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The Domain Name System
The DNS header
The `Transaction ID` (transaction identifier) is a 16-bits random value chosen by the client. When a client sends a question to a DNS server, it remembers the question and its identifier. When a server returns an answer, it returns in the `Transaction ID` field the identifier chosen by the client. Thanks to this identifier, the client can match the received answer with the question that it sent.
The `RD` (recursion desired) bit is set by a client when it sends a query to a resolver. Such a query is said to be `recursive` because the resolver will recursively traverse the DNS hierarchy to retrieve the answer on behalf of the client. In the past, all resolvers were configured to perform recursive queries on behalf of any Internet host. However, this exposes the resolvers to several security risks. The simplest one is that the resolver could become overloaded by having too many recursive queries to process. Most resolvers [#f8888]_ only allow recursive queries from clients belonging to their company or network and discard all other recursive queries. The `RA` bit indicates whether the server supports recursion. The `RCODE` is used to distinguish between different types of errors. See :rfc:`1035` for additional details. The last four fields indicate the size of the `Question`, `Answer`, `Authority` and `Additional` sections of the DNS message.
The last four sections of the DNS message contain `Resource Records` (RR). All RRs have the same top level format shown in the figure below.
DNS Resource Records
The `TTL` field indicates the lifetime of the `Resource Record` in seconds. This field is set by the server that returns an answer and indicates for how long a client or a resolver can store the `Resource Record` inside its cache. A long `TTL` indicates a stable `RR`. Some companies use short `TTL` values for mobile hosts and also for popular servers. For example, a web hosting company that wants to spread the load over a pool of hundred servers can configure its nameservers to return different answers to different clients. If each answer has a small `TTL`, the clients will be forced to send DNS queries regularly. The nameserver will reply to these queries by supplying the address of the less loaded server.
The `RDLength` field is the length of the `RData` field that contains the information of the type specified in the `Type` field.


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locale/cs/LC_MESSAGES/protocols/dns.po, string 1