tailcfg: add Node.SelfNodeV4MasqAddrForThisPeer

This only adds the field, to be used in a future commit.

Updates tailscale/corp#8020

Co-authored-by: Melanie Warrick <warrick@tailscale.com>
Signed-off-by: Maisem Ali <maisem@tailscale.com>

tailcfg/tailcfg.go

@@ -273,6 +273,21 @@ type Node struct {
 	// the client, this is calculated client-side based on a timestamp sent
 	// from control, to avoid clock skew issues.
 	Expired bool `json:",omitempty"`
+
+	// SelfNodeV4MasqAddrForThisPeer is the IPv4 that this peer knows the current node as.
+	// It may be empty if the peer knows the current node by its native
+	// IPv4 address.
+	// This field is only populated in a MapResponse for peers and not
+	// for the current node.
+	//
+	// If set, it should be used to masquerade traffic originating from the
+	// current node to this peer. The masquerade address is only relevant
+	// for this peer and not for other peers.
+	//
+	// This only applies to traffic originating from the current node to the
+	// peer or any of its subnets. Traffic originating from subnet routes will
+	// not be masqueraded (e.g. in case of --snat-subnet-routes).
+	SelfNodeV4MasqAddrForThisPeer netip.Addr `json:",omitempty"`
 }
 
 // DisplayName returns the user-facing name for a node which should

tailcfg/tailcfg_clone.go

@@ -98,6 +98,7 @@ var _NodeCloneNeedsRegeneration = Node(struct {
 	ComputedNameWithHost          string
 	DataPlaneAuditLogID           string
 	Expired                       bool
+	SelfNodeV4MasqAddrForThisPeer netip.Addr
 }{})
 
 // Clone makes a deep copy of Hostinfo.

tailcfg/tailcfg_view.go

@@ -176,6 +176,9 @@ func (v NodeView) ComputedName() string              { return v.ж.ComputedName
 func (v NodeView) ComputedNameWithHost() string      { return v.ж.ComputedNameWithHost }
 func (v NodeView) DataPlaneAuditLogID() string       { return v.ж.DataPlaneAuditLogID }
 func (v NodeView) Expired() bool                     { return v.ж.Expired }
+func (v NodeView) SelfNodeV4MasqAddrForThisPeer() netip.Addr {
+	return v.ж.SelfNodeV4MasqAddrForThisPeer
+}
 func (v NodeView) Equal(v2 NodeView) bool { return v.ж.Equal(v2.ж) }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
@@ -210,6 +213,7 @@ var _NodeViewNeedsRegeneration = Node(struct {
 	ComputedNameWithHost          string
 	DataPlaneAuditLogID           string
 	Expired                       bool
+	SelfNodeV4MasqAddrForThisPeer netip.Addr
 }{})
 
 // View returns a readonly view of Hostinfo.