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Sun Bear
Asked: 2024-10-24 17:26:20 +0800 CST

What does `;; SERVER: 127.0.0.53#53(127.0.0.53) (UDP)` mean?

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When I execute dig example.com, one of the returned line states:

;; SERVER: 127.0.0.53#53(127.0.0.53) (UDP)

Can you explain the meaning of each of the components in this line?

networking

1 Answers

  1. Best Answer
    ;; SERVER: 127.0.0.53#53(127.0.0.53) (UDP)
    • (127.0.0.53): the target DNS server for the query was 127.0.0.53;
    • 127.0.0.53#53: the target DNS server was resolved to 127.0.0.53 and contacted on port #53;
    • (UDP): the name resolution happened via UDP.

    Why 127.0.0.53 is picked as the target DNS server for the query is explained down below; but if you tried and switch things up by asking dig to resolve the name using a specific DNS server (specifying a domain name instead of an IP address) via TCP, such as in:

    dig @dns.google +tcp askubuntu.com

    That would've resulted in:

    ;; SERVER: 8.8.8.8#53(dns.google) (TCP)

    Note that 127.0.0.53 is actually your own machine (which can easily be told by the range the address falls in), and what's listening on port #53 is (most likely) systemd-resolved's DNS stub listener.

    Here 127.0.0.53 is queried because, as per man dig:

    If no server argument is provided, dig consults /etc/resolv.conf; if an address is found there, it queries the nameserver at that address.

    And, by deafult, /etc/resolv.conf contains an entry for 127.0.0.53:

    % grep nameserver /etc/resolv.conf
nameserver 127.0.0.53
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