I'm trying to enable ICMP ping response from my Android 4.2.2 Phone (Galaxy S4). I am only considering ping over LAN via Wi-Fi, not the cellular network. WLAN Network connectivity is not the problem: I can connect to an FTP server running on the phone from another machine on the LAN. Pinging the phone from the same host of the FTP client results in timeouts. The phone is not asleep because the FTP session can be active, but ping still times out. I can also ping an Android 4.4 tablet just fine, so it is not something specific with Android.
In the phone, I examined /system/etc/sysctl.conf and found the line:
sysctl -w net.ipv4.icmp_echo_ignore_all=1;
which I commented out as (editing with ES File Explorer Root Explorer, FS set to RW)
#sysctl -w net.ipv4.icmp_echo_ignore_all=1;
After rebooting the phone, there is still no ping response. What other settings could inhibit ICMP ping response?
Edit: I also tried the form
sysctl -w net.ipv4.icmp_echo_ignore_all=0;
That did not make a difference.
I can ping any other host on the LAN, including the router, from the phone using the Net Ping app.
I also discovered that I can ping the phone at its IPv6 link-local address (fe80::xxxx) It will respond to pings while the phone is awake, but as soon as the screen goes dark, ping responses stop. But I do not get any IPv4 ping response under any circumstances, so some setting is blocking it. I do not have any add-on firewall like DroidWall running.