Par défaut, le DNS utilise le port 53/udp pour les résolutions de noms. Si le volume de données à transférer est trop important, la librairie passe automatiquement en 53/tcp.

Il est donc nécessaire d'ouvrir les deux protocoles dans les pare-feux.
Exemple : host -t MX cybermailing.com renvoie

;; Truncated, retrying in TCP mode.

si le pare-feu est filtré
et

;; Truncated, retrying in TCP mode.
cybermailing.com mail is handled by 30 smtp8.cybermailing.com.
cybermailing.com mail is handled by 30 smtp13.cybermailing.com.
cybermailing.com mail is handled by 30 smtp14.cybermailing.com.
cybermailing.com mail is handled by 30 smtp15.cybermailing.com.
cybermailing.com mail is handled by 30 smtp16.cybermailing.com.
cybermailing.com mail is handled by 30 smtp17.cybermailing.com.
cybermailing.com mail is handled by 30 smtp18.cybermailing.com.
cybermailing.com mail is handled by 30 smtp19.cybermailing.com.
cybermailing.com mail is handled by 30 smtp20.cybermailing.com.
cybermailing.com mail is handled by 30 smtp21.cybermailing.com.
cybermailing.com mail is handled by 30 smtp22.cybermailing.com.
cybermailing.com mail is handled by 10 smtp9.cybermailing.com.
cybermailing.com mail is handled by 15 smtp10.cybermailing.com.
cybermailing.com mail is handled by 20 smtp11.cybermailing.com.
cybermailing.com mail is handled by 25 smtp12.cybermailing.com.
cybermailing.com mail is handled by 30 smtp1.cybermailing.com.
cybermailing.com mail is handled by 30 smtp2.cybermailing.com.
cybermailing.com mail is handled by 30 smtp3.cybermailing.com.
cybermailing.com mail is handled by 30 smtp4.cybermailing.com.
cybermailing.com mail is handled by 30 smtp5.cybermailing.com.
cybermailing.com mail is handled by 30 smtp6.cybermailing.com.
cybermailing.com mail is handled by 30 smtp7.cybermailing.com.

si le pare-feu est ouvert.