How to check the DHCP lease remaining time

Question

Hello Community,
I am troubleshooting a connectivity issue on my Macbook pro which APPARENTLY ignores the DHCP ACK messages sent in reply to Unicast DHCP Requests (the ones sent when T1 timer expires at 50% of the lease time) from a particular DHCP server.
Do you happen to know the command which shows the current remaining lease time for the assigned DHCP IP address? I need that to confirm my assumptiom.
By checking in the /leases folder you may only see the original lease Start date, which is the time the initial DHCP DORA process took place. When the 'refresh' DHCP requests are sent after 50% of the lease, and consequently the DHCP ACKs from the server are received the value in that file does not change. That means that this command is not good to check what I need.
I.e.:
xxxxxxx-mac:db xxxxxxi$ sudo cat /private/var/db/dhcpclient/leases/en1-1,<omitted-MAC>
Password:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
          <key>IPAddress</key>
          <string>192.168.1.14</string>
          <key>LeaseLength</key>
          <integer>600</integer>
          <key>LeaseStartDate</key>
          <date>2012-03-02T09:16:14Z</date>
          <key>PacketData</key>
          <data>
          AgEGAKA9KL0AAAAAAAAAAMCoAQ7AqAEBAAAAAAAmuxA9XwAAAAAAAAAAAAAAAAAAAAAA
          AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
          AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
          AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
          AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABjglNjNQEFNgTAqAEBMwQAAAJY
          AQT///8AAwTAqAEBBgTAqAEBDwRob21lKgTDDQGZ/wAAAAAAAAAAAAAAAAAA
          </data>
          <key>RouterHardwareAddress</key>
          <data>
          6L6BA24U
          </data>
          <key>RouterIPAddress</key>
          <string>192.168.1.1</string>
</dict>
</plist>
Even when a subsequent DHCP ACK message is received that value remains unchanged.
I need a command or application which shows instead the actual lease timer.
thanks,
Riccardo

Answer 1

apparently I am not getting lucky by putting the question this way.. let me try another approach giving more background details.
PROBLEM:
When I am in a particular location my mac get disconnected from the network every 30 minutes when my DHCP IP address expires. The DHCP server grants 30 minutes IP leases only.
I took some capture (I sniffed DHCP transactions) and I could see that when 50% of the lease time is reached (T1) my mac sends a new unicast DHCP request to the DHCP server which replies immediately with a DHCP ACK confirming 30 more minutes.
Unfortunately I did not find any command to see if the lease gets actually extended.
What I see is that after few minutes (matching the 87.5% T2 timer) my match fires a new DHCP request, which is also answered immediately with a ACK. Then in the last minutes before the 30 minutes threshold I see many other requests which are also all answered. Nevertheless when the origianal 30 minutes lease expires my MAC releases the IP and start a new Discovery process (sending a bcast request). After few seconds the DORA process completes and usually the mac gets the same IP address. The issue for me is that in the meantime my VPN session to my corporate network goes down and all the services associated with that go down too (softphone, presence, chat, email etc).
I did some test in my place and I configured my DHCP server to assign IP addresses with a 30 minutes lease time too. Strangely the issue does not happen. On my place every 15 minutes (again the famous T1 timer = 50% of the lease) my mac sends a request and the DHCP replies with an ACK. This time apparently the lease gets extended as I don't see all the other requests. Needless to say that the IP does not get released at any time.
I compared the wireshark capture to spot any difference between that DHCP ACK I got in that given location and the one I got in my place but they look the same to me.
So the question is, how would you troubleshoot this issue? Which kinds of logs, outputs or debugs should I take?
thanks,
Riccardo