diff --git a/cmd/geth/dao_test.go b/cmd/geth/dao_test.go
index 59730b17f..56e77ba0b 100644
--- a/cmd/geth/dao_test.go
+++ b/cmd/geth/dao_test.go
@@ -116,19 +116,19 @@ func TestDAOInitOldPrivnet(t *testing.T) {
 	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{}, nil, false)
 }
 func TestDAODefaultOldPrivnet(t *testing.T) {
-	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{false, false}}, params.MainNetDAOForkBlock, true)
+	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{false, false}}, nil, false)
 }
 func TestDAOSupportOldPrivnet(t *testing.T) {
-	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{true, false}}, params.MainNetDAOForkBlock, true)
+	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{true, false}}, nil, true)
 }
 func TestDAOOpposeOldPrivnet(t *testing.T) {
-	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{false, true}}, params.MainNetDAOForkBlock, false)
+	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{false, true}}, nil, false)
 }
 func TestDAOSwitchToSupportOldPrivnet(t *testing.T) {
-	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{false, true}, {true, false}}, params.MainNetDAOForkBlock, true)
+	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{false, true}, {true, false}}, nil, true)
 }
 func TestDAOSwitchToOpposeOldPrivnet(t *testing.T) {
-	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{true, false}, {false, true}}, params.MainNetDAOForkBlock, false)
+	testDAOForkBlockNewChain(t, false, daoOldGenesis, [][2]bool{{true, false}, {false, true}}, nil, false)
 }
 func TestDAOInitNoForkPrivnet(t *testing.T) {
 	testDAOForkBlockNewChain(t, false, daoNoForkGenesis, [][2]bool{}, daoGenesisForkBlock, false)
diff --git a/cmd/utils/flags.go b/cmd/utils/flags.go
index abfb4d1e7..edb0873f6 100644
--- a/cmd/utils/flags.go
+++ b/cmd/utils/flags.go
@@ -781,27 +781,43 @@ func MakeChainConfigFromDb(ctx *cli.Context, db ethdb.Database) *core.ChainConfi
 			Fatalf("Could not make chain configuration: %v", err)
 		}
 	}
-	// Set any missing fields due to them being unset or system upgrade
-	if config.HomesteadBlock == nil {
-		if ctx.GlobalBool(TestNetFlag.Name) {
-			config.HomesteadBlock = params.TestNetHomesteadBlock
-		} else {
-			config.HomesteadBlock = params.MainNetHomesteadBlock
+	// Check whether we are allowed to set default config params or not:
+	//  - If no genesis is set, we're running either mainnet or testnet (private nets use `geth init`)
+	//  - If a genesis is already set, ensure we have a configuration for it (mainnet or testnet)
+	defaults := genesis == nil ||
+		(genesis.Hash() == params.MainNetGenesisHash && !ctx.GlobalBool(TestNetFlag.Name)) ||
+		(genesis.Hash() == params.TestNetGenesisHash && ctx.GlobalBool(TestNetFlag.Name))
+
+	// Set any missing chainConfig fields due to them being unset or system upgrade
+	if defaults {
+		if config.HomesteadBlock == nil {
+			if ctx.GlobalBool(TestNetFlag.Name) {
+				config.HomesteadBlock = params.TestNetHomesteadBlock
+			} else {
+				config.HomesteadBlock = params.MainNetHomesteadBlock
+			}
 		}
-	}
-	if config.DAOForkBlock == nil {
-		if ctx.GlobalBool(TestNetFlag.Name) {
-			config.DAOForkBlock = params.TestNetDAOForkBlock
-		} else {
-			config.DAOForkBlock = params.MainNetDAOForkBlock
+		if config.DAOForkBlock == nil {
+			if ctx.GlobalBool(TestNetFlag.Name) {
+				config.DAOForkBlock = params.TestNetDAOForkBlock
+			} else {
+				config.DAOForkBlock = params.MainNetDAOForkBlock
+			}
+			config.DAOForkSupport = true
 		}
-		config.DAOForkSupport = true
-	}
-	if config.HomesteadGasRepriceBlock == nil {
-		if ctx.GlobalBool(TestNetFlag.Name) {
-			config.HomesteadGasRepriceBlock = params.TestNetHomesteadGasRepriceBlock
-		} else {
-			config.HomesteadGasRepriceBlock = params.MainNetHomesteadGasRepriceBlock
+		if config.HomesteadGasRepriceBlock == nil {
+			if ctx.GlobalBool(TestNetFlag.Name) {
+				config.HomesteadGasRepriceBlock = params.TestNetHomesteadGasRepriceBlock
+			} else {
+				config.HomesteadGasRepriceBlock = params.MainNetHomesteadGasRepriceBlock
+			}
+		}
+		if config.HomesteadGasRepriceHash == (common.Hash{}) {
+			if ctx.GlobalBool(TestNetFlag.Name) {
+				config.HomesteadGasRepriceHash = params.TestNetHomesteadGasRepriceHash
+			} else {
+				config.HomesteadGasRepriceHash = params.MainNetHomesteadGasRepriceHash
+			}
 		}
 	}
 	// Force override any existing configs if explicitly requested
diff --git a/core/block_validator.go b/core/block_validator.go
index e5bc6178b..1bb1a9713 100644
--- a/core/block_validator.go
+++ b/core/block_validator.go
@@ -248,7 +248,15 @@ func ValidateHeader(config *ChainConfig, pow pow.PoW, header *types.Header, pare
 		}
 	}
 	// If all checks passed, validate the extra-data field for hard forks
-	return ValidateDAOHeaderExtraData(config, header)
+	if err := ValidateDAOHeaderExtraData(config, header); err != nil {
+		return err
+	}
+	if config.HomesteadGasRepriceBlock != nil && config.HomesteadGasRepriceBlock.Cmp(header.Number) == 0 {
+		if config.HomesteadGasRepriceHash != (common.Hash{}) && config.HomesteadGasRepriceHash != header.Hash() {
+			return ValidationError("Homestead gas reprice fork hash mismatch: have 0x%x, want 0x%x", header.Hash(), config.HomesteadGasRepriceHash)
+		}
+	}
+	return nil
 }
 
 // CalcDifficulty is the difficulty adjustment algorithm. It returns
diff --git a/core/config.go b/core/config.go
index 3ab04e520..96e39ea3c 100644
--- a/core/config.go
+++ b/core/config.go
@@ -20,6 +20,7 @@ import (
 	"errors"
 	"math/big"
 
+	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/vm"
 	"github.com/ethereum/go-ethereum/params"
 )
@@ -36,7 +37,8 @@ type ChainConfig struct {
 	DAOForkBlock   *big.Int `json:"daoForkBlock"`   // TheDAO hard-fork switch block (nil = no fork)
 	DAOForkSupport bool     `json:"daoForkSupport"` // Whether the nodes supports or opposes the DAO hard-fork
 
-	HomesteadGasRepriceBlock *big.Int `json:"homesteadGasRepriceBlock"` // Homestead gas reprice switch block (nil = no fork)
+	HomesteadGasRepriceBlock *big.Int    `json:"homesteadGasRepriceBlock"` // Homestead gas reprice switch block (nil = no fork)
+	HomesteadGasRepriceHash  common.Hash `json:"homesteadGasRepriceHash"`  // Homestead gas reprice switch block hash (fast sync aid)
 
 	VmConfig vm.Config `json:"-"`
 }
diff --git a/params/util.go b/params/util.go
index 583cf03bb..63f571a17 100644
--- a/params/util.go
+++ b/params/util.go
@@ -16,11 +16,22 @@
 
 package params
 
-import "math/big"
+import (
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/common"
+)
 
 var (
-	TestNetHomesteadBlock           = big.NewInt(494000)  // Testnet homestead block
-	MainNetHomesteadBlock           = big.NewInt(1150000) // Mainnet homestead block
-	TestNetHomesteadGasRepriceBlock = big.NewInt(1783000) // Test net gas reprice block
-	MainNetHomesteadGasRepriceBlock = big.NewInt(2463000) // Main net gas reprice block
+	TestNetGenesisHash = common.HexToHash("0x0cd786a2425d16f152c658316c423e6ce1181e15c3295826d7c9904cba9ce303") // Testnet genesis hash to enforce below configs on
+	MainNetGenesisHash = common.HexToHash("0xd4e56740f876aef8c010b86a40d5f56745a118d0906a34e69aec8c0db1cb8fa3") // Mainnet genesis hash to enforce below configs on
+
+	TestNetHomesteadBlock = big.NewInt(494000)  // Testnet homestead block
+	MainNetHomesteadBlock = big.NewInt(1150000) // Mainnet homestead block
+
+	TestNetHomesteadGasRepriceBlock = big.NewInt(1783000) // Testnet gas reprice block
+	MainNetHomesteadGasRepriceBlock = big.NewInt(2463000) // Mainnet gas reprice block
+
+	TestNetHomesteadGasRepriceHash = common.HexToHash("0xf376243aeff1f256d970714c3de9fd78fa4e63cf63e32a51fe1169e375d98145") // Testnet gas reprice block hash (used by fast sync)
+	MainNetHomesteadGasRepriceHash = common.HexToHash("0x2086799aeebeae135c246c65021c82b4e15a2c451340993aacfd2751886514f0") // Mainnet gas reprice block hash (used by fast sync)
 )