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Basic networking/subnetting question. (lemmy.dbzer0.com)

submitted 1 year ago by marauding_gibberish142@lemmy.dbzer0.com to c/selfhosted@lemmy.world

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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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[–] nottelling@lemmy.world 1 points 1 year ago* (last edited 1 year ago) (1 children)

A VLAN is (theoretically) equivalent to a physically separated layer 2 domain. 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.

Edit: Should probably note that simply VLAN tagging from the endpoints on a trunk port isn't any better than subnetting, since an untrusted machine can just tag packets however it wants. You need to use an 802.1q aware switch and gateway to use VLANs effectively.

[–] marauding_gibberish142@lemmy.dbzer0.com 1 points 1 year ago

Thank you for the great comment.

This line cleared it up for me:

802.1q aware switch and gateway to use VLANs effectively.

It is indeed as you say. VLANs on a trunk port wouldn't really work for security either. This is making me reconsider my entire networking infrastructure since when I started I wasn't very invested in such things. Thanks for giving me material to think about