DNS Protocol – the Additional and Authoritative Section of the DNS Response

  DNS, DNS Protocols, DNS Theory

DNS servers have an additional relation between names and IP addresses. If you are asking some DNS server for NS or MX record (which contains the name) and the server has authoritative information about IP addresses for obtained names, it will return to us in the additional section of a DNS response packet (packets) these IP addresses.

In the next section, the so-called authority section, it also returns NS records for the domain we are asking for (and in the additional section we may also find IP addresses for these NS records). The purpose of these two sections is obvious, it will save us many more possible queries on IP addresses for all records obtained.

Let’s ask the DNS server ns.ms.mff.cuni.cz for MX records for the domain ms.mff.cuni.cz with a detailed statement of the result (for simplicity, some uninteresting rows and columns of statements have been omitted):

;ms.mff.cuni.cz.          MX

ms.mff.cuni.cz.           MX    20 smtp2.ms.mff.cuni.cz.
ms.mff.cuni.cz.           MX    40 smtp1.kolej.mff.cuni.cz.
ms.mff.cuni.cz.           MX    20 smtp1.ms.mff.cuni.cz.

ms.mff.cuni.cz.           NS    golias.ruk.cuni.cz.
ms.mff.cuni.cz.           NS    ns.ms.mff.cuni.cz.
ms.mff.cuni.cz.           NS    ns.kolej.mff.cuni.cz.
ms.mff.cuni.cz.           NS    sns.ms.mff.cuni.cz.

smtp1.ms.mff.cuni.cz.     A
smtp2.ms.mff.cuni.cz.     A
smtp1.kolej.mff.cuni.cz.  A
smtp1.kolej.mff.cuni.cz.  AAAA  2001:718:1e03:a01::4
ns.ms.mff.cuni.cz.        A
ns.kolej.mff.cuni.cz.     A
sns.ms.mff.cuni.cz.       A
golias.ruk.cuni.cz.       A


We see that the server really told us all the IP addresses known to it for all the names in the responses and saved us and itself a lot of extra work with other questions.

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