Tools/scripts/Networking

Host-side helpers for ArduPilot's SITL networking. The scripts here manage the host plumbing that lets SITL binaries with the optional AP_NETWORKING_BACKEND_SITL_TUN backend bridge their lwIP stack to a real TAP device and reach (or be reached from) the wider host network.

`sitl_network.sh`

Brings up a host-side TAP interface, assigns the host an address on it, and installs iptables rules so the SITL subnet has NAT'd outbound reachability through the host's default outbound interface.

Usage

Tools/scripts/Networking/sitl_network.sh up     # create TAP + NAT
sim_vehicle.py -v Plane -f quadplane-PPP        # SITL processes start
curl http://10.77.193.20/                       # browse the PPPGW web UI
Tools/scripts/Networking/sitl_network.sh down   # tear it back down

Both subcommands require sudo only for the privileged steps (TAP creation, iptables -t nat, sysctl net.ipv4.ip_forward). The TAP device itself is created with user $USER so the SITL binary opens /dev/net/tun without any extra privileges.

The script writes a small /tmp/sitl_network.state file on up so that down undoes exactly what was put in place (the outbound interface used for MASQUERADE, and the previous net.ipv4.ip_forward value).

What `up` does

Creates sitltap (TAP, persistent, owned by $USER) if it doesn't already exist, sets it up, and assigns the host 10.77.193.1/24.
Detects the default outbound interface from ip route.
Enables net.ipv4.ip_forward (if it wasn't on already).
Adds an iptables -t nat -A POSTROUTING ... -j MASQUERADE rule for 10.77.193.0/24 leaving via the outbound interface, plus matching FORWARD ACCEPTs so the kernel actually forwards.

What `down` does

Reverses everything up set up, in the opposite order. Safe to call even if nothing is up (it just prints "TAP does not exist").

Customising

A few environment variables let you change the defaults:

Variable	Default	Meaning
`DEV`	`sitltap`	TAP interface name
`HOST_IP`	`10.77.193.1/24`	Address assigned to the host on the TAP
`SUBNET`	`10.77.193.0/24`	Subnet the MASQUERADE rule covers
`PEER_IP`	`10.77.193.20`	Just used in the friendly "where to curl" message

Companion SITL build

This script is paired with the libraries/AP_HAL_SITL/hwdef/sitl_periph_PPP build target, which has AP_NETWORKING_BACKEND_SITL_TUN enabled and a periph-side NET_GWADDR=10.77.193.1 default so the periph's lwIP routes out-of-subnet traffic via the host TAP. With the script's NAT rules in place, both the periph and the ArduPlane behind it on the PPP link can reach the wider host network and the public internet; with NAT off, only the developer-side reachability (curl the PPPGW web UI on 10.77.193.20) still works.

If sitl_network.sh up has not been run, the SITL_TUN backend falls back to a "PPPGW-only, no host bridge" mode: PPP and IPCP still work, the periph is just not reachable from the host.

README.md