You need to explicitly reconfigure the iptables/nftables to prevent that from happening.
Some software, say LXD/Incus, enable forwarding automatically upon installation/startup, and do not configure firewall to block non-their traffic, making the machine an open router. I've reported that, the developers said that's by design (despite other virtualization/containerization systems block forwarding if they happen to enable the sysctl).
“The Linux box instantly turns into a router as soon as you run `sysctl net.ipv4.ip_forward=1`, because the default policy for FORWARD table is ACCEPT.”
In the setup I presented, we are bridging an Ethernet and a WiFi network. This would be desirable if you wanted to use an upstream dhcp server for your WiFi clients- or if you wanted to avoid double nat’ing.
In 802.11 infrastructure mode, a station can only send frames with its own MAC address. The AP won’t accept or forward frames from unknown MACs. So you can’t transparently bridge Ethernet devices’ MAC addresses through a WiFi client interface. This is why we need hostapd.
In every other circumstance- I think your statement holds.
I tried to do some weird alerting on new MAC addresses and ran into this weirdness. Bridging WiFi and Ethernet gets weird.
I think that is incorrect. hostapd handles the authentication side of things, but 4addr tuples are controlled by 'struct wireless_dev.use_4addr', and can be set by 'ip link set type bridge_slave ... proxy_arp_wifi on', `iw dev ... 4addr on', and if using systemd-networkd, with slave interface's
[Bridge]
ProxyARPWiFi=yes
Kernel then uses NL80211_IFTYPE_AP_VLAN and handles the proxy operation.
Then the actual title of the article mentions routing and switching but not a firewall, dhcp server or WiFi access point. Then at the end you seem to change the goal to being a WiFi router but really you have presented more steps than required for that. You have also setup switching, a firewall and a dhcp server which are not required to be a router with WiFi access point.
Man that is totally a fair point.
I feel like I’ve struggled with the tutorials on these configs so many times in my life that I’ve kind of munged several ideas together here. There’s so much subtlety to the iptables/nftables rules that I failed to understand for so long, that I forgot that some folks might not understand that WiFi has specific weirdness. You’re right- I open with routing as a topic, but I’m in a very specific nuance right away.
- router
- NAT gateway
- DHCP server
In a typical scenario, turning IP forwarding on will do nothing unless:
- DHCP has given the devices on the 'inside' IP addresses and told them the gateway address, and
- the router is set up to do IP masquerading
Your IP could also be used to run DoS attacks on the internet, although in this case (compared to containers/VM access) the attacker won't be able to receive replies (one-way communication only, like the address spoofing). But if you also happen to configure NAT (MASQUERADE) without additional limits, anyone in "LAN" could use your machine as a router (use your IP address to access websites).
Such misconfiguration is rarely told about in the how-tos and guides, and it's pretty common to have "additional free IPs" on your VPS/dedicated this way :D
This is also true in case of manually crafted home routers (such as in this article, and misconfigured advanced routers like Mikrotik, OpenWrt, Cisco, etc): if you happen to unconditionally enable forwarding without firewall, and think that NAT will somehow block the access to your home LAN, that's not true. Your neighbor, connected to the same ISP switch, could just add the routing record to 192.168.0.0/24 via your router's MAC address and access your 192.168.0.0/24 LAN devices without restrictions (unless ISP specifically blocks such access).
As for the GP's example, running VM's or containers* on your own machine? I'd say the default ACCEPT policy is fine. However, silently changing such a setting on software installation is a problem because if the machine is multi-homed (i.e. has more than one network interface), you've now created a network route outside of the network admin's control.
* The default for docker and podman is to use a private network, not a bridge anyway.
Setting that up with safe/fair bandwidth-sharing requires intermediate IT skill level. Still a great hobby project =3
after all, most routers/WAP/gateways that you buy today will have linux on the inside, configured similarly.
As a kid with no AI, no google, it was quite a feat and I’m still very proud of it
Was my introduction into how the internet works and I’ll never forget working with ipchains
I remember enduring a lot of people in forums calling me a noob, but only after spending collective hours answering my dumb questions
I credit a big part of my moderate success in tech, to being familiar with stuff at just a tad bit lower of a level than the average bear
To my friend Sam who I haven’t talked to in 20 years, thanks for the idea
The open friendly ~safe Internet died long ago.
There are so many places where no one even thinks to ask your age, they just help/troll/etc
If age verification were in place, you may be forbidden from posting to those places which is mind boggling stupid to me
I just don’t see the argument for age verification, it’s just yet another government overreach. It’s a well known thing to use children for any privacy reasons encroaching bills and they are always called “the save the children from online predators and other evil doers” so that you can be easily vilified if you oppose them
So now I do not do any funky stuff with firewalls anymore. Separate appliance with opnsense bare metal.
My router is a 16GB n150 mini PC with dual NICs. The actual router OS is within openwrt VM managed by Incus (VM/Container hypervisor) that has both NICs passed through.
One of the NICs is connected to another OpenWrt wifi access point, and the other is connected to the ISP modem.
The n150 also has a wifi card that I setup as an additional AP I can connect to if something goes wrong with the virtualization setup.
Been running this for at least 6 months and has been working pretty well.
For example, bandwidth rate-limiting may be inhibited for admin SSH or package updates, and LAN IPv4 private ranges for your host address pool are set.
Finally, your internal DHCP should statically bind your admin computer MAC to a fixed LAN host IP to further reduce issues.
Personally, I always build my NAS from scratch, as I have lost count of the number of problems web-GUI have caused over the years. =3
Btw you do also need to be careful with opnsense. I was years behind on updates for mine because every time I updated I assumed that it would bring me up to date with the latest version. But opnsense has to install the upgrades in order. After you reboot you need to check again for updates and repeat until there's no more to install.
An x86 mini PC can run all this without breaking a sweat; using separate appliances seems very wasteful. That being said, I configure everything in DIY mode, and don't rely on GUIs or other similar things that increase the attack surface considerably.
I think your initial setup was perfectly valid. Then you diagnosed a fault and fixed it with aplomb, in a way that you could verify. The key point is: "in a way you could verify" and you failed safe. Well played.
Proxmox itself has a useful firewall implementation too, although it takes a bit of getting used to because you can set it at the cluster, host and VM levels. I personally love it because it is easier to manage than individual host based firewalls, which I also do, but I'm a masochist! For smaller systems I generally use the cluster level to keep all the rules in one place.
One thing I’d add, is that the best explanation I’ve ever seen for this, is the famous diagram [0] on Wikipedia of the netfilter API — I remember when I saw that, everything clicked into place. I’m not sure how up to date it is now, but it’s really good.
[0] https://commons.wikimedia.org/wiki/File:Netfilter-packet-flo...
One thing I did not understand before: why SNAT must happen at POSTROUTING?
Because the exit interface is only known after the routing decision... before that kernel does not know which source IP to write
this visual schematic made it click for me - https://vectree.io/c/linux-netfilter-packet-flow-tables-chai...
I know there is OpenWrt, but my experience is that is more geared toward running on embedded wifi hardware than an x86 machine. The x86 install comes with a tiny root partition that's actually pretty difficult to resize, for example, and upgrades are quite brittle compared to standard Linux distros.
And there's also pfSense and OPNsense, but these run on FreeBSD which seems to lag behind Linux for hardware support. There's no support for the Aquantia AQC113 NIC, for example (although it looks like this may finally have been added in the last month or so).
Something like an Ubuntu Appliance [1] would be quite nice.
Keep in mind most network appliances have dedicated hardware hand-off adapters, and so the CPU isn't involved in routing once the connection is setup. It is why people can use a $30 SoC, and still be able to saturate several 10Gb/100Gb ports. =3
- net.ipv6.conf.all.forwarding=1
- nftables is default to `ip` family which only applies to IPv4. Setting it to `inet` will allow rules to apply to both IPv4 & 6; or `ip6` for IPv6 only. You can skip NAT rules, usually.
- dnsmasq: in addition to DNS and DHCP, turns on router advertisement with SLAAC. Some devices can get IPv6 address from stateful DHCPv6 server, others (e.g. Android) only work with SLAAC.
It explains steps I used to fumble through stabbing in the dark following piecemeal examples trying to bring up quick and dirty networking on an oddball Linux device (like a BPI-R4 or router VM).
Set Linux as router in one command. Support Internet sharing, redsocks, Wifi hotspot, IPv6. Can also be used for routing VM/containers
At the lowest level, it is impossible to have a default DROP for forwarding, because nftables is an optional piece of the kernel that often isn't loaded.