# IPv4 Routing (Linux) ## General Remarks Reaching a remote network or tunneling all the internet traffic via n2n are two common tasks which require a proper routing setup. n2n supports routing needs providing options for packet forwarding including broadcasts as well as modifying the routing table. In this context, the `server` is the edge node which provides access to the remote network/internet, whereas the `client` is the connecting edge node. In order to enable routing, the `server` must be configured as follows: 1. Add the `-r` option to the edge options to enable routing 2. Enable packet forwarding with `sudo sysctl -w net.ipv4.ip_forward=1` 3. Enable IP masquerading: `sudo iptables -t nat -A POSTROUTING -j MASQUERADE` On the client side, the easiest way to configure routing is via the `-n` option. For example: - In order to connect to the remote network `192.168.100.0/24`, use `-n 192.168.100.0/24:10.0.0.1` - In order to tunnel all the internet traffic, use `-n 0.0.0.0/0:10.0.0.1` 10.0.0.1 is the IP address of the gateway to use to route the specified network. It should correspond to the IP address of the `server` within n2n. Multiple `-n` options can be specified. As an alternative to the `-n` option, the `ip route` linux command can be manually used. See the [n2n_gateway.sh](doc/n2n_gateway.sh) script for an example. See also the following description of other use cases and in depth explanation. ## Special Scenarios ### Assumptions - There are two Local Area Networks, namely 10.11.12.0/24 (maybe at **h**ome) and 192.168.1.0/24 (maybe in **o**ffice). - These networks are connected to the internet via routers 10.11.12.1 and 192.168.1.1, respectively. - In each network, there is a computer running a successfully setup n2n node: 10.11.12.5 (**h**ickory) and 192.168.1.6 (**o**scar). They are connected to their networks through a device called _eth0_. Their n2n devices shall be called _n2n0_, and their n2n IP addresses are 10.99.99.50 (**h**ickory) and 10.99.99.60 (**o**scar) in the 10.99.99.0/24 network. - The _iptables_ are flushed. ### Prerequisites - Both, **h**ickory and **o**scar have ip forwarding enabled: `echo 1 > /proc/sys/net/ipv4/ip_forward` or `sysctl -w net.ipv4.ip_forward=1`. To make this setting persistent over reboot, a file containing the line `net.ipv4.ip_forward=1` could be added in /etc/sysctl.d/ – your distro may vary. - To allow n2n to forward packets, both edge nodes need to be started with `-r` option on their command line. All other regular network interfaces usually already allow packet forwarding and thus do not need any further configuration. ### Reach Complete Office Network from n2n Node at Home - To make **h**ickory send all packets with office destination via **o**scar, **h**ickory needs to be made aware of where to route this packets to. On **h**ickory: `ip route add 192.168.1.0/24 via 10.99.99.60 dev n2n0 src 10.11.12.5`. - **o**scar needs to know where to send packets to, too. So, on **o**scar: `ip route add 10.11.12.5 via 10.99.99.50 dev n2n0 src 192.168.1.6`. **o**scar and **h**ickory should now be able to exchange packets by using just their regular (non-n2n) IP addresses 10.11.12.5 and 192.168.1.6. To make the complete office network aware of how packets or answers are sent to **h**ickory, one more step is required: - Packets from any office computer to **h**ickory need to be directed to **o**scar that – thanks to enabled IP forwarding and the routing rule – already knows how to handle this kind of packets. - To handle it in a one-stop-shop, the office router 192.168.1.1 can be configured to direct those packets to **o**scar. Luckily, even most modern small-office-and-home routers allow to add static routing rules via a web interface. A rule like "All packets for host 10.11.12.5 (or network 10.11.12.0/24) need to be sent to another router, namely 192.168.1.5" will do. This is the **recommended** solution. - However, a **less recommended** but working option would be to add static routes to each single of those computers in the office network that shall be able to connect to or be accessed by **h**ickory. On those, e.g. **o**livia with IP address 192.168.1.123: `ip route add 10.11.12.5 via 192.168.1.5 dev eth0 src 192.168.1.123`. - Alternatively, in case the office router does not allow to have added own static routing rules, **o**scar needs to perform NAT for all connections initiated from **h**ickory: `iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE` `iptables -A FORWARD -i eth0 -o n2n0 -m state --state RELATED,ESTABLISHED -j ACCEPT` `iptables -A FORWARD -i n2n0 -o eth0 -j ACCEPT` There is a major drawback with this solution which thus is the **least recommended**: Connections between **h**ickory and the office network will only work if initiated by **h**ickory – not the other way 'round. By the way, in case _iptables_ are messed up, they can be flushed by: `iptables -F` `iptables -X` `iptables -t nat -F` `iptables -t nat -X` `iptables -t mangle -F` `iptables -t mangle -X` `iptables -t raw -F` `iptables -t raw -X` `iptables -t security -F` `iptables -t security -X` `iptables -P INPUT ACCEPT` `iptables -P FORWARD ACCEPT` `iptables -P OUTPUT ACCEPT` ### Reach n2n Node in Office from Whole Home Network This is easy: - Just exchange home and office IP addresses and the computer names in the instructions given above. ### Reach Whole Home Network from Whole Office Network This is not too complicated either. Basically, follow the given example above and apply the following changes: - The instructions used above need to be expanded from **h**ickory's IP 10.11.12.5 to its full network 10.11.12.0/24 in the route commands on **o**scar:, especially: `ip route add 10.11.12.0/24 via 10.99.99.50 dev n2n0 src 192.168.1.6`. - In case of adding a static route to the office network router 192.168.1.1, the home network 10.11.12.0/24 must be specified instead of **h**ickory's more specific IP address 11.11.12.5. Same for the less recommended static routes on other office computers. - Packets from home network's computers to the office network need to be sent through the n2n tunnel. The three alternatives given above can be used just with exchanged office and home IP addresses. One needs to be aware that NAT only (third alternative) on both sides will not allow any connection, i.e. at least on one side static routes need to be added either to the router (best option) or all those computers that shall be able to connect to the other network. ### Route All Internet Traffic from n2n Node at Home through Office Network This scenario could be considered a n2n-tunneled VPN connection which also would work for travelling users on their laptop. All external internet traffic will appear to originate from **o**scar and the office network. - First, one of the setups described above needs to be in place, with the following change: - NAT on **o**scar (see the three _iptables_ commands above) must be enabled. It will not work without because the office router 192.168.1.1 usually denies forwarding to packets not originating from its own network. It could be in addition to the eventually installed static routes for 10.11.12.0/24 in the router 192.168.1.1 or on other office computers – it will not interfere. However, **o**scar definitely needs the route given above: `ip route add 10.11.12.5 via 10.99.99.50 dev n2n0 src 192.168.1.6`. - To have **h**ickory's complete internet traffic going through the n2n tunnel, its default route needs to be changed: `ip route del default` `ip route add default via 10.99.99.60 dev n2n0 src 10.11.12.5` - **h**ickory's home network should still be reachable as usually, _eth0_ and the associated network 10.11.12.0/24 get their very own route. If not, i.e. it was only covered by default route before, it needs to be added: `ip route add 10.11.12.0/24 dev eth0 src 10.11.12.5`. - Unfortunately (unless the supernode is on **h**ickory's local network), n2n supernode becomes unreachable for **h**ickory. To fix it: `ip route add via 10.11.12.1 dev eth0 src 10.11.12.5` The supernode's IP address needs to be known to have this work. However, if the supernode's IP needs to be resolved from some domain name (FQDN), e.g. in case of using dynamic domain name services, a DNS server needs to remain reachable, too. Either the reachable home network router 10.11.12.1 is good enough for that (if it offers DNS) or another route could to be added and **h**ickory's DNS settings might be set accordingly, maybe to Google's 8.8.8.8. If [DNS leaks](https://en.wikipedia.org/wiki/DNS_leak) do not matter, this setup is complete. ### Preventing DNS Leaks Otherwise, there is more to it: Without changes, all future DNS queries go through the home router 10.11.12.1 to the ISP's servers or directly to Google (via the home router 10.11.12.1 along the configured route for 8.8.8.8 and not through the n2n tunnel) while the remaining traffic ships through the n2n tunnel. To prevent such a DNS leak, the supernode's IP address must be determined independently from **h**ickory's DNS configuration, e.g. by digesting `dig +short mysupernode.myfavoritednsservice.com @8.8.8.8`'s output in the n2n-edge's setup script for both, the edge node command line as well as the static route mentioned above. Without further additional work, dynamic address changes remain undetected. A static route to 8.8.8.8 is still required. **h**ickory's regular DNS configuration should query a different DNS server for its regular DNS needs, e.g. 9.9.9.9 or 1.1.1.1 or maybe the office DNS server, maybe 192.168.1.1. This guarantees the regular DNS queries also to get sent through the n2n tunnel. A test for DNS leaks can be found [here](https://www.dnsleaktest.com/).