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Do I really need a firewall for my server? (sopuli.xyz)
submitted 1 year ago by to c/selfhosted@lemmy.world
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I've been running my server without a firewall for quite some time now, I have a piped instance and snikket running on it. I've been meaning to get UFW on it but I've been too lazy to do so. Is it a necessary thing that I need to have or it's a huge security vulnerability? I can only SSH my server from only my local network and must use a VPN if I wanna SSH in outside so I'd say my server's pretty secure but not the furthest I could take it. Opinions please?

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[–] 7 points 1 year ago (3 children)

Op means, as they said, a firewall on the server itself.

NAT is, effectively, a firewall.

No it isn't. Stop giving advice on edge security.

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  • [–] 3 points 1 year ago (2 children)

    Are you saying that NAT isn’t effectively a firewall or that a NAT firewall isn’t effectively a firewall?

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  • [–] 3 points 1 year ago

    NAT simply maps IPS across subnet boundaries in such a way that upstream routing tables don't need updating.

    If you use destination NAT forward rules to facilitate specific destination port access, you are using a firewall.

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  • [–] 0 points 1 year ago (1 child)

    Assuming it's not a 1-1 NAT it does make for a functional unidirectional firewall. Now, a pure router in the sense of simply offering a gateway to another subnet doesn't do much, but the typical home router as most people think of it is creating a snat for multiple devices to reach out to the internet and without port forwarding effectively blocks off traffic from the outside in.

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  • [–] 6 points 1 year ago (2 children)

    Assuming it's not a 1-1 NAT it does make for a functional unidirectional firewall.

    That's like saying a router and firewall are the same thing. NAT appears to be a "firewall" because it's usually deployed with one. NAT itself has no filtering functions the way you're describing.

    Now, a pure router in the sense of simply offering a gateway to another subnet

    A "pure" router, as you put it, understands upstream subnets and routing tables. NAT does not, and is usually overlayed on top of an existing routing function.

    You can set up NAT between two subnets as an experiment with no iptables and it will do its job.

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  • [–] 2 points 1 year ago (1 child)

    In practice a stateful NAT is the same as a stateful Firewall. I've never heard of a NAT that isn't a Firewall. A port forward is the same as a Firewall allow rule.

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  • [–] 1 point 1 year ago

    What you might call a stateful NAT is really a 1-1 NAT, anything going out picks up an IP and anything retuned to that IP is routed back to the single address behind the NAT. Most home users a many to one source nat so their internal devices pick up a routable IP and multiple connections to a given dest are tracked by a source port map to route return traffic to the appropriate internal host.

    Basically yes to what you said, but a port forward technically is a route map inbound to a mapped IP. You could have an ACL or firewall rule to control access to the NAT but in itself the forward isn't a true firewall allow.

    Same basic result but if you trace a packet into a router without a port forward it'll be dropped before egress rather than being truly blocked. I think where some of the contention lies is that routing between private nets you have something like:

    0.0.0.0/0 > 192.168.1.1 10.0.0.0/8 > 192.168.2.1

    The more specific route would send everything for 10.x to the .2 route and it would be relayed as the routing tables dictate from that device. So a NAT in that case isn't a filter.

    From a routable address to non-route 1918 address as most would have from outside in though you can't make that jump without a map (forward) into the local subnet.

    So maybe more appropriate to say a NAT 'can' act as a firewall, but only by virtue of losing the route rather than blocking it.

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  • [–] -1 points 1 year ago

    NAT in the sense used when people talk about at home is a source nat, or as we like to call it in the office space a hide address, everyone going to the adjacent net appears to be the same source IP and the system maintains a table of connections to correlate return traffic to.

    The other direction though, if you where on that upstream net and tried to target traffic towards the SNAT address above the router has no idea where to send it to unless there's a map to designate where incoming connections need to be sent on the other side of the NAT so it ends up being dropped. I suppose in theory it could try and send it to everyone in the local side net, but if you get multiple responses everything is going to get hosed up.

    So from the perspective of session state initiation it can act as a firewall since without route maps it only will work from one side.

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  • [–] -2 points 1 year ago* (1 child)

    How is NAT not a firewall? Sure theoretically it isn't but I've yet to see a implementation of NAT that doesn't act as a Firewall

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  • [–] 6 points 1 year ago

    Because NAT acts as a firewall with a "default deny" policy for incoming packets, but no other rules. You cannot prevent a device on the private subnet side of a NAT from attempting to communicate with an "outside" ip with nat alone, nat doesnt understand the concepts of accept/deny/drop.

    All nat does is rewrite address headers.

    The machines behind a NAT box are not directly addressable because they have private IP addresses. Machines out on the general Internet cannot send IP packets to them directly. Instead, any packets will be sent to the address of the NAT box, and the NAT box looks at its records to see which outgoing packet an incoming packet is in reply to, to decide which internal address the packet should be forwarded to. If the packet is not in reply to an outgoing packet, there's no matching record, and the NAT box discards the packet.

    It's a confused topic because for a lot of people, nat does essentially everything they want. As soon as you get into more complex networking where a routing table needs to be updated, or bidirectional fw rules, it becomes apparent why routing + fw + nat is the most common combo.

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