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Need an alert when unauthorized device plugs into LAN

Posted on 2014-01-15
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Hello Experts - I'm looking for a way to generate an email alert anytime an unauthorized device is plugged into my LAN.  No other action needs to take place, just a simple email letting me know someone has plugged in, preferably along with details including what port and perhaps information about the device.  I'm using a Cisco 4510 switch, a 5510 ASA, and a 2811 gateway.  Thanks in advance for any advice!
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Cyclops3590 earned 500 total points
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Since its a small network I would look into arpwatch.  It sees when an IP/mac pair changes or a new one is seen and emails you immediately.  you can then look that mac address up using 'sh mac-address-table' (might be 'sh mac address-table' on your switch) to find the port its connected to
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Looks interesting but not quite what I am looking for.  I'd prefer something Windows based and that would only alert me when an unauthorized device was plugged in.  This might provide a little too much information since I'd get an email any time someone moved with a laptop as an example.  Still, if nothing else comes up this could be handy, thanks!
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actually it wouldn't alert just with a laptop moving.  that laptop must get a new IP as well.  arpwatch doesn't care about switch ports, only IP to mac mappings.

so do you only want to be alerted if an "unauthorized" client connects or prefer to block them to begin with?  also, define unauthorized.  most tools are not going to know the difference.
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Ah, ok, that's better than I thought then.  I don't want to block anyone, just get an email when a device that isn't plugged in now gets plugged in later.  I pictured doing a network scan and collecting a small database of what would constitute authorized devices. Any device connecting to the LAN not already in the database would trigger the alert.
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that would definitely work to.  and the database could easily be a flat file that way (since you're on windows) you could, in your script, use the find command to quickly compare.  then could just put it as a scheduled job that fires off every X minutes to do the comparison and email.  would be a simple batch or powershell script really.
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by:Craig Beck
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ARPWatch is good but it will need an interface to be present in each VLAN you want to monitor.

If you only have one VLAN it's not a problem, but ARP doesn't work across routers so you'd need the PC running ARPWatch to have multiple interfaces if you do.
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