Don't cleanup NDP state when enabling forwarding

...to match linux behaviour:

```
$ sudo sysctl net.ipv6.conf.eno1.forwarding
net.ipv6.conf.eno1.forwarding = 0
$ ip addr list dev eno1
2: eno1: <...> ...
  inet6 PREFIX:TEMP_IID/64 scope global temporary dynamic
     valid_lft 209363sec preferred_lft 64024sec
  inet6 PREFIX:GLOBAL_STABLE_IID/64 scope global dynamic mngtmpaddr ...
     valid_lft 209363sec preferred_lft 209363sec
  inet6 fe80::LINKLOCAL_STABLE_IID/64 scope link
     valid_lft forever preferred_lft forever

$ sudo sysctl -w "net.ipv6.conf.all.forwarding=1"
net.ipv6.conf.all.forwarding = 1
$ sudo sysctl net.ipv6.conf.eno1.forwarding
net.ipv6.conf.eno1.forwarding = 1
$ ip addr list dev eno1
2: eno1: <...> ...
  inet6 PREFIX:TEMP_IID/64 scope global temporary dynamic
     valid_lft 209339sec preferred_lft 64000sec
  inet6 PREFIX:GLOBAL_STABLE_IID/64 scope global dynamic mngtmpaddr ...
     valid_lft 209339sec preferred_lft 209339sec
  inet6 fe80::LINKLOCAL_STABLE_IID/64 scope link
     valid_lft forever preferred_lft forever
$ ip -6 route list
...
PREFIX::/64 dev eno1 proto ra metric 100 expires 209241sec pref medium
default via fe80::ROUTER_IID dev eno1 proto ra ...
```
PiperOrigin-RevId: 372146689

2 files changed, 5 insertions, 37 deletions

diff --git a/pkg/tcpip/network/ipv6/ipv6.go b/pkg/tcpip/network/ipv6/ipv6.go
index f7510c243..6974eae11 100644
--- a/pkg/tcpip/network/ipv6/ipv6.go
+++ b/pkg/tcpip/network/ipv6/ipv6.go
@@ -420,11 +420,7 @@ func (e *endpoint) transitionForwarding(forwarding bool) {
 	defer e.mu.Unlock()
 
 	if forwarding {
-		// When transitioning into an IPv6 router, host-only state (NDP discovered
-		// routers, discovered on-link prefixes, and auto-generated addresses) is
-		// cleaned up/invalidated and NDP router solicitations are stopped.
 		e.mu.ndp.stopSolicitingRouters()
-		e.mu.ndp.cleanupState(true /* hostOnly */)
 
 		// As per RFC 4291 section 2.8:
 		//
@@ -613,7 +609,7 @@ func (e *endpoint) disableLocked() {
 
 		return true
 	})
-	e.mu.ndp.cleanupState(false /* hostOnly */)
+	e.mu.ndp.cleanupState()
 
 	// The endpoint may have already left the multicast group.
 	switch err := e.leaveGroupLocked(header.IPv6AllNodesMulticastAddress).(type) {
diff --git a/pkg/tcpip/network/ipv6/ndp.go b/pkg/tcpip/network/ipv6/ndp.go
index d6e0a81a6..9064fa769 100644
--- a/pkg/tcpip/network/ipv6/ndp.go
+++ b/pkg/tcpip/network/ipv6/ndp.go
@@ -1609,44 +1609,16 @@ func (ndp *ndpState) cleanupTempSLAACAddrResourcesAndNotifyInner(tempAddrs map[t
 	delete(tempAddrs, tempAddr)
 }
 
-// removeSLAACAddresses removes all SLAAC addresses.
-//
-// If keepLinkLocal is false, the SLAAC generated link-local address is removed.
-//
-// The IPv6 endpoint that ndp belongs to MUST be locked.
-func (ndp *ndpState) removeSLAACAddresses(keepLinkLocal bool) {
-	linkLocalSubnet := header.IPv6LinkLocalPrefix.Subnet()
-	var linkLocalPrefixes int
-	for prefix, state := range ndp.slaacPrefixes {
-		// RFC 4862 section 5 states that routers are also expected to generate a
-		// link-local address so we do not invalidate them if we are cleaning up
-		// host-only state.
-		if keepLinkLocal && prefix == linkLocalSubnet {
-			linkLocalPrefixes++
-			continue
-		}
-
-		ndp.invalidateSLAACPrefix(prefix, state)
-	}
-
-	if got := len(ndp.slaacPrefixes); got != linkLocalPrefixes {
-		panic(fmt.Sprintf("ndp: still have non-linklocal SLAAC prefixes after cleaning up; found = %d prefixes, of which %d are link-local", got, linkLocalPrefixes))
-	}
-}
-
 // cleanupState cleans up ndp's state.
 //
-// If hostOnly is true, then only host-specific state is cleaned up.
-//
 // This function invalidates all discovered on-link prefixes, discovered
 // routers, and auto-generated addresses.
 //
-// If hostOnly is true, then the link-local auto-generated address aren't
-// invalidated as routers are also expected to generate a link-local address.
-//
 // The IPv6 endpoint that ndp belongs to MUST be locked.
-func (ndp *ndpState) cleanupState(hostOnly bool) {
-	ndp.removeSLAACAddresses(hostOnly /* keepLinkLocal */)
+func (ndp *ndpState) cleanupState() {
+	for prefix, state := range ndp.slaacPrefixes {
+		ndp.invalidateSLAACPrefix(prefix, state)
+	}
 
 	for prefix := range ndp.onLinkPrefixes {
 		ndp.invalidateOnLinkPrefix(prefix)