New: Internet forwarding notes

notes/linux/internet_forwarding.md

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+# Internet Forwarding
+
+This note explains how to use the internet connection on computer A, which is
+also connected to internet-less computer B, on computer B. This requires some
+background
+
+## IPv4 basics
+
+IPv4 is made of 32 bits, written in 4x8-bit numbers like `192.168.0.1`. A
+network mask specified at the end here determines which bits belong to this
+network. Often it's 24, written like `192.168.0.1/24`, which means the first 3
+8-bit numbers here are the local network
+
+Usually `x.x.x.1` is the router or whatever resolves the WAN
+
+Most ipv4 addresses are global on the WAN, though 3 are reserved for private
+networks
+
+ - `192.168.x.x`, most commonly seen on consumer networks
+ - `10.x.x.x`, used for large local networks with 2^24 addresses
+ - `172.[16 to 32].x.x`. The second number must be in the range 16-32
+
+To simplify things, make sure your internal LAN is using a different one from
+the WAN interface's LAN. `172.16.x.x` is often a good choice
+
+## Forwarding internet
+
+We have 3 interfaces involved:
+
+ - `wan0`: the interface connected to the wider internet, possibly through a
+   second LAN network that has a router
+ - `eth0`: the interface on the same computer that has `wan0`, though connected
+   to the internal LAN instead. This interface itself doesn't have internet
+ - `eth1`: an interface on the internal network on another computer
+
+We want to set up a network between `eth0` and `eth1`. Then we'll want a NAt
+between `eth0` and `wan0` to route all the ipv4 packets incoming from `eth0` to
+`wan0`
+
+### Wan0
+
+`wan0`'s configuration file is the easiest. It can use DHCP or a static IP, just
+make sure it has internet with `ping archlinux.org`
+
+Example:
+
+```systemd
+[Network]
+Address=192.168.0.98/24
+Gateway=192.168.0.1  # This is my router's IP
+DNS=1.1.1.1
+# The following is possibly useful, not sure
+IPForward=ipv4
+IPMasquerade=yes
+```
+
+Test your connection with the following:
+
+```bash
+sudo networkctl reload
+ping 192.168.0.1  # Fails? You messed up your config
+ping 1.1  # Fails? Your router doesn't have internet || your gateway is wrong
+ping archlinux.org  # Fails? Your DNS isn't working. Try setting it to 1.1.1.1
+```
+
+### Eth0
+
+This is the second interface on the computer with internet. This interface
+itself doesn't have internet though. See the tests below to check
+
+```systemd
+[Network]
+Address=172.16.0.1/24
+Gateway=172.16.0.1  # Gateway is self!
+DNS=1.1.1.1  # Again, optional, might not even do anything here
+```
+
+Testing if this interface has internet access:
+
+```bash
+networkctl list  # Read out the interface name here or in `ip a`. Assume eth0
+ping -I eth0 1.1
+ping 1.1  # If this fails too, then the computer just doesn't have internet
+```
+
+If the above ping succeeds, this interface also has internet. Otherwise, test
+your connection to `eth1`:
+
+```bash
+ping -I eth0 172.16.0.22  # Fails? eth0 or eth1 isn't connected
+ping 172.16.0.22  # Fails? Routing tables aren't using eth0 for 172.16.x.x
+```
+
+It should now be ready to forward internet. Run the following script as root:
+
+```bash
+declare -r WAN=wan0
+declare -r LAN=eth0
+
+# Reset iptables
+iptables -P INPUT ACCEPT
+iptables -P OUTPUT ACCEPT
+iptables -P FORWARD ACCEPT
+
+iptables -F INPUT
+iptables -F OUTPUT
+iptables -F FORWARD
+
+# Forward internet
+sysctl net.ipv4.ip_forward=1
+
+iptables -t nat -A POSTROUTING -o "$WAN" -j MASQUERADE
+iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
+iptables -A FORWARD -i "$LAN" -o "$WAN" -j ACCEPT
+```
+
+### Eth1
+
+This is the interface on the computer that wouldn't have internet otherwise. We
+need to point it at `eth0`'s IP, so that `eth0` forwards all its networking
+
+```systemd
+[Network]
+Address=172.16.0.22/24  # Choose anything >= 2 and <=254 for the last 8bits
+Gateway=172.16.0.1  # This part is important
+DNS=1.1.1.1  # Doesn't do anything, probably
+IPForward=ipv4  # Might be useless
+```
+
+Test with:
+
+```bash
+ping -I eth1 172.16.0.1  # Fails? Something is wrong with eth1 or eth0's connection
+ping 172.16.0.1  # Fails? Routing tables aren't using eth1 for 172.16.x.x
+ping 192.168.0.1  # Fails? IPv4 forwarding isn't working
+ping 1.1  # Fails? Might be a router issue
+ping archlinux.org  # Fails? Likely a DNS issue
+```
+
+## Debugging
+
+After setting this up, `ping 1.1` from my `eth1` computer would keep saying
+`Packet filtered`. Running `systemctl stop firewalld.service` on the `eth0`
+computer solved this. This is not a proper solution
+
+You can also check if the ports are being filtered with the following. If
+they're `unfiltered`, that's good. If they're being `filtered`, then `firewalld`
+is the problem
+
+```bash
+nmap -sA 172.16.0.1
+```
+
+## Further reading
+ - [Internet forwarding article on the
+   archwiki](https://wiki.archlinux.org/title/Internet_sharing)
+ - [systemd-networkd on the
+   archwiki](https://wiki.archlinux.org/title/systemd-networkd)
+ - [IP forwarding](https://bbs.archlinux.org/viewtopic.php?id=245264). Their
+   solution was not using the Gateway with DHCP. Gateway is required for static
+   IP setups though
+ - [Nmap filtering
+   check](https://www.redhat.com/sysadmin/troubleshoot-packet-filters-network)
+ - ChatGPT can help a bit, though it's rarely right enough to actually get the
+   whole network running