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package common
import (
"net"
"sort"
"strings"
)
// GetOutboundIP returns an outbound IP address of this machine.
// It tries to access the internet and returns the local IP address of the connection.
// If the machine cannot access the internet, it returns a preferred IP address from network interfaces.
// It returns nil if no IP address is found.
func GetOutboundIP() net.IP {
// See https://stackoverflow.com/a/37382208
conn, err := net.Dial("udp", "8.8.8.8:80")
if err == nil {
defer conn.Close()
return conn.LocalAddr().(*net.UDPAddr).IP
}
// So the machine cannot access the internet. Pick an IP address from network interfaces.
if ifs, err := net.Interfaces(); err == nil {
type IP struct {
net.IP
net.Interface
}
var ips []IP
for _, i := range ifs {
if addrs, err := i.Addrs(); err == nil {
for _, addr := range addrs {
var ip net.IP
switch v := addr.(type) {
case *net.IPNet:
ip = v.IP
case *net.IPAddr:
ip = v.IP
}
if ip.IsGlobalUnicast() {
ips = append(ips, IP{ip, i})
}
}
}
}
if len(ips) > 1 {
sort.Slice(ips, func(i, j int) bool {
ifi := ips[i].Interface
ifj := ips[j].Interface
// ethernet is preferred
if vi, vj := strings.HasPrefix(ifi.Name, "e"), strings.HasPrefix(ifj.Name, "e"); vi != vj {
return vi
}
ipi := ips[i].IP
ipj := ips[j].IP
// IPv4 is preferred
if vi, vj := ipi.To4() != nil, ipj.To4() != nil; vi != vj {
return vi
}
// en0 is preferred to en1
if ifi.Name != ifj.Name {
return ifi.Name < ifj.Name
}
// fallback
return ipi.String() < ipj.String()
})
return ips[0].IP
}
}
return nil
}
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