fix(qemu): open the adapter TAP without a vnet header

The TAP netdev kept QEMU's default vnet_hdr, so virtio-net, e1000e, igb
and vmxnet3 guests offload TCP/UDP checksums (and TSO) to the TAP: their
frames carry only the pseudo-header sum in the checksum field. A kernel
bridge link finishes them, but the uBridge relay (cloud, NAT, hub, ...)
reads the TAP with libpcap and forwards the frames as they are, so DNS
and TCP through a cloud fail while ICMP and DHCP work.

With vnet_hdr=off QEMU stops offering the offloads and the guest (or the
emulated NIC) sends finished frames, as with the 2.x socket netdev.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

gns3server/compute/qemu/qemu_vm.py

@@ -2715,8 +2715,14 @@ class QemuVM(KernelDatapathMixin, BaseNode):
                 tap = self._kernel_host_ifc(adapter_number) or self._tap_name(adapter_number)
             network_options.extend(["-device", f"{device_string},netdev=gns3-{adapter_number}"])
             if self._tap_datapath or isinstance(nio, NIOTAP):
+                # vnet_hdr=off: with QEMU's default virtio-net header the
+                # guest NIC offloads TCP/UDP checksums and TSO to the TAP.
+                # A kernel bridge finishes such frames, but the uBridge relay
+                # (cloud, NAT, ...) copies them off the TAP as they are, so
+                # receivers drop them. Without the header QEMU and the guest
+                # send finished frames, as with the 2.x socket netdev.
                 network_options.extend(
-                    ["-netdev", f"tap,id=gns3-{adapter_number},ifname={tap},script=no,downscript=no"]
+                    ["-netdev", f"tap,id=gns3-{adapter_number},ifname={tap},script=no,downscript=no,vnet_hdr=off"]
                 )
             else:
                 # Legacy datapath (uBridge without the tap module): QEMU talks

tests/compute/qemu/test_qemu_kernel_datapath.py

@@ -29,6 +29,7 @@ from unittest.mock import MagicMock, patch
 import pytest
 import pytest_asyncio
 
+from gns3server.compute.nios.nio_tap import NIOTAP
 from gns3server.compute.qemu import Qemu
 from gns3server.compute.qemu.qemu_error import QemuError
 from gns3server.compute.qemu.qemu_vm import QemuVM
@@ -143,11 +144,35 @@ async def test_network_options_use_the_tap_anchor(vm):
 
     options = await vm._network_options()
 
-    assert f"tap,id=gns3-0,ifname={TAP0},script=no,downscript=no" in options
+    assert f"tap,id=gns3-0,ifname={TAP0},script=no,downscript=no,vnet_hdr=off" in options
     assert not any(option.startswith("socket,") for option in options)
     assert not vm._local_udp_tunnels
 
 
+@pytest.mark.asyncio
+async def test_network_options_turn_off_the_tap_vnet_header(vm):
+    """
+    Every TAP netdev turns the virtio-net header off, the legacy NIOTAP one
+    included. With QEMU's default header the guest NIC hands its TCP/UDP
+    checksums (and TSO) to the TAP, and the uBridge relay copies those
+    frames off the TAP unfinished: receivers behind a cloud drop them.
+    """
+
+    vm._tap_datapath = True
+    vm.adapters = 2
+    vm.adapter_type = "virtio-net-pci"
+    vm._kernel_taps[(0, 0)] = TAP0
+    vm._ethernet_adapters[1].add_nio(0, NIOTAP("tap-legacy"))
+
+    options = await vm._network_options()
+
+    netdevs = [options[i + 1] for i, option in enumerate(options) if option == "-netdev"]
+    assert netdevs == [
+        f"tap,id=gns3-0,ifname={TAP0},script=no,downscript=no,vnet_hdr=off",
+        "tap,id=gns3-1,ifname=tap-legacy,script=no,downscript=no,vnet_hdr=off",
+    ]
+
+
 @pytest.mark.asyncio
 async def test_network_options_keep_the_legacy_socket_netdev(vm):
     """

tests/e2e/test_qemu_kernel_datapath.py

@@ -11,7 +11,7 @@ binary, the real IOS CLI behind the server's telnet console), driven through
 the controller REST API. QEMU is the node type whose guest leg *is* the
 anchor: uBridge creates one persistent TAP per adapter before launch and
 hands it to this user, and QEMU opens it by ifname as its netdev
-(``-netdev tap,ifname=...,script=no,downscript=no``) — no userspace hop, no
+(``-netdev tap,ifname=...,script=no,downscript=no,vnet_hdr=off``) — no userspace hop, no
 port bridge, the anchor is the NIC's host side.
 
 * two QEMU nodes on one compute report ``kernel_datapath`` when the