ICMP and InetAddress.isReachable()
java.net.ServerSocket) and datagram based ones (or UDP ones -
The open bug 4727550 asks to support other socket types, that will allow, for example, to perform an ICMP ping and, in general, to access the sockets in raw mode.
To implement an ICMP ping, a partial solution has been introduced in J2SE 5: a way to check if some address is reachable, via
The implementation of these methods goes native and tries to do its best to "ping" the address represented by the
Surprisingly, there are many differences between the Windows and the Linux/Unix implementation of
Windows, as strange as it seems, does not officially support an ICMP "ping" system call. The J2SE 5 implementation hence tries to open a TCP socket on port 7 (the echo service) and hopes to get some sort of reply.
Linux/Unix, instead, supports an ICMP "ping" system call. So the implementation of
java.net.InetAddress.isReachable()first tries to perform the "ping" system call; if this fails, it falls back trying to open a TCP socket on port 7, as in Windows.
It turns out that in Linux/Unix the ping system call requires root privileges, so most of the times
java.net.InetAddress.isReachable()will fail, because many Java programs are not run as root, and because the target address unlikely has the echo service up and running.
But why on Linux, for example, I can use the ping program without being root ?
It's because ping has the setuid bit set:
$> ls -l /bin/ping
-rwsr-xr-x 1 root root 30724 2005-11-11 01:15 /bin/ping
When a user invokes ping, it will run as root (see the "rws" permission for the owner ?).
But why on Windows there is a ping executable that does the job ?
It seems that this executable is relying on undocumented features, so everyone tries to avoid binding to those features.
So, for now, writing portable Java code that wants to send ICMP ping is quite an adventure.