cmd, eth: Add support for --whitelist <blocknum>=<hash>,... flag

* Rejects peers that respond with a different hash for any of the passed in block numbers.
* Meant for emergency situations when the network forks unexpectedly.

eth/config.go

@@ -87,6 +87,9 @@ type Config struct {
 	SyncMode  downloader.SyncMode
 	NoPruning bool
 
+	// Whitelist of required block number -> hash values to accept
+	Whitelist map[uint64]common.Hash `toml:"-"`
+
 	// Light client options
 	LightServ  int `toml:",omitempty"` // Maximum percentage of time allowed for serving LES requests
 	LightPeers int `toml:",omitempty"` // Maximum number of LES client peers