Basic networking/subnetting question.

Submitted ⁨⁨2⁩ ⁨weeks⁩ ago⁩ by ⁨marauding_gibberish142@lemmy.dbzer0.com⁩ to ⁨selfhosted@lemmy.world⁩

Sorry for being such a noob. My networking is not very strong, thought I’d ask the fine folks here.

Let’s say I have a Linux box working as a router and a dumb switch (I.e. L2 only). I have 2 PCs that I would like to keep separated and not let them talk to each other.

Can I plug these two PCs into the switch, configure their interfaces with IPs from different subnets, and configure the relevant sub-interfaces and ACLs (to prevent inter-subnet communication through the router) on the Linux router?

What I’m asking is; do I really need VLANs? I do need to segregate networks but I do not trust the operating systems running on these switches which can do L3 routing.

If you have a better solution than what I described which can scale with the number of computers, please let me know. Unfortunately, networking below L3 is still fuzzy in my head.

Thanks!

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Comments

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catloaf@lemm.ee ⁨2⁩ ⁨weeks⁩ ago
One of the PCs can spoof the MAC of the other and receive its Ethernet frames.

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  Thank you for that. I’d also like to ask you: is that a possibility too if one were to configure a trunk port on a switch and plug the PCs in?
  
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  - catloaf@lemm.ee ⁨2⁩ ⁨weeks⁩ ago
    That would be worse, because then it would send and receive traffic for multiple vlans.
    
    Unless your switch uses that to refer to link aggregation instead of vlan trunking. Network terminology like that can mean different things to different vendors.
    
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kylian0087@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
What is holding you back in regards to VLANs?

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  I’d either have to do it in the router (which would need a lot of PCIe network cards which can get expensive + difficult to accommodate enough physical PCIe lanes on consumer hardware) or run it on a switch running a proprietary OS that I can’t control and don’t know what it’s doing underneath.
  
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  - kylian0087@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
    Can you elaborate why you think you need much more PCIe lanes? Technically you can do with 1 single LAN port with all your VLANs.
    
    You configure the VLANs on the router then make a single trunk port to a switch. then have that switch divide the VLANs on the ports you desire. this can be a L2 switch.
    
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  - Clearwater@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
    As a heads up, almost all OpenWRT routers function as managed switches with vlan capabilities. Not truly all, but a very good number.
    
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Goingdown@sopuli.xyz ⁨2⁩ ⁨weeks⁩ ago
If computers are in same network, even with different ip addresses, they still can see all broadcast and multicast traffic. This means for example dhcp.

If you fully trust your computers, and are sure that no external party can access any of them, you should be fine. But if anyone can fain access any computer, it is trivial to gain access to all networks.

If you need best security, multiple switches and multiple nics are unfortunately only really secure solution.

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  No, I do not trust my computers that much. Quite unfortunate, really that I’ll have to build a whitebox switch to get what I want
  
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- nottelling@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
  Broadcast traffic (such as DHCP) doesn’t cross subnets without a router configured to forward it. It’s one of the reasons subnets exist.
  
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bane_killgrind@slrpnk.net ⁨2⁩ ⁨weeks⁩ ago

I do need to segregate networks but I do not trust the operating systems running on these switches which can do L3 routing.

Ok, so you are trusting the PCs which you need to keep separate.

There’s no way to know if one of them is hoovering all the traffic from the other, if they are both connected to the same unmanaged switch.

youtube.com/playlist?list=PLjVwd8FlHBASO5vLBtMYNO…

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  The computers will be running OpenBSD. I am researching hardening methods for them and also seeing if it is feasible for me to get Corebooted hardware. I didn’t mention it because I didn’t think it was important.
  
  I feel like my post is being taken very negatively with people finding faults in my words rather than in the networking concept. Would you happen to know why?
  
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  - bane_killgrind@slrpnk.net ⁨2⁩ ⁨weeks⁩ ago
    You are basically asking for people to solve a solved problem, there’s no actual need for keeping the PCs separate since you control them both, and oh and you want it done cheap. A bespoke custom solution will not scale regardless if you need it to or not, you should know that.
    
    hometechhacker.com/great-choices-for-opnsense-har…
    
    A firewall device with as many ports as you need is your best bet.
    
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cmnybo@discuss.tchncs.de ⁨2⁩ ⁨weeks⁩ ago
I would just get a basic layer 2 managed switch and use VLANs. The 5 port and 8 port switches are super cheap these days.

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  It’s not that they are expensive, it’s that they run archaic proprietary OSes which the consumer cannot control. I cannot trust such a switch when the rest of my network depends on it. Please let me know if something in the post didn’t make sense.
  
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  - cmnybo@discuss.tchncs.de ⁨2⁩ ⁨weeks⁩ ago
    Put a multi port NIC in your router PC and use a separate unmanaged switch for each network then.
    
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possiblylinux127@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
Yes you need vlans

Technically there would be some isolation at layer 3 but they would all still be in the same layer 2 network.

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nottelling@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
What you are asking will work. That’s the whole point of subnets. No you don’t need a VLAN to segregate traffic. It can be helpful for things like broadcast control.

However, you used the word “trust” which means that this is a security concern. If you are subnetting because of trust, then yes you absolutely do need to use VLANs.

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  Could you elaborate why the question of trust invalidates using just subnets?
  
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  - possiblylinux127@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
    Subnets are on layer 3 not layer 2. You can easy access other devices on layer 3 by finding the right subnet on layer 2. ARP is used to resolve IP addresses into MAC addresses and vis versa.
    
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  - nottelling@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
    A VLAN is (theoretically) equivalent to a physically separated switch. The only way for machines to communicate between vlans is via a gateway interface.
    
    If you don’t trust the operating system, then you don’t trust that it won’t change it’s IP/subnet to just hop onto the other network. Or even send packets with the other network’s header and spoof packets onto the other subnets.
    
    It’s trivially easy to malform broadcast traffic and hop subnets, or to use various arp table attacks to trick the switching device. If you need to segregate traffic, you need a VLAN.
    
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neidu3@sh.itjust.works ⁨2⁩ ⁨weeks⁩ ago
As others have said: It will work as you’ve planned it. The subletting will keep these two PCs separated (If they still need internet, just add a second IP in your router-PC to allow for communication with this subnet).

VLANs aren’t required, but are more relevant when you want to force network segregation based on individual ports. If you really want to, you can add tagged virtual interfaces on these two separated hosts so that the others hosts aren’t able to simply change the address to reach these. The switch should ignore the VLAN tag and pass it through anyway. But again, it’s not really needed, just something you can do if you really want to play with tagged VLAN interfaces

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  Thank you. In theory, is there a mechanism which will prevent other hosts from tagging the interface with a VLAN ID common with another host and spoof traffic that way? Sorry, I need to study more about this stuff
  
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  - neidu3@sh.itjust.works ⁨2⁩ ⁨weeks⁩ ago
    Yes, but that’s done on the switch. Basically VLAN tags are applied in one of two ways:
    
    Untagged (sometimes called Access) is something you apply on a switch port. For example, if you assign a port to Untagged VLAN 32, anything connected to that port will only be able to connect to port 32.
    
    Tagged (sometimesreferred to as Trunk), on the other hand, is for traffic that is already assigned a VLAN tag. For example Tagged 32 means that it will allow traffic that already has a VLAN tag of 32. It is possible to assign multiple VLANs to a Tagged port. Whatever is connected to that port will need to be able to talk to the associated VLAN(s).
    
    In your particular case, the best practice would be to assign two ports (One for each host, obviously) to Untagged 32 (arbitrarily chosen number, any VLAN ID will do, as long as you’re consistent), and all the other ports as Untagged to a different VLAN ID. That way the switch will effectively contain two segments that cannot talk to each other.
    
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  - possiblylinux127@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
    Usually you would configure that on the switch
    
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MITM0@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
You don’t have to apologize for being a noob, we were all once noobs (& we still are to some extent)

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non_burglar@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
For simple cases you might be able to use 802.1x authentication if “trust” is the issue. This doesnt scale well as a solution on a larger network though.

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  Hmm, I haven’t heard of that before. Could you explain?
  
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  - non_burglar@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
    en.m.wikipedia.org/wiki/IEEE_802.1X
    
    802.1x are a set of protocols that allow port access to be locked to specific devices, which would preclude your need for multiple subnets. You would likely need a few extra physical ports on your white box router, the unmanaged switch could later become overwhelmed passing traffic in a more complicated setup, and you would still need to keep trusted and untrusted traffic separate at the gateway subnet.
    
    Your use case is exactly why vlans were invented.
    
    However, I suspect from your other answers that you are actually looking for an open source managed switch so your entire networking stack is auditable.
    
    There are a few solutions like opx, but hardware supporting opx is prohibitively expensive and it is almost always cheaper to build a beige box and use Linux or get a 2nd hand supported device and use openwrt.
    
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pHr34kY@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
I’ve done this. I have 3 subnets on a single L2 switch without vlans, and the device isolation works. There’s a few caveats:

I used a 4-port NIC on my router so I could have each subnet on its own interface. They all go directly into the L2 switch.

You can only have one DHCP server broadcasting. If you have two, there is no way of predicting which subnet you land on.

My guest subnet is only accessible via Wifi. I have specifically set up my access points so that a particular SSID is assigned to a particular subnet. The access point can broadcast DHCP on a single SSID.

My third subnet is for my security cameras. It’s IPv6-only, and each camera has a static IP address. There is no DHCP. It means my cameras never physically use the same cables as my primary LAN, although they are on the same L2 switch.

All traffic between subnets seems to go through the router, so I have some nftables rules to ensure my guest wifi can only see its own subnet and the public internet.
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Lettuceeatlettuce@lemmy.ml ⁨2⁩ ⁨weeks⁩ ago
Have you looked into Tailscale or an equivalent solution like Netbird?

You could set up a tailnet, make create unique tags for each machine, add both machines to the tailnet, and then set up each machine’s network interface to only go through the tailnet.

Then you just use Tailscale’s ACLs with the tags to isolate those machines, making sure they can only talk to whatever central device(s) or services you want them to, but also stopping them from talking to or even seeing each other.

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- marauding_gibberish142@lemmy.dbzer0.com ⁨2⁩ ⁨weeks⁩ ago
  I never considered tailscale for my LAN, but it’s certainly an intriguing idea. I suppose running Headscale as a VM on my router isn’t that difficult. Thank you, I will think about it a bit more
  
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  - Lettuceeatlettuce@lemmy.ml ⁨2⁩ ⁨weeks⁩ ago
    Yeah, and it’s free for a basic account + up to 100 devices, so plenty for most home lab needs.
    
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