by justin@fathomdb.com:

I noticed that some interfaces did not return any IP addresses from Addrs(), but e.g. ip
addr show would show the IP addresses.

By hacking up the code, I believe the issue is here:

http://golang.org/src/pkg/net/interface_linux.go?#L152

if ifi.Flags&FlagPointToPoint != 0 && a.Attr.Type == syscall.IFA_LOCAL ||
   ifi.Flags&FlagPointToPoint == 0 && a.Attr.Type == syscall.IFA_ADDRESS {

I have an IP address that is up|pointtopoint|multicast, but the address type is
IFA_LOCAL.  This is an IPv6 address on a SSH-created tunnel device.  I added the IP
address myself using ip addr add, so I may well have done it wrong.

But, ip addr show does show the IP, whereas Go skips over it.

I'm not quite sure of the logic behind the skipping.

Here's how to reproduce...

sudo ssh -f -N -i ~username/.ssh/id_rsa -w99:99 root@SERVER_IP
sudo ip addr add 172.16.1.1/24 dev tun99
sudo ip addr add fd00::f00d/96 dev tun99
sudo ip link set tun99 up


A quick test program...

package main

import (
    "log"
    "net"
)

func main() {
    intfs, err := net.Interfaces()
    if err != nil {
        log.Panic(err)
    }

    for _, intf := range intfs {
        intf, err := InterfaceByName(intf.Name)
        if err != nil {
            log.Panic(err)
        }

        addrs, err := intf.Addrs()
        if err != nil {
            log.Panic(err)
        }

        log.Printf("%v", intf.Name)
        for _, addr := range addrs {
            log.Printf("\t%s", addr)
        }
    }

    log.Printf("DONE")
}

This prints (the relevant bit for tun99):

2013/09/20 11:42:31 tun99
2013/09/20 11:42:31     172.16.1.1/24


The IPv6 address is missing.  "ip addr show" includes it:

ip addr show dev tun99

63: tun99: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UP qlen 500
    link/none 
    inet 172.16.1.1/24 scope global tun99
    inet6 fd00::f00d/128 scope global 
       valid_lft forever preferred_lft forever



By hacking up newAddr to insert a println when the loop condition is not hit:

    log.Printf("Skipped: %v %v", ifi, a.Attr.Type)

2013/09/20 11:44:44 Skipped: &{63 1500 tun99  up|pointtopoint|multicast} 1
2013/09/20 11:44:44 Skipped: &{63 1500 tun99  up|pointtopoint|multicast} 1
2013/09/20 11:44:44 Skipped: &{63 1500 tun99  up|pointtopoint|multicast} 6



So I think the problem is that - for the Ipv6 address on the pointtopoint tun device,
we're only getting an IFA_ADDRESS record, and no IFA_LOCAL record.