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How does netstat determine symbolic hostname?

I was trying to understand what the --numeric/-n flag of netstat does? Manual says the following about --numeric/-n

–numeric , -n

Show numerical addresses instead of trying to determine symbolic host, port or user names.

  1. Following is a line of output with “-n” option

    tcp  0  0 :::8080    :::*   LISTEN      -

  2. Following is the same line as in A but without “-n” option

    tcp  0  0 *:terabase *:*    LISTEN      -

port 8080 in my case is associated with solr. I have no idea why it’s being listed as terabase. That’s why I am wondering how netstat determines symbolic host. It would be helpful if someone can throw light on this.

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Answer

You’re mixing ports and hosts I believe.

The symbolic host is determined by DNS lookup, whereas the port usage is what you’ll find in /etc/services (why you have teradata for port 8080 I don’t know – it’s usually http-alt – but go have a look)

So, for example with -n you could have

tcp        0      0 127.0.0.1:5432          0.0.0.0:*               LISTEN

meaning something is listening on port 5432 on IP 127.0.0.1. Without -n it would be

tcp        0      0 localhost:postgresql    *:*                     LISTEN

which makes us much wiser, since we can now see it’s PostgreSQL on localhost.

The price for that information is that the DNS roundtrips take time…!

Cheers,

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