p2p: DNS resolution for static nodes (#30822)

Closes #23210 

# Context 
When deploying Geth in Kubernetes with ReplicaSets, we encountered two
DNS-related issues affecting node connectivity. First, during startup,
Geth tries to resolve DNS names for static nodes too early in the config
unmarshaling phase. If peer nodes aren't ready yet (which is common in
Kubernetes rolling deployments), this causes an immediate failure:


```
INFO [11-26|10:03:42.816] Starting Geth on Ethereum mainnet...
INFO [11-26|10:03:42.817] Bumping default cache on mainnet         provided=1024 updated=4096
Fatal: config.toml, line 81: (p2p.Config.StaticNodes) lookup idontexist.geth.node: no such host
``` 

The second issue comes up when pods get rescheduled to different nodes -
their IPs change but peers keep using the initially resolved IP, never
updating the DNS mapping.

This PR adds proper DNS support for enode:// URLs by deferring resolution
to connection time. It also handles DNS failures gracefully instead of failing
fatally during startup, making it work better in container environments where
IPs are dynamic and peers come and go during rollouts.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

p2p/dial.go

@@ -24,6 +24,7 @@ import (
 	"fmt"
 	mrand "math/rand"
 	"net"
+	"net/netip"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -31,6 +32,7 @@ import (
 	"github.com/ethereum/go-ethereum/common/mclock"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/p2p/enode"
+	"github.com/ethereum/go-ethereum/p2p/enr"
 	"github.com/ethereum/go-ethereum/p2p/netutil"
 )
 
@@ -77,6 +79,7 @@ var (
 	errRecentlyDialed   = errors.New("recently dialed")
 	errNetRestrict      = errors.New("not contained in netrestrict list")
 	errNoPort           = errors.New("node does not provide TCP port")
+	errNoResolvedIP     = errors.New("node does not provide a resolved IP")
 )
 
 // dialer creates outbound connections and submits them into Server.
@@ -90,16 +93,17 @@ var (
 //     to create peer connections to nodes arriving through the iterator.
 type dialScheduler struct {
 	dialConfig
-	setupFunc   dialSetupFunc
-	wg          sync.WaitGroup
-	cancel      context.CancelFunc
-	ctx         context.Context
-	nodesIn     chan *enode.Node
-	doneCh      chan *dialTask
-	addStaticCh chan *enode.Node
-	remStaticCh chan *enode.Node
-	addPeerCh   chan *conn
-	remPeerCh   chan *conn
+	setupFunc     dialSetupFunc
+	dnsLookupFunc func(ctx context.Context, network string, name string) ([]netip.Addr, error)
+	wg            sync.WaitGroup
+	cancel        context.CancelFunc
+	ctx           context.Context
+	nodesIn       chan *enode.Node
+	doneCh        chan *dialTask
+	addStaticCh   chan *enode.Node
+	remStaticCh   chan *enode.Node
+	addPeerCh     chan *conn
+	remPeerCh     chan *conn
 
 	// Everything below here belongs to loop and
 	// should only be accessed by code on the loop goroutine.
@@ -159,18 +163,19 @@ func (cfg dialConfig) withDefaults() dialConfig {
 func newDialScheduler(config dialConfig, it enode.Iterator, setupFunc dialSetupFunc) *dialScheduler {
 	cfg := config.withDefaults()
 	d := &dialScheduler{
-		dialConfig:   cfg,
-		historyTimer: mclock.NewAlarm(cfg.clock),
-		setupFunc:    setupFunc,
-		dialing:      make(map[enode.ID]*dialTask),
-		static:       make(map[enode.ID]*dialTask),
-		peers:        make(map[enode.ID]struct{}),
-		doneCh:       make(chan *dialTask),
-		nodesIn:      make(chan *enode.Node),
-		addStaticCh:  make(chan *enode.Node),
-		remStaticCh:  make(chan *enode.Node),
-		addPeerCh:    make(chan *conn),
-		remPeerCh:    make(chan *conn),
+		dialConfig:    cfg,
+		historyTimer:  mclock.NewAlarm(cfg.clock),
+		setupFunc:     setupFunc,
+		dnsLookupFunc: net.DefaultResolver.LookupNetIP,
+		dialing:       make(map[enode.ID]*dialTask),
+		static:        make(map[enode.ID]*dialTask),
+		peers:         make(map[enode.ID]struct{}),
+		doneCh:        make(chan *dialTask),
+		nodesIn:       make(chan *enode.Node),
+		addStaticCh:   make(chan *enode.Node),
+		remStaticCh:   make(chan *enode.Node),
+		addPeerCh:     make(chan *conn),
+		remPeerCh:     make(chan *conn),
 	}
 	d.lastStatsLog = d.clock.Now()
 	d.ctx, d.cancel = context.WithCancel(context.Background())
@@ -274,7 +279,7 @@ loop:
 		case node := <-d.addStaticCh:
 			id := node.ID()
 			_, exists := d.static[id]
-			d.log.Trace("Adding static node", "id", id, "ip", node.IPAddr(), "added", !exists)
+			d.log.Trace("Adding static node", "id", id, "endpoint", nodeEndpointForLog(node), "added", !exists)
 			if exists {
 				continue loop
 			}
@@ -433,10 +438,68 @@ func (d *dialScheduler) removeFromStaticPool(idx int) {
 	task.staticPoolIndex = -1
 }
 
+// dnsResolveHostname updates the given node from its DNS hostname.
+// This is used to resolve static dial targets.
+func (d *dialScheduler) dnsResolveHostname(n *enode.Node) (*enode.Node, error) {
+	if n.Hostname() == "" {
+		return n, nil
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	foundIPs, err := d.dnsLookupFunc(ctx, "ip", n.Hostname())
+	if err != nil {
+		return n, err
+	}
+
+	// Check for IP updates.
+	var (
+		nodeIP4, nodeIP6   netip.Addr
+		foundIP4, foundIP6 netip.Addr
+	)
+	n.Load((*enr.IPv4Addr)(&nodeIP4))
+	n.Load((*enr.IPv6Addr)(&nodeIP6))
+	for _, ip := range foundIPs {
+		if ip.Is4() && !foundIP4.IsValid() {
+			foundIP4 = ip
+		}
+		if ip.Is6() && !foundIP6.IsValid() {
+			foundIP6 = ip
+		}
+	}
+
+	if !foundIP4.IsValid() && !foundIP6.IsValid() {
+		// Lookup failed.
+		return n, errNoResolvedIP
+	}
+	if foundIP4 == nodeIP4 && foundIP6 == nodeIP6 {
+		// No updates necessary.
+		d.log.Trace("Node DNS lookup had no update", "id", n.ID(), "name", n.Hostname(), "ip", foundIP4, "ip6", foundIP6)
+		return n, nil
+	}
+
+	// Update the node. Note this invalidates the ENR signature, because we use SignNull
+	// to create a modified copy. But this should be OK, since we just use the node as a
+	// dial target. And nodes will usually only have a DNS hostname if they came from a
+	// enode:// URL, which has no signature anyway. If it ever becomes a problem, the
+	// resolved IP could also be stored into dialTask instead of the node.
+	rec := n.Record()
+	if foundIP4.IsValid() {
+		rec.Set(enr.IPv4Addr(foundIP4))
+	}
+	if foundIP6.IsValid() {
+		rec.Set(enr.IPv6Addr(foundIP6))
+	}
+	rec.SetSeq(n.Seq()) // ensure seq not bumped by update
+	newNode := enode.SignNull(rec, n.ID()).WithHostname(n.Hostname())
+	d.log.Debug("Node updated from DNS lookup", "id", n.ID(), "name", n.Hostname(), "ip", newNode.IP())
+	return newNode, nil
+}
+
 // startDial runs the given dial task in a separate goroutine.
 func (d *dialScheduler) startDial(task *dialTask) {
 	node := task.dest()
-	d.log.Trace("Starting p2p dial", "id", node.ID(), "ip", node.IPAddr(), "flag", task.flags)
+	d.log.Trace("Starting p2p dial", "id", node.ID(), "endpoint", nodeEndpointForLog(node), "flag", task.flags)
 	hkey := string(node.ID().Bytes())
 	d.history.add(hkey, d.clock.Now().Add(dialHistoryExpiration))
 	d.dialing[node.ID()] = task
@@ -473,14 +536,29 @@ func (t *dialTask) dest() *enode.Node {
 }
 
 func (t *dialTask) run(d *dialScheduler) {
-	if t.needResolve() && !t.resolve(d) {
-		return
+	if t.isStatic() {
+		// Resolve DNS.
+		if n := t.dest(); n.Hostname() != "" {
+			resolved, err := d.dnsResolveHostname(n)
+			if err != nil {
+				d.log.Warn("DNS lookup of static node failed", "id", n.ID(), "name", n.Hostname(), "err", err)
+			} else {
+				t.destPtr.Store(resolved)
+			}
+		}
+		// Try resolving node ID through the DHT if there is no IP address.
+		if !t.dest().IPAddr().IsValid() {
+			if !t.resolve(d) {
+				return // DHT resolve failed, skip dial.
+			}
+		}
 	}
 
 	err := t.dial(d, t.dest())
 	if err != nil {
 		// For static nodes, resolve one more time if dialing fails.
-		if _, ok := err.(*dialError); ok && t.flags&staticDialedConn != 0 {
+		var dialErr *dialError
+		if errors.As(err, &dialErr) && t.isStatic() {
 			if t.resolve(d) {
 				t.dial(d, t.dest())
 			}
@@ -488,8 +566,8 @@ func (t *dialTask) run(d *dialScheduler) {
 	}
 }
 
-func (t *dialTask) needResolve() bool {
-	return t.flags&staticDialedConn != 0 && !t.dest().IPAddr().IsValid()
+func (t *dialTask) isStatic() bool {
+	return t.flags&staticDialedConn != 0
 }
 
 // resolve attempts to find the current endpoint for the destination
@@ -553,3 +631,10 @@ func cleanupDialErr(err error) error {
 	}
 	return err
 }
+
+func nodeEndpointForLog(n *enode.Node) string {
+	if n.Hostname() != "" {
+		return n.Hostname()
+	}
+	return n.IPAddr().String()
+}

p2p/dial_test.go

@@ -22,6 +22,7 @@ import (
 	"fmt"
 	"math/rand"
 	"net"
+	"net/netip"
 	"reflect"
 	"sync"
 	"testing"
@@ -394,6 +395,34 @@ func TestDialSchedResolve(t *testing.T) {
 	})
 }
 
+func TestDialSchedDNSHostname(t *testing.T) {
+	t.Parallel()
+
+	config := dialConfig{
+		maxActiveDials: 1,
+		maxDialPeers:   1,
+	}
+	node := newNode(uintID(0x01), ":30303").WithHostname("node-hostname")
+	resolved := newNode(uintID(0x01), "1.2.3.4:30303").WithHostname("node-hostname")
+	runDialTest(t, config, []dialTestRound{
+		{
+			update: func(d *dialScheduler) {
+				d.dnsLookupFunc = func(ctx context.Context, network string, name string) ([]netip.Addr, error) {
+					if name != "node-hostname" {
+						t.Error("wrong hostname in DNS lookup:", name)
+					}
+					result := []netip.Addr{netip.MustParseAddr("1.2.3.4")}
+					return result, nil
+				}
+				d.addStatic(node)
+			},
+			wantNewDials: []*enode.Node{
+				resolved,
+			},
+		},
+	})
+}
+
 // -------
 // Code below here is the framework for the tests above.
 

p2p/enode/node.go

@@ -37,6 +37,10 @@ var errMissingPrefix = errors.New("missing 'enr:' prefix for base64-encoded reco
 type Node struct {
 	r  enr.Record
 	id ID
+
+	// hostname tracks the DNS name of the node.
+	hostname string
+
 	// endpoint information
 	ip  netip.Addr
 	udp uint16
@@ -77,6 +81,8 @@ func newNodeWithID(r *enr.Record, id ID) *Node {
 		n.setIP4(ip4)
 	case valid6:
 		n.setIP6(ip6)
+	default:
+		n.setIPv4Ports()
 	}
 	return n
 }
@@ -103,6 +109,10 @@ func localityScore(ip netip.Addr) int {
 
 func (n *Node) setIP4(ip netip.Addr) {
 	n.ip = ip
+	n.setIPv4Ports()
+}
+
+func (n *Node) setIPv4Ports() {
 	n.Load((*enr.UDP)(&n.udp))
 	n.Load((*enr.TCP)(&n.tcp))
 }
@@ -184,6 +194,18 @@ func (n *Node) TCP() int {
 	return int(n.tcp)
 }
 
+// WithHostname adds a DNS hostname to the node.
+func (n *Node) WithHostname(hostname string) *Node {
+	cpy := *n
+	cpy.hostname = hostname
+	return &cpy
+}
+
+// Hostname returns the DNS name assigned by WithHostname.
+func (n *Node) Hostname() string {
+	return n.hostname
+}
+
 // UDPEndpoint returns the announced UDP endpoint.
 func (n *Node) UDPEndpoint() (netip.AddrPort, bool) {
 	if !n.ip.IsValid() || n.ip.IsUnspecified() || n.udp == 0 {

p2p/enode/node_test.go

@@ -74,6 +74,7 @@ func TestNodeEndpoints(t *testing.T) {
 		wantUDP  int
 		wantTCP  int
 		wantQUIC int
+		wantDNS  string
 	}
 	tests := []endpointTest{
 		{
@@ -90,6 +91,7 @@ func TestNodeEndpoints(t *testing.T) {
 				r.Set(enr.UDP(9000))
 				return SignNull(&r, id)
 			}(),
+			wantUDP: 9000,
 		},
 		{
 			name: "tcp-only",
@@ -98,6 +100,7 @@ func TestNodeEndpoints(t *testing.T) {
 				r.Set(enr.TCP(9000))
 				return SignNull(&r, id)
 			}(),
+			wantTCP: 9000,
 		},
 		{
 			name: "quic-only",
@@ -268,6 +271,19 @@ func TestNodeEndpoints(t *testing.T) {
 			wantIP:   netip.MustParseAddr("2001::ff00:0042:8329"),
 			wantQUIC: 9001,
 		},
+		{
+			name: "dns-only",
+			node: func() *Node {
+				var r enr.Record
+				r.Set(enr.UDP(30303))
+				r.Set(enr.TCP(30303))
+				n := SignNull(&r, id).WithHostname("example.com")
+				return n
+			}(),
+			wantTCP: 30303,
+			wantUDP: 30303,
+			wantDNS: "example.com",
+		},
 	}
 
 	for _, test := range tests {
@@ -284,6 +300,9 @@ func TestNodeEndpoints(t *testing.T) {
 			if quic, _ := test.node.QUICEndpoint(); test.wantQUIC != int(quic.Port()) {
 				t.Errorf("node has wrong QUIC port %d, want %d", quic.Port(), test.wantQUIC)
 			}
+			if test.wantDNS != test.node.Hostname() {
+				t.Errorf("node has wrong DNS name %s, want %s", test.node.Hostname(), test.wantDNS)
+			}
 		})
 	}
 }

p2p/enode/urlv4.go

@@ -33,7 +33,6 @@ import (
 
 var (
 	incompleteNodeURL = regexp.MustCompile("(?i)^(?:enode://)?([0-9a-f]+)$")
-	lookupIPFunc      = net.LookupIP
 )
 
 // MustParseV4 parses a node URL. It panics if the URL is not valid.
@@ -126,20 +125,9 @@ func parseComplete(rawurl string) (*Node, error) {
 	if id, err = parsePubkey(u.User.String()); err != nil {
 		return nil, fmt.Errorf("invalid public key (%v)", err)
 	}
-	// Parse the IP address.
+
+	// Parse the IP and ports.
 	ip := net.ParseIP(u.Hostname())
-	if ip == nil {
-		ips, err := lookupIPFunc(u.Hostname())
-		if err != nil {
-			return nil, err
-		}
-		ip = ips[0]
-	}
-	// Ensure the IP is 4 bytes long for IPv4 addresses.
-	if ipv4 := ip.To4(); ipv4 != nil {
-		ip = ipv4
-	}
-	// Parse the port numbers.
 	if tcpPort, err = strconv.ParseUint(u.Port(), 10, 16); err != nil {
 		return nil, errors.New("invalid port")
 	}
@@ -151,7 +139,13 @@ func parseComplete(rawurl string) (*Node, error) {
 			return nil, errors.New("invalid discport in query")
 		}
 	}
-	return NewV4(id, ip, int(tcpPort), int(udpPort)), nil
+
+	// Create the node.
+	node := NewV4(id, ip, int(tcpPort), int(udpPort))
+	if ip == nil && u.Hostname() != "" {
+		node = node.WithHostname(u.Hostname())
+	}
+	return node, nil
 }
 
 // parsePubkey parses a hex-encoded secp256k1 public key.
@@ -181,15 +175,23 @@ func (n *Node) URLv4() string {
 		nodeid = fmt.Sprintf("%s.%x", scheme, n.id[:])
 	}
 	u := url.URL{Scheme: "enode"}
-	if !n.ip.IsValid() {
-		u.Host = nodeid
-	} else {
+	if n.Hostname() != "" {
+		// For nodes with a DNS name: include DNS name, TCP port, and optional UDP port
+		u.User = url.User(nodeid)
+		u.Host = fmt.Sprintf("%s:%d", n.Hostname(), n.TCP())
+		if n.UDP() != n.TCP() {
+			u.RawQuery = "discport=" + strconv.Itoa(n.UDP())
+		}
+	} else if n.ip.IsValid() {
+		// For IP-based nodes: include IP address, TCP port, and optional UDP port
 		addr := net.TCPAddr{IP: n.IP(), Port: n.TCP()}
 		u.User = url.User(nodeid)
 		u.Host = addr.String()
 		if n.UDP() != n.TCP() {
 			u.RawQuery = "discport=" + strconv.Itoa(n.UDP())
 		}
+	} else {
+		u.Host = nodeid
 	}
 	return u.String()
 }

p2p/enode/urlv4_test.go

@@ -18,7 +18,6 @@ package enode
 
 import (
 	"crypto/ecdsa"
-	"errors"
 	"net"
 	"reflect"
 	"strings"
@@ -28,15 +27,6 @@ import (
 	"github.com/ethereum/go-ethereum/p2p/enr"
 )
 
-func init() {
-	lookupIPFunc = func(name string) ([]net.IP, error) {
-		if name == "node.example.org" {
-			return []net.IP{{33, 44, 55, 66}}, nil
-		}
-		return nil, errors.New("no such host")
-	}
-}
-
 var parseNodeTests = []struct {
 	input      string
 	wantError  string
@@ -70,10 +60,6 @@ var parseNodeTests = []struct {
 		wantError: enr.ErrInvalidSig.Error(),
 	},
 	// Complete node URLs with IP address and ports
-	{
-		input:     "enode://1dd9d65c4552b5eb43d5ad55a2ee3f56c6cbc1c64a5c8d659f51fcd51bace24351232b8d7821617d2b29b54b81cdefb9b3e9c37d7fd5f63270bcc9e1a6f6a439@invalid.:3",
-		wantError: `no such host`,
-	},
 	{
 		input:     "enode://1dd9d65c4552b5eb43d5ad55a2ee3f56c6cbc1c64a5c8d659f51fcd51bace24351232b8d7821617d2b29b54b81cdefb9b3e9c37d7fd5f63270bcc9e1a6f6a439@127.0.0.1:foo",
 		wantError: `invalid port`,
@@ -91,6 +77,15 @@ var parseNodeTests = []struct {
 			52150,
 		),
 	},
+	{
+		input: "enode://1dd9d65c4552b5eb43d5ad55a2ee3f56c6cbc1c64a5c8d659f51fcd51bace24351232b8d7821617d2b29b54b81cdefb9b3e9c37d7fd5f63270bcc9e1a6f6a439@valid.:3",
+		wantResult: NewV4(
+			hexPubkey("1dd9d65c4552b5eb43d5ad55a2ee3f56c6cbc1c64a5c8d659f51fcd51bace24351232b8d7821617d2b29b54b81cdefb9b3e9c37d7fd5f63270bcc9e1a6f6a439"),
+			nil,
+			3,
+			3,
+		).WithHostname("valid."),
+	},
 	{
 		input: "enode://1dd9d65c4552b5eb43d5ad55a2ee3f56c6cbc1c64a5c8d659f51fcd51bace24351232b8d7821617d2b29b54b81cdefb9b3e9c37d7fd5f63270bcc9e1a6f6a439@[::]:52150",
 		wantResult: NewV4(

p2p/peer_test.go

@@ -60,22 +60,22 @@ func uintID(i uint16) enode.ID {
 // newNode creates a node record with the given address.
 func newNode(id enode.ID, addr string) *enode.Node {
 	var r enr.Record
-	if addr != "" {
-		// Set the port if present.
-		if strings.Contains(addr, ":") {
-			hs, ps, err := net.SplitHostPort(addr)
-			if err != nil {
-				panic(fmt.Errorf("invalid address %q", addr))
-			}
-			port, err := strconv.Atoi(ps)
-			if err != nil {
-				panic(fmt.Errorf("invalid port in %q", addr))
-			}
-			r.Set(enr.TCP(port))
-			r.Set(enr.UDP(port))
-			addr = hs
+	// Set the port if present.
+	if strings.Contains(addr, ":") {
+		hs, ps, err := net.SplitHostPort(addr)
+		if err != nil {
+			panic(fmt.Errorf("invalid address %q", addr))
+		}
+		port, err := strconv.Atoi(ps)
+		if err != nil {
+			panic(fmt.Errorf("invalid port in %q", addr))
 		}
-		// Set the IP.
+		r.Set(enr.TCP(port))
+		r.Set(enr.UDP(port))
+		addr = hs
+	}
+	// Set the IP.
+	if addr != "" {
 		ip := net.ParseIP(addr)
 		if ip == nil {
 			panic(fmt.Errorf("invalid IP %q", addr))