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Per this comment, I'm going to take advice and ask this as a separate question. I am trying to learn more about networking and security and want to play with tools to help increase my understanding.

Fing seems like a pretty cool tool - finding devices on the network and their associated MAC address. One could easily implement any of the solutions that provides detection and alerting, but I would like to know how these tools are implemented? Is this a combination of low level linux utilities, or is there some custom programming going on?

If it is the second - what would that algorithm look like?

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I just ran Fing against my wireless network. Using tcpdump, it appears that Fing generates Address Resolution Protocol (ARP) request packets. ARP is a pretty simple protocol that runs at the Ethernet Protocol level (Data Link, OSI level 2). An ARP request packet has the broadcast address (ff:ff:ff:ff:ff:ff) as the "to" address, the Android phone's MAC and IP address as the "from" information, and an IP address that Fing wants to know about. It appears that Fing just marches through whatever subnet it's on, in my case 172.31.0.0/24, so 255 IP addresses, from 172.31.0.1 to 172.31.0.254. After the march, it appears to try IP addresses that haven't responded a second time. This looks to me like Fing tries IP addresses in batches, and relies on the underlying Linux kernel to buffer ARP replies, for a Fing thread to deal with as fast as it can. If Fing decides that there's a timeout, it resends. It's not clear to me how Fing (a Java program) gets the phone's Linux kernel to generate ARP packets.

The notorious nmap, invoked with -sn, the "ping scan" flag, does the same thing. I did an strace on nmap -sn 172.31.0.0/24 to see how it gets the kernel to send ARP requests. It looks like nmap creates an ordinary TCP/IP socket, and calls connect() on the socket to TCP port 80, with an IP address. nmap must be doing this in non-blocking mode, as it does a large number of connect() calls sequentially, faster than it would take for Linux to decide to time out a connect() when there's no host with the IP address.

So there's your answer: create a TCP/IP socket, call connect() with a particular port and IP address, then see what the error is. If the error is ECONNREFUSED, it's a live IP address, but nothing is listening on that port. If you get a TCP connection, that IP address has a host. IP addresses that the connect() call times out for, don't have a machine associated. You need to batch the connect() calls for speed, and you need to wait for connect() calls to timeout to decide that an IP address does not have a machine associated with it.

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    connect() is not part of it. Nmap uses the platform-dependent implementation of eth_send in libdnet. For Linux, this uses sendto on a PF_PACkET raw socket. – bonsaiviking Mar 10 '16 at 19:56
  • @bonsaiviking - connect() system calls show up for consecutive IP addresses in the strace output. I was looking for some weird raw socket thing, and I was surprised to see the connect() calls show up. I did not look through nmap source code, however, and I also just ran "nmap -sn 172.31.0.0/24", so it's possible that I missed something important. – Bruce Ediger Mar 10 '16 at 20:35
  • @bruce - thanks. This looks like a solid approach, I guess my lack of experience in the area limited my ability to peice together the tools to solve the problem - despite having used tcpdump in the past. I just ran a quick test and see some of the ARP requests coming from FING. When I get back to the office - I will realy dig into this. – akaphenom Mar 10 '16 at 22:10
  • @bonsaiviking - you are correct, in that if you run nmap as root, it does do the raw socket things. After looking over the source code in nmap..c, there's a value o.isr00t that when set to false, causes nmap to create regular TCP sockets, and call connect() on each IP address. When true, it does the raw socket thing. – Bruce Ediger Mar 10 '16 at 23:02

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