I have a computer (CentOS) with 2 network cards. I want to use one network card (A) to retrieve time from NTP server (internet). This would then set [this] local computer's time. I then want to point all other clients, (computers, and other devices) to the second network (B) card, using it as a LOCAL NTP. The B network must remain unto itself. This is all this computer will be used for. No other programs or functions. Can this be achieved?


  • network A gets time via NTP Ethernet.
  • network B then provides time for incoming queries from local clients.
  • No cross network communication can occur.

I'm obviously new to networking, and any help is appreciated.


A computer with two or more network cards, unless it is specifically configured as a router or NAT gateway or something, will not automatically provide access to network A from computers on network B. However, a program (such as ntpd) running on your computer can talk to both networks. Each network interface on your computer will have its own IP address.

You probably don't have to do anything special to set up ntpd. As long as your computer's default gateway points toward the Internet connection, requests from ntpd to other time servers will go out on network A. Clients on network B can contact ntpd using the IP address specific to network B.

If you are concerned about cross-network communication, you may want to set up firewall rules that restrict the possible communications to NTP packets. However, this is not required and I would recommend getting it working correctly before introducing a firewall.

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