I hope I'm posting this in the right board... maybe it should be networking, but It's more Linux.  I am running Linux Mint 18.1 (Essentially Ubuntu/Debian), as a desktop machine. I installed a 4 port Intel NIC on the box with the objective of having the NICs work as follows:NIC on Motherboardeth0 - Normal use - regular linux desktop http/s, dns, ntp, etc. (All except classes of traffic below)4 Port NIC Cardeth1 - on VLAN1 (For assignment to a Virtualbox VM in bridged mode)eth2 - on VLAN2 (For assignment to a Virtualbox VM in bridged mode)eth3 - on VLAN3 (For assignment to a Virtualbox VM in bridged mode)eth4 - Floating - to be used by wireshark or possibly another VM for testing.  I would put my SG300 switch into monitor mode and pipe traffic from an interface or VLAN for testing.eth1,2,3 are cabled to the switch, and get their own IP address from DHCP and are automatically assigned to their own VLAN.  (Working)So far I'm only using eth1 (bridged mode to Virtual Box), and eth4 (with Wireshark) and they work fine.My problem is that I have noticed occasionally traffic that should be going out eth0 is going out eth1 (I have only seen DNS, just because of sheer volume-easy to see in the logs, but there may be other traffic).  It appears that I don't have isolation between these interfaces within Linux.  Leakage between interfaces MUST NOT happen.  eth1,2,3 shouldn't have any traffic util I connect them to a VM with Virtualbox or to Wireshark.I've tried google, but my use case is unique, and all the multi-home questions seem to relate to very different enterprise applications.  Any help would be much appreciated-even precise terms to google that would help me find a more relevant answer.ThanksEdit #1 - RE DNS (put here becuase I need to format it)I'm not sure how to answer that question-networking is pretty much the same as a fresh install.  Just for clarification, the base machine is physical hardware with a total of 5 real nics... one on the MB, and one 4 port intel card that I picked up on kijiji.  So far just running a Win8.1 instance and one linux lite instance (again more or less out of the box installs) that I bind to eth1.  That being said, I noticed the leakage when the VMs were down (may have been hapening when they were up, but I didn't notice.)Here's resolv.conf (lan is the local internal domain that I've set in my pfsense router):cat /etc/resolv.confnameserver 127.0.1.1search lanEdit #2 - The VMs are down and the NICs are not being used at the time I noticed the leakage.  VMs are out of the box installs with no special configuration.  I would doubt if they leak.  I have done dnsleak.com checks with them and they have been fine.  I am almost 100% certain that the leakage is occurring within the linux box.What determines which traffic goes through which interface?Does that give you the info you need? If you need anything else, let me know what to check.Edit #3@Celada correctly identified the likely source of the problem - IP address are assigned to eth1,2,3 by the switch  and by the VMs.  I have an SG300 switch, and each port of the muti-port nic goes to one port on the switch.  Each switchport is configured as a separate VLAN, and IP address is assigned by DHCP.  Based on this input, I turned DHCP off for eth3, set IP configuration to manual and configured the IP address as 172.18.0.2/255.255.255.0 and gateway of 172.18.0.1 (172.18.x.x is not configured anywhere in my network).  The VM still gets it's DHCP allocation correctly, so I suspect this will solve the problem. There were two other configuration options:Link-Local OnlyShared to Other ComputersI'm not sure exactly what they do, or if one of these would be better option than the one I have chosen.  I did briefly try Shared to Other Computers and I got some 10. IP address that I don't know where it came from.  Might have been Virtualbox's network system, or it may have come from one of the VPN tunnels that I had open at the time.  So to be on the safe side, I decided to go with a manual configuration that should be a dead end.Am I correct that using Link-Local could end up with some unwanted interaction with the host system?As for Shared to Other Computers, I have no idea exactly what it does, or how it ended up with the IP address that it did.Can someone please advise me if I made the best choice for how to handle the situation?I will monitor for a week or so, and then close the question and assign points.  I think I may also have an upstream firewall issue as well, but if that is the case I will open another question if I can't resolve it.
How can I isolate NICs?
linux;networking;linux mint;ethernet