core/forkid: implement the forkid EIP, announce via ENR (#19738)

* eth: chain config (genesis + fork) ENR entry

* core/forkid, eth: protocol independent fork ID, update to CRC32 spec

* core/forkid, eth: make forkid a struct, next uint64, enr struct, RLP

* core/forkid: change forkhash rlp encoding from int to [4]byte

* eth: fixup eth entry a bit and update it every block

* eth: fix lint

* eth: fix crash in ethclient tests

11 files changed, 571 insertions, 96 deletions

diff --git a/core/blockchain.go b/core/blockchain.go
index 8b7c2e615..59be35589 100644
--- a/core/blockchain.go
+++ b/core/blockchain.go
@@ -2146,7 +2146,7 @@ func (bc *BlockChain) GetHeaderByNumber(number uint64) *types.Header {
 	return bc.hc.GetHeaderByNumber(number)
 }
 
-// Config retrieves the blockchain's chain configuration.
+// Config retrieves the chain's fork configuration.
 func (bc *BlockChain) Config() *params.ChainConfig { return bc.chainConfig }
 
 // Engine retrieves the blockchain's consensus engine.
diff --git a/core/forkid/forkid.go b/core/forkid/forkid.go
new file mode 100644
index 000000000..8c1700879
--- /dev/null
+++ b/core/forkid/forkid.go
@@ -0,0 +1,236 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// Package forkid implements EIP-2124 (https://eips.ethereum.org/EIPS/eip-2124).
+package forkid
+
+import (
+	"encoding/binary"
+	"errors"
+	"hash/crc32"
+	"math"
+	"math/big"
+	"reflect"
+	"strings"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core"
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/params"
+)
+
+var (
+	// ErrRemoteStale is returned by the validator if a remote fork checksum is a
+	// subset of our already applied forks, but the announced next fork block is
+	// not on our already passed chain.
+	ErrRemoteStale = errors.New("remote needs update")
+
+	// ErrLocalIncompatibleOrStale is returned by the validator if a remote fork
+	// checksum does not match any local checksum variation, signalling that the
+	// two chains have diverged in the past at some point (possibly at genesis).
+	ErrLocalIncompatibleOrStale = errors.New("local incompatible or needs update")
+)
+
+// ID is a fork identifier as defined by EIP-2124.
+type ID struct {
+	Hash [4]byte // CRC32 checksum of the genesis block and passed fork block numbers
+	Next uint64  // Block number of the next upcoming fork, or 0 if no forks are known
+}
+
+// NewID calculates the Ethereum fork ID from the chain config and head.
+func NewID(chain *core.BlockChain) ID {
+	return newID(
+		chain.Config(),
+		chain.Genesis().Hash(),
+		chain.CurrentHeader().Number.Uint64(),
+	)
+}
+
+// newID is the internal version of NewID, which takes extracted values as its
+// arguments instead of a chain. The reason is to allow testing the IDs without
+// having to simulate an entire blockchain.
+func newID(config *params.ChainConfig, genesis common.Hash, head uint64) ID {
+	// Calculate the starting checksum from the genesis hash
+	hash := crc32.ChecksumIEEE(genesis[:])
+
+	// Calculate the current fork checksum and the next fork block
+	var next uint64
+	for _, fork := range gatherForks(config) {
+		if fork <= head {
+			// Fork already passed, checksum the previous hash and the fork number
+			hash = checksumUpdate(hash, fork)
+			continue
+		}
+		next = fork
+		break
+	}
+	return ID{Hash: checksumToBytes(hash), Next: next}
+}
+
+// NewFilter creates an filter that returns if a fork ID should be rejected or not
+// based on the local chain's status.
+func NewFilter(chain *core.BlockChain) func(id ID) error {
+	return newFilter(
+		chain.Config(),
+		chain.Genesis().Hash(),
+		func() uint64 {
+			return chain.CurrentHeader().Number.Uint64()
+		},
+	)
+}
+
+// newFilter is the internal version of NewFilter, taking closures as its arguments
+// instead of a chain. The reason is to allow testing it without having to simulate
+// an entire blockchain.
+func newFilter(config *params.ChainConfig, genesis common.Hash, headfn func() uint64) func(id ID) error {
+	// Calculate the all the valid fork hash and fork next combos
+	var (
+		forks = gatherForks(config)
+		sums  = make([][4]byte, len(forks)+1) // 0th is the genesis
+	)
+	hash := crc32.ChecksumIEEE(genesis[:])
+	sums[0] = checksumToBytes(hash)
+	for i, fork := range forks {
+		hash = checksumUpdate(hash, fork)
+		sums[i+1] = checksumToBytes(hash)
+	}
+	// Add two sentries to simplify the fork checks and don't require special
+	// casing the last one.
+	forks = append(forks, math.MaxUint64) // Last fork will never be passed
+
+	// Create a validator that will filter out incompatible chains
+	return func(id ID) error {
+		// Run the fork checksum validation ruleset:
+		//   1. If local and remote FORK_CSUM matches, connect.
+		//        The two nodes are in the same fork state currently. They might know
+		//        of differing future forks, but that's not relevant until the fork
+		//        triggers (might be postponed, nodes might be updated to match).
+		//   2. If the remote FORK_CSUM is a subset of the local past forks and the
+		//      remote FORK_NEXT matches with the locally following fork block number,
+		//      connect.
+		//        Remote node is currently syncing. It might eventually diverge from
+		//        us, but at this current point in time we don't have enough information.
+		//   3. If the remote FORK_CSUM is a superset of the local past forks and can
+		//      be completed with locally known future forks, connect.
+		//        Local node is currently syncing. It might eventually diverge from
+		//        the remote, but at this current point in time we don't have enough
+		//        information.
+		//   4. Reject in all other cases.
+		head := headfn()
+		for i, fork := range forks {
+			// If our head is beyond this fork, continue to the next (we have a dummy
+			// fork of maxuint64 as the last item to always fail this check eventually).
+			if head > fork {
+				continue
+			}
+			// Found the first unpassed fork block, check if our current state matches
+			// the remote checksum (rule #1).
+			if sums[i] == id.Hash {
+				// Yay, fork checksum matched, ignore any upcoming fork
+				return nil
+			}
+			// The local and remote nodes are in different forks currently, check if the
+			// remote checksum is a subset of our local forks (rule #2).
+			for j := 0; j < i; j++ {
+				if sums[j] == id.Hash {
+					// Remote checksum is a subset, validate based on the announced next fork
+					if forks[j] != id.Next {
+						return ErrRemoteStale
+					}
+					return nil
+				}
+			}
+			// Remote chain is not a subset of our local one, check if it's a superset by
+			// any chance, signalling that we're simply out of sync (rule #3).
+			for j := i + 1; j < len(sums); j++ {
+				if sums[j] == id.Hash {
+					// Yay, remote checksum is a superset, ignore upcoming forks
+					return nil
+				}
+			}
+			// No exact, subset or superset match. We are on differing chains, reject.
+			return ErrLocalIncompatibleOrStale
+		}
+		log.Error("Impossible fork ID validation", "id", id)
+		return nil // Something's very wrong, accept rather than reject
+	}
+}
+
+// checksum calculates the IEEE CRC32 checksum of a block number.
+func checksum(fork uint64) uint32 {
+	var blob [8]byte
+	binary.BigEndian.PutUint64(blob[:], fork)
+	return crc32.ChecksumIEEE(blob[:])
+}
+
+// checksumUpdate calculates the next IEEE CRC32 checksum based on the previous
+// one and a fork block number (equivalent to CRC32(original-blob || fork)).
+func checksumUpdate(hash uint32, fork uint64) uint32 {
+	var blob [8]byte
+	binary.BigEndian.PutUint64(blob[:], fork)
+	return crc32.Update(hash, crc32.IEEETable, blob[:])
+}
+
+// checksumToBytes converts a uint32 checksum into a [4]byte array.
+func checksumToBytes(hash uint32) [4]byte {
+	var blob [4]byte
+	binary.BigEndian.PutUint32(blob[:], hash)
+	return blob
+}
+
+// gatherForks gathers all the known forks and creates a sorted list out of them.
+func gatherForks(config *params.ChainConfig) []uint64 {
+	// Gather all the fork block numbers via reflection
+	kind := reflect.TypeOf(params.ChainConfig{})
+	conf := reflect.ValueOf(config).Elem()
+
+	var forks []uint64
+	for i := 0; i < kind.NumField(); i++ {
+		// Fetch the next field and skip non-fork rules
+		field := kind.Field(i)
+		if !strings.HasSuffix(field.Name, "Block") {
+			continue
+		}
+		if field.Type != reflect.TypeOf(new(big.Int)) {
+			continue
+		}
+		// Extract the fork rule block number and aggregate it
+		rule := conf.Field(i).Interface().(*big.Int)
+		if rule != nil {
+			forks = append(forks, rule.Uint64())
+		}
+	}
+	// Sort the fork block numbers to permit chronologival XOR
+	for i := 0; i < len(forks); i++ {
+		for j := i + 1; j < len(forks); j++ {
+			if forks[i] > forks[j] {
+				forks[i], forks[j] = forks[j], forks[i]
+			}
+		}
+	}
+	// Deduplicate block numbers applying multiple forks
+	for i := 1; i < len(forks); i++ {
+		if forks[i] == forks[i-1] {
+			forks = append(forks[:i], forks[i+1:]...)
+			i--
+		}
+	}
+	// Skip any forks in block 0, that's the genesis ruleset
+	if len(forks) > 0 && forks[0] == 0 {
+		forks = forks[1:]
+	}
+	return forks
+}
diff --git a/core/forkid/forkid_test.go b/core/forkid/forkid_test.go
new file mode 100644
index 000000000..b33f85bec
--- /dev/null
+++ b/core/forkid/forkid_test.go
@@ -0,0 +1,205 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package forkid
+
+import (
+	"bytes"
+	"math"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/params"
+	"github.com/ethereum/go-ethereum/rlp"
+)
+
+// TestCreation tests that different genesis and fork rule combinations result in
+// the correct fork ID.
+func TestCreation(t *testing.T) {
+	type testcase struct {
+		head uint64
+		want ID
+	}
+	tests := []struct {
+		config  *params.ChainConfig
+		genesis common.Hash
+		cases   []testcase
+	}{
+		// Mainnet test cases
+		{
+			params.MainnetChainConfig,
+			params.MainnetGenesisHash,
+			[]testcase{
+				{0, ID{Hash: checksumToBytes(0xfc64ec04), Next: 1150000}},       // Unsynced
+				{1149999, ID{Hash: checksumToBytes(0xfc64ec04), Next: 1150000}}, // Last Frontier block
+				{1150000, ID{Hash: checksumToBytes(0x97c2c34c), Next: 1920000}}, // First Homestead block
+				{1919999, ID{Hash: checksumToBytes(0x97c2c34c), Next: 1920000}}, // Last Homestead block
+				{1920000, ID{Hash: checksumToBytes(0x91d1f948), Next: 2463000}}, // First DAO block
+				{2462999, ID{Hash: checksumToBytes(0x91d1f948), Next: 2463000}}, // Last DAO block
+				{2463000, ID{Hash: checksumToBytes(0x7a64da13), Next: 2675000}}, // First Tangerine block
+				{2674999, ID{Hash: checksumToBytes(0x7a64da13), Next: 2675000}}, // Last Tangerine block
+				{2675000, ID{Hash: checksumToBytes(0x3edd5b10), Next: 4370000}}, // First Spurious block
+				{4369999, ID{Hash: checksumToBytes(0x3edd5b10), Next: 4370000}}, // Last Spurious block
+				{4370000, ID{Hash: checksumToBytes(0xa00bc324), Next: 7280000}}, // First Byzantium block
+				{7279999, ID{Hash: checksumToBytes(0xa00bc324), Next: 7280000}}, // Last Byzantium block
+				{7280000, ID{Hash: checksumToBytes(0x668db0af), Next: 0}},       // First and last Constantinople, first Petersburg block
+				{7987396, ID{Hash: checksumToBytes(0x668db0af), Next: 0}},       // Today Petersburg block
+			},
+		},
+		// Ropsten test cases
+		{
+			params.TestnetChainConfig,
+			params.TestnetGenesisHash,
+			[]testcase{
+				{0, ID{Hash: checksumToBytes(0x30c7ddbc), Next: 10}},            // Unsynced, last Frontier, Homestead and first Tangerine block
+				{9, ID{Hash: checksumToBytes(0x30c7ddbc), Next: 10}},            // Last Tangerine block
+				{10, ID{Hash: checksumToBytes(0x63760190), Next: 1700000}},      // First Spurious block
+				{1699999, ID{Hash: checksumToBytes(0x63760190), Next: 1700000}}, // Last Spurious block
+				{1700000, ID{Hash: checksumToBytes(0x3ea159c7), Next: 4230000}}, // First Byzantium block
+				{4229999, ID{Hash: checksumToBytes(0x3ea159c7), Next: 4230000}}, // Last Byzantium block
+				{4230000, ID{Hash: checksumToBytes(0x97b544f3), Next: 4939394}}, // First Constantinople block
+				{4939393, ID{Hash: checksumToBytes(0x97b544f3), Next: 4939394}}, // Last Constantinople block
+				{4939394, ID{Hash: checksumToBytes(0xd6e2149b), Next: 0}},       // First Petersburg block
+				{5822692, ID{Hash: checksumToBytes(0xd6e2149b), Next: 0}},       // Today Petersburg block
+			},
+		},
+		// Rinkeby test cases
+		{
+			params.RinkebyChainConfig,
+			params.RinkebyGenesisHash,
+			[]testcase{
+				{0, ID{Hash: checksumToBytes(0x3b8e0691), Next: 1}},             // Unsynced, last Frontier block
+				{1, ID{Hash: checksumToBytes(0x60949295), Next: 2}},             // First and last Homestead block
+				{2, ID{Hash: checksumToBytes(0x8bde40dd), Next: 3}},             // First and last Tangerine block
+				{3, ID{Hash: checksumToBytes(0xcb3a64bb), Next: 1035301}},       // First Spurious block
+				{1035300, ID{Hash: checksumToBytes(0xcb3a64bb), Next: 1035301}}, // Last Spurious block
+				{1035301, ID{Hash: checksumToBytes(0x8d748b57), Next: 3660663}}, // First Byzantium block
+				{3660662, ID{Hash: checksumToBytes(0x8d748b57), Next: 3660663}}, // Last Byzantium block
+				{3660663, ID{Hash: checksumToBytes(0xe49cab14), Next: 4321234}}, // First Constantinople block
+				{4321233, ID{Hash: checksumToBytes(0xe49cab14), Next: 4321234}}, // Last Constantinople block
+				{4321234, ID{Hash: checksumToBytes(0xafec6b27), Next: 0}},       // First Petersburg block
+				{4586649, ID{Hash: checksumToBytes(0xafec6b27), Next: 0}},       // Today Petersburg block
+			},
+		},
+		// Goerli test cases
+		{
+			params.GoerliChainConfig,
+			params.GoerliGenesisHash,
+			[]testcase{
+				{0, ID{Hash: checksumToBytes(0xa3f5ab08), Next: 0}},      // Unsynced, last Frontier, Homestead, Tangerine, Spurious, Byzantium, Constantinople and first Petersburg block
+				{795329, ID{Hash: checksumToBytes(0xa3f5ab08), Next: 0}}, // Today Petersburg block
+			},
+		},
+	}
+	for i, tt := range tests {
+		for j, ttt := range tt.cases {
+			if have := newID(tt.config, tt.genesis, ttt.head); have != ttt.want {
+				t.Errorf("test %d, case %d: fork ID mismatch: have %x, want %x", i, j, have, ttt.want)
+			}
+		}
+	}
+}
+
+// TestValidation tests that a local peer correctly validates and accepts a remote
+// fork ID.
+func TestValidation(t *testing.T) {
+	tests := []struct {
+		head uint64
+		id   ID
+		err  error
+	}{
+		// Local is mainnet Petersburg, remote announces the same. No future fork is announced.
+		{7987396, ID{Hash: checksumToBytes(0x668db0af), Next: 0}, nil},
+
+		// Local is mainnet Petersburg, remote announces the same. Remote also announces a next fork
+		// at block 0xffffffff, but that is uncertain.
+		{7987396, ID{Hash: checksumToBytes(0x668db0af), Next: math.MaxUint64}, nil},
+
+		// Local is mainnet currently in Byzantium only (so it's aware of Petersburg), remote announces
+		// also Byzantium, but it's not yet aware of Petersburg (e.g. non updated node before the fork).
+		// In this case we don't know if Petersburg passed yet or not.
+		{7279999, ID{Hash: checksumToBytes(0xa00bc324), Next: 0}, nil},
+
+		// Local is mainnet currently in Byzantium only (so it's aware of Petersburg), remote announces
+		// also Byzantium, and it's also aware of Petersburg (e.g. updated node before the fork). We
+		// don't know if Petersburg passed yet (will pass) or not.
+		{7279999, ID{Hash: checksumToBytes(0xa00bc324), Next: 7280000}, nil},
+
+		// Local is mainnet currently in Byzantium only (so it's aware of Petersburg), remote announces
+		// also Byzantium, and it's also aware of some random fork (e.g. misconfigured Petersburg). As
+		// neither forks passed at neither nodes, they may mismatch, but we still connect for now.
+		{7279999, ID{Hash: checksumToBytes(0xa00bc324), Next: math.MaxUint64}, nil},
+
+		// Local is mainnet Petersburg, remote announces Byzantium + knowledge about Petersburg. Remote
+		// is simply out of sync, accept.
+		{7987396, ID{Hash: checksumToBytes(0x668db0af), Next: 7280000}, nil},
+
+		// Local is mainnet Petersburg, remote announces Spurious + knowledge about Byzantium. Remote
+		// is definitely out of sync. It may or may not need the Petersburg update, we don't know yet.
+		{7987396, ID{Hash: checksumToBytes(0x3edd5b10), Next: 4370000}, nil},
+
+		// Local is mainnet Byzantium, remote announces Petersburg. Local is out of sync, accept.
+		{7279999, ID{Hash: checksumToBytes(0x668db0af), Next: 0}, nil},
+
+		// Local is mainnet Spurious, remote announces Byzantium, but is not aware of Petersburg. Local
+		// out of sync. Local also knows about a future fork, but that is uncertain yet.
+		{4369999, ID{Hash: checksumToBytes(0xa00bc324), Next: 0}, nil},
+
+		// Local is mainnet Petersburg. remote announces Byzantium but is not aware of further forks.
+		// Remote needs software update.
+		{7987396, ID{Hash: checksumToBytes(0xa00bc324), Next: 0}, ErrRemoteStale},
+
+		// Local is mainnet Petersburg, and isn't aware of more forks. Remote announces Petersburg +
+		// 0xffffffff. Local needs software update, reject.
+		{7987396, ID{Hash: checksumToBytes(0x5cddc0e1), Next: 0}, ErrLocalIncompatibleOrStale},
+
+		// Local is mainnet Byzantium, and is aware of Petersburg. Remote announces Petersburg +
+		// 0xffffffff. Local needs software update, reject.
+		{7279999, ID{Hash: checksumToBytes(0x5cddc0e1), Next: 0}, ErrLocalIncompatibleOrStale},
+
+		// Local is mainnet Petersburg, remote is Rinkeby Petersburg.
+		{7987396, ID{Hash: checksumToBytes(0xafec6b27), Next: 0}, ErrLocalIncompatibleOrStale},
+	}
+	for i, tt := range tests {
+		filter := newFilter(params.MainnetChainConfig, params.MainnetGenesisHash, func() uint64 { return tt.head })
+		if err := filter(tt.id); err != tt.err {
+			t.Errorf("test %d: validation error mismatch: have %v, want %v", i, err, tt.err)
+		}
+	}
+}
+
+// Tests that IDs are properly RLP encoded (specifically important because we
+// use uint32 to store the hash, but we need to encode it as [4]byte).
+func TestEncoding(t *testing.T) {
+	tests := []struct {
+		id   ID
+		want []byte
+	}{
+		{ID{Hash: checksumToBytes(0), Next: 0}, common.Hex2Bytes("c6840000000080")},
+		{ID{Hash: checksumToBytes(0xdeadbeef), Next: 0xBADDCAFE}, common.Hex2Bytes("ca84deadbeef84baddcafe,")},
+		{ID{Hash: checksumToBytes(math.MaxUint32), Next: math.MaxUint64}, common.Hex2Bytes("ce84ffffffff88ffffffffffffffff")},
+	}
+	for i, tt := range tests {
+		have, err := rlp.EncodeToBytes(tt.id)
+		if err != nil {
+			t.Errorf("test %d: failed to encode forkid: %v", i, err)
+			continue
+		}
+		if !bytes.Equal(have, tt.want) {
+			t.Errorf("test %d: RLP mismatch: have %x, want %x", i, have, tt.want)
+		}
+	}
+}
diff --git a/eth/backend.go b/eth/backend.go
index a3275caca..dc4ff8ade 100644
--- a/eth/backend.go
+++ b/eth/backend.go
@@ -47,6 +47,7 @@ import (
 	"github.com/ethereum/go-ethereum/miner"
 	"github.com/ethereum/go-ethereum/node"
 	"github.com/ethereum/go-ethereum/p2p"
+	"github.com/ethereum/go-ethereum/p2p/enr"
 	"github.com/ethereum/go-ethereum/params"
 	"github.com/ethereum/go-ethereum/rlp"
 	"github.com/ethereum/go-ethereum/rpc"
@@ -66,7 +67,9 @@ type Ethereum struct {
 	config *Config
 
 	// Channel for shutting down the service
-	shutdownChan chan bool // Channel for shutting down the Ethereum
+	shutdownChan chan bool
+
+	server *p2p.Server
 
 	// Handlers
 	txPool          *core.TxPool
@@ -496,7 +499,7 @@ func (s *Ethereum) EventMux() *event.TypeMux           { return s.eventMux }
 func (s *Ethereum) Engine() consensus.Engine           { return s.engine }
 func (s *Ethereum) ChainDb() ethdb.Database            { return s.chainDb }
 func (s *Ethereum) IsListening() bool                  { return true } // Always listening
-func (s *Ethereum) EthVersion() int                    { return int(s.protocolManager.SubProtocols[0].Version) }
+func (s *Ethereum) EthVersion() int                    { return int(ProtocolVersions[0]) }
 func (s *Ethereum) NetVersion() uint64                 { return s.networkID }
 func (s *Ethereum) Downloader() *downloader.Downloader { return s.protocolManager.downloader }
 func (s *Ethereum) Synced() bool                       { return atomic.LoadUint32(&s.protocolManager.acceptTxs) == 1 }
@@ -505,15 +508,22 @@ func (s *Ethereum) ArchiveMode() bool                  { return s.config.NoPruni
 // Protocols implements node.Service, returning all the currently configured
 // network protocols to start.
 func (s *Ethereum) Protocols() []p2p.Protocol {
-	if s.lesServer == nil {
-		return s.protocolManager.SubProtocols
+	protos := make([]p2p.Protocol, len(ProtocolVersions))
+	for i, vsn := range ProtocolVersions {
+		protos[i] = s.protocolManager.makeProtocol(vsn)
+		protos[i].Attributes = []enr.Entry{s.currentEthEntry()}
+	}
+	if s.lesServer != nil {
+		protos = append(protos, s.lesServer.Protocols()...)
 	}
-	return append(s.protocolManager.SubProtocols, s.lesServer.Protocols()...)
+	return protos
 }
 
 // Start implements node.Service, starting all internal goroutines needed by the
 // Ethereum protocol implementation.
 func (s *Ethereum) Start(srvr *p2p.Server) error {
+	s.startEthEntryUpdate(srvr.LocalNode())
+
 	// Start the bloom bits servicing goroutines
 	s.startBloomHandlers(params.BloomBitsBlocks)
 
diff --git a/eth/enr_entry.go b/eth/enr_entry.go
new file mode 100644
index 000000000..d9e7b9578
--- /dev/null
+++ b/eth/enr_entry.go
@@ -0,0 +1,61 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package eth
+
+import (
+	"github.com/ethereum/go-ethereum/core"
+	"github.com/ethereum/go-ethereum/core/forkid"
+	"github.com/ethereum/go-ethereum/p2p/enode"
+	"github.com/ethereum/go-ethereum/rlp"
+)
+
+// ethEntry is the "eth" ENR entry which advertises eth protocol
+// on the discovery network.
+type ethEntry struct {
+	ForkID forkid.ID // Fork identifier per EIP-2124
+
+	// Ignore additional fields (for forward compatibility).
+	Rest []rlp.RawValue `rlp:"tail"`
+}
+
+// ENRKey implements enr.Entry.
+func (e ethEntry) ENRKey() string {
+	return "eth"
+}
+
+func (eth *Ethereum) startEthEntryUpdate(ln *enode.LocalNode) {
+	var newHead = make(chan core.ChainHeadEvent, 10)
+	sub := eth.blockchain.SubscribeChainHeadEvent(newHead)
+
+	go func() {
+		defer sub.Unsubscribe()
+		for {
+			select {
+			case <-newHead:
+				ln.Set(eth.currentEthEntry())
+			case <-sub.Err():
+				// Would be nice to sync with eth.Stop, but there is no
+				// good way to do that.
+				return
+			}
+		}
+	}()
+}
+
+func (eth *Ethereum) currentEthEntry() *ethEntry {
+	return &ethEntry{ForkID: forkid.NewID(eth.blockchain)}
+}
diff --git a/eth/handler.go b/eth/handler.go
index fe9f8b53b..4ce2d1c82 100644
--- a/eth/handler.go
+++ b/eth/handler.go
@@ -58,10 +58,6 @@ var (
 	syncChallengeTimeout = 15 * time.Second // Time allowance for a node to reply to the sync progress challenge
 )
 
-// errIncompatibleConfig is returned if the requested protocols and configs are
-// not compatible (low protocol version restrictions and high requirements).
-var errIncompatibleConfig = errors.New("incompatible configuration")
-
 func errResp(code errCode, format string, v ...interface{}) error {
 	return fmt.Errorf("%v - %v", code, fmt.Sprintf(format, v...))
 }
@@ -75,17 +71,14 @@ type ProtocolManager struct {
 	checkpointNumber uint64      // Block number for the sync progress validator to cross reference
 	checkpointHash   common.Hash // Block hash for the sync progress validator to cross reference
 
-	txpool      txPool
-	blockchain  *core.BlockChain
-	chainconfig *params.ChainConfig
-	maxPeers    int
+	txpool     txPool
+	blockchain *core.BlockChain
+	maxPeers   int
 
 	downloader *downloader.Downloader
 	fetcher    *fetcher.Fetcher
 	peers      *peerSet
 
-	SubProtocols []p2p.Protocol
-
 	eventMux      *event.TypeMux
 	txsCh         chan core.NewTxsEvent
 	txsSub        event.Subscription
@@ -113,7 +106,6 @@ func NewProtocolManager(config *params.ChainConfig, checkpoint *params.TrustedCh
 		eventMux:    mux,
 		txpool:      txpool,
 		blockchain:  blockchain,
-		chainconfig: config,
 		peers:       newPeerSet(),
 		whitelist:   whitelist,
 		newPeerCh:   make(chan *peer),
@@ -149,45 +141,7 @@ func NewProtocolManager(config *params.ChainConfig, checkpoint *params.TrustedCh
 		manager.checkpointNumber = (checkpoint.SectionIndex+1)*params.CHTFrequency - 1
 		manager.checkpointHash = checkpoint.SectionHead
 	}
-	// Initiate a sub-protocol for every implemented version we can handle
-	manager.SubProtocols = make([]p2p.Protocol, 0, len(ProtocolVersions))
-	for i, version := range ProtocolVersions {
-		// Skip protocol version if incompatible with the mode of operation
-		// TODO(karalabe): hard-drop eth/62 from the code base
-		if atomic.LoadUint32(&manager.fastSync) == 1 && version < eth63 {
-			continue
-		}
-		// Compatible; initialise the sub-protocol
-		version := version // Closure for the run
-		manager.SubProtocols = append(manager.SubProtocols, p2p.Protocol{
-			Name:    ProtocolName,
-			Version: version,
-			Length:  ProtocolLengths[i],
-			Run: func(p *p2p.Peer, rw p2p.MsgReadWriter) error {
-				peer := manager.newPeer(int(version), p, rw)
-				select {
-				case manager.newPeerCh <- peer:
-					manager.wg.Add(1)
-					defer manager.wg.Done()
-					return manager.handle(peer)
-				case <-manager.quitSync:
-					return p2p.DiscQuitting
-				}
-			},
-			NodeInfo: func() interface{} {
-				return manager.NodeInfo()
-			},
-			PeerInfo: func(id enode.ID) interface{} {
-				if p := manager.peers.Peer(fmt.Sprintf("%x", id[:8])); p != nil {
-					return p.Info()
-				}
-				return nil
-			},
-		})
-	}
-	if len(manager.SubProtocols) == 0 {
-		return nil, errIncompatibleConfig
-	}
+
 	// Construct the downloader (long sync) and its backing state bloom if fast
 	// sync is requested. The downloader is responsible for deallocating the state
 	// bloom when it's done.
@@ -235,6 +189,39 @@ func NewProtocolManager(config *params.ChainConfig, checkpoint *params.TrustedCh
 	return manager, nil
 }
 
+func (pm *ProtocolManager) makeProtocol(version uint) p2p.Protocol {
+	length, ok := protocolLengths[version]
+	if !ok {
+		panic("makeProtocol for unknown version")
+	}
+
+	return p2p.Protocol{
+		Name:    protocolName,
+		Version: version,
+		Length:  length,
+		Run: func(p *p2p.Peer, rw p2p.MsgReadWriter) error {
+			peer := pm.newPeer(int(version), p, rw)
+			select {
+			case pm.newPeerCh <- peer:
+				pm.wg.Add(1)
+				defer pm.wg.Done()
+				return pm.handle(peer)
+			case <-pm.quitSync:
+				return p2p.DiscQuitting
+			}
+		},
+		NodeInfo: func() interface{} {
+			return pm.NodeInfo()
+		},
+		PeerInfo: func(id enode.ID) interface{} {
+			if p := pm.peers.Peer(fmt.Sprintf("%x", id[:8])); p != nil {
+				return p.Info()
+			}
+			return nil
+		},
+	}
+}
+
 func (pm *ProtocolManager) removePeer(id string) {
 	// Short circuit if the peer was already removed
 	peer := pm.peers.Peer(id)
@@ -381,8 +368,8 @@ func (pm *ProtocolManager) handleMsg(p *peer) error {
 	if err != nil {
 		return err
 	}
-	if msg.Size > ProtocolMaxMsgSize {
-		return errResp(ErrMsgTooLarge, "%v > %v", msg.Size, ProtocolMaxMsgSize)
+	if msg.Size > protocolMaxMsgSize {
+		return errResp(ErrMsgTooLarge, "%v > %v", msg.Size, protocolMaxMsgSize)
 	}
 	defer msg.Discard()
 
diff --git a/eth/handler_test.go b/eth/handler_test.go
index 445d52afc..0f1672fd4 100644
--- a/eth/handler_test.go
+++ b/eth/handler_test.go
@@ -38,35 +38,6 @@ import (
 	"github.com/ethereum/go-ethereum/params"
 )
 
-// Tests that protocol versions and modes of operations are matched up properly.
-func TestProtocolCompatibility(t *testing.T) {
-	// Define the compatibility chart
-	tests := []struct {
-		version    uint
-		mode       downloader.SyncMode
-		compatible bool
-	}{
-		{61, downloader.FullSync, true}, {62, downloader.FullSync, true}, {63, downloader.FullSync, true},
-		{61, downloader.FastSync, false}, {62, downloader.FastSync, false}, {63, downloader.FastSync, true},
-	}
-	// Make sure anything we screw up is restored
-	backup := ProtocolVersions
-	defer func() { ProtocolVersions = backup }()
-
-	// Try all available compatibility configs and check for errors
-	for i, tt := range tests {
-		ProtocolVersions = []uint{tt.version}
-
-		pm, _, err := newTestProtocolManager(tt.mode, 0, nil, nil)
-		if pm != nil {
-			defer pm.Stop()
-		}
-		if (err == nil && !tt.compatible) || (err != nil && tt.compatible) {
-			t.Errorf("test %d: compatibility mismatch: have error %v, want compatibility %v", i, err, tt.compatible)
-		}
-	}
-}
-
 // Tests that block headers can be retrieved from a remote chain based on user queries.
 func TestGetBlockHeaders62(t *testing.T) { testGetBlockHeaders(t, 62) }
 func TestGetBlockHeaders63(t *testing.T) { testGetBlockHeaders(t, 63) }
diff --git a/eth/peer.go b/eth/peer.go
index 208badc5e..814c787b8 100644
--- a/eth/peer.go
+++ b/eth/peer.go
@@ -394,8 +394,8 @@ func (p *peer) readStatus(network uint64, status *statusData, genesis common.Has
 	if msg.Code != StatusMsg {
 		return errResp(ErrNoStatusMsg, "first msg has code %x (!= %x)", msg.Code, StatusMsg)
 	}
-	if msg.Size > ProtocolMaxMsgSize {
-		return errResp(ErrMsgTooLarge, "%v > %v", msg.Size, ProtocolMaxMsgSize)
+	if msg.Size > protocolMaxMsgSize {
+		return errResp(ErrMsgTooLarge, "%v > %v", msg.Size, protocolMaxMsgSize)
 	}
 	// Decode the handshake and make sure everything matches
 	if err := msg.Decode(&status); err != nil {
diff --git a/eth/protocol.go b/eth/protocol.go
index 5beb562f8..de0c979d8 100644
--- a/eth/protocol.go
+++ b/eth/protocol.go
@@ -34,16 +34,16 @@ const (
 	eth63 = 63
 )
 
-// ProtocolName is the official short name of the protocol used during capability negotiation.
-var ProtocolName = "eth"
+// protocolName is the official short name of the protocol used during capability negotiation.
+const protocolName = "eth"
 
 // ProtocolVersions are the supported versions of the eth protocol (first is primary).
-var ProtocolVersions = []uint{eth63, eth62}
+var ProtocolVersions = []uint{eth63}
 
-// ProtocolLengths are the number of implemented message corresponding to different protocol versions.
-var ProtocolLengths = []uint64{17, 8}
+// protocolLengths are the number of implemented message corresponding to different protocol versions.
+var protocolLengths = map[uint]uint64{eth63: 17, eth62: 8}
 
-const ProtocolMaxMsgSize = 10 * 1024 * 1024 // Maximum cap on the size of a protocol message
+const protocolMaxMsgSize = 10 * 1024 * 1024 // Maximum cap on the size of a protocol message
 
 // eth protocol message codes
 const (
diff --git a/p2p/enr/enr_test.go b/p2p/enr/enr_test.go
index 434685e0b..96a9ced5c 100644
--- a/p2p/enr/enr_test.go
+++ b/p2p/enr/enr_test.go
@@ -70,7 +70,7 @@ func TestGetSetIPv6(t *testing.T) {
 	assert.Equal(t, ip, ip2)
 }
 
-// TestGetSetDiscPort tests encoding/decoding and setting/getting of the DiscPort key.
+// TestGetSetUDP tests encoding/decoding and setting/getting of the UDP key.
 func TestGetSetUDP(t *testing.T) {
 	port := UDP(30309)
 	var r Record
diff --git a/p2p/server.go b/p2p/server.go
index 12b4c05c1..692c9eb7d 100644
--- a/p2p/server.go
+++ b/p2p/server.go
@@ -286,6 +286,11 @@ func (c *conn) set(f connFlag, val bool) {
 	}
 }
 
+// LocalNode returns the local node record.
+func (srv *Server) LocalNode() *enode.LocalNode {
+	return srv.localnode
+}
+
 // Peers returns all connected peers.
 func (srv *Server) Peers() []*Peer {
 	var ps []*Peer