On Sat, Apr 10, 2021 at 7:35 AM David Woodhouse <[email protected]> wrote: > On Sat, 2021-04-10 at 13:38 +0000, Simon Rozman wrote: > > Hi David,This is my proposal: > > https://git.zx2c4.com/wintun/commit/?id=eebd6aea4f75551f6e847a1d4fff857450bac6e9 > > Awaiting review and zx2c4 approval. > > Regards, Simon > > > Looks good to me; thanks. Just need to work out how to cross-build it > (I can muster up a Windows VM for testing, but *building* on it is > beyond my tolerance of Windows for now).
+1 to all that. > We'll also need to be able to WintunAllocateSendPacket() of the full > possible MTU, then receive and decrypt into that, and send only the > actual size of the packet we received. > > A per-packet tail would have let us do that, but I agree that we don't > want to expand the TUN_PACKET header if we can avoid doing so. > > Perhaps a WintunShrinkAndSendPacket() — which can only *shrink*, of > course, and which can only be used on the *last* packet allocated, > checking that its tail *is* the Session->Receive.Tail before adjusting > the latter accordingly. In addition to the use case for chopping ESP trailers and less-than-full-size packets, OpenConnect has the case of "PPP packets in HDLC-like framing" which need to be "un-HDLC-ed" in a way that can only cause them to shrink. (https://gitlab.com/openconnect/openconnect/blob/master/ppp.c#L102-158) There are two cases worth considering where the packet size could actually *expand*: 1) Some VPN protocols support compression of the tunneled packets. It would be bad behavior to use this to stuff a packet of >(advertised MTU) bytes in <(advertised MTU) bytes, but it wouldn't surprise me if it exists in the wild. We now deal with receipt of larger-than-expected-MTU packets in OpenConnect in a relatively uniform way: allocate MAX(mtu, 16384) bytes for packets coming from the VPN (if using TLS transport) or MAX(mtu, 2048) if using DTLS. 2) Some VPN protocols concatenate multiple packets into a single aggregate on the wire. On Linux we can decrypt, truncate, and send to the tunnel interface without further copying. Case (1) can be handled with overallocate-and-shrink. Case (2) is pretty rare among the protocols that OpenConnect supports, so fallback to memcpy seems fine. Dan
