prober: icmp: Initialize ID and sequence to random values (#412)

This should help preventing issues with some network devices that have
trouble NATing ICMP packets with the same ID and sequence nubmer but a
different source IP address.

Currently, this can happen if the blackbox_exporter runs in a container
(the ID is set to the PID, which is typically 1 in a container), and
serveral blackbox_exporters are restarted at the same time (the sequence
numbers are reset to zero and stay in sync).

This commit sets the ICMP echo ID to a random value if the PID is 1, and
initializes the sequence number at a random offset.

See #411 for details.

Signed-off-by: Benoît Knecht <[email protected]>

prober/icmp.go

@@ -16,6 +16,7 @@ package prober
 import (
 	"bytes"
 	"context"
+	"math/rand"
 	"net"
 	"os"
 	"sync"
@@ -32,10 +33,27 @@ import (
 )
 
 var (
+	icmpID            int
 	icmpSequence      uint16
 	icmpSequenceMutex sync.Mutex
 )
 
+func init() {
+	// PID is typically 1 when running in a container; in that case, set
+	// the ICMP echo ID to a random value to avoid potential clashes with
+	// other blackbox_exporter instances. See #411.
+	if pid := os.Getpid(); pid == 1 {
+		icmpID = rand.Intn(1 << 16)
+	} else {
+		icmpID = pid & 0xffff
+	}
+
+	// Start the ICMP echo sequence at a random offset to prevent them from
+	// being in sync when several blackbox_exporter instances are restarted
+	// at the same time. See #411.
+	icmpSequence = uint16(rand.Intn(1 << 16))
+}
+
 func getICMPSequence() uint16 {
 	icmpSequenceMutex.Lock()
 	defer icmpSequenceMutex.Unlock()
@@ -129,7 +147,7 @@ func ProbeICMP(ctx context.Context, target string, module config.Module, registr
 	}
 
 	body := &icmp.Echo{
-		ID:   os.Getpid() & 0xffff,
+		ID:   icmpID,
 		Seq:  int(getICMPSequence()),
 		Data: data,
 	}