fix: ensure domain/server resolve to same ipv4

See coop-cloud/organising#227 (comment).

pkg/dns/common.go

@@ -64,3 +64,33 @@ func EnsureIPv4(domainName string) (string, error) {
 
 	return ipv4, nil
 }
+
+// EnsureDomainsResolveSameIPv4 ensures that domains resolve to the same ipv4 address
+func EnsureDomainsResolveSameIPv4(domainName, server string) (string, error) {
+	var ipv4 string
+
+	domainIPv4, err := EnsureIPv4(domainName)
+	if err != nil {
+		return ipv4, err
+	}
+
+	if domainIPv4 == "" {
+		return ipv4, fmt.Errorf("cannot resolve ipv4 for %s?", domainName)
+	}
+
+	serverIPv4, err := EnsureIPv4(server)
+	if err != nil {
+		return ipv4, err
+	}
+
+	if serverIPv4 == "" {
+		return ipv4, fmt.Errorf("cannot resolve ipv4 for %s?", server)
+	}
+
+	if domainIPv4 != serverIPv4 {
+		err := "app domain %s (%s) does not appear to resolve to app server %s (%s)?"
+		return ipv4, fmt.Errorf(err, domainName, domainIPv4, server, serverIPv4)
+	}
+
+	return ipv4, nil
+}