes, 0)
+		var err error
+
+		if len(rpcReqs) > 0 {
+			res, err = back.Forward(ctx, rpcReqs, isBatch)
+			if errors.Is(err, ErrMethodNotWhitelisted) {
+				return nil, err
+			}
+			if errors.Is(err, ErrBackendOffline) {
+				log.Warn(
+					"skipping offline backend",
+					"name", back.Name,
+					"auth", GetAuthCtx(ctx),
+					"req_id", GetReqID(ctx),
+				)
+				continue
+			}
+			if errors.Is(err, ErrBackendOverCapacity) {
+				log.Warn(
+					"skipping over-capacity backend",
+					"name", back.Name,
+					"auth", GetAuthCtx(ctx),
+					"req_id", GetReqID(ctx),
+				)
+				continue
+			}
+			if err != nil {
+				log.Error(
+					"error forwarding request to backend",
+					"name", back.Name,
+					"req_id", GetReqID(ctx),
+					"auth", GetAuthCtx(ctx),
+					"err", err,
+				)
+				continue
+			}
 		}
-		if errors.Is(err, ErrBackendOffline) {
-			log.Warn(
-				"skipping offline backend",
-				"name", back.Name,
-				"auth", GetAuthCtx(ctx),
-				"req_id", GetReqID(ctx),
-			)
-			continue
-		}
-		if errors.Is(err, ErrBackendOverCapacity) {
-			log.Warn(
-				"skipping over-capacity backend",
-				"name", back.Name,
-				"auth", GetAuthCtx(ctx),
-				"req_id", GetReqID(ctx),
-			)
-			continue
-		}
-		if err != nil {
-			log.Error(
-				"error forwarding request to backend",
-				"name", back.Name,
-				"req_id", GetReqID(ctx),
-				"auth", GetAuthCtx(ctx),
-				"err", err,
-			)
-			continue
+
+		// re-apply overridden responses
+		for _, ov := range overriddenResponses {
+			if len(res) > 0 {
+				// insert ov.res at position ov.index
+				res = append(res[:ov.index], append([]*RPCRes{ov.res}, res[ov.index:]...)...)
+			} else {
+				res = append(res, ov.res)
+			}
 		}
+
 		return res, nil
 	}
 

proxyd/proxyd/integration_tests/consensus_test.go

@@ -433,6 +433,184 @@ func TestConsensus(t *testing.T) {
 		require.Equal(t, len(node1.Requests()), 0, msg)
 		require.Equal(t, len(node2.Requests()), 10, msg)
 	})
+
+	t.Run("rewrite response of eth_blockNumber", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+		node1.Reset()
+		node2.Reset()
+		bg.Consensus.Unban()
+
+		// establish the consensus
+
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildGetBlockResponse("0x2", "hash2"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildGetBlockResponse("0x2", "hash2"),
+		})
+
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		totalRequests := len(node1.Requests()) + len(node2.Requests())
+
+		require.Equal(t, 2, len(bg.Consensus.GetConsensusGroup()))
+
+		// pretend backends advanced in consensus, but we are still serving the latest value of the consensus
+		// until it gets updated again
+
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildGetBlockResponse("0x3", "hash3"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildGetBlockResponse("0x3", "hash3"),
+		})
+
+		resRaw, statusCode, err := client.SendRPC("eth_blockNumber", nil)
+		require.NoError(t, err)
+		require.Equal(t, 200, statusCode)
+
+		var jsonMap map[string]interface{}
+		err = json.Unmarshal(resRaw, &jsonMap)
+		require.NoError(t, err)
+		require.Equal(t, "0x2", jsonMap["result"])
+
+		// no extra request hit the backends
+		require.Equal(t, totalRequests, len(node1.Requests())+len(node2.Requests()))
+	})
+
+	t.Run("rewrite request of eth_getBlockByNumber", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+		bg.Consensus.Unban()
+
+		// establish the consensus and ban node2 for now
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildGetBlockResponse("0x2", "hash2"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "net_peerCount",
+			Block:    "",
+			Response: buildPeerCountResponse(1),
+		})
+
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		require.Equal(t, 1, len(bg.Consensus.GetConsensusGroup()))
+
+		node1.Reset()
+
+		_, statusCode, err := client.SendRPC("eth_getBlockByNumber", []interface{}{"latest"})
+		require.NoError(t, err)
+		require.Equal(t, 200, statusCode)
+
+		var jsonMap map[string]interface{}
+		err = json.Unmarshal(node1.Requests()[0].Body, &jsonMap)
+		require.NoError(t, err)
+		require.Equal(t, "0x2", jsonMap["params"].([]interface{})[0])
+	})
+
+	t.Run("rewrite request of eth_getBlockByNumber - out of range", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+		bg.Consensus.Unban()
+
+		// establish the consensus and ban node2 for now
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildGetBlockResponse("0x2", "hash2"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "net_peerCount",
+			Block:    "",
+			Response: buildPeerCountResponse(1),
+		})
+
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		require.Equal(t, 1, len(bg.Consensus.GetConsensusGroup()))
+
+		node1.Reset()
+
+		resRaw, statusCode, err := client.SendRPC("eth_getBlockByNumber", []interface{}{"0x10"})
+		require.NoError(t, err)
+		require.Equal(t, 400, statusCode)
+
+		var jsonMap map[string]interface{}
+		err = json.Unmarshal(resRaw, &jsonMap)
+		require.NoError(t, err)
+		require.Equal(t, -32019, int(jsonMap["error"].(map[string]interface{})["code"].(float64)))
+		require.Equal(t, "block is out of range", jsonMap["error"].(map[string]interface{})["message"])
+	})
+
+	t.Run("batched rewrite", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+		bg.Consensus.Unban()
+
+		// establish the consensus and ban node2 for now
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildGetBlockResponse("0x2", "hash2"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "net_peerCount",
+			Block:    "",
+			Response: buildPeerCountResponse(1),
+		})
+
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		require.Equal(t, 1, len(bg.Consensus.GetConsensusGroup()))
+
+		node1.Reset()
+
+		resRaw, statusCode, err := client.SendBatchRPC(
+			NewRPCReq("1", "eth_getBlockByNumber", []interface{}{"latest"}),
+			NewRPCReq("2", "eth_getBlockByNumber", []interface{}{"0x10"}),
+			NewRPCReq("3", "eth_getBlockByNumber", []interface{}{"0x1"}))
+		require.NoError(t, err)
+		require.Equal(t, 200, statusCode)
+
+		var jsonMap []map[string]interface{}
+		err = json.Unmarshal(resRaw, &jsonMap)
+		require.NoError(t, err)
+		require.Equal(t, 3, len(jsonMap))
+
+		// rewrite latest to 0x2
+		require.Equal(t, "0x2", jsonMap[0]["result"].(map[string]interface{})["number"])
+
+		// out of bounds for block 0x10
+		require.Equal(t, -32019, int(jsonMap[1]["error"].(map[string]interface{})["code"].(float64)))
+		require.Equal(t, "block is out of range", jsonMap[1]["error"].(map[string]interface{})["message"])
+
+		// dont rewrite for 0x1
+		require.Equal(t, "0x1", jsonMap[2]["result"].(map[string]interface{})["number"])
+	})
 }
 
 func backend(bg *proxyd.BackendGroup, name string) *proxyd.Backend {

proxyd/proxyd/rewriter.go

@@ -0,0 +1,175 @@
+package proxyd
+
+import (
+	"encoding/json"
+	"errors"
+	"strings"
+
+	"github.com/ethereum/go-ethereum/common/hexutil"
+)
+
+type RewriteContext struct {
+	latest hexutil.Uint64
+}
+
+type RewriteResult uint8
+
+const (
+	// RewriteNone means request should be forwarded as-is
+	RewriteNone RewriteResult = iota
+
+	// RewriteOverrideError means there was an error attempting to rewrite
+	RewriteOverrideError
+
+	// RewriteOverrideRequest means the modified request should be forwarded to the backend
+	RewriteOverrideRequest
+
+	// RewriteOverrideResponse means to skip calling the backend and serve the overridden response
+	RewriteOverrideResponse
+)
+
+var (
+	ErrRewriteBlockOutOfRange = errors.New("block is out of range")
+)
+
+// RewriteTags modifies the request and the response based on block tags
+func RewriteTags(rctx RewriteContext, req *RPCReq, res *RPCRes) (RewriteResult, error) {
+	rw, err := RewriteResponse(rctx, req, res)
+	if rw == RewriteOverrideResponse {
+		return rw, err
+	}
+	return RewriteRequest(rctx, req, res)
+}
+
+// RewriteResponse modifies the response object to comply with the rewrite context
+// after the method has been called at the backend
+// RewriteResult informs the decision of the rewrite
+func RewriteResponse(rctx RewriteContext, req *RPCReq, res *RPCRes) (RewriteResult, error) {
+	switch req.Method {
+	case "eth_blockNumber":
+		res.Result = rctx.latest
+		return RewriteOverrideResponse, nil
+	}
+	return RewriteNone, nil
+}
+
+// RewriteRequest modifies the request object to comply with the rewrite context
+// before the method has been called at the backend
+// it returns false if nothing was changed
+func RewriteRequest(rctx RewriteContext, req *RPCReq, res *RPCRes) (RewriteResult, error) {
+	switch req.Method {
+	case "eth_getLogs",
+		"eth_newFilter":
+		return rewriteRange(rctx, req, res, 0)
+	case "eth_getBalance",
+		"eth_getCode",
+		"eth_getTransactionCount",
+		"eth_call":
+		return rewriteParam(rctx, req, res, 1)
+	case "eth_getStorageAt":
+		return rewriteParam(rctx, req, res, 2)
+	case "eth_getBlockTransactionCountByNumber",
+		"eth_getUncleCountByBlockNumber",
+		"eth_getBlockByNumber",
+		"eth_getTransactionByBlockNumberAndIndex",
+		"eth_getUncleByBlockNumberAndIndex":
+		return rewriteParam(rctx, req, res, 0)
+	}
+	return RewriteNone, nil
+}
+
+func rewriteParam(rctx RewriteContext, req *RPCReq, res *RPCRes, pos int) (RewriteResult, error) {
+	var p []interface{}
+	err := json.Unmarshal(req.Params, &p)
+	if err != nil {
+		return RewriteOverrideError, err
+	}
+
+	if len(p) <= pos {
+		p = append(p, "latest")
+	}
+
+	val, rw, err := rewriteTag(rctx, p[pos].(string))
+	if err != nil {
+		return RewriteOverrideError, err
+	}
+
+	if rw {
+		p[pos] = val
+		paramRaw, err := json.Marshal(p)
+		if err != nil {
+			return RewriteOverrideError, err
+		}
+		req.Params = paramRaw
+		return RewriteOverrideRequest, nil
+	}
+	return RewriteNone, nil
+}
+
+func rewriteRange(rctx RewriteContext, req *RPCReq, res *RPCRes, pos int) (RewriteResult, error) {
+	var p []map[string]interface{}
+	err := json.Unmarshal(req.Params, &p)
+	if err != nil {
+		return RewriteOverrideError, err
+	}
+
+	modifiedFrom, err := rewriteTagMap(rctx, p[pos], "fromBlock")
+	if err != nil {
+		return RewriteOverrideError, err
+	}
+
+	modifiedTo, err := rewriteTagMap(rctx, p[pos], "toBlock")
+	if err != nil {
+		return RewriteOverrideError, err
+	}
+
+	// if any of the fields the request have been changed, re-marshal the params
+	if modifiedFrom || modifiedTo {
+		paramsRaw, err := json.Marshal(p)
+		req.Params = paramsRaw
+		if err != nil {
+			return RewriteOverrideError, err
+		}
+		return RewriteOverrideRequest, nil
+	}
+
+	return RewriteNone, nil
+}
+
+func rewriteTagMap(rctx RewriteContext, m map[string]interface{}, key string) (bool, error) {
+	if m[key] == nil || m[key] == "" {
+		return false, nil
+	}
+
+	current, ok := m[key].(string)
+	if !ok {
+		return false, errors.New("expected string")
+	}
+
+	val, rw, err := rewriteTag(rctx, current)
+	if err != nil {
+		return false, err
+	}
+	if rw {
+		m[key] = val
+		return true, nil
+	}
+
+	return false, nil
+}
+
+func rewriteTag(rctx RewriteContext, current string) (string, bool, error) {
+	if current == "latest" {
+		return rctx.latest.String(), true, nil
+	} else if strings.HasPrefix(current, "0x") {
+		decode, err := hexutil.DecodeUint64(current)
+		if err != nil {
+			return current, false, err
+		}
+		b := hexutil.Uint64(decode)
+		if b > rctx.latest {
+			return "", false, ErrRewriteBlockOutOfRange
+		}
+	}
+	return current, false, nil
+}

proxyd/proxyd/rewriter_test.go

@@ -0,0 +1,441 @@
+package proxyd
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/stretchr/testify/require"
+)
+
+type args struct {
+	rctx RewriteContext
+	req  *RPCReq
+	res  *RPCRes
+}
+
+type rewriteTest struct {
+	name        string
+	args        args
+	expected    RewriteResult
+	expectedErr error
+	check       func(*testing.T, args)
+}
+
+func TestRewriteRequest(t *testing.T) {
+	tests := []rewriteTest{
+		/* range scoped */
+		{
+			name: "eth_getLogs fromBlock latest",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"fromBlock": "latest"}})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []map[string]interface{}
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, hexutil.Uint64(100).String(), p[0]["fromBlock"])
+			},
+		},
+		{
+			name: "eth_getLogs fromBlock within range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"fromBlock": hexutil.Uint64(55).String()}})},
+				res:  nil,
+			},
+			expected: RewriteNone,
+			check: func(t *testing.T, args args) {
+				var p []map[string]interface{}
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, hexutil.Uint64(55).String(), p[0]["fromBlock"])
+			},
+		},
+		{
+			name: "eth_getLogs fromBlock out of range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"fromBlock": hexutil.Uint64(111).String()}})},
+				res:  nil,
+			},
+			expected:    RewriteOverrideError,
+			expectedErr: ErrRewriteBlockOutOfRange,
+		},
+		{
+			name: "eth_getLogs toBlock latest",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"toBlock": "latest"}})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []map[string]interface{}
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, hexutil.Uint64(100).String(), p[0]["toBlock"])
+			},
+		},
+		{
+			name: "eth_getLogs toBlock within range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"toBlock": hexutil.Uint64(55).String()}})},
+				res:  nil,
+			},
+			expected: RewriteNone,
+			check: func(t *testing.T, args args) {
+				var p []map[string]interface{}
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, hexutil.Uint64(55).String(), p[0]["toBlock"])
+			},
+		},
+		{
+			name: "eth_getLogs toBlock out of range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"toBlock": hexutil.Uint64(111).String()}})},
+				res:  nil,
+			},
+			expected:    RewriteOverrideError,
+			expectedErr: ErrRewriteBlockOutOfRange,
+		},
+		{
+			name: "eth_getLogs fromBlock, toBlock latest",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"fromBlock": "latest", "toBlock": "latest"}})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []map[string]interface{}
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, hexutil.Uint64(100).String(), p[0]["fromBlock"])
+				require.Equal(t, hexutil.Uint64(100).String(), p[0]["toBlock"])
+			},
+		},
+		{
+			name: "eth_getLogs fromBlock, toBlock within range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"fromBlock": hexutil.Uint64(55).String(), "toBlock": hexutil.Uint64(77).String()}})},
+				res:  nil,
+			},
+			expected: RewriteNone,
+			check: func(t *testing.T, args args) {
+				var p []map[string]interface{}
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, hexutil.Uint64(55).String(), p[0]["fromBlock"])
+				require.Equal(t, hexutil.Uint64(77).String(), p[0]["toBlock"])
+			},
+		},
+		{
+			name: "eth_getLogs fromBlock, toBlock out of range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getLogs", Params: mustMarshalJSON([]map[string]interface{}{{"fromBlock": hexutil.Uint64(111).String(), "toBlock": hexutil.Uint64(222).String()}})},
+				res:  nil,
+			},
+			expected:    RewriteOverrideError,
+			expectedErr: ErrRewriteBlockOutOfRange,
+		},
+		/* default block parameter */
+		{
+			name: "eth_getCode omit block, should add",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getCode", Params: mustMarshalJSON([]string{"0x123"})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 2, len(p))
+				require.Equal(t, "0x123", p[0])
+				require.Equal(t, hexutil.Uint64(100).String(), p[1])
+			},
+		},
+		{
+			name: "eth_getCode latest",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getCode", Params: mustMarshalJSON([]string{"0x123", "latest"})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 2, len(p))
+				require.Equal(t, "0x123", p[0])
+				require.Equal(t, hexutil.Uint64(100).String(), p[1])
+			},
+		},
+		{
+			name: "eth_getCode within range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getCode", Params: mustMarshalJSON([]string{"0x123", hexutil.Uint64(55).String()})},
+				res:  nil,
+			},
+			expected: RewriteNone,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 2, len(p))
+				require.Equal(t, "0x123", p[0])
+				require.Equal(t, hexutil.Uint64(55).String(), p[1])
+			},
+		},
+		{
+			name: "eth_getCode out of range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getCode", Params: mustMarshalJSON([]string{"0x123", hexutil.Uint64(111).String()})},
+				res:  nil,
+			},
+			expected:    RewriteOverrideError,
+			expectedErr: ErrRewriteBlockOutOfRange,
+		},
+		/* default block parameter, at position 2 */
+		{
+			name: "eth_getStorageAt omit block, should add",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getStorageAt", Params: mustMarshalJSON([]string{"0x123", "5"})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 3, len(p))
+				require.Equal(t, "0x123", p[0])
+				require.Equal(t, "5", p[1])
+				require.Equal(t, hexutil.Uint64(100).String(), p[2])
+			},
+		},
+		{
+			name: "eth_getStorageAt latest",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getStorageAt", Params: mustMarshalJSON([]string{"0x123", "5", "latest"})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 3, len(p))
+				require.Equal(t, "0x123", p[0])
+				require.Equal(t, "5", p[1])
+				require.Equal(t, hexutil.Uint64(100).String(), p[2])
+			},
+		},
+		{
+			name: "eth_getStorageAt within range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getStorageAt", Params: mustMarshalJSON([]string{"0x123", "5", hexutil.Uint64(55).String()})},
+				res:  nil,
+			},
+			expected: RewriteNone,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 3, len(p))
+				require.Equal(t, "0x123", p[0])
+				require.Equal(t, "5", p[1])
+				require.Equal(t, hexutil.Uint64(55).String(), p[2])
+			},
+		},
+		{
+			name: "eth_getStorageAt out of range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getStorageAt", Params: mustMarshalJSON([]string{"0x123", "5", hexutil.Uint64(111).String()})},
+				res:  nil,
+			},
+			expected:    RewriteOverrideError,
+			expectedErr: ErrRewriteBlockOutOfRange,
+		},
+		/* default block parameter, at position 0 */
+		{
+			name: "eth_getBlockByNumber omit block, should add",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getBlockByNumber", Params: mustMarshalJSON([]string{})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 1, len(p))
+				require.Equal(t, hexutil.Uint64(100).String(), p[0])
+			},
+		},
+		{
+			name: "eth_getBlockByNumber latest",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getBlockByNumber", Params: mustMarshalJSON([]string{"latest"})},
+				res:  nil,
+			},
+			expected: RewriteOverrideRequest,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 1, len(p))
+				require.Equal(t, hexutil.Uint64(100).String(), p[0])
+			},
+		},
+		{
+			name: "eth_getBlockByNumber within range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getBlockByNumber", Params: mustMarshalJSON([]string{hexutil.Uint64(55).String()})},
+				res:  nil,
+			},
+			expected: RewriteNone,
+			check: func(t *testing.T, args args) {
+				var p []string
+				err := json.Unmarshal(args.req.Params, &p)
+				require.Nil(t, err)
+				require.Equal(t, 1, len(p))
+				require.Equal(t, hexutil.Uint64(55).String(), p[0])
+			},
+		},
+		{
+			name: "eth_getBlockByNumber out of range",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_getBlockByNumber", Params: mustMarshalJSON([]string{hexutil.Uint64(111).String()})},
+				res:  nil,
+			},
+			expected:    RewriteOverrideError,
+			expectedErr: ErrRewriteBlockOutOfRange,
+		},
+	}
+
+	// generalize tests for other methods with same interface and behavior
+	tests = generalize(tests, "eth_getLogs", "eth_newFilter")
+	tests = generalize(tests, "eth_getCode", "eth_getBalance")
+	tests = generalize(tests, "eth_getCode", "eth_getTransactionCount")
+	tests = generalize(tests, "eth_getCode", "eth_call")
+	tests = generalize(tests, "eth_getBlockByNumber", "eth_getBlockTransactionCountByNumber")
+	tests = generalize(tests, "eth_getBlockByNumber", "eth_getUncleCountByBlockNumber")
+	tests = generalize(tests, "eth_getBlockByNumber", "eth_getTransactionByBlockNumberAndIndex")
+	tests = generalize(tests, "eth_getBlockByNumber", "eth_getUncleByBlockNumberAndIndex")
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result, err := RewriteRequest(tt.args.rctx, tt.args.req, tt.args.res)
+			if result != RewriteOverrideError {
+				require.Nil(t, err)
+				require.Equal(t, tt.expected, result)
+			} else {
+				require.Equal(t, tt.expectedErr, err)
+			}
+			if tt.check != nil {
+				tt.check(t, tt.args)
+			}
+		})
+	}
+}
+
+func generalize(tests []rewriteTest, baseMethod string, generalizedMethod string) []rewriteTest {
+	newCases := make([]rewriteTest, 0)
+	for _, t := range tests {
+		if t.args.req.Method == baseMethod {
+			newName := strings.Replace(t.name, baseMethod, generalizedMethod, -1)
+			var req *RPCReq
+			var res *RPCRes
+
+			if t.args.req != nil {
+				req = &RPCReq{
+					JSONRPC: t.args.req.JSONRPC,
+					Method:  generalizedMethod,
+					Params:  t.args.req.Params,
+					ID:      t.args.req.ID,
+				}
+			}
+
+			if t.args.res != nil {
+				res = &RPCRes{
+					JSONRPC: t.args.res.JSONRPC,
+					Result:  t.args.res.Result,
+					Error:   t.args.res.Error,
+					ID:      t.args.res.ID,
+				}
+			}
+			newCases = append(newCases, rewriteTest{
+				name: newName,
+				args: args{
+					rctx: t.args.rctx,
+					req:  req,
+					res:  res,
+				},
+				expected:    t.expected,
+				expectedErr: t.expectedErr,
+				check:       t.check,
+			})
+		}
+	}
+	return append(tests, newCases...)
+}
+
+func TestRewriteResponse(t *testing.T) {
+	type args struct {
+		rctx RewriteContext
+		req  *RPCReq
+		res  *RPCRes
+	}
+	tests := []struct {
+		name     string
+		args     args
+		expected RewriteResult
+		check    func(*testing.T, args)
+	}{
+		{
+			name: "eth_blockNumber latest",
+			args: args{
+				rctx: RewriteContext{latest: hexutil.Uint64(100)},
+				req:  &RPCReq{Method: "eth_blockNumber"},
+				res:  &RPCRes{Result: hexutil.Uint64(200)},
+			},
+			expected: RewriteOverrideResponse,
+			check: func(t *testing.T, args args) {
+				require.Equal(t, args.res.Result, hexutil.Uint64(100))
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result, err := RewriteResponse(tt.args.rctx, tt.args.req, tt.args.res)
+			require.Nil(t, err)
+			require.Equal(t, tt.expected, result)
+			if tt.check != nil {
+				tt.check(t, tt.args)
+			}
+		})
+	}
+}

proxyd/proxyd/tools/mockserver/handler/handler.go

@@ -6,6 +6,9 @@ import (
 	"io"
 	"net/http"
 	"os"
+	"strings"
+
+	"github.com/ethereum-optimism/optimism/proxyd"
 
 	"github.com/gorilla/mux"
 	"github.com/pkg/errors"
@@ -46,12 +49,6 @@ func (mh *MockedHandler) Handler(w http.ResponseWriter, req *http.Request) {
 		fmt.Printf("error reading request: %v\n", err)
 	}
 
-	var j map[string]interface{}
-	err = json.Unmarshal(body, &j)
-	if err != nil {
-		fmt.Printf("error reading request: %v\n", err)
-	}
-
 	var template []*MethodTemplate
 	if mh.Autoload {
 		template = append(template, mh.LoadFromFile(mh.AutoloadFile)...)
@@ -60,23 +57,51 @@ func (mh *MockedHandler) Handler(w http.ResponseWriter, req *http.Request) {
 		template = append(template, mh.Overrides...)
 	}
 
-	method := j["method"]
-	block := ""
-	if method == "eth_getBlockByNumber" {
-		block = (j["params"].([]interface{})[0]).(string)
+	batched := proxyd.IsBatch(body)
+	var requests []map[string]interface{}
+	if batched {
+		err = json.Unmarshal(body, &requests)
+		if err != nil {
+			fmt.Printf("error reading request: %v\n", err)
+		}
+	} else {
+		var j map[string]interface{}
+		err = json.Unmarshal(body, &j)
+		if err != nil {
+			fmt.Printf("error reading request: %v\n", err)
+		}
+		requests = append(requests, j)
 	}
 
-	var selectedResponse *string
-	for _, r := range template {
-		if r.Method == method && r.Block == block {
-			selectedResponse = &r.Response
+	var responses []string
+	for _, r := range requests {
+		method := r["method"]
+		block := ""
+		if method == "eth_getBlockByNumber" {
+			block = (r["params"].([]interface{})[0]).(string)
+		}
+
+		var selectedResponse string
+		for _, r := range template {
+			if r.Method == method && r.Block == block {
+				selectedResponse = r.Response
+			}
+		}
+		if selectedResponse != "" {
+			responses = append(responses, selectedResponse)
 		}
 	}
-	if selectedResponse != nil {
-		_, err := fmt.Fprintf(w, *selectedResponse)
-		if err != nil {
-			fmt.Printf("error writing response: %v\n", err)
-		}
+
+	resBody := ""
+	if batched {
+		resBody = "[" + strings.Join(responses, ",") + "]"
+	} else {
+		resBody = responses[0]
+	}
+
+	_, err = fmt.Fprint(w, resBody)
+	if err != nil {
+		fmt.Printf("error writing response: %v\n", err)
 	}
 }