ostp/ostp-tun/src/windows_route.rs

/// Windows routing table utilities for OSTP split tunneling.
///
/// The approach used here matches how sing-box/v2rayN implement split tunneling on Windows:
/// - A high-priority default route (metric=1) via ostp_tun captures ALL traffic.
/// - Per-host /32 routes via the REAL gateway with an even lower metric (=0, auto-managed by OS)
///   force excluded IPs to bypass the TUN.
/// - Process-based exclusions are NOT supported via pure routing — they would require WFP.
///   Instead, we surface a diagnostic warning in logs.

#[cfg(target_os = "windows")]
pub mod sys {
    use std::mem;
    use std::net::Ipv4Addr;
    use std::ptr;

    use winapi::shared::ipmib::{MIB_IPFORWARDROW, MIB_IPFORWARDTABLE};
    use winapi::shared::minwindef::{DWORD, ULONG};
    use winapi::shared::winerror::{ERROR_INSUFFICIENT_BUFFER, NO_ERROR};
    use winapi::um::iphlpapi::{
        CreateIpForwardEntry, DeleteIpForwardEntry, GetAdaptersAddresses, GetIpForwardTable,
    };
    use winapi::um::iptypes::{
        GAA_FLAG_SKIP_ANYCAST, GAA_FLAG_SKIP_DNS_SERVER, GAA_FLAG_SKIP_MULTICAST, IP_ADAPTER_ADDRESSES,
    };
    use winapi::shared::ws2def::AF_INET;

    fn ipv4_to_dword(ip: Ipv4Addr) -> DWORD {
        u32::from_ne_bytes(ip.octets())
    }

    fn dword_to_ipv4(dw: DWORD) -> Ipv4Addr {
        Ipv4Addr::from(dw.to_ne_bytes())
    }

    /// Returns the (gateway_ip, interface_index) of the physical default IPv4 route,
    /// excluding any route that goes through an interface named "ostp_tun".
    pub fn get_default_ipv4_route() -> Option<(Ipv4Addr, u32)> {
        // Enumerate adapters to find the ostp_tun interface index, so we can skip it.
        let tun_index = get_interface_index("ostp_tun");

        unsafe {
            let mut size: ULONG = 0;
            let mut ret = GetIpForwardTable(ptr::null_mut(), &mut size, 0);
            if ret != ERROR_INSUFFICIENT_BUFFER {
                return None;
            }

            let mut buf: Vec<u8> = vec![0; size as usize];
            let table = buf.as_mut_ptr() as *mut MIB_IPFORWARDTABLE;

            ret = GetIpForwardTable(table, &mut size, 0);
            if ret != NO_ERROR {
                return None;
            }

            let entries = std::slice::from_raw_parts((*table).table.as_ptr(), (*table).dwNumEntries as usize);

            let mut best_gw = None;
            let mut best_metric = u32::MAX;
            let mut best_ifindex = 0u32;

            for row in entries {
                // Only consider default routes (0.0.0.0/0)
                if row.dwForwardDest == 0 && row.dwForwardMask == 0 {
                    // Skip the TUN interface
                    if let Some(ti) = tun_index {
                        if row.dwForwardIfIndex == ti {
                            continue;
                        }
                    }
                    let metric = row.dwForwardMetric1;
                    if metric < best_metric {
                        best_metric = metric;
                        best_gw = Some(dword_to_ipv4(row.dwForwardNextHop));
                        best_ifindex = row.dwForwardIfIndex;
                    }
                }
            }

            best_gw.map(|gw| (gw, best_ifindex))
        }
    }

    pub fn add_ipv4_route(
        dest: Ipv4Addr,
        mask: Ipv4Addr,
        nexthop: Ipv4Addr,
        if_index: u32,
        metric: u32,
    ) -> Result<(), String> {
        let mut row: MIB_IPFORWARDROW = unsafe { mem::zeroed() };
        row.dwForwardDest = ipv4_to_dword(dest);
        row.dwForwardMask = ipv4_to_dword(mask);
        row.dwForwardNextHop = ipv4_to_dword(nexthop);
        row.dwForwardIfIndex = if_index;
        row.ForwardType = if nexthop == Ipv4Addr::UNSPECIFIED || dest == nexthop { 3 } else { 4 };
        row.ForwardProto = 3; // MIB_IPPROTO_NETMGMT
        row.dwForwardMetric1 = metric;

        let ret = unsafe { CreateIpForwardEntry(&mut row) };
        if ret == NO_ERROR {
            Ok(())
        } else {
            Err(format!("CreateIpForwardEntry failed: {}", ret))
        }
    }

    pub fn delete_ipv4_route(
        dest: Ipv4Addr,
        mask: Ipv4Addr,
        nexthop: Ipv4Addr,
        if_index: u32,
    ) -> Result<(), String> {
        let mut row: MIB_IPFORWARDROW = unsafe { mem::zeroed() };
        row.dwForwardDest = ipv4_to_dword(dest);
        row.dwForwardMask = ipv4_to_dword(mask);
        row.dwForwardNextHop = ipv4_to_dword(nexthop);
        row.dwForwardIfIndex = if_index;

        let ret = unsafe { DeleteIpForwardEntry(&mut row) };
        if ret == NO_ERROR || ret == 2 {
            Ok(())
        } else {
            Err(format!("DeleteIpForwardEntry failed: {}", ret))
        }
    }

    pub fn get_interface_index(name: &str) -> Option<u32> {
        unsafe {
            let mut size: ULONG = 0;
            let flags = GAA_FLAG_SKIP_ANYCAST | GAA_FLAG_SKIP_MULTICAST | GAA_FLAG_SKIP_DNS_SERVER;
            let mut ret = GetAdaptersAddresses(
                AF_INET as u32,
                flags,
                ptr::null_mut(),
                ptr::null_mut(),
                &mut size,
            );
            if ret != ERROR_INSUFFICIENT_BUFFER {
                return None;
            }

            let mut buf: Vec<u8> = vec![0; size as usize];
            let addresses = buf.as_mut_ptr() as *mut IP_ADAPTER_ADDRESSES;

            ret = GetAdaptersAddresses(AF_INET as u32, flags, ptr::null_mut(), addresses, &mut size);
            if ret != NO_ERROR {
                return None;
            }

            let mut curr = addresses;
            while !curr.is_null() {
                let friendly_name_ptr = (*curr).FriendlyName;
                if !friendly_name_ptr.is_null() {
                    let mut len = 0;
                    while *friendly_name_ptr.offset(len) != 0 {
                        len += 1;
                    }
                    let slice = std::slice::from_raw_parts(friendly_name_ptr, len as usize);
                    let friendly_name = String::from_utf16_lossy(slice);
                    if friendly_name == name {
                        return Some((*(*curr).u.s()).IfIndex);
                    }
                }
                curr = (*curr).Next;
            }
            None
        }
    }

    /// Add bypass routes for a list of resolved IP addresses (typically from exclusion config).
    /// Each IP gets a /32 host route via the physical gateway so it bypasses the TUN.
    /// Returns list of (ip, gw, if_index) that were successfully added, for later cleanup.
    pub fn add_bypass_routes(
        ips: &[Ipv4Addr],
        gw: Ipv4Addr,
        if_index: u32,
        metric: u32,
    ) -> Vec<(Ipv4Addr, Ipv4Addr, u32)> {
        let mut added = Vec::new();
        for &ip in ips {
            let mask = Ipv4Addr::new(255, 255, 255, 255);
            match add_ipv4_route(ip, mask, gw, if_index, metric) {
                Ok(()) => {
                    added.push((ip, gw, if_index));
                }
                Err(e) => {
                    // 87 = ERROR_INVALID_PARAMETER (route may already exist)
                    tracing::debug!("bypass route add {ip}/32 via {gw}: {e}");
                }
            }
        }
        added
    }

    /// Remove all bypass routes previously added by add_bypass_routes.
    pub fn remove_bypass_routes(routes: &[(Ipv4Addr, Ipv4Addr, u32)]) {
        for &(ip, gw, if_index) in routes {
            let mask = Ipv4Addr::new(255, 255, 255, 255);
            let _ = delete_ipv4_route(ip, mask, gw, if_index);
        }
    }
}