Find internal IP address with BASH - bash

Question

I am already aware of many ways of getting your internal IP (ifconfig, ip addr, /etc/hosts, etc), but I am trying to write a bash script that will always return the internal IP. The problem is, many one-liners (/sbin/ifconfig|grep inet|head -1|sed 's/\:/ /'|awk '{print $3}') can return multiple IPs, and I need to distinguish the internal one manually.
I suspect that to the computer, there is no difference between and an external IP and an internal IP, and thus no 100%, guaranteed way to get the right IP.
The end result is that this script will return the internal IP, no matter if its a 192 address or a 204 address, etc.
Thanks in advance.

Answer 1

"hostname -i" should hopefully give you the same result

Answer 2

As others have mentioned, a machine is not really guaranteed, or even likely, to have a single IP address. I'm not sure exactly what you mean by "internal IP"; sometimes this can mean "IP address on the local network", i.e. the interface which connects to a NAT-enabled firewall.
I'm thinking that the best way to do this is to connect to a host on the network you want and use the address from which that connection originates. This will be the interface which the machine normally uses to connect to that network. The user Unkwntech had the same idea on this thread. The code below is just taken from that answer.
I don't know if this really qualifies as a "bash" solution, since it's just an inline Python script, but anyway this will get you the local ip address used to reach google.com. So this will give you the IP address of whichever interface the machine uses to reach Internet hosts.
$ python -c 'import socket
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.connect(("google.com", 80))
print s.getsockname()[0]'
A more bash-y solution might use tracepath or some similar utility.

Answer 3

Systems can have multiple private IPs too though. You would have to limit your searching on IPs to private IPs. 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

Answer 4

Within the RFC 1918 private address spaces, a machine could conceivably have every address in the 10/8 range, the 172.16/12 range, and the 192.168/16 range, for a total of 17891328 IP addresses, and all of them would be legal "internal" IPs.
Oh yes, don't forget IPv6 :) 2^64 possible addresses per network for a single machine, which might participate in multiple networks.
This isn't exactly academic, either: it is quite common for VMWare, VirtualBox, QEMU, etc. host systems to have multiple RFC 1918 addresses assigned; one for the 'usual use', and one that is used specifically to communicate with guest operating systems. Or routers / firewalls, they might have a dozen internal IPs specifically to subnet a network for access control reasons.