Rate limit free-to-relay transactions.

Closes #40

ok @davecgh

config.go

@@ -37,6 +37,7 @@ const (
 	defaultMaxRPCWebsockets = 25
 	defaultVerifyEnabled    = false
 	defaultDbType           = "leveldb"
+	defaultFreeTxRelayLimit = 15.0
 )
 
 var (
@@ -93,6 +94,7 @@ type config struct {
 	CpuProfile         string        `long:"cpuprofile" description:"Write CPU profile to the specified file"`
 	DebugLevel         string        `short:"d" long:"debuglevel" description:"Logging level for all subsystems {trace, debug, info, warn, error, critical} -- You may also specify <subsystem>=<level>,<subsystem2>=<level>,... to set the log level for individual subsystems -- Use show to list available subsystems"`
 	Upnp               bool          `long:"upnp" description:"Use UPnP to map our listening port outside of NAT"`
+	FreeTxRelayLimit   float64       `long:"limitfreerelay" description:"Limit relay of transactions with no transaction fee to the given amount in thousands of bytes per minute"`
 	onionlookup        func(string) ([]net.IP, error)
 	lookup             func(string) ([]net.IP, error)
 	oniondial          func(string, string) (net.Conn, error)
@@ -293,6 +295,7 @@ func loadConfig() (*config, []string, error) {
 		DbType:           defaultDbType,
 		RPCKey:           defaultRPCKeyFile,
 		RPCCert:          defaultRPCCertFile,
+		FreeTxRelayLimit: defaultFreeTxRelayLimit,
 	}
 
 	// Service options which are only added on Windows.

doc.go

@@ -74,6 +74,9 @@ Application Options:
                            the log level for individual subsystems -- Use show
                            to list available subsystems (info)
       --upnp               Use UPnP to map our listening port outside of NAT
+      --limitfreerelay=    Limit relay of transactions with no transaction fee
+                           to the given amount in thousands of bytes per minute
+                           (15)
 
 Help Options:
   -h, --help           Show this help message

mempool.go

@@ -97,6 +97,9 @@ type txMemPool struct {
 	orphans       map[btcwire.ShaHash]*btcutil.Tx
 	orphansByPrev map[btcwire.ShaHash]*list.List
 	outpoints     map[btcwire.OutPoint]*btcutil.Tx
+	pennyTotal    float64 // exponentially decaying total for penny spends.
+	lastPennyUnix int64   // unix time of last ``penny spend''
+
 }
 
 // isDust returns whether or not the passed transaction output amount is
@@ -858,9 +861,29 @@ func (mp *txMemPool) maybeAcceptTransaction(tx *btcutil.Tx, isOrphan *bool, isNe
 		return TxRuleError(str)
 	}
 
-	// TODO(davec): Rate-limit 'free' transactions.  That is to say
-	// transactions which are less than the minimum relay fee and are
-	// therefore considered free.
+	// Free-to-relay transactions are rate limited here to prevent
+	// penny-flooding with tiny transactions as a form of attack.
+	if minRequiredFee == 0 {
+		nowUnix := time.Now().Unix()
+		// we decay passed data with an exponentially decaying ~10
+		// minutes window - matches bitcoind handling.
+		mp.pennyTotal *= math.Pow(1.0-1.0/600.0,
+			float64(nowUnix-mp.lastPennyUnix))
+		mp.lastPennyUnix = nowUnix
+
+		// Are we still over the limit?
+		if mp.pennyTotal >= cfg.FreeTxRelayLimit*10*1000 {
+			str := fmt.Sprintf("transaction %v has 0 fees and has "+
+				"been rejected by the rate limiter", txHash)
+			return TxRuleError(str)
+		}
+		oldTotal := mp.pennyTotal
+
+		mp.pennyTotal += float64(tx.MsgTx().SerializeSize())
+		txmpLog.Debugf("rate limit: curTotal %v, nextTotal: %v, "+
+			"limit %v", oldTotal, mp.pennyTotal,
+			cfg.FreeTxRelayLimit*10*1000)
+	}
 
 	// Verify crypto signatures for each input and reject the transaction if
 	// any don't verify.