Add vsock-backed SSH session reminders

Remind users when a VM is still running after 	hanger vm ssh exits instead of silently dropping them back to the host shell.\n\nAttach a Firecracker vsock device to each VM, persist the host vsock path/CID,\nadd a new guest-side banger-vsock-pingd responder to the runtime bundle and both\nimage-build paths, and expose a vm.ping RPC that the CLI and TUI call after SSH\nreturns. Doctor and start/build preflight now validate the helper plus\n/dev/vhost-vsock so the feature fails early and clearly.\n\nValidated with go mod tidy, bash -n customize.sh, git diff --check, make build,\nand GOCACHE=/tmp/banger-gocache go test ./... outside the sandbox because the\ndaemon tests need real Unix/UDP sockets. Rebuild the image/rootfs used for new\nVMs so the guest ping service is present.

AGENTS.md

@@ -9,11 +9,11 @@
 - The daemon keeps state under XDG directories rather than the old repo-local `state/` layout.
 
 ## Build, Test, and Development Commands
-- `make build` builds `./banger` and `./bangerd`.
+- `make build` builds `./banger`, `./bangerd`, and the bundled `./runtime/banger-vsock-pingd` guest helper.
 - `make runtime-bundle` bootstraps `./runtime/` from the archive referenced by `RUNTIME_MANIFEST`; the checked-in `runtime-bundle.toml` is only a template.
 - `banger` validates required host tools per command and reports actionable missing-tool errors; do not assume one workstation's package set.
 - `./banger vm create --name testbox` creates and starts a VM.
-- `./banger vm ssh testbox` connects to a running guest.
+- `./banger vm ssh testbox` connects to a running guest and reminds the user if the VM is still running when the session exits.
 - `./banger vm stop testbox` stops a VM while preserving its disks.
 - `./banger vm stop vm-a vm-b vm-c` and `./banger vm set --nat web-1 web-2` are supported; multi-VM lifecycle and `set` actions fan out concurrently through the CLI.
 - `./banger doctor` reports runtime bundle, host tool, feature, and image-build readiness from the same Go checks used by the daemon.
@@ -31,7 +31,7 @@
 - Primary automated coverage is `go test ./...`.
 - Manual verification for VM lifecycle changes: `./banger vm create`, confirm SSH access, then stop/delete the VM.
 - For host-integration changes, run `./banger doctor` as a quick readiness check before the live VM smoke.
-- Rebuilt images now include `mise`, `opencode`, and `tmux-resurrect`/`tmux-continuum` defaults for `root`; if you change guest provisioning, document whether users need to rebuild `./runtime/rootfs-docker.ext4` or another base image to pick it up.
+- Rebuilt images now include `mise`, `opencode`, `tmux-resurrect`/`tmux-continuum` defaults for `root`, and the `banger-vsock-pingd` service used by the SSH reminder path; if you change guest provisioning, document whether users need to rebuild `./runtime/rootfs-docker.ext4` or another base image to pick it up.
 - If you add a new operational workflow, document how to exercise it in `README.md`.
 - For NAT changes, verify both guest outbound access and host rule cleanup, for example with `./verify.sh --nat`.
 

Makefile

@@ -12,8 +12,9 @@ RUNTIME_MANIFEST ?= runtime-bundle.toml
 RUNTIME_SOURCE_DIR ?= runtime
 RUNTIME_ARCHIVE ?= dist/banger-runtime.tar.gz
 BINARIES := banger bangerd
+RUNTIME_HELPERS := $(RUNTIME_SOURCE_DIR)/banger-vsock-pingd
 GO_SOURCES := $(shell find cmd internal -type f -name '*.go' | sort)
-RUNTIME_EXECUTABLES := firecracker customize.sh packages.sh namegen
+RUNTIME_EXECUTABLES := firecracker customize.sh packages.sh namegen banger-vsock-pingd
 RUNTIME_DATA_FILES := packages.apt id_ed25519 rootfs-docker.ext4
 RUNTIME_OPTIONAL_DATA_FILES := rootfs.ext4 bundle.json
 RUNTIME_BOOT_FILES := wtf/root/boot/vmlinux-6.8.0-94-generic wtf/root/boot/initrd.img-6.8.0-94-generic
@@ -36,7 +37,7 @@ help:
 	  '  make clean   Remove built Go binaries' \
 	  '  make rootfs  Rebuild the source-checkout default rootfs image in ./runtime'
 
-build: $(BINARIES)
+build: $(BINARIES) $(RUNTIME_HELPERS)
 
 banger: $(GO_SOURCES) go.mod go.sum
 	$(GO) build -o ./banger ./cmd/banger
@@ -44,6 +45,10 @@ banger: $(GO_SOURCES) go.mod go.sum
 bangerd: $(GO_SOURCES) go.mod go.sum
 	$(GO) build -o ./bangerd ./cmd/bangerd
 
+$(RUNTIME_SOURCE_DIR)/banger-vsock-pingd: $(GO_SOURCES) go.mod go.sum
+	mkdir -p "$(RUNTIME_SOURCE_DIR)"
+	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 $(GO) build -o "$(RUNTIME_SOURCE_DIR)/banger-vsock-pingd" ./cmd/banger-vsock-pingd
+
 test:
 	$(GO) test ./...
 

README.md

@@ -4,6 +4,7 @@ Persistent Firecracker development VMs managed through a Go daemon, CLI, and TUI
 
 ## Requirements
 - Linux host with KVM (`/dev/kvm` access)
+- Vsock support for post-SSH liveness reminders (`/dev/vhost-vsock`)
 - Core VM lifecycle: `sudo`, `ip`, `dmsetup`, `losetup`, `blockdev`, `truncate`, `pgrep`, `chown`, `chmod`, `kill`
 - Guest rootfs patching: `e2cp`, `e2rm`, `debugfs`
 - Guest work disk creation/resizing: `mkfs.ext4`, `e2fsck`, `resize2fs`, `mount`, `umount`, `cp`
@@ -21,6 +22,7 @@ generated `./runtime/` bundle, while installed binaries use
 
 The bundle contains:
 - `firecracker`
+- `banger-vsock-pingd` for the guest-side SSH reminder responder
 - `bundle.json` with the bundle's default kernel/initrd/modules/rootfs paths
 - a kernel, initrd, and modules tree referenced by `bundle.json`
 - `rootfs-docker.ext4`
@@ -64,7 +66,8 @@ URL. `make install` will not fetch artifacts for you.
 make build
 ```
 
-Run `make build` after `./runtime/` has been bootstrapped.
+Run `make build` after `./runtime/` has been bootstrapped. It also rebuilds the
+bundled `banger-vsock-pingd` guest helper in `./runtime/`.
 
 Install into `~/.local/bin` by default, with the runtime bundle under
 `~/.local/lib/banger`:
@@ -102,6 +105,9 @@ SSH into a running VM:
 banger vm ssh calm-otter
 ```
 
+When the SSH session exits normally, `banger` checks the guest over vsock and
+reminds you if the VM is still running.
+
 Stop, restart, kill, or delete it:
 ```bash
 banger vm stop calm-otter
@@ -160,6 +166,7 @@ Useful config keys:
 - `ssh_key_path`
 - `namegen_path`
 - `customize_script` (manual helper compatibility; `banger image build` is Go-native)
+- `vsock_ping_helper_path`
 - `default_rootfs`
 - `default_base_rootfs`
 - `default_kernel`
@@ -197,9 +204,10 @@ banger image build --name docker-dev --docker
 ```
 
 Rebuilt images install a pinned `mise` at `/usr/local/bin/mise`, activate it
-for bash login and interactive shells, install `opencode` through `mise`, and
+for bash login and interactive shells, install `opencode` through `mise`,
 configure `tmux-resurrect` plus `tmux-continuum` for `root` with periodic
-autosaves and manual-only restore by default.
+autosaves and manual-only restore by default, and bake in the
+`banger-vsock-pingd` systemd service used by the post-SSH reminder path.
 
 Show or delete images:
 ```bash

cmd/banger-vsock-pingd/main.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	sdkvsock "github.com/firecracker-microvm/firecracker-go-sdk/vsock"
+	"github.com/sirupsen/logrus"
+
+	"banger/internal/vsockping"
+)
+
+func main() {
+	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer cancel()
+
+	logger := logrus.New()
+	logger.SetOutput(io.Discard)
+	listener, err := sdkvsock.Listener(ctx, logrus.NewEntry(logger), vsockping.Port)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "banger-vsock-pingd: %v\n", err)
+		os.Exit(1)
+	}
+	defer listener.Close()
+
+	for {
+		conn, err := listener.Accept()
+		if err != nil {
+			if ctx.Err() != nil || errors.Is(err, net.ErrClosed) {
+				return
+			}
+			fmt.Fprintf(os.Stderr, "banger-vsock-pingd: accept: %v\n", err)
+			time.Sleep(200 * time.Millisecond)
+			continue
+		}
+		go func(conn net.Conn) {
+			if err := vsockping.ServeConn(conn); err != nil {
+				fmt.Fprintf(os.Stderr, "banger-vsock-pingd: %v\n", err)
+			}
+		}(conn)
+	}
+}

customize.sh

@@ -68,6 +68,7 @@ FC_BIN="$RUNTIME_DIR/firecracker"
 KERNEL="$(bundle_path default_kernel "$RUNTIME_DIR/wtf/root/boot/vmlinux-6.8.0-94-generic")"
 INITRD="$(bundle_path default_initrd "$RUNTIME_DIR/wtf/root/boot/initrd.img-6.8.0-94-generic")"
 SSH_KEY="$RUNTIME_DIR/id_ed25519"
+VSOCK_PING_HELPER="$(bundle_path vsock_ping_helper_path "$RUNTIME_DIR/banger-vsock-pingd")"
 
 BR_DEV="br-fc"
 BR_IP="172.16.0.1"
@@ -207,6 +208,11 @@ if [[ ! -f "$PACKAGES_FILE" ]]; then
   log "package manifest not found: $PACKAGES_FILE"
   exit 1
 fi
+if [[ ! -x "$VSOCK_PING_HELPER" ]]; then
+  log "vsock ping helper not found or not executable: $VSOCK_PING_HELPER"
+  log "run 'make build' or refresh the runtime bundle"
+  exit 1
+fi
 
 APT_PACKAGES=()
 if ! banger_packages_read_array APT_PACKAGES "$PACKAGES_FILE"; then
@@ -382,6 +388,10 @@ if [[ "$SSH_READY" -ne 1 ]]; then
 fi
 
 log "configuring guest"
+log "installing vsock ping helper"
+scp -i "$SSH_KEY" -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null \
+  "$VSOCK_PING_HELPER" "root@${GUEST_IP}:/usr/local/bin/banger-vsock-pingd" >/dev/null
+
 ssh -i "$SSH_KEY" -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null \
   "root@${GUEST_IP}" bash -lc "set -e
 printf 'nameserver %s\n' \"$DNS_SERVER\" > /etc/resolv.conf
@@ -421,6 +431,32 @@ if [[ \"$INSTALL_DOCKER\" == \"1\" ]]; then
   fi
 fi
 rm -f /root/get-docker /root/get-docker.sh /tmp/get-docker /tmp/get-docker.sh
+chmod 0755 /usr/local/bin/banger-vsock-pingd
+mkdir -p /etc/modules-load.d /etc/systemd/system
+cat > /etc/modules-load.d/banger-vsock.conf <<'EOF'
+vsock
+vmw_vsock_virtio_transport
+EOF
+chmod 0644 /etc/modules-load.d/banger-vsock.conf
+cat > /etc/systemd/system/banger-vsock-pingd.service <<'EOF'
+[Unit]
+Description=Banger vsock ping responder
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/banger-vsock-pingd
+Restart=on-failure
+RestartSec=1
+
+[Install]
+WantedBy=multi-user.target
+EOF
+chmod 0644 /etc/systemd/system/banger-vsock-pingd.service
+if command -v systemctl >/dev/null 2>&1; then
+  systemctl daemon-reload || true
+  systemctl enable --now banger-vsock-pingd.service || true
+fi
 git config --system init.defaultBranch main
 "
 

go.mod

@@ -46,6 +46,8 @@ require (
 	github.com/lucasb-eyer/go-colorful v1.2.0 // indirect
 	github.com/mailru/easyjson v0.7.7 // indirect
 	github.com/mattn/go-runewidth v0.0.13 // indirect
+	github.com/mdlayher/socket v0.2.0 // indirect
+	github.com/mdlayher/vsock v1.1.1 // indirect
 	github.com/mitchellh/mapstructure v1.4.3 // indirect
 	github.com/muesli/ansi v0.0.0-20211018074035-2e021307bc4b // indirect
 	github.com/muesli/cancelreader v0.2.1 // indirect

go.sum

@@ -514,7 +514,9 @@ github.com/mattn/go-shellwords v1.0.3/go.mod h1:3xCvwCdWdlDJUrvuMn7Wuy9eWs4pE8vq
 github.com/mattn/go-shellwords v1.0.12/go.mod h1:EZzvwXDESEeg03EKmM+RmDnNOPKG4lLtQsUlTZDWQ8Y=
 github.com/matttproud/golang_protobuf_extensions v1.0.1/go.mod h1:D8He9yQNgCq6Z5Ld7szi9bcBfOoFv/3dc6xSMkL2PC0=
 github.com/matttproud/golang_protobuf_extensions v1.0.2-0.20181231171920-c182affec369/go.mod h1:BSXmuO+STAnVfrANrmjBb36TMTDstsz7MSK+HVaYKv4=
+github.com/mdlayher/socket v0.2.0 h1:EY4YQd6hTAg2tcXF84p5DTHazShE50u5HeBzBaNgjkA=
 github.com/mdlayher/socket v0.2.0/go.mod h1:QLlNPkFR88mRUNQIzRBMfXxwKal8H7u1h3bL1CV+f0E=
+github.com/mdlayher/vsock v1.1.1 h1:8lFuiXQnmICBrCIIA9PMgVSke6Fg6V4+r0v7r55k88I=
 github.com/mdlayher/vsock v1.1.1/go.mod h1:Y43jzcy7KM3QB+/FK15pfqGxDMCMzUXWegEfIbSM18U=
 github.com/miekg/dns v1.1.72 h1:vhmr+TF2A3tuoGNkLDFK9zi36F2LS+hKTRW0Uf8kbzI=
 github.com/miekg/dns v1.1.72/go.mod h1:+EuEPhdHOsfk6Wk5TT2CzssZdqkmFhf8r+aVyDEToIs=

internal/api/types.go

@@ -63,6 +63,11 @@ type VMSSHResult struct {
 	GuestIP string `json:"guest_ip"`
 }
 
+type VMPingResult struct {
+	Name  string `json:"name"`
+	Alive bool   `json:"alive"`
+}
+
 type ImageBuildParams struct {
 	Name       string `json:"name,omitempty"`
 	BaseRootfs string `json:"base_rootfs,omitempty"`

internal/cli/banger.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -23,6 +24,7 @@ import (
 	"banger/internal/rpc"
 	"banger/internal/system"
 	"banger/internal/vmdns"
+	"banger/internal/vsockping"
 
 	"github.com/spf13/cobra"
 )
@@ -32,7 +34,17 @@ var (
 	daemonExePath   = func(pid int) string {
 		return filepath.Join("/proc", fmt.Sprintf("%d", pid), "exe")
 	}
-	doctorFunc = daemon.Doctor
+	doctorFunc  = daemon.Doctor
+	sshExecFunc = func(ctx context.Context, stdin io.Reader, stdout, stderr io.Writer, args []string) error {
+		sshCmd := exec.CommandContext(ctx, "ssh", args...)
+		sshCmd.Stdout = stdout
+		sshCmd.Stderr = stderr
+		sshCmd.Stdin = stdin
+		return sshCmd.Run()
+	}
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return rpc.Call[api.VMPingResult](ctx, socketPath, "vm.ping", api.VMRefParams{IDOrName: idOrName})
+	}
 )
 
 func NewBangerCommand() *cobra.Command {
@@ -454,11 +466,7 @@ func newVMSSHCommand() *cobra.Command {
 			if err != nil {
 				return err
 			}
-			sshCmd := exec.CommandContext(cmd.Context(), "ssh", sshArgs...)
+			return runSSHSession(cmd.Context(), layout.SocketPath, result.Name, cmd.InOrStdin(), cmd.OutOrStdout(), cmd.ErrOrStderr(), sshArgs)
-			sshCmd.Stdout = cmd.OutOrStdout()
-			sshCmd.Stderr = cmd.ErrOrStderr()
-			sshCmd.Stdin = cmd.InOrStdin()
-			return sshCmd.Run()
 		},
 	}
 }
@@ -953,6 +961,36 @@ func validatePositiveSetting(label string, value int) error {
 	return nil
 }
 
+func runSSHSession(ctx context.Context, socketPath, vmRef string, stdin io.Reader, stdout, stderr io.Writer, sshArgs []string) error {
+	sshErr := sshExecFunc(ctx, stdin, stdout, stderr, sshArgs)
+	if !shouldCheckSSHReminder(sshErr) || ctx.Err() != nil {
+		return sshErr
+	}
+	pingCtx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+	ping, err := vmPingFunc(pingCtx, socketPath, vmRef)
+	if err != nil {
+		_, _ = fmt.Fprintln(stderr, vsockping.WarningMessage(vmRef, err))
+		return sshErr
+	}
+	if ping.Alive {
+		name := ping.Name
+		if strings.TrimSpace(name) == "" {
+			name = vmRef
+		}
+		_, _ = fmt.Fprintln(stderr, vsockping.ReminderMessage(name))
+	}
+	return sshErr
+}
+
+func shouldCheckSSHReminder(err error) bool {
+	if err == nil {
+		return true
+	}
+	var exitErr *exec.ExitError
+	return errors.As(err, &exitErr)
+}
+
 func sshCommandArgs(cfg model.DaemonConfig, guestIP string, extra []string) ([]string, error) {
 	if guestIP == "" {
 		return nil, errors.New("vm has no guest IP")

internal/cli/cli_test.go

@@ -4,7 +4,9 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"io"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"reflect"
 	"strings"
@@ -209,6 +211,56 @@ func TestVMSetParamsFromFlagsRejectsNonPositiveCPUAndMemory(t *testing.T) {
 	}
 }
 
+func TestRunSSHSessionPrintsReminderWhenPingAlive(t *testing.T) {
+	origSSHExec := sshExecFunc
+	origPing := vmPingFunc
+	t.Cleanup(func() {
+		sshExecFunc = origSSHExec
+		vmPingFunc = origPing
+	})
+
+	sshExecFunc = func(ctx context.Context, stdin io.Reader, stdout, stderr io.Writer, args []string) error {
+		return nil
+	}
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return api.VMPingResult{Name: "devbox", Alive: true}, nil
+	}
+
+	var stderr bytes.Buffer
+	if err := runSSHSession(context.Background(), "/tmp/bangerd.sock", "devbox", strings.NewReader(""), &bytes.Buffer{}, &stderr, []string{"root@127.0.0.1"}); err != nil {
+		t.Fatalf("runSSHSession: %v", err)
+	}
+	if !strings.Contains(stderr.String(), "devbox is still running") {
+		t.Fatalf("stderr = %q, want reminder", stderr.String())
+	}
+}
+
+func TestRunSSHSessionPreservesSSHExitStatusOnPingWarning(t *testing.T) {
+	origSSHExec := sshExecFunc
+	origPing := vmPingFunc
+	t.Cleanup(func() {
+		sshExecFunc = origSSHExec
+		vmPingFunc = origPing
+	})
+
+	sshExecFunc = func(ctx context.Context, stdin io.Reader, stdout, stderr io.Writer, args []string) error {
+		return &exec.ExitError{}
+	}
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return api.VMPingResult{}, errors.New("dial failed")
+	}
+
+	var stderr bytes.Buffer
+	err := runSSHSession(context.Background(), "/tmp/bangerd.sock", "devbox", strings.NewReader(""), &bytes.Buffer{}, &stderr, []string{"root@127.0.0.1"})
+	var exitErr *exec.ExitError
+	if !errors.As(err, &exitErr) {
+		t.Fatalf("runSSHSession error = %v, want exit error", err)
+	}
+	if !strings.Contains(stderr.String(), "failed to check whether devbox is still running") {
+		t.Fatalf("stderr = %q, want warning", stderr.String())
+	}
+}
+
 func TestResolveVMTargetsDeduplicatesAndReportsErrors(t *testing.T) {
 	vms := []model.VMRecord{
 		testCLIResolvedVM("alpha-id", "alpha"),

internal/cli/tui.go

@@ -16,6 +16,7 @@ import (
 	"banger/internal/paths"
 	"banger/internal/rpc"
 	"banger/internal/system"
+	"banger/internal/vsockping"
 
 	"github.com/charmbracelet/bubbles/help"
 	"github.com/charmbracelet/bubbles/key"
@@ -104,6 +105,7 @@ type externalPreparedMsg struct {
 	action     actionRequest
 	command    *exec.Cmd
 	doneStatus string
+	done       func(error) tea.Msg
 	refresh    bool
 	err        error
 }
@@ -716,10 +718,14 @@ func (m tuiModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			break
 		}
 		cmds = append(cmds, tea.ExecProcess(msg.command, func(err error) tea.Msg {
+			err = normalizeExecError(err)
+			if msg.done != nil {
+				return msg.done(err)
+			}
 			return actionResultMsg{
 				action:  msg.action,
 				status:  msg.doneStatus,
-				err:     normalizeExecError(err),
+				err:     err,
 				refresh: msg.refresh,
 				focusID: m.selectedID,
 			}
@@ -1439,14 +1445,55 @@ func prepareSSHCmd(layout paths.Layout, cfg model.DaemonConfig, action actionReq
 			return externalPreparedMsg{action: action, err: err}
 		}
 		return externalPreparedMsg{
-			action:     action,
+			action:  action,
-			command:    exec.Command("ssh", args...),
+			command: exec.Command("ssh", args...),
-			doneStatus: fmt.Sprintf("ssh session ended for %s", result.Name),
+			done: func(execErr error) tea.Msg {
-			refresh:    true,
+				return sshDoneMsg(layout, action, result.Name, execErr)
+			},
+			refresh: true,
 		}
 	}
 }
 
+func sshDoneMsg(layout paths.Layout, action actionRequest, name string, execErr error) tea.Msg {
+	if execErr != nil {
+		return actionResultMsg{
+			action:  action,
+			err:     execErr,
+			refresh: true,
+			focusID: action.id,
+		}
+	}
+	pingCtx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+	ping, err := vmPingFunc(pingCtx, layout.SocketPath, name)
+	if err != nil {
+		return actionResultMsg{
+			action:  action,
+			status:  vsockping.WarningMessage(name, err),
+			refresh: true,
+			focusID: action.id,
+		}
+	}
+	if ping.Alive {
+		if strings.TrimSpace(ping.Name) != "" {
+			name = ping.Name
+		}
+		return actionResultMsg{
+			action:  action,
+			status:  vsockping.ReminderMessage(name),
+			refresh: true,
+			focusID: action.id,
+		}
+	}
+	return actionResultMsg{
+		action:  action,
+		status:  fmt.Sprintf("ssh session ended for %s", name),
+		refresh: true,
+		focusID: action.id,
+	}
+}
+
 func prepareLogsCmd(layout paths.Layout, action actionRequest) tea.Cmd {
 	return func() tea.Msg {
 		result, err := rpc.Call[api.VMLogsResult](context.Background(), layout.SocketPath, "vm.logs", api.VMRefParams{IDOrName: action.id})

internal/cli/tui_test.go

@@ -2,12 +2,14 @@ package cli
 
 import (
 	"context"
+	"errors"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
 	"time"
 
+	"banger/internal/api"
 	"banger/internal/model"
 	"banger/internal/paths"
 
@@ -236,6 +238,41 @@ func TestTUIStatusIncludesStageDurationsAfterInitialLoad(t *testing.T) {
 	}
 }
 
+func TestSSHDoneMsgShowsReminderWhenPingAlive(t *testing.T) {
+	origPing := vmPingFunc
+	t.Cleanup(func() {
+		vmPingFunc = origPing
+	})
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return api.VMPingResult{Name: "devbox", Alive: true}, nil
+	}
+
+	msg := sshDoneMsg(paths.Layout{SocketPath: "/tmp/bangerd.sock"}, actionRequest{id: "devbox", name: "devbox"}, "devbox", nil)
+	result, ok := msg.(actionResultMsg)
+	if !ok {
+		t.Fatalf("msg = %T, want actionResultMsg", msg)
+	}
+	if !strings.Contains(result.status, "devbox is still running") {
+		t.Fatalf("status = %q, want reminder", result.status)
+	}
+}
+
+func TestSSHDoneMsgShowsWarningWhenPingFails(t *testing.T) {
+	origPing := vmPingFunc
+	t.Cleanup(func() {
+		vmPingFunc = origPing
+	})
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return api.VMPingResult{}, errors.New("dial failed")
+	}
+
+	msg := sshDoneMsg(paths.Layout{SocketPath: "/tmp/bangerd.sock"}, actionRequest{id: "devbox", name: "devbox"}, "devbox", nil)
+	result := msg.(actionResultMsg)
+	if !strings.Contains(result.status, "failed to check whether devbox is still running") {
+		t.Fatalf("status = %q, want warning", result.status)
+	}
+}
+
 func TestAggregateRunningVMResources(t *testing.T) {
 	t.Parallel()
 

internal/config/config.go

@@ -21,6 +21,7 @@ type fileConfig struct {
 	SSHKeyPath         string `toml:"ssh_key_path"`
 	NamegenPath        string `toml:"namegen_path"`
 	CustomizeScript    string `toml:"customize_script"`
+	VSockPingHelper    string `toml:"vsock_ping_helper_path"`
 	DefaultImageName   string `toml:"default_image_name"`
 	DefaultRootfs      string `toml:"default_rootfs"`
 	DefaultBaseRootfs  string `toml:"default_base_rootfs"`
@@ -87,6 +88,9 @@ func Load(layout paths.Layout) (model.DaemonConfig, error) {
 	if file.CustomizeScript != "" {
 		cfg.CustomizeScript = file.CustomizeScript
 	}
+	if file.VSockPingHelper != "" {
+		cfg.VSockPingHelperPath = file.VSockPingHelper
+	}
 	if file.DefaultImageName != "" {
 		cfg.DefaultImageName = file.DefaultImageName
 	}
@@ -180,6 +184,7 @@ func applyBundleMetadataDefaults(cfg *model.DaemonConfig, runtimeDir string, met
 	cfg.SSHKeyPath = defaultRuntimePath(cfg.SSHKeyPath, runtimeDir, meta.SSHKeyPath)
 	cfg.NamegenPath = defaultRuntimePath(cfg.NamegenPath, runtimeDir, meta.NamegenPath)
 	cfg.CustomizeScript = defaultRuntimePath(cfg.CustomizeScript, runtimeDir, meta.CustomizeScript)
+	cfg.VSockPingHelperPath = defaultRuntimePath(cfg.VSockPingHelperPath, runtimeDir, meta.VSockPingHelperPath)
 	cfg.DefaultKernel = defaultRuntimePath(cfg.DefaultKernel, runtimeDir, meta.DefaultKernel)
 	cfg.DefaultInitrd = defaultRuntimePath(cfg.DefaultInitrd, runtimeDir, meta.DefaultInitrd)
 	cfg.DefaultModulesDir = defaultRuntimePath(cfg.DefaultModulesDir, runtimeDir, meta.DefaultModulesDir)
@@ -193,6 +198,7 @@ func applyLegacyRuntimeDefaults(cfg *model.DaemonConfig) {
 	cfg.SSHKeyPath = defaultRuntimePath(cfg.SSHKeyPath, cfg.RuntimeDir, "id_ed25519")
 	cfg.NamegenPath = defaultRuntimePath(cfg.NamegenPath, cfg.RuntimeDir, "namegen")
 	cfg.CustomizeScript = defaultRuntimePath(cfg.CustomizeScript, cfg.RuntimeDir, "customize.sh")
+	cfg.VSockPingHelperPath = defaultRuntimePath(cfg.VSockPingHelperPath, cfg.RuntimeDir, "banger-vsock-pingd")
 	cfg.DefaultKernel = defaultRuntimePath(cfg.DefaultKernel, cfg.RuntimeDir, "wtf/root/boot/vmlinux-6.8.0-94-generic")
 	cfg.DefaultInitrd = defaultRuntimePath(cfg.DefaultInitrd, cfg.RuntimeDir, "wtf/root/boot/initrd.img-6.8.0-94-generic")
 	cfg.DefaultModulesDir = defaultRuntimePath(cfg.DefaultModulesDir, cfg.RuntimeDir, "wtf/root/lib/modules/6.8.0-94-generic")

internal/config/config_test.go

@@ -13,21 +13,23 @@ import (
 func TestLoadDerivesArtifactPathsFromRuntimeDir(t *testing.T) {
 	runtimeDir := t.TempDir()
 	meta := runtimebundle.BundleMetadata{
-		FirecrackerBin:    "bin/firecracker",
+		FirecrackerBin:      "bin/firecracker",
-		SSHKeyPath:        "keys/id_ed25519",
+		SSHKeyPath:          "keys/id_ed25519",
-		NamegenPath:       "bin/namegen",
+		NamegenPath:         "bin/namegen",
-		CustomizeScript:   "scripts/customize.sh",
+		CustomizeScript:     "scripts/customize.sh",
-		DefaultPackages:   "config/packages.apt",
+		VSockPingHelperPath: "bin/banger-vsock-pingd",
-		DefaultRootfs:     "images/rootfs-docker.ext4",
+		DefaultPackages:     "config/packages.apt",
-		DefaultKernel:     "kernels/vmlinux",
+		DefaultRootfs:       "images/rootfs-docker.ext4",
-		DefaultInitrd:     "kernels/initrd.img",
+		DefaultKernel:       "kernels/vmlinux",
-		DefaultModulesDir: "modules/current",
+		DefaultInitrd:       "kernels/initrd.img",
+		DefaultModulesDir:   "modules/current",
 	}
 	for _, rel := range []string{
 		meta.FirecrackerBin,
 		meta.SSHKeyPath,
 		meta.NamegenPath,
 		meta.CustomizeScript,
+		meta.VSockPingHelperPath,
 		meta.DefaultPackages,
 		meta.DefaultRootfs,
 		meta.DefaultKernel,
@@ -71,6 +73,9 @@ func TestLoadDerivesArtifactPathsFromRuntimeDir(t *testing.T) {
 	if cfg.CustomizeScript != filepath.Join(runtimeDir, meta.CustomizeScript) {
 		t.Fatalf("CustomizeScript = %q", cfg.CustomizeScript)
 	}
+	if cfg.VSockPingHelperPath != filepath.Join(runtimeDir, meta.VSockPingHelperPath) {
+		t.Fatalf("VSockPingHelperPath = %q", cfg.VSockPingHelperPath)
+	}
 	if cfg.DefaultRootfs != filepath.Join(runtimeDir, meta.DefaultRootfs) {
 		t.Fatalf("DefaultRootfs = %q", cfg.DefaultRootfs)
 	}
@@ -98,6 +103,7 @@ func TestLoadFallsBackToLegacyRuntimeLayoutWithoutBundleMetadata(t *testing.T) {
 		"id_ed25519",
 		"namegen",
 		"customize.sh",
+		"banger-vsock-pingd",
 		"packages.apt",
 		"rootfs-docker.ext4",
 		"wtf/root/boot/vmlinux-6.8.0-94-generic",
@@ -122,6 +128,9 @@ func TestLoadFallsBackToLegacyRuntimeLayoutWithoutBundleMetadata(t *testing.T) {
 	if cfg.FirecrackerBin != filepath.Join(runtimeDir, "firecracker") {
 		t.Fatalf("FirecrackerBin = %q", cfg.FirecrackerBin)
 	}
+	if cfg.VSockPingHelperPath != filepath.Join(runtimeDir, "banger-vsock-pingd") {
+		t.Fatalf("VSockPingHelperPath = %q", cfg.VSockPingHelperPath)
+	}
 	if cfg.DefaultKernel != filepath.Join(runtimeDir, "wtf/root/boot/vmlinux-6.8.0-94-generic") {
 		t.Fatalf("DefaultKernel = %q", cfg.DefaultKernel)
 	}

internal/daemon/daemon.go

@@ -323,6 +323,13 @@ func (d *Daemon) dispatch(ctx context.Context, req rpc.Request) rpc.Response {
 			return rpc.NewError("not_running", fmt.Sprintf("vm %s is not running", vm.Name))
 		}
 		return marshalResultOrError(api.VMSSHResult{Name: vm.Name, GuestIP: vm.Runtime.GuestIP}, nil)
+	case "vm.ping":
+		params, err := rpc.DecodeParams[api.VMRefParams](req)
+		if err != nil {
+			return rpc.NewError("bad_request", err.Error())
+		}
+		result, err := d.PingVM(ctx, params.IDOrName)
+		return marshalResultOrError(result, err)
 	case "image.list":
 		images, err := d.store.ListImages(ctx)
 		return marshalResultOrError(api.ImageListResult{Images: images}, err)

internal/daemon/doctor.go

@@ -33,6 +33,7 @@ func (d *Daemon) doctorReport(ctx context.Context) system.Report {
 
 	report.AddPreflight("runtime bundle", d.runtimeBundleChecks(), runtimeBundleStatus(d.config))
 	report.AddPreflight("core vm lifecycle", d.coreVMLifecycleChecks(), "required host tools available")
+	report.AddPreflight("vsock ssh reminder", d.vsockChecks(), "vsock reminder prerequisites available")
 	d.addCapabilityDoctorChecks(ctx, &report)
 	report.AddPreflight("image build", d.imageBuildChecks(ctx), "image build prerequisites available")
 
@@ -44,6 +45,7 @@ func (d *Daemon) runtimeBundleChecks() *system.Preflight {
 	hint := paths.RuntimeBundleHint()
 	checks.RequireExecutable(d.config.FirecrackerBin, "firecracker binary", hint)
 	checks.RequireFile(d.config.SSHKeyPath, "ssh private key", `set "ssh_key_path" or refresh the runtime bundle`)
+	checks.RequireExecutable(d.config.VSockPingHelperPath, "vsock ping helper", `run 'make build' or refresh the runtime bundle`)
 	checks.RequireFile(d.config.DefaultRootfs, "default rootfs image", `set "default_rootfs" or refresh the runtime bundle`)
 	checks.RequireFile(d.config.DefaultKernel, "kernel image", `set "default_kernel" or refresh the runtime bundle`)
 	if strings.TrimSpace(d.config.DefaultInitrd) != "" {
@@ -75,6 +77,13 @@ func (d *Daemon) imageBuildChecks(ctx context.Context) *system.Preflight {
 	return checks
 }
 
+func (d *Daemon) vsockChecks() *system.Preflight {
+	checks := system.NewPreflight()
+	checks.RequireExecutable(d.config.VSockPingHelperPath, "vsock ping helper", `run 'make build' or refresh the runtime bundle`)
+	checks.RequireFile(vsockHostDevicePath, "vsock host device", "load the vhost_vsock kernel module on the host")
+	return checks
+}
+
 func runtimeBundleStatus(cfg model.DaemonConfig) string {
 	if strings.TrimSpace(cfg.RuntimeDir) == "" {
 		return "runtime dir not configured"

internal/daemon/imagebuild.go

@@ -17,6 +17,7 @@ import (
 	"banger/internal/hostnat"
 	"banger/internal/model"
 	"banger/internal/system"
+	"banger/internal/vsockping"
 )
 
 const (
@@ -103,6 +104,16 @@ func (d *Daemon) runImageBuildNative(ctx context.Context, spec imageBuildSpec) (
 	}
 	defer client.Close()
 
+	helperBytes, err := os.ReadFile(d.config.VSockPingHelperPath)
+	if err != nil {
+		return err
+	}
+	if err := writeBuildLog(spec.BuildLog, "installing vsock ping helper"); err != nil {
+		return err
+	}
+	if err := client.UploadFile(ctx, vsockping.GuestInstallPath, 0o755, helperBytes, spec.BuildLog); err != nil {
+		return err
+	}
 	if err := writeBuildLog(spec.BuildLog, "configuring guest"); err != nil {
 		return err
 	}
@@ -207,7 +218,7 @@ func (d *Daemon) startImageBuildVM(ctx context.Context, spec imageBuildSpec) (im
 		return imageBuildVM{}, nil, err
 	}
 	vm.PID = d.resolveFirecrackerPID(firecrackerCtx, machine, vm.APISock)
-	if err := d.ensureSocketAccess(ctx, vm.APISock); err != nil {
+	if err := d.ensureSocketAccess(ctx, vm.APISock, "firecracker api socket"); err != nil {
 		_ = d.killVMProcess(context.Background(), vm.PID)
 		_ = hostnat.Ensure(ctx, d.runner, vm.GuestIP, vm.TapDevice, false)
 		_, _ = d.runner.RunSudo(ctx, "ip", "link", "del", vm.TapDevice)
@@ -255,6 +266,7 @@ func buildProvisionScript(vmName, dnsServer string, packages []string, installDo
 	script.WriteString("DEBIAN_FRONTEND=noninteractive apt-get -y install \"${PACKAGES[@]}\"\n")
 	appendMiseSetup(&script)
 	appendTmuxSetup(&script)
+	appendVSockPingSetup(&script)
 	if installDocker {
 		script.WriteString("DEBIAN_FRONTEND=noninteractive apt-get -y remove containerd || true\n")
 		script.WriteString("if ! DEBIAN_FRONTEND=noninteractive apt-get -y install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin; then\n")
@@ -318,6 +330,19 @@ func appendTmuxSetup(script *bytes.Buffer) {
 	script.WriteString("chmod 0644 \"$TMUX_CONF\"\n")
 }
 
+func appendVSockPingSetup(script *bytes.Buffer) {
+	script.WriteString("mkdir -p /etc/modules-load.d /etc/systemd/system\n")
+	script.WriteString("cat > /etc/modules-load.d/banger-vsock.conf <<'EOF'\n")
+	script.WriteString(vsockping.ModulesLoadConfig())
+	script.WriteString("EOF\n")
+	script.WriteString("chmod 0644 /etc/modules-load.d/banger-vsock.conf\n")
+	script.WriteString("cat > /etc/systemd/system/" + vsockping.ServiceName + " <<'EOF'\n")
+	script.WriteString(vsockping.ServiceUnit())
+	script.WriteString("EOF\n")
+	script.WriteString("chmod 0644 /etc/systemd/system/" + vsockping.ServiceName + "\n")
+	script.WriteString("if command -v systemctl >/dev/null 2>&1; then systemctl daemon-reload || true; systemctl enable --now " + vsockping.ServiceName + " || true; fi\n")
+}
+
 func appendGitRepo(script *bytes.Buffer, dir, repo string) {
 	fmt.Fprintf(script, "if [[ -d \"%s/.git\" ]]; then\n", dir)
 	fmt.Fprintf(script, "  git -C \"%s\" fetch --depth 1 origin\n", dir)

internal/daemon/imagebuild_test.go

@@ -26,6 +26,11 @@ func TestBuildProvisionScriptInstallsDefaultTools(t *testing.T) {
 		"set -g @continuum-restore 'off'",
 		"set -g @resurrect-dir '/root/.tmux/resurrect'",
 		"run '~/.tmux/plugins/tpm/tpm'",
+		"cat > /etc/modules-load.d/banger-vsock.conf <<'EOF'",
+		"vmw_vsock_virtio_transport",
+		"cat > /etc/systemd/system/banger-vsock-pingd.service <<'EOF'",
+		"ExecStart=/usr/local/bin/banger-vsock-pingd",
+		"systemctl enable --now banger-vsock-pingd.service || true",
 		"rm -f /root/get-docker /root/get-docker.sh /tmp/get-docker /tmp/get-docker.sh",
 	} {
 		if !strings.Contains(script, snippet) {

internal/daemon/logger_test.go

@@ -43,6 +43,11 @@ func TestNewDaemonLoggerEmitsJSONAtConfiguredLevel(t *testing.T) {
 
 func TestStartVMLockedLogsBridgeFailure(t *testing.T) {
 	ctx := context.Background()
+	origVsockHostDevicePath := vsockHostDevicePath
+	vsockHostDevicePath = filepath.Join(t.TempDir(), "vhost-vsock")
+	t.Cleanup(func() {
+		vsockHostDevicePath = origVsockHostDevicePath
+	})
 	binDir := t.TempDir()
 	for _, name := range []string{
 		"sudo", "ip", "dmsetup", "losetup", "blockdev", "truncate", "pgrep", "ps",
@@ -54,9 +59,16 @@ func TestStartVMLockedLogsBridgeFailure(t *testing.T) {
 	t.Setenv("PATH", binDir)
 
 	firecrackerBin := filepath.Join(t.TempDir(), "firecracker")
+	vsockHelper := filepath.Join(t.TempDir(), "banger-vsock-pingd")
 	if err := os.WriteFile(firecrackerBin, []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
 		t.Fatalf("write firecracker: %v", err)
 	}
+	if err := os.WriteFile(vsockHostDevicePath, []byte{}, 0o644); err != nil {
+		t.Fatalf("write vsock host device: %v", err)
+	}
+	if err := os.WriteFile(vsockHelper, []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
+		t.Fatalf("write vsock helper: %v", err)
+	}
 	rootfsPath := filepath.Join(t.TempDir(), "rootfs.ext4")
 	kernelPath := filepath.Join(t.TempDir(), "vmlinux")
 	for _, path := range []string{rootfsPath, kernelPath} {
@@ -93,11 +105,12 @@ func TestStartVMLockedLogsBridgeFailure(t *testing.T) {
 	d := &Daemon{
 		layout: paths.Layout{RuntimeDir: filepath.Join(t.TempDir(), "runtime")},
 		config: model.DaemonConfig{
-			BridgeName:        "br-fc",
+			BridgeName:          "br-fc",
-			BridgeIP:          model.DefaultBridgeIP,
+			BridgeIP:            model.DefaultBridgeIP,
-			DefaultDNS:        model.DefaultDNS,
+			DefaultDNS:          model.DefaultDNS,
-			FirecrackerBin:    firecrackerBin,
+			FirecrackerBin:      firecrackerBin,
-			StatsPollInterval: model.DefaultStatsPollInterval,
+			VSockPingHelperPath: vsockHelper,
+			StatsPollInterval:   model.DefaultStatsPollInterval,
 		},
 		runner: runner,
 		logger: logger,
@@ -138,11 +151,15 @@ func TestBuildImagePreservesBuildLogOnFailure(t *testing.T) {
 	packagesPath := filepath.Join(t.TempDir(), "packages.apt")
 	sshKeyPath := filepath.Join(t.TempDir(), "id_ed25519")
 	firecrackerBin := filepath.Join(t.TempDir(), "firecracker")
+	vsockHelper := filepath.Join(t.TempDir(), "banger-vsock-pingd")
 	for _, path := range []string{baseRootfs, kernelPath, packagesPath, sshKeyPath} {
 		if err := os.WriteFile(path, []byte("artifact"), 0o644); err != nil {
 			t.Fatalf("write %s: %v", path, err)
 		}
 	}
+	if err := os.WriteFile(vsockHelper, []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
+		t.Fatalf("write %s: %v", vsockHelper, err)
+	}
 	if err := os.WriteFile(firecrackerBin, []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
 		t.Fatalf("write %s: %v", firecrackerBin, err)
 	}
@@ -169,6 +186,7 @@ func TestBuildImagePreservesBuildLogOnFailure(t *testing.T) {
 			DefaultPackagesFile: packagesPath,
 			SSHKeyPath:          sshKeyPath,
 			FirecrackerBin:      firecrackerBin,
+			VSockPingHelperPath: vsockHelper,
 		},
 		store:  store,
 		runner: runner,

internal/daemon/preflight.go

@@ -9,6 +9,8 @@ import (
 	"banger/internal/system"
 )
 
+var vsockHostDevicePath = "/dev/vhost-vsock"
+
 func (d *Daemon) validateStartPrereqs(ctx context.Context, vm model.VMRecord, image model.Image) error {
 	checks := system.NewPreflight()
 	d.addBaseStartPrereqs(checks, image)
@@ -52,6 +54,8 @@ func (d *Daemon) addBaseStartPrereqs(checks *system.Preflight, image model.Image
 
 	d.addBaseStartCommandPrereqs(checks)
 	checks.RequireExecutable(d.config.FirecrackerBin, "firecracker binary", hint)
+	checks.RequireExecutable(d.config.VSockPingHelperPath, "vsock ping helper", `run 'make build' or refresh the runtime bundle`)
+	checks.RequireFile(vsockHostDevicePath, "vsock host device", "load the vhost_vsock kernel module on the host")
 	checks.RequireFile(image.RootfsPath, "rootfs image", "select a valid image or rebuild the runtime bundle")
 	checks.RequireFile(image.KernelPath, "kernel image", `set "default_kernel" or refresh the runtime bundle`)
 	if strings.TrimSpace(image.InitrdPath) != "" {
@@ -73,6 +77,7 @@ func (d *Daemon) addImageBuildPrereqs(ctx context.Context, checks *system.Prefli
 	}
 	checks.RequireExecutable(d.config.FirecrackerBin, "firecracker binary", hint)
 	checks.RequireFile(d.config.SSHKeyPath, "ssh private key", `set "ssh_key_path" or refresh the runtime bundle`)
+	checks.RequireExecutable(d.config.VSockPingHelperPath, "vsock ping helper", `run 'make build' or refresh the runtime bundle`)
 	checks.RequireFile(baseRootfs, "base rootfs image", `pass --base-rootfs or set "default_base_rootfs"`)
 	checks.RequireFile(kernelPath, "kernel image", `pass --kernel or set "default_kernel"`)
 	checks.RequireFile(d.config.DefaultPackagesFile, "package manifest", `set "default_packages_file" or refresh the runtime bundle`)

internal/daemon/vm.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"net"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -75,6 +76,10 @@ func (d *Daemon) CreateVM(ctx context.Context, params api.VMCreateParams) (vm mo
 	if err := os.MkdirAll(vmDir, 0o755); err != nil {
 		return model.VMRecord{}, err
 	}
+	vsockCID, err := defaultVSockCID(guestIP)
+	if err != nil {
+		return model.VMRecord{}, err
+	}
 	systemOverlaySize := int64(model.DefaultSystemOverlaySize)
 	if params.SystemOverlaySize != "" {
 		systemOverlaySize, err = model.ParseSize(params.SystemOverlaySize)
@@ -111,6 +116,8 @@ func (d *Daemon) CreateVM(ctx context.Context, params api.VMCreateParams) (vm mo
 			GuestIP:       guestIP,
 			DNSName:       vmdns.RecordName(name),
 			VMDir:         vmDir,
+			VSockPath:     defaultVSockPath(d.layout.RuntimeDir, id),
+			VSockCID:      vsockCID,
 			SystemOverlay: filepath.Join(vmDir, "system.cow"),
 			WorkDiskPath:  filepath.Join(vmDir, "root.ext4"),
 			LogPath:       filepath.Join(vmDir, "firecracker.log"),
@@ -183,9 +190,21 @@ func (d *Daemon) startVMLocked(ctx context.Context, vm model.VMRecord, image mod
 	apiSock := filepath.Join(d.layout.RuntimeDir, "fc-"+shortID+".sock")
 	tap := "tap-fc-" + shortID
 	dmName := "fc-rootfs-" + shortID
+	if strings.TrimSpace(vm.Runtime.VSockPath) == "" {
+		vm.Runtime.VSockPath = defaultVSockPath(d.layout.RuntimeDir, vm.ID)
+	}
+	if vm.Runtime.VSockCID == 0 {
+		vm.Runtime.VSockCID, err = defaultVSockCID(vm.Runtime.GuestIP)
+		if err != nil {
+			return model.VMRecord{}, err
+		}
+	}
 	if err := os.RemoveAll(apiSock); err != nil && !os.IsNotExist(err) {
 		return model.VMRecord{}, err
 	}
+	if err := os.RemoveAll(vm.Runtime.VSockPath); err != nil && !os.IsNotExist(err) {
+		return model.VMRecord{}, err
+	}
 
 	op.stage("system_overlay", "overlay_path", vm.Runtime.SystemOverlay)
 	if err := d.ensureSystemOverlay(ctx, &vm); err != nil {
@@ -260,6 +279,8 @@ func (d *Daemon) startVMLocked(ctx context.Context, vm model.VMRecord, image mod
 			IsRoot:   true,
 		}},
 		TapDevice: tap,
+		VSockPath: vm.Runtime.VSockPath,
+		VSockCID:  vm.Runtime.VSockCID,
 		VCPUCount: vm.Spec.VCPUCount,
 		MemoryMiB: vm.Spec.MemoryMiB,
 		Logger:    d.logger,
@@ -276,7 +297,11 @@ func (d *Daemon) startVMLocked(ctx context.Context, vm model.VMRecord, image mod
 	vm.Runtime.PID = d.resolveFirecrackerPID(firecrackerCtx, machine, apiSock)
 	op.debugStage("firecracker_started", "pid", vm.Runtime.PID)
 	op.stage("socket_access", "api_socket", apiSock)
-	if err := d.ensureSocketAccess(ctx, apiSock); err != nil {
+	if err := d.ensureSocketAccess(ctx, apiSock, "firecracker api socket"); err != nil {
+		return cleanupOnErr(err)
+	}
+	op.stage("vsock_access", "vsock_path", vm.Runtime.VSockPath, "vsock_cid", vm.Runtime.VSockCID)
+	if err := d.ensureSocketAccess(ctx, vm.Runtime.VSockPath, "firecracker vsock socket"); err != nil {
 		return cleanupOnErr(err)
 	}
 	op.stage("post_start_features")
@@ -556,6 +581,33 @@ func (d *Daemon) GetVMStats(ctx context.Context, idOrName string) (model.VMRecor
 	return vm, vm.Stats, nil
 }
 
+func (d *Daemon) PingVM(ctx context.Context, idOrName string) (result api.VMPingResult, err error) {
+	_, err = d.withVMLockByRef(ctx, idOrName, func(vm model.VMRecord) (model.VMRecord, error) {
+		result.Name = vm.Name
+		if vm.State != model.VMStateRunning || !system.ProcessRunning(vm.Runtime.PID, vm.Runtime.APISockPath) {
+			result.Alive = false
+			return vm, nil
+		}
+		if strings.TrimSpace(vm.Runtime.VSockPath) == "" {
+			return model.VMRecord{}, errors.New("vm has no vsock path")
+		}
+		if vm.Runtime.VSockCID == 0 {
+			return model.VMRecord{}, errors.New("vm has no vsock cid")
+		}
+		if err := d.ensureSocketAccess(ctx, vm.Runtime.VSockPath, "firecracker vsock socket"); err != nil {
+			return model.VMRecord{}, err
+		}
+		pingCtx, cancel := context.WithTimeout(ctx, 3*time.Second)
+		defer cancel()
+		if err := firecracker.PingVSock(pingCtx, d.logger, vm.Runtime.VSockPath); err != nil {
+			return model.VMRecord{}, err
+		}
+		result.Alive = true
+		return vm, nil
+	})
+	return result, err
+}
+
 func (d *Daemon) getVMStatsLocked(ctx context.Context, vm model.VMRecord) (model.VMRecord, error) {
 	stats, err := d.collectStats(ctx, vm)
 	if err == nil {
@@ -812,11 +864,14 @@ func (d *Daemon) firecrackerBinary() (string, error) {
 	return path, nil
 }
 
-func (d *Daemon) ensureSocketAccess(ctx context.Context, apiSock string) error {
+func (d *Daemon) ensureSocketAccess(ctx context.Context, socketPath, label string) error {
-	if _, err := d.runner.RunSudo(ctx, "chown", fmt.Sprintf("%d:%d", os.Getuid(), os.Getgid()), apiSock); err != nil {
+	if err := waitForPath(ctx, socketPath, 5*time.Second, label); err != nil {
 		return err
 	}
-	_, err := d.runner.RunSudo(ctx, "chmod", "600", apiSock)
+	if _, err := d.runner.RunSudo(ctx, "chown", fmt.Sprintf("%d:%d", os.Getuid(), os.Getgid()), socketPath); err != nil {
+		return err
+	}
+	_, err := d.runner.RunSudo(ctx, "chmod", "600", socketPath)
 	return err
 }
 
@@ -841,7 +896,7 @@ func (d *Daemon) resolveFirecrackerPID(ctx context.Context, machine *firecracker
 }
 
 func (d *Daemon) sendCtrlAltDel(ctx context.Context, vm model.VMRecord) error {
-	if err := d.ensureSocketAccess(ctx, vm.Runtime.APISockPath); err != nil {
+	if err := d.ensureSocketAccess(ctx, vm.Runtime.APISockPath, "firecracker api socket"); err != nil {
 		return err
 	}
 	client := firecracker.New(vm.Runtime.APISockPath, d.logger)
@@ -887,6 +942,9 @@ func (d *Daemon) cleanupRuntime(ctx context.Context, vm model.VMRecord, preserve
 	if vm.Runtime.APISockPath != "" {
 		_ = os.Remove(vm.Runtime.APISockPath)
 	}
+	if vm.Runtime.VSockPath != "" {
+		_ = os.Remove(vm.Runtime.VSockPath)
+	}
 	snapshotErr := d.cleanupDMSnapshot(ctx, dmSnapshotHandles{
 		BaseLoop: vm.Runtime.BaseLoop,
 		COWLoop:  vm.Runtime.COWLoop,
@@ -910,6 +968,37 @@ func clearRuntimeHandles(vm *model.VMRecord) {
 	vm.Runtime.DMDev = ""
 }
 
+func defaultVSockPath(runtimeDir, vmID string) string {
+	return filepath.Join(runtimeDir, "fc-"+system.ShortID(vmID)+".vsock")
+}
+
+func defaultVSockCID(guestIP string) (uint32, error) {
+	ip := net.ParseIP(strings.TrimSpace(guestIP)).To4()
+	if ip == nil {
+		return 0, fmt.Errorf("guest IP is not IPv4: %q", guestIP)
+	}
+	return 10000 + uint32(ip[3]), nil
+}
+
+func waitForPath(ctx context.Context, path string, timeout time.Duration, label string) error {
+	deadline := time.Now().Add(timeout)
+	for {
+		if _, err := os.Stat(path); err == nil {
+			return nil
+		} else if err != nil && !os.IsNotExist(err) {
+			return err
+		}
+		if time.Now().After(deadline) {
+			return fmt.Errorf("%s not ready: %s: %w", label, path, context.DeadlineExceeded)
+		}
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-time.After(100 * time.Millisecond):
+		}
+	}
+}
+
 func (d *Daemon) setDNS(ctx context.Context, vmName, guestIP string) error {
 	if d.vmDNS == nil {
 		return nil

internal/daemon/vm_test.go

@@ -261,6 +261,114 @@ func TestSetVMRejectsStoppedOnlyChangesForRunningVM(t *testing.T) {
 	}
 }
 
+func TestPingVMReturnsAliveForRunningGuest(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	db := openDaemonStore(t)
+	apiSock := filepath.Join(t.TempDir(), "fc.sock")
+	fake := startFakeFirecrackerProcess(t, apiSock)
+	t.Cleanup(func() {
+		_ = fake.Process.Kill()
+		_ = fake.Wait()
+	})
+
+	vsockSock := filepath.Join(t.TempDir(), "fc.vsock")
+	listener, err := net.Listen("unix", vsockSock)
+	if err != nil {
+		t.Fatalf("listen vsock: %v", err)
+	}
+	t.Cleanup(func() {
+		_ = listener.Close()
+		_ = os.Remove(vsockSock)
+	})
+	serverDone := make(chan error, 1)
+	go func() {
+		conn, err := listener.Accept()
+		if err != nil {
+			serverDone <- err
+			return
+		}
+		defer conn.Close()
+		buf := make([]byte, 128)
+		n, err := conn.Read(buf)
+		if err != nil {
+			serverDone <- err
+			return
+		}
+		if got := string(buf[:n]); got != "CONNECT 42070\n" {
+			serverDone <- fmt.Errorf("unexpected connect message %q", got)
+			return
+		}
+		if _, err := conn.Write([]byte("OK 1\n")); err != nil {
+			serverDone <- err
+			return
+		}
+		n, err = conn.Read(buf)
+		if err != nil {
+			serverDone <- err
+			return
+		}
+		if got := string(buf[:n]); got != "PING\n" {
+			serverDone <- fmt.Errorf("unexpected ping payload %q", got)
+			return
+		}
+		_, err = conn.Write([]byte("PONG\n"))
+		serverDone <- err
+	}()
+
+	vm := testVM("alive", "image-alive", "172.16.0.41")
+	vm.State = model.VMStateRunning
+	vm.Runtime.State = model.VMStateRunning
+	vm.Runtime.PID = fake.Process.Pid
+	vm.Runtime.APISockPath = apiSock
+	vm.Runtime.VSockPath = vsockSock
+	vm.Runtime.VSockCID = 10041
+	if err := db.UpsertVM(ctx, vm); err != nil {
+		t.Fatalf("UpsertVM: %v", err)
+	}
+
+	runner := &scriptedRunner{
+		t: t,
+		steps: []runnerStep{
+			sudoStep("", nil, "chown", fmt.Sprintf("%d:%d", os.Getuid(), os.Getgid()), vsockSock),
+			sudoStep("", nil, "chmod", "600", vsockSock),
+		},
+	}
+	d := &Daemon{store: db, runner: runner}
+	result, err := d.PingVM(ctx, vm.Name)
+	if err != nil {
+		t.Fatalf("PingVM: %v", err)
+	}
+	if !result.Alive || result.Name != vm.Name {
+		t.Fatalf("PingVM result = %+v, want alive %s", result, vm.Name)
+	}
+	runner.assertExhausted()
+	if err := <-serverDone; err != nil {
+		t.Fatalf("server: %v", err)
+	}
+}
+
+func TestPingVMReturnsFalseForStoppedVM(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	db := openDaemonStore(t)
+	vm := testVM("stopped-ping", "image-stopped", "172.16.0.42")
+	if err := db.UpsertVM(ctx, vm); err != nil {
+		t.Fatalf("UpsertVM: %v", err)
+	}
+
+	d := &Daemon{store: db}
+	result, err := d.PingVM(ctx, vm.Name)
+	if err != nil {
+		t.Fatalf("PingVM: %v", err)
+	}
+	if result.Alive {
+		t.Fatalf("PingVM result = %+v, want not alive", result)
+	}
+}
+
 func TestSetVMDiskResizeFailsPreflightWhenToolsMissing(t *testing.T) {
 	ctx := context.Background()
 	db := openDaemonStore(t)

internal/firecracker/client.go

@@ -1,17 +1,23 @@
 package firecracker
 
 import (
+	"bufio"
 	"context"
+	"fmt"
 	"io"
 	"log/slog"
 	"os"
 	"os/exec"
 	"strings"
 	"sync"
+	"time"
 
 	sdk "github.com/firecracker-microvm/firecracker-go-sdk"
 	models "github.com/firecracker-microvm/firecracker-go-sdk/client/models"
+	sdkvsock "github.com/firecracker-microvm/firecracker-go-sdk/vsock"
 	"github.com/sirupsen/logrus"
+
+	"banger/internal/vsockping"
 )
 
 type MachineConfig struct {
@@ -25,6 +31,8 @@ type MachineConfig struct {
 	KernelArgs      string
 	Drives          []DriveConfig
 	TapDevice       string
+	VSockPath       string
+	VSockCID        uint32
 	VCPUCount       int
 	MemoryMiB       int
 	Logger          *slog.Logger
@@ -132,6 +140,7 @@ func buildConfig(cfg MachineConfig) sdk.Config {
 				HostDevName: cfg.TapDevice,
 			},
 		}},
+		VsockDevices: buildVsockDevices(cfg),
 		MachineCfg: models.MachineConfiguration{
 			VcpuCount:  sdk.Int64(int64(cfg.VCPUCount)),
 			MemSizeMib: sdk.Int64(int64(cfg.MemoryMiB)),
@@ -141,6 +150,17 @@ func buildConfig(cfg MachineConfig) sdk.Config {
 	}
 }
 
+func buildVsockDevices(cfg MachineConfig) []sdk.VsockDevice {
+	if strings.TrimSpace(cfg.VSockPath) == "" || cfg.VSockCID == 0 {
+		return nil
+	}
+	return []sdk.VsockDevice{{
+		ID:   "vsock",
+		Path: cfg.VSockPath,
+		CID:  cfg.VSockCID,
+	}}
+}
+
 func splitDrives(drives []DriveConfig) (DriveConfig, []DriveConfig) {
 	root := DriveConfig{ID: "rootfs"}
 	var extras []DriveConfig
@@ -192,6 +212,39 @@ func newLogger(base *slog.Logger) *logrus.Entry {
 	return logrus.NewEntry(logger)
 }
 
+func PingVSock(ctx context.Context, logger *slog.Logger, socketPath string) error {
+	conn, err := sdkvsock.DialContext(
+		ctx,
+		socketPath,
+		vsockping.Port,
+		sdkvsock.WithRetryTimeout(3*time.Second),
+		sdkvsock.WithRetryInterval(100*time.Millisecond),
+		sdkvsock.WithLogger(newLogger(logger)),
+	)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	if deadline, ok := ctx.Deadline(); ok {
+		_ = conn.SetDeadline(deadline)
+	} else {
+		_ = conn.SetDeadline(time.Now().Add(3 * time.Second))
+	}
+
+	if _, err := io.WriteString(conn, vsockping.RequestLine); err != nil {
+		return err
+	}
+	line, err := bufio.NewReader(conn).ReadString('\n')
+	if err != nil {
+		return err
+	}
+	if strings.TrimSpace(line) != strings.TrimSpace(vsockping.ResponseLine) {
+		return fmt.Errorf("unexpected vsock response %q", strings.TrimSpace(line))
+	}
+	return nil
+}
+
 type slogHook struct {
 	logger *slog.Logger
 }

internal/firecracker/client_test.go

@@ -2,9 +2,13 @@ package firecracker
 
 import (
 	"bytes"
+	"context"
 	"log/slog"
+	"net"
+	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestBuildConfig(t *testing.T) {
@@ -21,6 +25,8 @@ func TestBuildConfig(t *testing.T) {
 			{ID: "work", Path: "/var/lib/banger/root.ext4"},
 		},
 		TapDevice: "tap-fc-1",
+		VSockPath: "/tmp/fc.vsock",
+		VSockCID:  10042,
 		VCPUCount: 4,
 		MemoryMiB: 2048,
 	})
@@ -46,6 +52,12 @@ func TestBuildConfig(t *testing.T) {
 	if len(cfg.NetworkInterfaces) != 1 {
 		t.Fatalf("interface count = %d, want 1", len(cfg.NetworkInterfaces))
 	}
+	if len(cfg.VsockDevices) != 1 {
+		t.Fatalf("vsock count = %d, want 1", len(cfg.VsockDevices))
+	}
+	if cfg.VsockDevices[0].Path != "/tmp/fc.vsock" || cfg.VsockDevices[0].CID != 10042 {
+		t.Fatalf("unexpected vsock config: %+v", cfg.VsockDevices[0])
+	}
 	if got := cfg.NetworkInterfaces[0].StaticConfiguration.HostDevName; got != "tap-fc-1" {
 		t.Fatalf("host dev name = %q", got)
 	}
@@ -115,3 +127,81 @@ func TestSDKLoggerBridgeSuppressesDebugAtInfoLevel(t *testing.T) {
 		t.Fatalf("expected info-level logger to suppress sdk debug chatter, got %q", buf.String())
 	}
 }
+
+func TestPingVSock(t *testing.T) {
+	dir := t.TempDir()
+	socketPath := filepath.Join(dir, "fc.vsock")
+	listener, err := net.Listen("unix", socketPath)
+	if err != nil {
+		t.Fatalf("Listen: %v", err)
+	}
+	defer listener.Close()
+
+	done := make(chan error, 1)
+	go func() {
+		conn, err := listener.Accept()
+		if err != nil {
+			done <- err
+			return
+		}
+		defer conn.Close()
+		buf := make([]byte, 0, 64)
+		tmp := make([]byte, 64)
+		for {
+			n, err := conn.Read(tmp)
+			if err != nil {
+				done <- err
+				return
+			}
+			buf = append(buf, tmp[:n]...)
+			if strings.Contains(string(buf), "\n") {
+				break
+			}
+		}
+		if got := string(buf); got != "CONNECT 42070\n" {
+			done <- errUnexpectedString(got)
+			return
+		}
+		if _, err := conn.Write([]byte("OK 55\n")); err != nil {
+			done <- err
+			return
+		}
+		buf = buf[:0]
+		for {
+			n, err := conn.Read(tmp)
+			if err != nil {
+				done <- err
+				return
+			}
+			buf = append(buf, tmp[:n]...)
+			if strings.Contains(string(buf), "\n") {
+				break
+			}
+		}
+		if got := string(buf); got != "PING\n" {
+			done <- errUnexpectedString(got)
+			return
+		}
+		_, err = conn.Write([]byte("PONG\n"))
+		done <- err
+	}()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	if err := PingVSock(ctx, nil, socketPath); err != nil {
+		t.Fatalf("PingVSock: %v", err)
+	}
+	if err := <-done; err != nil {
+		t.Fatalf("server: %v", err)
+	}
+}
+
+type unexpectedStringError string
+
+func (e unexpectedStringError) Error() string {
+	return "unexpected string: " + string(e)
+}
+
+func errUnexpectedString(value string) error {
+	return unexpectedStringError(value)
+}

internal/guest/ssh.go

@@ -2,6 +2,7 @@ package guest
 
 import (
 	"archive/tar"
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -73,6 +74,11 @@ func (c *Client) RunScript(ctx context.Context, script string, logWriter io.Writ
 	return c.runSession(ctx, "bash -se", strings.NewReader(script), logWriter)
 }
 
+func (c *Client) UploadFile(ctx context.Context, remotePath string, mode os.FileMode, data []byte, logWriter io.Writer) error {
+	command := fmt.Sprintf("install -D -m %04o /dev/stdin %s", mode.Perm(), shellQuote(remotePath))
+	return c.runSession(ctx, command, bytes.NewReader(data), logWriter)
+}
+
 func (c *Client) StreamTar(ctx context.Context, sourceDir, remoteCommand string, logWriter io.Writer) error {
 	reader, writer := io.Pipe()
 	writeErr := make(chan error, 1)
@@ -123,6 +129,10 @@ func privateKeySigner(path string) (ssh.Signer, error) {
 	return ssh.ParsePrivateKey(data)
 }
 
+func shellQuote(value string) string {
+	return "'" + strings.ReplaceAll(value, "'", `'"'"'`) + "'"
+}
+
 func writeTarArchive(dst io.Writer, sourceDir string) error {
 	tw := tar.NewWriter(dst)
 	defer tw.Close()

internal/model/types.go

@@ -41,6 +41,7 @@ type DaemonConfig struct {
 	SSHKeyPath          string
 	NamegenPath         string
 	CustomizeScript     string
+	VSockPingHelperPath string
 	AutoStopStaleAfter  time.Duration
 	StatsPollInterval   time.Duration
 	MetricsPollInterval time.Duration
@@ -87,6 +88,8 @@ type VMRuntime struct {
 	GuestIP       string  `json:"guest_ip"`
 	TapDevice     string  `json:"tap_device,omitempty"`
 	APISockPath   string  `json:"api_sock_path,omitempty"`
+	VSockPath     string  `json:"vsock_path,omitempty"`
+	VSockCID      uint32  `json:"vsock_cid,omitempty"`
 	LogPath       string  `json:"log_path,omitempty"`
 	MetricsPath   string  `json:"metrics_path,omitempty"`
 	DNSName       string  `json:"dns_name,omitempty"`

internal/paths/paths_test.go

@@ -56,19 +56,21 @@ func TestResolveRuntimeDirUsesSourceCheckoutRuntimeSubdir(t *testing.T) {
 func createRuntimeBundle(t *testing.T, runtimeDir string) {
 	t.Helper()
 	metadata := runtimebundle.BundleMetadata{
-		FirecrackerBin:  "bin/firecracker",
+		FirecrackerBin:      "bin/firecracker",
-		SSHKeyPath:      "keys/id_ed25519",
+		SSHKeyPath:          "keys/id_ed25519",
-		NamegenPath:     "bin/namegen",
+		NamegenPath:         "bin/namegen",
-		CustomizeScript: "scripts/customize.sh",
+		CustomizeScript:     "scripts/customize.sh",
-		DefaultPackages: "config/packages.apt",
+		VSockPingHelperPath: "bin/banger-vsock-pingd",
-		DefaultRootfs:   "images/rootfs-docker.ext4",
+		DefaultPackages:     "config/packages.apt",
-		DefaultKernel:   "kernels/vmlinux",
+		DefaultRootfs:       "images/rootfs-docker.ext4",
+		DefaultKernel:       "kernels/vmlinux",
 	}
 	for _, rel := range []string{
 		metadata.FirecrackerBin,
 		metadata.SSHKeyPath,
 		metadata.NamegenPath,
 		metadata.CustomizeScript,
+		metadata.VSockPingHelperPath,
 		metadata.DefaultPackages,
 		metadata.DefaultRootfs,
 		metadata.DefaultKernel,

internal/runtimebundle/bundle.go

@@ -30,16 +30,17 @@ type Manifest struct {
 }
 
 type BundleMetadata struct {
-	FirecrackerBin    string `json:"firecracker_bin" toml:"firecracker_bin"`
+	FirecrackerBin      string `json:"firecracker_bin" toml:"firecracker_bin"`
-	SSHKeyPath        string `json:"ssh_key_path" toml:"ssh_key_path"`
+	SSHKeyPath          string `json:"ssh_key_path" toml:"ssh_key_path"`
-	NamegenPath       string `json:"namegen_path" toml:"namegen_path"`
+	NamegenPath         string `json:"namegen_path" toml:"namegen_path"`
-	CustomizeScript   string `json:"customize_script" toml:"customize_script"`
+	CustomizeScript     string `json:"customize_script" toml:"customize_script"`
-	DefaultPackages   string `json:"default_packages_file" toml:"default_packages_file"`
+	VSockPingHelperPath string `json:"vsock_ping_helper_path" toml:"vsock_ping_helper_path"`
-	DefaultRootfs     string `json:"default_rootfs" toml:"default_rootfs"`
+	DefaultPackages     string `json:"default_packages_file" toml:"default_packages_file"`
-	DefaultBaseRootfs string `json:"default_base_rootfs,omitempty" toml:"default_base_rootfs"`
+	DefaultRootfs       string `json:"default_rootfs" toml:"default_rootfs"`
-	DefaultKernel     string `json:"default_kernel" toml:"default_kernel"`
+	DefaultBaseRootfs   string `json:"default_base_rootfs,omitempty" toml:"default_base_rootfs"`
-	DefaultInitrd     string `json:"default_initrd,omitempty" toml:"default_initrd"`
+	DefaultKernel       string `json:"default_kernel" toml:"default_kernel"`
-	DefaultModulesDir string `json:"default_modules_dir,omitempty" toml:"default_modules_dir"`
+	DefaultInitrd       string `json:"default_initrd,omitempty" toml:"default_initrd"`
+	DefaultModulesDir   string `json:"default_modules_dir,omitempty" toml:"default_modules_dir"`
 }
 
 const BundleMetadataFile = "bundle.json"
@@ -209,6 +210,7 @@ func validateBundleMetadata(runtimeDir string, meta BundleMetadata) error {
 		{meta.SSHKeyPath, "ssh_key_path"},
 		{meta.NamegenPath, "namegen_path"},
 		{meta.CustomizeScript, "customize_script"},
+		{meta.VSockPingHelperPath, "vsock_ping_helper_path"},
 		{meta.DefaultPackages, "default_packages_file"},
 		{meta.DefaultRootfs, "default_rootfs"},
 		{meta.DefaultKernel, "default_kernel"},
@@ -227,6 +229,7 @@ func validateBundleMetadata(runtimeDir string, meta BundleMetadata) error {
 		{meta.SSHKeyPath, "ssh_key_path", true},
 		{meta.NamegenPath, "namegen_path", true},
 		{meta.CustomizeScript, "customize_script", true},
+		{meta.VSockPingHelperPath, "vsock_ping_helper_path", true},
 		{meta.DefaultPackages, "default_packages_file", true},
 		{meta.DefaultRootfs, "default_rootfs", true},
 		{meta.DefaultBaseRootfs, "default_base_rootfs", false},
@@ -264,6 +267,7 @@ func metadataArchiveBytes(runtimeDir string, meta BundleMetadata) ([]byte, error
 		strings.TrimSpace(meta.SSHKeyPath) == "" &&
 		strings.TrimSpace(meta.NamegenPath) == "" &&
 		strings.TrimSpace(meta.CustomizeScript) == "" &&
+		strings.TrimSpace(meta.VSockPingHelperPath) == "" &&
 		strings.TrimSpace(meta.DefaultPackages) == "" &&
 		strings.TrimSpace(meta.DefaultRootfs) == "" &&
 		strings.TrimSpace(meta.DefaultBaseRootfs) == "" &&
@@ -283,6 +287,7 @@ func normalizeBundleMetadata(meta BundleMetadata) BundleMetadata {
 	meta.SSHKeyPath = strings.TrimSpace(meta.SSHKeyPath)
 	meta.NamegenPath = strings.TrimSpace(meta.NamegenPath)
 	meta.CustomizeScript = strings.TrimSpace(meta.CustomizeScript)
+	meta.VSockPingHelperPath = strings.TrimSpace(meta.VSockPingHelperPath)
 	meta.DefaultPackages = strings.TrimSpace(meta.DefaultPackages)
 	meta.DefaultRootfs = strings.TrimSpace(meta.DefaultRootfs)
 	meta.DefaultBaseRootfs = strings.TrimSpace(meta.DefaultBaseRootfs)

internal/runtimebundle/bundle_test.go

@@ -20,6 +20,7 @@ func TestBootstrapExtractsBundleAndValidatesChecksum(t *testing.T) {
 		"runtime/firecracker":                                       "fc",
 		"runtime/id_ed25519":                                        "key",
 		"runtime/namegen":                                           "namegen",
+		"runtime/banger-vsock-pingd":                                "pingd",
 		"runtime/customize.sh":                                      "#!/bin/bash\n",
 		"runtime/packages.sh":                                       "#!/bin/bash\n",
 		"runtime/packages.apt":                                      "vim\n",
@@ -27,7 +28,7 @@ func TestBootstrapExtractsBundleAndValidatesChecksum(t *testing.T) {
 		"runtime/wtf/root/boot/vmlinux-6.8.0-94-generic":            "kernel",
 		"runtime/wtf/root/boot/initrd.img-6.8.0-94-generic":         "initrd",
 		"runtime/wtf/root/lib/modules/6.8.0-94-generic/modules.dep": "dep",
-		"runtime/bundle.json":                                       mustJSON(t, BundleMetadata{FirecrackerBin: "firecracker", SSHKeyPath: "id_ed25519", NamegenPath: "namegen", CustomizeScript: "customize.sh", DefaultPackages: "packages.apt", DefaultRootfs: "rootfs-docker.ext4", DefaultKernel: "wtf/root/boot/vmlinux-6.8.0-94-generic", DefaultInitrd: "wtf/root/boot/initrd.img-6.8.0-94-generic", DefaultModulesDir: "wtf/root/lib/modules/6.8.0-94-generic"}),
+		"runtime/bundle.json":                                       mustJSON(t, BundleMetadata{FirecrackerBin: "firecracker", SSHKeyPath: "id_ed25519", NamegenPath: "namegen", CustomizeScript: "customize.sh", VSockPingHelperPath: "banger-vsock-pingd", DefaultPackages: "packages.apt", DefaultRootfs: "rootfs-docker.ext4", DefaultKernel: "wtf/root/boot/vmlinux-6.8.0-94-generic", DefaultInitrd: "wtf/root/boot/initrd.img-6.8.0-94-generic", DefaultModulesDir: "wtf/root/lib/modules/6.8.0-94-generic"}),
 	})
 	archivePath := filepath.Join(manifestDir, "bundle.tar.gz")
 	if err := os.WriteFile(archivePath, bundleData, 0o644); err != nil {
@@ -38,7 +39,7 @@ func TestBootstrapExtractsBundleAndValidatesChecksum(t *testing.T) {
 		URL:           "./bundle.tar.gz",
 		SHA256:        sha256Hex(bundleData),
 		BundleRoot:    "runtime",
-		RequiredPaths: []string{"firecracker", "customize.sh", "packages.apt", "rootfs-docker.ext4", "wtf/root/boot/vmlinux-6.8.0-94-generic", "wtf/root/lib/modules/6.8.0-94-generic"},
+		RequiredPaths: []string{"firecracker", "banger-vsock-pingd", "customize.sh", "packages.apt", "rootfs-docker.ext4", "wtf/root/boot/vmlinux-6.8.0-94-generic", "wtf/root/lib/modules/6.8.0-94-generic"},
 	}
 	outDir := filepath.Join(t.TempDir(), "runtime")
 	if err := Bootstrap(context.Background(), manifest, filepath.Join(manifestDir, "runtime-bundle.toml"), outDir); err != nil {
@@ -99,6 +100,7 @@ func TestPackageWritesArchive(t *testing.T) {
 		"firecracker",
 		"id_ed25519",
 		"namegen",
+		"banger-vsock-pingd",
 		"customize.sh",
 		"packages.apt",
 		"rootfs-docker.ext4",
@@ -126,20 +128,22 @@ func TestPackageWritesArchive(t *testing.T) {
 	manifest := Manifest{
 		BundleRoot: "runtime",
 		BundleMeta: BundleMetadata{
-			FirecrackerBin:    "firecracker",
+			FirecrackerBin:      "firecracker",
-			SSHKeyPath:        "id_ed25519",
+			SSHKeyPath:          "id_ed25519",
-			NamegenPath:       "namegen",
+			NamegenPath:         "namegen",
-			CustomizeScript:   "customize.sh",
+			CustomizeScript:     "customize.sh",
-			DefaultPackages:   "packages.apt",
+			VSockPingHelperPath: "banger-vsock-pingd",
-			DefaultRootfs:     "rootfs-docker.ext4",
+			DefaultPackages:     "packages.apt",
-			DefaultKernel:     "wtf/root/boot/vmlinux-6.8.0-94-generic",
+			DefaultRootfs:       "rootfs-docker.ext4",
-			DefaultInitrd:     "wtf/root/boot/initrd.img-6.8.0-94-generic",
+			DefaultKernel:       "wtf/root/boot/vmlinux-6.8.0-94-generic",
-			DefaultModulesDir: "wtf/root/lib/modules/6.8.0-94-generic",
+			DefaultInitrd:       "wtf/root/boot/initrd.img-6.8.0-94-generic",
+			DefaultModulesDir:   "wtf/root/lib/modules/6.8.0-94-generic",
 		},
 		RequiredPaths: []string{
 			"firecracker",
 			"id_ed25519",
 			"namegen",
+			"banger-vsock-pingd",
 			"customize.sh",
 			"packages.apt",
 			"rootfs-docker.ext4",
@@ -182,7 +186,7 @@ func TestPackageWritesArchive(t *testing.T) {
 
 func TestLoadBundleMetadataRejectsMissingRequiredPath(t *testing.T) {
 	runtimeDir := t.TempDir()
-	for _, rel := range []string{"firecracker", "id_ed25519", "namegen", "customize.sh", "packages.apt", "rootfs-docker.ext4"} {
+	for _, rel := range []string{"firecracker", "id_ed25519", "namegen", "banger-vsock-pingd", "customize.sh", "packages.apt", "rootfs-docker.ext4"} {
 		path := filepath.Join(runtimeDir, rel)
 		if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 			t.Fatalf("MkdirAll: %v", err)
@@ -192,13 +196,14 @@ func TestLoadBundleMetadataRejectsMissingRequiredPath(t *testing.T) {
 		}
 	}
 	data := mustJSON(t, BundleMetadata{
-		FirecrackerBin:  "firecracker",
+		FirecrackerBin:      "firecracker",
-		SSHKeyPath:      "id_ed25519",
+		SSHKeyPath:          "id_ed25519",
-		NamegenPath:     "namegen",
+		NamegenPath:         "namegen",
-		CustomizeScript: "customize.sh",
+		CustomizeScript:     "customize.sh",
-		DefaultPackages: "packages.apt",
+		VSockPingHelperPath: "banger-vsock-pingd",
-		DefaultRootfs:   "rootfs-docker.ext4",
+		DefaultPackages:     "packages.apt",
-		DefaultKernel:   "missing-kernel",
+		DefaultRootfs:       "rootfs-docker.ext4",
+		DefaultKernel:       "missing-kernel",
 	})
 	if err := os.WriteFile(filepath.Join(runtimeDir, BundleMetadataFile), []byte(data), 0o644); err != nil {
 		t.Fatalf("WriteFile: %v", err)

internal/vsockping/vsockping.go

@@ -0,0 +1,105 @@
+package vsockping
+
+import (
+	"bufio"
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"net"
+	"strings"
+	"time"
+
+	sdkvsock "github.com/firecracker-microvm/firecracker-go-sdk/vsock"
+)
+
+const (
+	Port             uint32 = 42070
+	RequestLine             = "PING\n"
+	ResponseLine            = "PONG\n"
+	GuestBinaryName         = "banger-vsock-pingd"
+	GuestInstallPath        = "/usr/local/bin/" + GuestBinaryName
+	ServiceName             = "banger-vsock-pingd.service"
+	serviceUnit             = `[Unit]
+Description=Banger vsock ping responder
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/banger-vsock-pingd
+Restart=on-failure
+RestartSec=1
+
+[Install]
+WantedBy=multi-user.target
+`
+	modulesLoadConfig = "vsock\nvmw_vsock_virtio_transport\n"
+)
+
+func Ping(ctx context.Context, logger *slog.Logger, socketPath string) error {
+	conn, err := sdkvsock.DialContext(
+		ctx,
+		socketPath,
+		Port,
+		sdkvsock.WithRetryTimeout(3*time.Second),
+		sdkvsock.WithRetryInterval(100*time.Millisecond),
+	)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	if deadline, ok := ctx.Deadline(); ok {
+		_ = conn.SetDeadline(deadline)
+	} else {
+		_ = conn.SetDeadline(time.Now().Add(3 * time.Second))
+	}
+
+	if _, err := io.WriteString(conn, RequestLine); err != nil {
+		return err
+	}
+	line, err := bufio.NewReader(conn).ReadString('\n')
+	if err != nil {
+		return err
+	}
+	if strings.TrimSpace(line) != strings.TrimSpace(ResponseLine) {
+		return fmt.Errorf("unexpected vsock ping response %q", strings.TrimSpace(line))
+	}
+	if logger != nil {
+		logger.Debug("vsock ping ok", "vsock_path", socketPath, "vsock_port", Port)
+	}
+	return nil
+}
+
+func ServiceUnit() string {
+	return serviceUnit
+}
+
+func ModulesLoadConfig() string {
+	return modulesLoadConfig
+}
+
+func ReminderMessage(name string) string {
+	return fmt.Sprintf("session ended; %s is still running (stop it with 'banger vm stop %s')", name, name)
+}
+
+func WarningMessage(name string, err error) string {
+	if err == nil {
+		return ""
+	}
+	return fmt.Sprintf("warning: failed to check whether %s is still running: %v", name, err)
+}
+
+func ServeConn(conn net.Conn) error {
+	defer conn.Close()
+	_ = conn.SetDeadline(time.Now().Add(5 * time.Second))
+	line, err := bufio.NewReader(conn).ReadString('\n')
+	if err != nil {
+		return err
+	}
+	if strings.TrimSpace(line) != strings.TrimSpace(RequestLine) {
+		return fmt.Errorf("unexpected request %q", strings.TrimSpace(line))
+	}
+	_, err = io.WriteString(conn, ResponseLine)
+	return err
+}

runtime-bundle.toml

@@ -10,6 +10,7 @@ required_paths = [
   "customize.sh",
   "packages.sh",
   "namegen",
+  "banger-vsock-pingd",
   "packages.apt",
   "id_ed25519",
   "rootfs-docker.ext4",
@@ -23,6 +24,7 @@ firecracker_bin = "firecracker"
 ssh_key_path = "id_ed25519"
 namegen_path = "namegen"
 customize_script = "customize.sh"
+vsock_ping_helper_path = "banger-vsock-pingd"
 default_packages_file = "packages.apt"
 default_rootfs = "rootfs-docker.ext4"
 default_kernel = "wtf/root/boot/vmlinux-6.8.0-94-generic"