DHCP lease log on flash memory with timestamp

I want to keep logs of my dhcp server.Also need to sign the files using timestamp.

I want to keep as txt files and example like below

IP Address Lease Start Date Lease End Date MAC

How can i do it?

I’m planning to use 450g for logs and memory card for storage.
Also I want to delete the files older than 6 months.

It would better to use an external syslog server. If you want build one in a small form factor.

Actually.I don’t not exactly but I think I need a log system like CALEA.Keeping logs of dhcp server without access logs is meaningless.If there is a crime.Access logs would help much for officers.As far as I know CALEA uses timestamp or sth else to verify the exact time.
I want to create a all in one solution.Such as
RB411UAHR have usb port that means we can connect for extra storage to our RB.Generally customers want plug and play solutions.Files older than 6 month will be deleted because service provider obligated to keep records for 6 months.

Is it possible?