On Wed, Feb 22, 2017 at 02:22:17PM +0000, Laurie Brown wrote:
On 22/02/17 13:50, Chris Green wrote:
I want a fast (as in less than a second, not 'computer fast') way of checking if a particular private IP is present on the LAN.
[SNIP]
---- cut here ---- ~ $ nmap -sn 10.0.1.254
Starting Nmap 6.40 ( http://nmap.org ) at 2017-02-22 14:16 GMT Nmap scan report for 10.0.1.254 Host is up (0.00078s latency). Nmap done: 1 IP address (1 host up) scanned in 0.00 seconds
~ $ nmap -sn 10.0.1.80
Starting Nmap 6.40 ( http://nmap.org ) at 2017-02-22 14:17 GMT Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn Nmap done: 1 IP address (0 hosts up) scanned in 3.00 seconds
---- cut here ----
If it's there it's faster, so just change your logic...
You've illustrated my problem perfectly! That 3 seconds delay is the same delay I see using my original ping method.
I need to find out, fast, if 192.168.0.104 isn't on the LAN I'm connected to. To do this I have to run your second case above which takes, like my ping, 3 seconds to execute.
It's not possible to 'change the logic'. The ssh config file has the following in it:-
Match host odin exec "hostNotLocal 192.168.0.104" ProxyCommand ssh cheddar nc -q0 localhost 51234
Host odin HostName 192.168.0.104
Simply reversing the logic to:-
Match host odin exec "hostIsLocal 192.168.0.104" HostName 192.168.0.104
Host odin ProxyCommand ssh cheddar nc -q0 localhost 51234
Makes no difference at all. The hostIsLocal command will take 3 seconds to fail when it's not local and I'm no better off.