From c483bebba8ceadd9ac9ca8e81d9c86c5c6d6d500 Mon Sep 17 00:00:00 2001
From: Kristoffer Dalby <kristoffer@tailscale.com>
Date: Sun, 7 Jun 2026 07:37:50 +0000
Subject: [PATCH] mapper: guard nil Hostinfo in addNextDNSMetadata

addNextDNSMetadata dereferenced node.Hostinfo().OS() without the .Valid() guard its siblings (RequestTags, TailNode) apply, so building the DNS config for a node with nil Hostinfo (a legacy NULL host_info row) panicked whenever a NextDNS resolver was configured. Guard the OS() dereference.
---
 hscontrol/mapper/mapper.go      |  8 +++++++-
 hscontrol/mapper/mapper_test.go | 24 ++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/hscontrol/mapper/mapper.go b/hscontrol/mapper/mapper.go
index 82aed5af..d52200eb 100644
--- a/hscontrol/mapper/mapper.go
+++ b/hscontrol/mapper/mapper.go
@@ -256,7 +256,13 @@ func addNextDNSMetadata(resolvers []*dnstype.Resolver, node types.NodeView) {
 
 		q := u.Query()
 		q.Set("device_name", node.Hostname())
-		q.Set("device_model", node.Hostinfo().OS())
+
+		// Guard Hostinfo().Valid() before dereferencing OS(): a node loaded
+		// from a legacy NULL host_info row has a nil Hostinfo, and OS() would
+		// panic. Mirrors the .Valid() guard in RequestTags/TailNode.
+		if node.Hostinfo().Valid() {
+			q.Set("device_model", node.Hostinfo().OS())
+		}
 
 		if ips := node.IPs(); len(ips) > 0 {
 			q.Set("device_ip", ips[0].String())
diff --git a/hscontrol/mapper/mapper_test.go b/hscontrol/mapper/mapper_test.go
index 236c0094..b8baefc6 100644
--- a/hscontrol/mapper/mapper_test.go
+++ b/hscontrol/mapper/mapper_test.go
@@ -350,3 +350,27 @@ func TestBuildFromChangeFiltersUserProfilesByVisibility(t *testing.T) {
 			"n1 must not receive user2's profile; n2 is not ACL-visible to n1")
 	}
 }
+
+// TestGenerateDNSConfigNilHostinfoNoPanic proves generateDNSConfig does not
+// panic when a node's Hostinfo is nil (e.g. a legacy DB row with a NULL
+// host_info column). addNextDNSMetadata dereferenced node.Hostinfo().OS()
+// without the .Valid() guard its siblings (RequestTags, TailNode) apply, so
+// building such a node's map crashed the server whenever a NextDNS resolver
+// was configured.
+func TestGenerateDNSConfigNilHostinfoNoPanic(t *testing.T) {
+	node := (&types.Node{
+		Hostname: "legacy-node",
+		IPv4:     iap("100.64.0.1"),
+		// Hostinfo intentionally nil, as a legacy NULL host_info row loads.
+	}).View()
+
+	cfg := &types.Config{
+		TailcfgDNSConfig: &tailcfg.DNSConfig{
+			Resolvers: []*dnstype.Resolver{{Addr: "https://dns.nextdns.io/abc"}},
+		},
+	}
+
+	require.NotPanics(t, func() {
+		generateDNSConfig(cfg, node, nil)
+	}, "generateDNSConfig must not panic when a node has nil Hostinfo")
+}