Biscuit128 Biscuit128 - 3 years ago 188
Java Question

Refresh ARP tables programmatically

I have been reading around the topic of getting a list of devices connected to my WIFI hotspot on my phone programmatically.

I found a useful post on stack overflow with the following code and a comment saying


This is incorrect as the ARP tables don't necessarily refresh when a
device disconnects. One say say, that you could simply ping all the
devices in the table to see which IPs are active but it is quite
possible that a device is firewalled to not respond to ICMP request
and your ping would fail. This solution, although a working one, isn't
robust and accurate.


I was wondering, if reading proc/net/arp isn't accurate enough due to refresh issues, how can you force the system to do a refresh to get accurate data?

private int countNumMac()
{
int macCount = 0;
BufferedReader br = null;
try {
br = new BufferedReader(new FileReader("/proc/net/arp"));
String line;
while ((line = br.readLine()) != null) {
String[] splitted = line.split(" +");
if (splitted != null && splitted.length >= 4) {
// Basic sanity check
String mac = splitted[3];
if (mac.matches("..:..:..:..:..:..")) {
macCount++;
}
}
}
}
catch (Exception e) {
e.printStackTrace();
}
finally {
try {
br.close();
}
catch (IOException e) {
e.printStackTrace();
}
}

if (macCount == 0)
return 0;
else
return macCount-1; //One MAC address entry will be for the host.
}

Answer Source

The ARP table is not a distributed list of "hosts that are online." It's a local-only list of "hosts that we've seen on this network."

Once you realize this, you'll note that there's no way of removing entries when someone disconnects other than trying to connect to them. Just have a continuous loop in the background, connecting to each host and sending a single ICMP packet (kinda hard to pull off from Java but Android does come with a ping command-line tool).

Granted, hosts that don't reply to ping will be filtered out but depending on the application that might be acceptable.

Alternatively, just let the user try to connect to any of the listed hosts and gracefully time out when the other party is offline. That's what most networking applications do anyway and is the only foolproof way of checking if the host is online.

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