net/portmapper: check returned epoch from PMP and PCP protocols

If the epoch that we see during a Probe is less than the existing epoch,
it means that the gateway has either restarted or reset its
configuration, and an existing mapping is no longer valid. Reset any
saved mapping(s) if we detect this case so that a future
createOrGetMapping will not attempt to re-use it.

Updates #10597

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ie3cddaf625cb94a29885f7a1eeea25dbf6b97b47

net/portmapper/pcp.go

@@ -54,8 +54,7 @@ type pcpMapping struct {
 	renewAfter time.Time
 	goodUntil  time.Time
 
-	// TODO should this also contain an epoch?
-	// Doesn't seem to be used elsewhere, but can use it for validation at some point.
+	epoch uint32
 }
 
 func (p *pcpMapping) MappingType() string      { return "pcp" }
@@ -140,6 +139,7 @@ func parsePCPMapResponse(resp []byte) (*pcpMapping, error) {
 		external:   external,
 		renewAfter: now.Add(lifetime / 2),
 		goodUntil:  now.Add(lifetime),
+		epoch:      res.Epoch,
 	}
 
 	return mapping, nil