MTU value error

[vnt-dev]

wintun-bindings/src/util.rs

Line 497 in 41ff615

pub(crate) fn get_adapter_mtu(luid: &NET_LUID_LH) -> std::io::Result<usize> {

The MTU retrieved is for IPv6, but the MTU is set for IPv4 in the command line. Therefore, the retrieved MTU is always 65535.

[ssrlive]

It looks like Microsoft's API is a mess. It is impossible to get the MTU correctly.
The right side of the figure below shows the result of running the function GetAdaptersAddresses.

The current solution may only be through the command line and then parse the results.
After obtaining the MTU of ipv4 and ipv6 respectively, they are returned in the form of tuples.

(Some(1450), None)

Maybe this feature will be completed sometime in the future.

netsh interface ipv4 set interface "wintun" mtu=1450
netsh interface ipv6 set interface "wintun" mtu=2450

netsh interface ipv4 show subinterface
netsh interface ipv6 show subinterface

[vnt-dev]

This method can retrieve the MTU as expected.

pub fn get_mtu_by_index(index: u32, is_v4: bool) -> io::Result<u32> {
    // https://learn.microsoft.com/en-us/windows/win32/api/netioapi/nf-netioapi-getipinterfacetable#examples
    let mut if_table: *mut MIB_IPINTERFACE_TABLE = ptr::null_mut();
    let mut mtu = None;
    unsafe {
        if GetIpInterfaceTable(if is_v4 { AF_INET } else { AF_INET6 }, &mut if_table as _)
            != NO_ERROR
        {
            return Err(io::Error::last_os_error());
        }
        let ifaces = unsafe {
            std::slice::from_raw_parts::<MIB_IPINTERFACE_ROW>(
                &(*if_table).Table[0],
                (*if_table).NumEntries as usize,
            )
        };
        for x in ifaces {
            if x.InterfaceIndex == index {
                mtu = Some(x.NlMtu);
                break;
            }
        }
        windows_sys::Win32::NetworkManagement::IpHelper::FreeMibTable(if_table as _);
    }
    if let Some(mtu) = mtu {
        Ok(mtu)
    } else {
        Err(io::Error::from(io::ErrorKind::NotFound))
    }
}

[xmh0511]

The correct MTU value can be obtained either by https://crates.io/crates/mtu or copy the code from this crates into wintun-bindings