Logging all Connections

[dschmid]

Hello, I'm searching for a way to log all connections from internal IPs to external ones with date and time information and vice versa. The background is that the Lan is used from about 50 users and we want to record connections for a certain time in case that some users do illegal stuff in the internet and the police knocks on our door. We use the captive portal solution with user and pass-through mac adressess.
Any tip or help is appreciated.