Hostsfile is a go (golang) package for parsing hosts files and performing reverse IP -> hostname lookups which are file-based (e.g. via /etc/hosts).

This can be helpful to determine if there is a prettier (or known) hostname available for an IP address.

These lookups are "extremely inexpensive" compared to normal IP reverse DNS lookups because no network communication is required, as these lookups are all file-based! Naturally, the (obvious) tradeoff/downside is that this only works in cases where the IP mapping exists in the hostsfile.

go get -u github.com/jaytaylor/go-hostsfile

package main

import (
    "fmt"

    "github.com/jaytaylor/go-hostsfile"
)

func main() {
    res, err := hostsfile.ReverseLookup("127.0.0.1")
    if err != nil {
        panic(err)
    }
    fmt.Printf("%v maps 127.0.0.1 to the following names: %v", hostsfile.HostsPath, res)
}

Output:

/etc/hosts maps 127.0.0.1 to the following names: [localhost]

Tested and verified working on:

• Linux
• Mac OS X
• Windows

It should also support Plan9, though admittedly I don't have a plan9 installation available to verify that one ;)

Running the unit-tests is straightforward and standard:

go test

Permissive MIT license.

You are more than welcome to open issues and send pull requests if you find a bug or want a new feature.

If you appreciate this library please feel free to drop me a line and tell me! It's always nice to hear from people who have benefitted from my work.

Email: jay at (my github username).com

Twitter: @jtaylor