fix: force lowercase DNS hostnames for VPN (#16613)

ethanndickson · web-flow · commit 92870f06422c · 2025-02-20T13:02:45.000+11:00
Closes coder/coder-desktop-macos#54 I've also double checked that agents with hyphens & underscores play nice once programmed, as do workspaces with hyphens: ``` $ ping6 main_agent-1.main-workspace.admin.coder PING6(56=40+8+8 bytes) fd60:627a:a42b:4e91:88c0:da4a:df4f:b54e --> fd60:627a:a42b:46d4:8b55:e549:e498:e6f5 ``` also fine in Firefox & Safari, though I'm a little surprised underscores work.
diff --git a/tailnet/controllers.go b/tailnet/controllers.go
@@ -883,23 +883,30 @@ type Workspace struct {
 }
 
 // updateDNSNames updates the DNS names for all agents in the workspace.
+// DNS hosts must be all lowercase, or the resolver won't be able to find them.
+// Usernames are globally unique & case-insensitive.
+// Workspace names are unique per-user & case-insensitive.
+// Agent names are unique per-workspace & case-insensitive.
 func (w *Workspace) updateDNSNames() error {
+	wsName := strings.ToLower(w.Name)
+	username := strings.ToLower(w.ownerUsername)
 	for id, a := range w.agents {
+		agentName := strings.ToLower(a.Name)
 		names := make(map[dnsname.FQDN][]netip.Addr)
 		// TODO: technically, DNS labels cannot start with numbers, but the rules are often not
 		//       strictly enforced.
-		fqdn, err := dnsname.ToFQDN(fmt.Sprintf("%s.%s.me.coder.", a.Name, w.Name))
+		fqdn, err := dnsname.ToFQDN(fmt.Sprintf("%s.%s.me.coder.", agentName, wsName))
 		if err != nil {
 			return err
 		}
 		names[fqdn] = []netip.Addr{CoderServicePrefix.AddrFromUUID(a.ID)}
-		fqdn, err = dnsname.ToFQDN(fmt.Sprintf("%s.%s.%s.coder.", a.Name, w.Name, w.ownerUsername))
+		fqdn, err = dnsname.ToFQDN(fmt.Sprintf("%s.%s.%s.coder.", agentName, wsName, username))
 		if err != nil {
 			return err
 		}
 		names[fqdn] = []netip.Addr{CoderServicePrefix.AddrFromUUID(a.ID)}
 		if len(w.agents) == 1 {
-			fqdn, err := dnsname.ToFQDN(fmt.Sprintf("%s.coder.", w.Name))
+			fqdn, err := dnsname.ToFQDN(fmt.Sprintf("%s.coder.", wsName))
 			if err != nil {
 				return err
 			}
