// Use freely with source code level attribution (a comment in the source code linking to this page is fine)

package foo

import (
    "fmt"
    "net"
    "os"
    "strings"
)

// GetFQDN gets a fully qualified domain name for a given host (use empty string for the local host)
//
// GetFQDN follows the same logic as Python's `socket.getfqdn([name])`
// (https://docs.python.org/3.5/library/socket.html#socket.getfqdn)
// in selecting the first name that contains a period.
func GetFQDN(hostname string) string {
    var err error
    
    if hostname == "" {
        hostname, err = os.Hostname()
        if err != nil { panic("cannot look up hostname") }
    }
    
    ips, err := net.LookupIP(hostname)
    if err != nil { return hostname }
    
    var results = make([]string, 0)
    
    for _, ip := range ips {
        hosts, err := net.LookupAddr(ip.String())
        if err != nil { continue }
        
        for _, host := range hosts {
            results = append(results, fmt.Sprintf("%s: %s", ip.String(), host))
            if -1 != strings.LastIndexByte(host, '.') {
                return strings.TrimSuffix(host, ".")
            }
        }
    }
    
    return hostname
}