cmd/devp2p: add --mode flag selecting crawl iterator

Wire the new discover.CrawlIterator into devp2p discv4/discv5 crawl
behind a --mode flag (default 'lookup', i.e. existing behaviour).

  devp2p discv4 crawl --mode=fast --timeout 30s nodes.json
  devp2p discv5 crawl --mode=fast --timeout 30s nodes.json

Smoke test against mainnet bootnodes for 30s on a residential link
yields ~2.4x more nodes under --mode=fast (587 vs 240 in one run),
with the new per-tick LogDist log showing a much more uniform
distribution of query distances. Workers default to the existing
--parallel value (16); pacing is RTT-driven.

The 'lookup' default keeps existing behaviour byte-identical for any
operator running the saved devp2p discv4 crawl from a script.

cmd/devp2p/discv4cmd.go

@@ -91,7 +91,7 @@ var (
 		Name:   "crawl",
 		Usage:  "Updates a nodes.json file with random nodes found in the DHT",
 		Action: discv4Crawl,
-		Flags:  slices.Concat(discoveryNodeFlags, []cli.Flag{crawlTimeoutFlag, crawlParallelismFlag}),
+		Flags:  slices.Concat(discoveryNodeFlags, []cli.Flag{crawlTimeoutFlag, crawlParallelismFlag, crawlModeFlag}),
 	}
 	discv4TestCommand = &cli.Command{
 		Name:   "test",
@@ -135,9 +135,14 @@ var (
 	}
 	crawlParallelismFlag = &cli.IntFlag{
 		Name:  "parallel",
-		Usage: "How many parallel discoveries to attempt.",
+		Usage: "How many parallel discoveries to attempt. Used both as the crawler harness's RequestENR worker count and (under -mode=fast) as the FINDNODE iterator's worker count.",
 		Value: 16,
 	}
+	crawlModeFlag = &cli.StringFlag{
+		Name:  "mode",
+		Usage: "Crawl iterator mode: 'lookup' (alpha-bounded Kademlia lookup) or 'fast' (one FINDNODE per peer with rotating prefix; sized by -parallel).",
+		Value: "lookup",
+	}
 	remoteEnodeFlag = &cli.StringFlag{
 		Name:    "remote",
 		Usage:   "Enode of the remote node under test",
@@ -259,7 +264,11 @@ func discv4Crawl(ctx *cli.Context) error {
 	disc, config := startV4(ctx)
 	defer disc.Close()
 
-	c, err := newCrawler(inputSet, config.Bootnodes, disc, disc.RandomNodes())
+	iter, err := newDiscv4CrawlIterator(disc, config.Bootnodes, ctx.String(crawlModeFlag.Name), ctx.Int(crawlParallelismFlag.Name))
+	if err != nil {
+		return err
+	}
+	c, err := newCrawler(inputSet, config.Bootnodes, disc, iter)
 	if err != nil {
 		return err
 	}
@@ -269,6 +278,20 @@ func discv4Crawl(ctx *cli.Context) error {
 	return nil
 }
 
+func newDiscv4CrawlIterator(disc *discover.UDPv4, bootnodes []*enode.Node, mode string, parallel int) (enode.Iterator, error) {
+	switch mode {
+	case "", "lookup":
+		return disc.RandomNodes(), nil
+	case "fast":
+		return disc.CrawlIterator(discover.CrawlOptions{
+			Workers: parallel,
+			Seeds:   bootnodes,
+		}), nil
+	default:
+		return nil, fmt.Errorf("unknown -%s value %q (want 'lookup' or 'fast')", crawlModeFlag.Name, mode)
+	}
+}
+
 // discv4Test runs the protocol test suite.
 func discv4Test(ctx *cli.Context) error {
 	// Configure test package globals.

cmd/devp2p/discv5cmd.go

@@ -25,6 +25,7 @@ import (
 	"github.com/ethereum/go-ethereum/cmd/devp2p/internal/v5test"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/p2p/discover"
+	"github.com/ethereum/go-ethereum/p2p/enode"
 	"github.com/urfave/cli/v2"
 )
 
@@ -58,6 +59,8 @@ var (
 		Action: discv5Crawl,
 		Flags: slices.Concat(discoveryNodeFlags, []cli.Flag{
 			crawlTimeoutFlag,
+			crawlParallelismFlag,
+			crawlModeFlag,
 		}),
 	}
 	discv5TestCommand = &cli.Command{
@@ -111,7 +114,11 @@ func discv5Crawl(ctx *cli.Context) error {
 	disc, config := startV5(ctx)
 	defer disc.Close()
 
-	c, err := newCrawler(inputSet, config.Bootnodes, disc, disc.RandomNodes())
+	iter, err := newDiscv5CrawlIterator(disc, config.Bootnodes, ctx.String(crawlModeFlag.Name), ctx.Int(crawlParallelismFlag.Name))
+	if err != nil {
+		return err
+	}
+	c, err := newCrawler(inputSet, config.Bootnodes, disc, iter)
 	if err != nil {
 		return err
 	}
@@ -121,6 +128,20 @@ func discv5Crawl(ctx *cli.Context) error {
 	return nil
 }
 
+func newDiscv5CrawlIterator(disc *discover.UDPv5, bootnodes []*enode.Node, mode string, parallel int) (enode.Iterator, error) {
+	switch mode {
+	case "", "lookup":
+		return disc.RandomNodes(), nil
+	case "fast":
+		return disc.CrawlIterator(discover.CrawlOptions{
+			Workers: parallel,
+			Seeds:   bootnodes,
+		}), nil
+	default:
+		return nil, fmt.Errorf("unknown -%s value %q (want 'lookup' or 'fast')", crawlModeFlag.Name, mode)
+	}
+}
+
 // discv5Test runs the protocol test suite.
 func discv5Test(ctx *cli.Context) error {
 	suite := &v5test.Suite{