ipn/ipnlocal: add start of inter-user Taildrop

Controlled by server-sent capability policy.

To be initially used for SSH servers to record sessions to other
nodes. Not yet productized into something user-accessible. (Notably,
the list of Taildrop targets from the sender side isn't augmented
yet.) This purely permits expanding the set of expands a node will
accept a drop from.

Updates #3802
Updates #4217

Change-Id: Id7a5bccd686490f8ef2cdc7dae7c07c440dc0085
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/peerapi.go

@@ -620,9 +620,31 @@ func (f *incomingFile) PartialFile() ipn.PartialFile {
 	}
 }
 
+// canPutFile reports whether h can put a file ("Taildrop") to this node.
+func (h *peerAPIHandler) canPutFile() bool {
+	if h.isSelf {
+		return true
+	}
+	if h.peerNode == nil {
+		// Shouldn't happen, but in case.
+		return false
+	}
+	for _, addr := range h.peerNode.Addresses {
+		if !addr.IsSingleIP() {
+			continue
+		}
+		for _, cap := range h.ps.b.PeerCaps(addr.IP()) {
+			if cap == tailcfg.CapabilityFileSharingSend {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 func (h *peerAPIHandler) handlePeerPut(w http.ResponseWriter, r *http.Request) {
-	if !h.isSelf {
-		http.Error(w, "not owner", http.StatusForbidden)
+	if !h.canPutFile() {
+		http.Error(w, "Taildrop access denied", http.StatusForbidden)
 		return
 	}
 	if !h.ps.b.hasCapFileSharing() {

ipn/ipnlocal/peerapi_test.go

@@ -158,7 +158,7 @@ func TestHandlePeerAPI(t *testing.T) {
 			req:        httptest.NewRequest("PUT", "/v0/put/foo", nil),
 			checks: checks(
 				httpStatus(http.StatusForbidden),
-				bodyContains("not owner"),
+				bodyContains("Taildrop access denied"),
 			),
 		},
 		{

tailcfg/tailcfg.go

@@ -1577,8 +1577,16 @@ type Oauth2Token struct {
 }
 
 const (
+	// MapResponse.Node self capabilities.
+
 	CapabilityFileSharing = "https://tailscale.com/cap/file-sharing"
 	CapabilityAdmin       = "https://tailscale.com/cap/is-admin"
+
+	// Inter-node capabilities.
+
+	// CapabilityFileSharingSend grants the ability to receive files from a
+	// node that's owned by a different user.
+	CapabilityFileSharingSend = "https://tailscale.com/cap/file-send"
 )
 
 // SetDNSRequest is a request to add a DNS record.