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	net/tstun: block looped disco traffic, take 17
It was in the wrong filter direction before, per CPU profiles we now have. Updates #1526 (maybe fixes? time will tell) Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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			| @@ -413,6 +413,16 @@ func (t *Wrapper) filterOut(p *packet.Parsed) filter.Response { | ||||
| 		return filter.DropSilently // don't pass on to OS; already handled | ||||
| 	} | ||||
|  | ||||
| 	// Issue 1526 workaround: if we sent disco packets over | ||||
| 	// Tailscale from ourselves, then drop them, as that shouldn't | ||||
| 	// happen unless a networking stack is confused, as it seems | ||||
| 	// macOS in Network Extension mode might be. | ||||
| 	if p.IPProto == ipproto.UDP && // disco is over UDP; avoid isSelfDisco call for TCP/etc | ||||
| 		t.isSelfDisco(p) { | ||||
| 		t.logf("[unexpected] received self disco out packet over tstun; dropping") | ||||
| 		return filter.DropSilently | ||||
| 	} | ||||
|  | ||||
| 	if t.PreFilterOut != nil { | ||||
| 		if res := t.PreFilterOut(p, t); res.IsDrop() { | ||||
| 			return res | ||||
| @@ -517,7 +527,7 @@ func (t *Wrapper) filterIn(buf []byte) filter.Response { | ||||
| 	// macOS in Network Extension mode might be. | ||||
| 	if p.IPProto == ipproto.UDP && // disco is over UDP; avoid isSelfDisco call for TCP/etc | ||||
| 		t.isSelfDisco(p) { | ||||
| 		t.logf("[unexpected] received self disco package over tstun; dropping") | ||||
| 		t.logf("[unexpected] received self disco in packet over tstun; dropping") | ||||
| 		return filter.DropSilently | ||||
| 	} | ||||
|  | ||||
|   | ||||
| @@ -514,7 +514,18 @@ func TestFilterDiscoLoop(t *testing.T) { | ||||
| 	if got != filter.DropSilently { | ||||
| 		t.Errorf("got %v; want DropSilently", got) | ||||
| 	} | ||||
| 	if got, want := memLog.String(), "[unexpected] received self disco package over tstun; dropping\n"; got != want { | ||||
| 	if got, want := memLog.String(), "[unexpected] received self disco in packet over tstun; dropping\n"; got != want { | ||||
| 		t.Errorf("log output mismatch\n got: %q\nwant: %q\n", got, want) | ||||
| 	} | ||||
|  | ||||
| 	memLog.Reset() | ||||
| 	pp := new(packet.Parsed) | ||||
| 	pp.Decode(pkt) | ||||
| 	got = tw.filterOut(pp) | ||||
| 	if got != filter.DropSilently { | ||||
| 		t.Errorf("got %v; want DropSilently", got) | ||||
| 	} | ||||
| 	if got, want := memLog.String(), "[unexpected] received self disco out packet over tstun; dropping\n"; got != want { | ||||
| 		t.Errorf("log output mismatch\n got: %q\nwant: %q\n", got, want) | ||||
| 	} | ||||
| } | ||||
|   | ||||
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