Add SSH source support with dialstdio and UI components

- Implement SSH dial via stdio for remote connections
- Add sources API and storage layer for managing connection sources
- Add SourcePanel and SshFields web components for SSH configuration
- Update app structure to support source-based connections
- Update handlers and server for new sources endpoint

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

README.md

@@ -15,11 +15,16 @@ Two ways to move things:
 
 The target needs **nothing installed**: no agent, no Python, no Go — just `sshd`, `docker`, `bash` and `gzip`.
 
+The **source** is picked in the UI: the daemon DockMV runs next to, another daemon by address, or a
+remote host over SSH — which needs nothing installed either. One DockMV can therefore move containers
+between any two of your hosts.
+
 ---
 
 ## Install
 
-Run DockMV on the **source** host (the one holding the containers to move).
+Run DockMV on the **source** host (the one holding the containers to move), or anywhere that can
+reach it — see [sources](#sources).
 
 ### Docker Compose — recommended
 
@@ -66,7 +71,7 @@ make build        # rebuilds the UI, then the binary
 
 ## Dependencies
 
-### Source host — where DockMV runs
+### Source host — where the containers are
 
 | Requirement | Notes |
 | --- | --- |
@@ -75,6 +80,9 @@ make build        # rebuilds the UI, then the binary
 
 Nothing else. The binary is static: no libc, no runtime, no Python.
 
+A **remote** source needs the same as a target — `sshd` and a `docker` CLI of 18.09 or newer, since
+the Engine API is tunnelled through `docker system dial-stdio`. Nothing is installed there either.
+
 ### Target host — where containers land
 
 | Requirement | Why |
@@ -104,17 +112,37 @@ on the Go side; React 19, Vite 7 and TypeScript 5.9 on the UI side. That is the
 
 ## Using it
 
-1. **Containers tab** — everything on the source host, grouped by compose project. Tick what to move.
+1. **Source host** — top of the right panel. Defaults to the daemon DockMV runs next to; pick another
-2. **Expand a row** (`▸`) for per-container details: target name, image pulled or transferred,
+   one to read a different host. See [sources](#sources).
+2. **Containers tab** — everything on the source host, grouped by compose project. Tick what to move.
+3. **Expand a row** (`▸`) for per-container details: target name, image pulled or transferred,
    networks and ports, and — per mount — **copy the data**, **create it empty**, or **do not mount it**.
    Bind mounts can be relocated; named volumes renamed.
-3. **Apply to selected** does the same thing to every selected container at once.
+4. **Apply to selected** does the same thing to every selected container at once.
-4. **Right panel** — add the target host, *connect*, then **migrate over SSH** or **build a package**.
+5. **Right panel** — add the target host, *connect*, then **migrate over SSH** or **build a package**.
-5. **Jobs tab** — live progress per container and per mount, with the full command log.
+6. **Jobs tab** — live progress per container and per mount, with the full command log.
 
 Start with **dry run** ticked: it runs every check and prints every command without touching the target.
 **Preview the commands** shows the exact `docker` invocations that will run. Nothing is hidden.
 
+### Sources
+
+Three kinds, all interchangeable once selected — the container list, the preview, the migration and
+the package build all read from whichever source is active:
+
+| Kind | How it is reached | Notes |
+| --- | --- | --- |
+| **this host** | the socket in `DOCKER_HOST`, or `--docker-host` | always present; cannot be edited or removed |
+| **docker address** | `tcp://host:2375`, or another `unix://` socket | TLS uses the certificates from `DOCKER_CERT_PATH` in DockMV's own environment. A plain `tcp://` daemon is unauthenticated — anyone who reaches that port is root on that host |
+| **ssh** | the remote host's own docker CLI, through `docker system dial-stdio` | host keys are verified and credentials handled exactly like a target's |
+
+The selected source is remembered in `<data-dir>/sources.json` and reselected on the next start; an
+explicit `--docker-host` on the command line overrides it for that run. Saved sources whose
+credentials you chose not to remember ask for them again after a restart.
+
+With a remote source the data relays through DockMV — source → this host → target — so it crosses the
+network twice. Running DockMV on the source host keeps it to one hop.
+
 <details>
 <summary><b>How the data is actually moved</b></summary>
 
@@ -163,10 +191,11 @@ The tool can stop containers and read every volume on the host, so it is treated
 admin tool:
 
 - Binds to **`127.0.0.1` by default**. Binding elsewhere auto-generates an access token and prints it.
-- **SSH host keys are verified** like OpenSSH. An unknown key is refused until you approve the
+- **SSH host keys are verified** like OpenSSH, for sources as well as targets. An unknown key is
-  fingerprint in the UI; a *changed* key is refused outright. Trusted keys go to `<data-dir>/known_hosts`.
+  refused until you approve the fingerprint in the UI; a *changed* key is refused outright. Trusted
+  keys go to `<data-dir>/known_hosts`.
 - **Credentials are not persisted unless you ask.** *Remember* writes them to
-  `<data-dir>/connections.json`, mode `0600`.
+  `<data-dir>/connections.json` for targets and `<data-dir>/sources.json` for sources, mode `0600`.
 - **Nothing on the target is overwritten by default.** An existing container name fails the item;
   you pick *skip*, *rename* or *replace*. An existing volume is reused and merged into, never
   silently deleted, unless you pick *replace*.
@@ -227,9 +256,10 @@ dockmv version
 ```
 --addr string          address to listen on (default "127.0.0.1:8080")
 --token string         require this token on every request; "auto" generates one
---data-dir string      connections and trusted host keys (default: OS config dir)
+--data-dir string      sources, connections and trusted host keys (default: OS config dir)
 --package-dir string   where migration packages are written (default <data-dir>/packages)
---docker-host string   source docker daemon (default: the DOCKER_HOST environment)
+--docker-host string   local source docker daemon (default: the DOCKER_HOST environment);
+                       given explicitly, it overrides the remembered source
 -v                     verbose logging
 ```
 
@@ -245,6 +275,12 @@ Everything the UI does is available over HTTP. Pass the token as `X-Auth-Token`
 GET    /api/health
 GET    /api/source                          inventory + default selections
 GET    /api/source/sizes                    volume sizes (slow)
+GET    /api/sources                         known sources + which one is selected
+POST   /api/sources
+DELETE /api/sources/{id}
+POST   /api/sources/{id}/select              switch the source everything reads from
+POST   /api/sources/{id}/probe               read the SSH host key fingerprint
+POST   /api/sources/{id}/trust               approve that fingerprint
 GET    /api/connections
 POST   /api/connections
 DELETE /api/connections/{id}
@@ -270,7 +306,9 @@ GET    /api/packages, /api/packages/{name}/download
 web/                 React + TypeScript UI (vite)
 internal/spec/       the transport model: a container, and how to render it back into docker flags
 internal/dkr/        source Docker daemon: inventory, archive streams, image save
-internal/sshx/       SSH transport, host key trust, driving the target's docker CLI
+internal/sshx/       SSH transport, host key trust, driving the target's docker CLI, and
+                     tunnelling a remote source's API through `docker system dial-stdio`
+internal/store/      saved sources and target connections, and which source is selected
 internal/migrate/    the two engines: SSH streaming, and package + installer generation
 internal/job/        progress tracking for long-running work
 internal/api/        HTTP handlers and SSE

internal/api/handlers.go

@@ -21,24 +21,40 @@ import (
 )
 
 func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
-	ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
+	ctx, cancel := context.WithTimeout(r.Context(), 45*time.Second)
 	defer cancel()
 
 	body := map[string]any{
 		"ok":           true,
-		"dockerHost":   s.docker.Endpoint,
 		"packageDir":   s.cfg.PackageDir,
 		"dataDir":      s.cfg.DataDir,
 		"knownHosts":   s.hosts.Path(),
 		"authRequired": s.cfg.Token != "",
 	}
-	if v, err := s.docker.Ping(ctx); err != nil {
+
+	// Health is also what connects to the selected source on a fresh start, so
+	// the UI learns straight away when the remembered source is unreachable.
+	conn, release, err := s.source(ctx)
+	if err != nil {
+		selected, _ := s.sources.Get(s.sources.Selected())
 		body["ok"] = false
 		body["dockerError"] = err.Error()
-	} else {
+		endpoint := sourceEndpoint(selected)
-		body["dockerVersion"] = v
+		body["dockerHost"] = endpoint
+		body["source"] = sourceStatus{
+			ID: selected.ID, Name: selected.Name, Kind: selected.Kind,
+			Endpoint: endpoint, Error: err.Error(),
 		}
 		writeJSON(w, http.StatusOK, body)
+		return
+	}
+	defer release()
+
+	st := conn.status()
+	body["dockerHost"] = st.Endpoint
+	body["dockerVersion"] = st.DockerVersion
+	body["source"] = st
+	writeJSON(w, http.StatusOK, body)
 }
 
 // handleSource returns the full inventory of the source daemon, plus the
@@ -47,7 +63,14 @@ func (s *Server) handleSource(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := context.WithTimeout(r.Context(), 2*time.Minute)
 	defer cancel()
 
-	inv, err := s.docker.Inventory(ctx)
+	conn, release, err := s.source(ctx)
+	if err != nil {
+		s.writeDialError(w, err)
+		return
+	}
+	defer release()
+
+	inv, err := conn.docker.Inventory(ctx)
 	if err != nil {
 		writeError(w, http.StatusBadGateway, "%v", err)
 		return
@@ -69,7 +92,14 @@ func (s *Server) handleSourceSizes(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := context.WithTimeout(r.Context(), 5*time.Minute)
 	defer cancel()
 
-	sizes, err := s.docker.VolumeSizes(ctx)
+	conn, release, err := s.source(ctx)
+	if err != nil {
+		s.writeDialError(w, err)
+		return
+	}
+	defer release()
+
+	sizes, err := conn.docker.VolumeSizes(ctx)
 	if err != nil {
 		writeError(w, http.StatusBadGateway, "%v", err)
 		return
@@ -209,7 +239,14 @@ func (s *Server) handlePreview(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := context.WithTimeout(r.Context(), 2*time.Minute)
 	defer cancel()
 
-	inv, err := s.docker.Inventory(ctx)
+	conn, release, err := s.source(ctx)
+	if err != nil {
+		s.writeDialError(w, err)
+		return
+	}
+	defer release()
+
+	inv, err := conn.docker.Inventory(ctx)
 	if err != nil {
 		writeError(w, http.StatusBadGateway, "%v", err)
 		return
@@ -299,12 +336,23 @@ func (s *Server) handleMigrateSSH(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// The source is held for the whole job: picking another source in the UI
+	// while this runs must not close the socket it is reading from.
+	srcCtx, srcCancel := context.WithTimeout(r.Context(), 60*time.Second)
+	src, release, err := s.source(srcCtx)
+	srcCancel()
+	if err != nil {
+		s.writeDialError(w, err)
+		return
+	}
+
 	// The inventory is re-read now so the plan is applied to current state
 	// rather than to whatever the browser last loaded.
 	invCtx, cancel := context.WithTimeout(r.Context(), 2*time.Minute)
-	inv, err := s.docker.Inventory(invCtx)
+	inv, err := src.docker.Inventory(invCtx)
 	cancel()
 	if err != nil {
+		release()
 		writeError(w, http.StatusBadGateway, "%v", err)
 		return
 	}
@@ -315,13 +363,14 @@ func (s *Server) handleMigrateSSH(w http.ResponseWriter, r *http.Request) {
 	client, err := sshx.Dial(dialCtx, cfg, s.hosts)
 	dialCancel()
 	if err != nil {
+		release()
 		s.writeDialError(w, err)
 		return
 	}
 
-	title := fmt.Sprintf("%d container(s) to %s", countIncluded(req.Plan), cfg.Name)
+	title := fmt.Sprintf("%d container(s) from %s to %s", countIncluded(req.Plan), src.src.Name, cfg.Name)
 	runner := &migrate.SSHRunner{
-		Src:        s.docker,
+		Src:        src.docker,
 		Dst:        sshx.NewRemoteDocker(client),
 		Containers: inv.Containers,
 		Volumes:    inv.Volumes,
@@ -331,8 +380,10 @@ func (s *Server) handleMigrateSSH(w http.ResponseWriter, r *http.Request) {
 
 	j := s.jobs.Run(context.Background(), job.KindSSH, title, req.Plan.Options.DryRun,
 		func(ctx context.Context, j *job.Job) error {
+			defer release()
 			defer client.Close()
-			j.Logf(job.LevelInfo, "", "migrating to %s@%s over ssh", cfg.User, cfg.Host)
+			j.Logf(job.LevelInfo, "", "migrating from %s to %s@%s over ssh",
+				src.docker.Endpoint, cfg.User, cfg.Host)
 			return runner.Run(ctx, j)
 		})
 
@@ -362,16 +413,25 @@ func (s *Server) handleBuildPackage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	srcCtx, srcCancel := context.WithTimeout(r.Context(), 60*time.Second)
+	src, release, err := s.source(srcCtx)
+	srcCancel()
+	if err != nil {
+		s.writeDialError(w, err)
+		return
+	}
+
 	invCtx, cancel := context.WithTimeout(r.Context(), 2*time.Minute)
-	inv, err := s.docker.Inventory(invCtx)
+	inv, err := src.docker.Inventory(invCtx)
 	cancel()
 	if err != nil {
+		release()
 		writeError(w, http.StatusBadGateway, "%v", err)
 		return
 	}
 
 	packager := &migrate.Packager{
-		Src:        s.docker,
+		Src:        src.docker,
 		Containers: inv.Containers,
 		Volumes:    inv.Volumes,
 		Networks:   inv.Networks,
@@ -384,6 +444,8 @@ func (s *Server) handleBuildPackage(w http.ResponseWriter, r *http.Request) {
 	title := fmt.Sprintf("package of %d container(s)", countIncluded(req.Plan))
 	j := s.jobs.Run(context.Background(), job.KindPackage, title, req.Plan.Options.DryRun,
 		func(ctx context.Context, j *job.Job) error {
+			defer release()
+			j.Logf(job.LevelInfo, "", "reading from %s", src.docker.Endpoint)
 			res, err := packager.Run(ctx, j)
 			if err != nil {
 				return err

internal/api/server.go

@@ -11,9 +11,9 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"sync"
 	"time"
 
-	"github.com/arescom/dockmv/internal/dkr"
 	"github.com/arescom/dockmv/internal/job"
 	"github.com/arescom/dockmv/internal/sshx"
 	"github.com/arescom/dockmv/internal/store"
@@ -29,23 +29,31 @@ type Config struct {
 	DataDir string
 	// PackageDir is where migration packages are written.
 	PackageDir string
-	// DockerHost overrides the source daemon address.
+	// DockerHost overrides the local source daemon address.
 	DockerHost string
+	// DockerHostSet reports that DockerHost was given explicitly on the command
+	// line. It then wins over the source remembered from the last run.
+	DockerHostSet bool
 	// UI is the embedded web app; nil disables the UI.
 	UI fs.FS
 	// Logger receives request and error logs.
 	Logger *slog.Logger
 }
 
-// Server ties the Docker client, connection store and job manager to HTTP.
+// Server ties the source daemon, connection store and job manager to HTTP.
 type Server struct {
 	cfg     Config
 	log     *slog.Logger
-	docker *dkr.Client
+	sources *store.Sources
 	conns   *store.Connections
 	hosts   *sshx.KnownHosts
 	jobs    *job.Manager
 	mux     *http.ServeMux
+
+	// srcMu guards cur, the connection to the selected source. It is opened on
+	// first use and replaced when the operator picks another source.
+	srcMu sync.Mutex
+	cur   *sourceConn
 }
 
 // New builds the server and everything it owns.
@@ -60,10 +68,6 @@ func New(cfg Config) (*Server, error) {
 		return nil, fmt.Errorf("create package directory: %w", err)
 	}
 
-	docker, err := dkr.New(cfg.DockerHost)
-	if err != nil {
-		return nil, err
-	}
 	conns, err := store.NewConnections(filepath.Join(cfg.DataDir, "connections.json"))
 	if err != nil {
 		return nil, err
@@ -72,17 +76,35 @@ func New(cfg Config) (*Server, error) {
 	if err != nil {
 		return nil, err
 	}
+	local := store.Source{Name: "this host", DockerHost: cfg.DockerHost}
+	if local.DockerHost == "" {
+		local.DockerHost = os.Getenv("DOCKER_HOST")
+	}
+	sources, err := store.NewSources(filepath.Join(cfg.DataDir, "sources.json"), local)
+	if err != nil {
+		return nil, err
+	}
+	// An explicit --docker-host is an instruction for this run, so it overrides
+	// the source remembered from the last one.
+	if cfg.DockerHostSet {
+		if err := sources.Select(store.LocalSourceID); err != nil {
+			return nil, err
+		}
+	}
 
 	s := &Server{
-		cfg: cfg, log: cfg.Logger, docker: docker,
+		cfg: cfg, log: cfg.Logger, sources: sources,
 		conns: conns, hosts: hosts, jobs: job.NewManager(), mux: http.NewServeMux(),
 	}
 	s.routes()
 	return s, nil
 }
 
-// Close releases the Docker connection.
+// Close releases the connection to the current source.
-func (s *Server) Close() error { return s.docker.Close() }
+func (s *Server) Close() error {
+	s.invalidateSource("")
+	return nil
+}
 
 // Handler returns the root HTTP handler.
 func (s *Server) Handler() http.Handler {
@@ -96,6 +118,13 @@ func (s *Server) routes() {
 	m.HandleFunc("GET /api/source", s.handleSource)
 	m.HandleFunc("GET /api/source/sizes", s.handleSourceSizes)
 
+	m.HandleFunc("GET /api/sources", s.handleListSources)
+	m.HandleFunc("POST /api/sources", s.handleSaveSource)
+	m.HandleFunc("DELETE /api/sources/{id}", s.handleDeleteSource)
+	m.HandleFunc("POST /api/sources/{id}/select", s.handleSelectSource)
+	m.HandleFunc("POST /api/sources/{id}/probe", s.handleSourceProbe)
+	m.HandleFunc("POST /api/sources/{id}/trust", s.handleSourceTrust)
+
 	m.HandleFunc("GET /api/connections", s.handleListConnections)
 	m.HandleFunc("POST /api/connections", s.handleSaveConnection)
 	m.HandleFunc("DELETE /api/connections/{id}", s.handleDeleteConnection)

internal/api/sources.go

@@ -0,0 +1,328 @@
+package api
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/arescom/dockmv/internal/dkr"
+	"github.com/arescom/dockmv/internal/sshx"
+	"github.com/arescom/dockmv/internal/store"
+)
+
+// sourceConn is a live connection to one source daemon.
+//
+// It is reference counted rather than closed eagerly: a migration holds its
+// source for as long as it runs, so switching source in the UI half way through
+// a transfer must not pull the socket out from under it. The connection is
+// closed once it is both replaced and unused.
+type sourceConn struct {
+	src     store.Source
+	docker  *dkr.Client
+	ssh     *sshx.Client
+	version string
+
+	refs  int
+	stale bool
+}
+
+func (c *sourceConn) close() {
+	if c.docker != nil {
+		_ = c.docker.Close()
+	}
+	if c.ssh != nil {
+		_ = c.ssh.Close()
+	}
+}
+
+// sourceStatus is what the UI shows about the source it is looking at.
+type sourceStatus struct {
+	ID            string           `json:"id"`
+	Name          string           `json:"name"`
+	Kind          store.SourceKind `json:"kind"`
+	Endpoint      string           `json:"endpoint"`
+	DockerVersion string           `json:"dockerVersion,omitempty"`
+	Connected     bool             `json:"connected"`
+	Error         string           `json:"error,omitempty"`
+}
+
+func (c *sourceConn) status() sourceStatus {
+	return sourceStatus{
+		ID: c.src.ID, Name: c.src.Name, Kind: c.src.Kind,
+		Endpoint: c.docker.Endpoint, DockerVersion: c.version, Connected: true,
+	}
+}
+
+// source returns the current source, connecting to it on first use. The
+// returned release function must be called when the caller is done with it —
+// for a job, when the job finishes.
+func (s *Server) source(ctx context.Context) (*sourceConn, func(), error) {
+	s.srcMu.Lock()
+	if cur := s.cur; cur != nil {
+		cur.refs++
+		s.srcMu.Unlock()
+		return cur, func() { s.releaseSource(cur) }, nil
+	}
+	id := s.sources.Selected()
+	s.srcMu.Unlock()
+
+	conn, err := s.dialSource(ctx, id)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	s.srcMu.Lock()
+	defer s.srcMu.Unlock()
+	// Another request may have connected while this one was dialling; one
+	// connection is enough, so the loser is dropped.
+	if s.cur != nil {
+		conn.close()
+		conn = s.cur
+	} else {
+		s.cur = conn
+	}
+	conn.refs++
+	return conn, func() { s.releaseSource(conn) }, nil
+}
+
+func (s *Server) releaseSource(c *sourceConn) {
+	s.srcMu.Lock()
+	defer s.srcMu.Unlock()
+	c.refs--
+	if c.refs <= 0 && c.stale {
+		c.close()
+	}
+}
+
+// selectSource connects to a source and, once that worked, makes it the current
+// one and records the choice for the next run.
+func (s *Server) selectSource(ctx context.Context, id string) (sourceStatus, error) {
+	conn, err := s.dialSource(ctx, id)
+	if err != nil {
+		return sourceStatus{}, err
+	}
+	st := conn.status()
+
+	s.srcMu.Lock()
+	old := s.cur
+	s.cur = conn
+	if old != nil {
+		old.stale = true
+		if old.refs <= 0 {
+			old.close()
+		}
+	}
+	s.srcMu.Unlock()
+
+	if err := s.sources.Select(conn.src.ID); err != nil {
+		return st, fmt.Errorf("remember the selected source: %w", err)
+	}
+	s.log.Info("source selected", "id", conn.src.ID, "endpoint", conn.docker.Endpoint)
+	return st, nil
+}
+
+// invalidateSource drops the cached connection when the source behind it has
+// been edited or removed. An empty id invalidates whatever is current.
+func (s *Server) invalidateSource(id string) {
+	s.srcMu.Lock()
+	defer s.srcMu.Unlock()
+	if s.cur == nil {
+		return
+	}
+	if id != "" && s.cur.src.ID != id {
+		return
+	}
+	s.cur.stale = true
+	if s.cur.refs <= 0 {
+		s.cur.close()
+	}
+	s.cur = nil
+}
+
+// dialSource opens a connection to one source and verifies the daemon answers.
+func (s *Server) dialSource(ctx context.Context, id string) (*sourceConn, error) {
+	src, err := s.sources.Get(id)
+	if err != nil {
+		return nil, err
+	}
+
+	conn := &sourceConn{src: src}
+	switch src.Kind {
+	case store.SourceLocal, store.SourceDocker:
+		c, err := dkr.New(src.DockerHost)
+		if err != nil {
+			return nil, err
+		}
+		conn.docker = c
+	case store.SourceSSH:
+		if src.SSH == nil {
+			return nil, errors.New("source has no ssh configuration")
+		}
+		client, err := sshx.Dial(ctx, *src.SSH, s.hosts)
+		if err != nil {
+			return nil, err
+		}
+		rd := sshx.NewRemoteDocker(client)
+		// The CLI is checked first: a missing binary or a user outside the
+		// docker group is a readable error here, and an unexplained broken
+		// socket if it is left to the tunnel.
+		if _, err := rd.ProbeCLI(ctx); err != nil {
+			client.Close()
+			return nil, err
+		}
+		c, err := dkr.NewTunnel(sourceEndpoint(src), func(ctx context.Context, _, _ string) (net.Conn, error) {
+			return rd.DialAPI(ctx)
+		})
+		if err != nil {
+			client.Close()
+			return nil, err
+		}
+		conn.ssh, conn.docker = client, c
+	default:
+		return nil, fmt.Errorf("unknown source kind %q", src.Kind)
+	}
+
+	pingCtx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+	version, err := conn.docker.Ping(pingCtx)
+	if err != nil {
+		conn.close()
+		return nil, fmt.Errorf("connect to docker at %s: %w", conn.docker.Endpoint, err)
+	}
+	conn.version = version
+	return conn, nil
+}
+
+// sourceEndpoint describes where a source lives, before and after it is
+// connected to.
+func sourceEndpoint(src store.Source) string {
+	if src.Kind == store.SourceSSH && src.SSH != nil {
+		port := src.SSH.Port
+		if port == 0 {
+			port = 22
+		}
+		return fmt.Sprintf("ssh://%s@%s", src.SSH.User, net.JoinHostPort(src.SSH.Host, strconv.Itoa(port)))
+	}
+	return src.DockerHost
+}
+
+// handleListSources lists the sources without connecting to any of them; the
+// state of the current one comes from /api/health.
+func (s *Server) handleListSources(w http.ResponseWriter, r *http.Request) {
+	s.srcMu.Lock()
+	var current *sourceStatus
+	if s.cur != nil {
+		st := s.cur.status()
+		current = &st
+	}
+	s.srcMu.Unlock()
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"sources":  s.sources.List(),
+		"selected": s.sources.Selected(),
+		"current":  current,
+	})
+}
+
+func (s *Server) handleSaveSource(w http.ResponseWriter, r *http.Request) {
+	var src store.Source
+	if err := decode(r, &src); err != nil {
+		writeError(w, http.StatusBadRequest, "%v", err)
+		return
+	}
+	saved, err := s.sources.Save(src)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "%v", err)
+		return
+	}
+	// Editing the source in use means the live connection describes the old
+	// settings; drop it so the next call reconnects.
+	s.invalidateSource(saved.ID)
+	writeJSON(w, http.StatusOK, saved)
+}
+
+func (s *Server) handleDeleteSource(w http.ResponseWriter, r *http.Request) {
+	id := r.PathValue("id")
+	if err := s.sources.Delete(id); err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			writeError(w, http.StatusNotFound, "%v", err)
+			return
+		}
+		writeError(w, http.StatusBadRequest, "%v", err)
+		return
+	}
+	s.invalidateSource(id)
+	w.WriteHeader(http.StatusNoContent)
+}
+
+// handleSelectSource switches the source the whole UI works against.
+func (s *Server) handleSelectSource(w http.ResponseWriter, r *http.Request) {
+	ctx, cancel := context.WithTimeout(r.Context(), 60*time.Second)
+	defer cancel()
+
+	st, err := s.selectSource(ctx, r.PathValue("id"))
+	if err != nil {
+		s.writeDialError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, st)
+}
+
+// handleSourceProbe reads the SSH host key of a source host, so its fingerprint
+// can be approved the same way a target's is.
+func (s *Server) handleSourceProbe(w http.ResponseWriter, r *http.Request) {
+	cfg, err := s.sourceSSH(r.PathValue("id"))
+	if err != nil {
+		writeError(w, http.StatusNotFound, "%v", err)
+		return
+	}
+	ctx, cancel := context.WithTimeout(r.Context(), 30*time.Second)
+	defer cancel()
+
+	info, err := sshx.Probe(ctx, cfg, s.hosts)
+	if err != nil {
+		writeError(w, http.StatusBadGateway, "%v", err)
+		return
+	}
+	writeJSON(w, http.StatusOK, info)
+}
+
+func (s *Server) handleSourceTrust(w http.ResponseWriter, r *http.Request) {
+	cfg, err := s.sourceSSH(r.PathValue("id"))
+	if err != nil {
+		writeError(w, http.StatusNotFound, "%v", err)
+		return
+	}
+	var body struct {
+		Fingerprint string `json:"fingerprint"`
+	}
+	if err := decode(r, &body); err != nil {
+		writeError(w, http.StatusBadRequest, "%v", err)
+		return
+	}
+	ctx, cancel := context.WithTimeout(r.Context(), 30*time.Second)
+	defer cancel()
+
+	if err := sshx.TrustFromProbe(ctx, cfg, s.hosts, body.Fingerprint); err != nil {
+		writeError(w, http.StatusBadRequest, "%v", err)
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]any{"trusted": true})
+}
+
+// sourceSSH returns the SSH configuration of a source, refusing the ones that
+// are not reached over SSH.
+func (s *Server) sourceSSH(id string) (sshx.Config, error) {
+	src, err := s.sources.Get(id)
+	if err != nil {
+		return sshx.Config{}, err
+	}
+	if src.Kind != store.SourceSSH || src.SSH == nil {
+		return sshx.Config{}, fmt.Errorf("source %s is not reached over ssh", src.Name)
+	}
+	return *src.SSH, nil
+}

internal/dkr/client.go

@@ -6,6 +6,9 @@ package dkr
 import (
 	"context"
 	"fmt"
+	"net"
+	"net/http"
+	"time"
 
 	"github.com/docker/docker/api/types/system"
 	"github.com/docker/docker/client"
@@ -33,6 +36,43 @@ func New(host string) (*Client, error) {
 	return &Client{api: api, Endpoint: api.DaemonHost()}, nil
 }
 
+// Dialer opens one connection to a daemon's API socket.
+type Dialer func(ctx context.Context, network, addr string) (net.Conn, error)
+
+// NewTunnel connects to a daemon that is only reachable through dial, such as a
+// remote daemon behind an SSH connection. The HTTP host is a placeholder: every
+// connection comes from dial, so the address is never resolved.
+//
+// endpoint is what the UI displays, e.g. ssh://root@10.0.0.5.
+func NewTunnel(endpoint string, dial Dialer) (*Client, error) {
+	// The transport is ours so that WithHost cannot leave a TCP dialer or the
+	// environment's HTTP proxy in place; either would send API calls somewhere
+	// other than through the tunnel.
+	tr := &http.Transport{
+		DisableCompression: true,
+		// Every connection through the tunnel costs one SSH channel, and sshd
+		// allows ten per connection by default (MaxSessions). Capping the pool
+		// keeps a parallel migration from exhausting them; extra calls wait.
+		MaxConnsPerHost:     8,
+		MaxIdleConnsPerHost: 4,
+		IdleConnTimeout:     5 * time.Minute,
+	}
+	api, err := client.NewClientWithOpts(
+		client.WithHTTPClient(&http.Client{Transport: tr}),
+		client.WithHost("http://docker.tunnel.invalid"),
+		client.WithAPIVersionNegotiation(),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("create docker client: %w", err)
+	}
+	tr.Proxy = nil
+	tr.DialContext = dial
+	if endpoint == "" {
+		endpoint = "tunnel"
+	}
+	return &Client{api: api, Endpoint: endpoint}, nil
+}
+
 // API exposes the underlying SDK client for callers that need an operation
 // this package does not wrap.
 func (c *Client) API() *client.Client { return c.api }

internal/sshx/dialstdio.go

@@ -0,0 +1,170 @@
+package sshx
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/crypto/ssh"
+)
+
+// DialAPI opens one connection to the remote daemon's API by running
+// `docker system dial-stdio` over SSH and treating that session's stdin and
+// stdout as a socket. It is the same mechanism `docker -H ssh://…` uses, so the
+// remote host still needs nothing but sshd and the docker CLI.
+//
+// The returned connection is what dkr.NewTunnel dials through: from there on the
+// whole Docker Engine API — inventory, archive streams, image save — is
+// available on a remote source host exactly as it is on a local one.
+func (r *RemoteDocker) DialAPI(ctx context.Context) (net.Conn, error) {
+	sess, err := r.c.conn.NewSession()
+	if err != nil {
+		return nil, fmt.Errorf("open ssh session: %w", err)
+	}
+	stdin, err := sess.StdinPipe()
+	if err != nil {
+		sess.Close()
+		return nil, fmt.Errorf("attach to remote stdin: %w", err)
+	}
+	stdout, err := sess.StdoutPipe()
+	if err != nil {
+		sess.Close()
+		return nil, fmt.Errorf("attach to remote stdout: %w", err)
+	}
+	errBuf := &syncBuffer{}
+	sess.Stderr = errBuf
+
+	cmd := r.Cmd("system", "dial-stdio")
+	if err := sess.Start(cmd); err != nil {
+		sess.Close()
+		return nil, fmt.Errorf("start %q: %w", cmd, err)
+	}
+
+	cfg := r.c.Config()
+	// The connection deliberately outlives ctx: the HTTP transport keeps it in
+	// its idle pool between API calls, and closes it itself when a request is
+	// cancelled or the client is closed.
+	return &apiConn{
+		sess: sess, stdin: stdin, stdout: stdout, stderr: errBuf,
+		remote: apiAddr(fmt.Sprintf("%s@%s", cfg.User, cfg.addr())),
+	}, nil
+}
+
+// ProbeCLI checks that the remote docker CLI is usable before the API is
+// tunnelled through it, so a missing binary or a permission problem is reported
+// as itself rather than as a broken socket. It returns the daemon version.
+func (r *RemoteDocker) ProbeCLI(ctx context.Context) (string, error) {
+	out, ok, err := r.Try(ctx, "version", "--format", "{{.Server.Version}}")
+	if err != nil {
+		return "", err
+	}
+	if ok {
+		if v := strings.TrimSpace(out); v != "" {
+			return v, nil
+		}
+	}
+	res, err := r.c.Run(ctx, r.Cmd("version"))
+	if err != nil {
+		return "", err
+	}
+	msg := strings.TrimSpace(res.Stderr)
+	switch {
+	case strings.Contains(msg, "permission denied"):
+		return "", errors.New("the login user cannot talk to the docker daemon; " +
+			"add it to the docker group, or enable sudo -n for this source")
+	case msg != "":
+		return "", errors.New("docker is not usable on that host: " + firstLine(msg))
+	default:
+		return "", errors.New("docker is not installed or not on PATH on that host")
+	}
+}
+
+// apiConn adapts an SSH session to net.Conn.
+type apiConn struct {
+	sess   *ssh.Session
+	stdin  io.WriteCloser
+	stdout io.Reader
+	stderr *syncBuffer
+	remote apiAddr
+
+	once sync.Once
+	err  error
+}
+
+func (c *apiConn) Read(p []byte) (int, error) {
+	n, err := c.stdout.Read(p)
+	if err != nil {
+		return n, c.wrap(err)
+	}
+	return n, nil
+}
+
+func (c *apiConn) Write(p []byte) (int, error) {
+	n, err := c.stdin.Write(p)
+	if err != nil {
+		return n, c.wrap(err)
+	}
+	return n, nil
+}
+
+// wrap replaces the bare EOF a failed remote command produces with whatever it
+// printed on stderr, which is the only place the reason appears.
+func (c *apiConn) wrap(err error) error {
+	if msg := strings.TrimSpace(c.stderr.String()); msg != "" {
+		return fmt.Errorf("docker system dial-stdio on the remote host failed: %s", firstLine(msg))
+	}
+	return err
+}
+
+func (c *apiConn) Close() error {
+	c.once.Do(func() {
+		// Closing stdin lets the remote docker exit cleanly; the session is torn
+		// down straight after either way.
+		_ = c.stdin.Close()
+		c.err = c.sess.Close()
+		if errors.Is(c.err, io.EOF) {
+			c.err = nil
+		}
+	})
+	return c.err
+}
+
+func (c *apiConn) LocalAddr() net.Addr  { return apiAddr("dockmv") }
+func (c *apiConn) RemoteAddr() net.Addr { return c.remote }
+
+// The deadline calls are no-ops: an SSH channel has no deadline of its own, and
+// the Docker client relies on context cancellation rather than on these. This
+// mirrors what the docker CLI's own ssh:// transport does.
+func (c *apiConn) SetDeadline(time.Time) error      { return nil }
+func (c *apiConn) SetReadDeadline(time.Time) error  { return nil }
+func (c *apiConn) SetWriteDeadline(time.Time) error { return nil }
+
+type apiAddr string
+
+func (a apiAddr) Network() string { return "ssh" }
+func (a apiAddr) String() string  { return string(a) }
+
+// syncBuffer collects remote stderr, which the ssh session writes from its own
+// goroutine while the connection is being read.
+type syncBuffer struct {
+	mu  sync.Mutex
+	buf bytes.Buffer
+}
+
+func (b *syncBuffer) Write(p []byte) (int, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.Write(p)
+}
+
+func (b *syncBuffer) String() string {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.String()
+}

internal/sshx/dialstdio_test.go

@@ -0,0 +1,79 @@
+package sshx
+
+import (
+	"errors"
+	"io"
+	"net"
+	"strings"
+	"testing"
+	"time"
+)
+
+// TestDialAPICommand pins the command the tunnel runs on the source host: it is
+// the same one `docker -H ssh://…` uses, and the sudo / custom binary settings
+// have to reach it.
+func TestDialAPICommand(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		rd   *RemoteDocker
+		want string
+	}{
+		{"plain", &RemoteDocker{binary: "docker"}, "docker system dial-stdio"},
+		{"sudo", &RemoteDocker{binary: "docker", sudo: true}, "sudo -n docker system dial-stdio"},
+		{"podman", &RemoteDocker{binary: "podman"}, "podman system dial-stdio"},
+		{"path with a space", &RemoteDocker{binary: "/opt/my docker/bin/docker"}, "'/opt/my docker/bin/docker' system dial-stdio"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := tc.rd.Cmd("system", "dial-stdio"); got != tc.want {
+				t.Fatalf("command = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+// TestAPIConnSurfacesRemoteStderr covers the failure that would otherwise reach
+// the Docker client as a bare EOF: the remote docker printing a reason and
+// exiting.
+func TestAPIConnSurfacesRemoteStderr(t *testing.T) {
+	errBuf := &syncBuffer{}
+	errBuf.Write([]byte("docker: 'system dial-stdio' is not a docker command\n"))
+	c := &apiConn{
+		stdout: strings.NewReader(""),
+		stdin:  nopWriteCloser{io.Discard},
+		stderr: errBuf,
+	}
+	_, err := c.Read(make([]byte, 8))
+	if err == nil {
+		t.Fatal("a closed stream with remote stderr should be an error")
+	}
+	if !strings.Contains(err.Error(), "is not a docker command") {
+		t.Fatalf("error = %v, want the remote stderr in it", err)
+	}
+	// Without stderr the plain EOF must survive, or the HTTP transport cannot
+	// tell a finished response from a broken one.
+	quiet := &apiConn{stdout: strings.NewReader(""), stdin: nopWriteCloser{io.Discard}, stderr: &syncBuffer{}}
+	if _, err := quiet.Read(make([]byte, 8)); !errors.Is(err, io.EOF) {
+		t.Fatalf("error = %v, want io.EOF", err)
+	}
+}
+
+func TestAPIConnDeadlinesAreNoops(t *testing.T) {
+	var c net.Conn = &apiConn{stdout: strings.NewReader(""), stdin: nopWriteCloser{io.Discard}, stderr: &syncBuffer{}}
+	now := time.Now()
+	if err := c.SetDeadline(now); err != nil {
+		t.Fatalf("SetDeadline: %v", err)
+	}
+	if err := c.SetReadDeadline(now); err != nil {
+		t.Fatalf("SetReadDeadline: %v", err)
+	}
+	if err := c.SetWriteDeadline(now); err != nil {
+		t.Fatalf("SetWriteDeadline: %v", err)
+	}
+	if c.RemoteAddr().Network() != "ssh" {
+		t.Fatalf("network = %q, want ssh", c.RemoteAddr().Network())
+	}
+}
+
+type nopWriteCloser struct{ io.Writer }
+
+func (nopWriteCloser) Close() error { return nil }

internal/store/sources.go

@@ -0,0 +1,315 @@
+package store
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"sync"
+
+	"github.com/arescom/dockmv/internal/sshx"
+)
+
+// SourceKind says how a source daemon is reached.
+type SourceKind string
+
+const (
+	// SourceLocal is the daemon this process talks to by default: the socket in
+	// DOCKER_HOST, or whatever --docker-host was given.
+	SourceLocal SourceKind = "local"
+	// SourceDocker is an explicit daemon address, e.g. tcp://10.0.0.5:2375.
+	SourceDocker SourceKind = "docker"
+	// SourceSSH is a remote daemon reached over SSH, driven through the remote
+	// host's own docker CLI.
+	SourceSSH SourceKind = "ssh"
+)
+
+// LocalSourceID identifies the built-in local source. It is always listed, and
+// cannot be edited or deleted.
+const LocalSourceID = "local"
+
+// Source is one place containers can be read from.
+type Source struct {
+	ID   string     `json:"id"`
+	Name string     `json:"name"`
+	Kind SourceKind `json:"kind"`
+	// DockerHost is the daemon address for SourceDocker, and the address the
+	// local source resolved to for SourceLocal (read-only in that case).
+	DockerHost string `json:"dockerHost,omitempty"`
+	// SSH describes the remote host for SourceSSH.
+	SSH *sshx.Config `json:"ssh,omitempty"`
+}
+
+// Sources is a JSON-backed list of source daemons plus the one currently
+// selected, so a restart comes back to the host the operator was working on.
+//
+// Like Connections, SSH credentials only reach the file when the operator ticks
+// "remember"; otherwise they live in memory for this process only.
+type Sources struct {
+	path  string
+	local Source
+
+	mu       sync.RWMutex
+	items    map[string]Source
+	secrets  map[string]secret
+	selected string
+}
+
+// sourcesFile is the on-disk shape.
+type sourcesFile struct {
+	Selected string   `json:"selected,omitempty"`
+	Sources  []Source `json:"sources"`
+}
+
+// NewSources loads (or creates) the source file at path. local describes the
+// built-in local source, which is not persisted.
+func NewSources(path string, local Source) (*Sources, error) {
+	local.ID = LocalSourceID
+	local.Kind = SourceLocal
+	if local.Name == "" {
+		local.Name = "this host"
+	}
+	s := &Sources{
+		path: path, local: local,
+		items: map[string]Source{}, secrets: map[string]secret{},
+		selected: LocalSourceID,
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		return nil, fmt.Errorf("create data directory: %w", err)
+	}
+	b, err := os.ReadFile(path)
+	if errors.Is(err, os.ErrNotExist) {
+		return s, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("read sources: %w", err)
+	}
+	var f sourcesFile
+	if err := json.Unmarshal(b, &f); err != nil {
+		return nil, fmt.Errorf("parse sources file %s: %w", path, err)
+	}
+	for _, src := range f.Sources {
+		if src.ID == "" || src.ID == LocalSourceID {
+			continue
+		}
+		s.items[src.ID] = src
+	}
+	if f.Selected != "" {
+		if _, ok := s.items[f.Selected]; ok || f.Selected == LocalSourceID {
+			s.selected = f.Selected
+		}
+	}
+	return s, nil
+}
+
+// Local returns the built-in local source.
+func (s *Sources) Local() Source { return s.local }
+
+// List returns the local source followed by the saved ones, credentials
+// stripped, in name order.
+func (s *Sources) List() []Source {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	out := make([]Source, 0, len(s.items)+1)
+	for _, src := range s.items {
+		out = append(out, redactSource(src))
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Name < out[j].Name })
+	return append([]Source{s.local}, out...)
+}
+
+// Get returns a source ready to connect to, with credentials filled back in.
+func (s *Sources) Get(id string) (Source, error) {
+	if id == "" || id == LocalSourceID {
+		return s.local, nil
+	}
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	src, ok := s.items[id]
+	if !ok {
+		return Source{}, ErrNotFound
+	}
+	if src.SSH != nil {
+		cfg := *src.SSH
+		if sec, ok := s.secrets[id]; ok {
+			if cfg.Password == "" {
+				cfg.Password = sec.Password
+			}
+			if cfg.PrivateKey == "" {
+				cfg.PrivateKey = sec.PrivateKey
+			}
+			if cfg.Passphrase == "" {
+				cfg.Passphrase = sec.Passphrase
+			}
+		}
+		src.SSH = &cfg
+	}
+	return src, nil
+}
+
+// Selected returns the id of the current source, falling back to the local one.
+func (s *Sources) Selected() string {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.selected
+}
+
+// Select records which source is in use. It does not connect: that is the
+// caller's job, so a selection is only stored once it has been shown to work.
+func (s *Sources) Select(id string) error {
+	if id == "" {
+		id = LocalSourceID
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if id != LocalSourceID {
+		if _, ok := s.items[id]; !ok {
+			return ErrNotFound
+		}
+	}
+	if s.selected == id {
+		return nil
+	}
+	s.selected = id
+	return s.flush()
+}
+
+// Save inserts or updates a source and returns the stored, redacted form.
+//
+// As with connections, an update that omits credentials keeps the ones already
+// held, so a source can be edited without re-entering a password.
+func (s *Sources) Save(src Source) (Source, error) {
+	if src.ID == LocalSourceID {
+		return Source{}, errors.New("the local source cannot be edited")
+	}
+	switch src.Kind {
+	case SourceDocker:
+		src.DockerHost = strings.TrimSpace(src.DockerHost)
+		if src.DockerHost == "" {
+			return Source{}, errors.New("a docker address is required, e.g. tcp://10.0.0.5:2375")
+		}
+		if !strings.Contains(src.DockerHost, "://") {
+			return Source{}, fmt.Errorf("%q is not a docker address; it needs a scheme, e.g. tcp://%s",
+				src.DockerHost, src.DockerHost)
+		}
+		src.SSH = nil
+		if src.Name == "" {
+			src.Name = src.DockerHost
+		}
+	case SourceSSH:
+		if src.SSH == nil || src.SSH.Host == "" {
+			return Source{}, errors.New("host is required")
+		}
+		if src.SSH.User == "" {
+			return Source{}, errors.New("user is required")
+		}
+		if src.SSH.Port == 0 {
+			src.SSH.Port = 22
+		}
+		src.DockerHost = ""
+		if src.Name == "" {
+			src.Name = src.SSH.Host
+		}
+	case SourceLocal:
+		return Source{}, errors.New("there is only one local source")
+	default:
+		return Source{}, fmt.Errorf("unknown source kind %q", src.Kind)
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if src.ID == "" {
+		src.ID = newID()
+	}
+	if src.SSH != nil {
+		prev := s.items[src.ID]
+		prevSecret := s.secrets[src.ID]
+		var prevSSH sshx.Config
+		if prev.SSH != nil {
+			prevSSH = *prev.SSH
+		}
+		if src.SSH.Password == "" {
+			src.SSH.Password = firstNonEmpty(prevSSH.Password, prevSecret.Password)
+		}
+		if src.SSH.PrivateKey == "" {
+			src.SSH.PrivateKey = firstNonEmpty(prevSSH.PrivateKey, prevSecret.PrivateKey)
+		}
+		if src.SSH.Passphrase == "" {
+			src.SSH.Passphrase = firstNonEmpty(prevSSH.Passphrase, prevSecret.Passphrase)
+		}
+		// The SSH id is only meaningful inside the source that owns it.
+		src.SSH.ID = src.ID
+		src.SSH.Name = src.Name
+
+		if src.SSH.SaveSecrets {
+			delete(s.secrets, src.ID)
+			s.items[src.ID] = src
+		} else {
+			s.secrets[src.ID] = secret{
+				Password:   src.SSH.Password,
+				PrivateKey: src.SSH.PrivateKey,
+				Passphrase: src.SSH.Passphrase,
+			}
+			s.items[src.ID] = redactSource(src)
+		}
+	} else {
+		s.items[src.ID] = src
+	}
+
+	if err := s.flush(); err != nil {
+		return Source{}, err
+	}
+	return redactSource(s.items[src.ID]), nil
+}
+
+// Delete removes a source, falling back to the local one when the source being
+// removed is the selected one.
+func (s *Sources) Delete(id string) error {
+	if id == LocalSourceID {
+		return errors.New("the local source cannot be deleted")
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if _, ok := s.items[id]; !ok {
+		return ErrNotFound
+	}
+	delete(s.items, id)
+	delete(s.secrets, id)
+	if s.selected == id {
+		s.selected = LocalSourceID
+	}
+	return s.flush()
+}
+
+// flush writes the file. The caller must hold the write lock.
+func (s *Sources) flush() error {
+	f := sourcesFile{Selected: s.selected, Sources: make([]Source, 0, len(s.items))}
+	for _, src := range s.items {
+		f.Sources = append(f.Sources, src)
+	}
+	sort.Slice(f.Sources, func(i, j int) bool { return f.Sources[i].ID < f.Sources[j].ID })
+	b, err := json.MarshalIndent(f, "", "  ")
+	if err != nil {
+		return err
+	}
+	tmp := s.path + ".tmp"
+	if err := os.WriteFile(tmp, b, 0o600); err != nil {
+		return fmt.Errorf("write sources: %w", err)
+	}
+	if err := os.Rename(tmp, s.path); err != nil {
+		return fmt.Errorf("replace sources file: %w", err)
+	}
+	return nil
+}
+
+func redactSource(src Source) Source {
+	if src.SSH != nil {
+		cfg := redact(*src.SSH)
+		src.SSH = &cfg
+	}
+	return src
+}

internal/store/sources_test.go

@@ -0,0 +1,202 @@
+package store
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/arescom/dockmv/internal/sshx"
+)
+
+func newTestSources(t *testing.T) (*Sources, string) {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "sources.json")
+	s, err := NewSources(path, Source{Name: "this host", DockerHost: "unix:///var/run/docker.sock"})
+	if err != nil {
+		t.Fatalf("NewSources: %v", err)
+	}
+	return s, path
+}
+
+func TestSourcesLocalIsAlwaysPresent(t *testing.T) {
+	s, _ := newTestSources(t)
+
+	list := s.List()
+	if len(list) != 1 || list[0].ID != LocalSourceID || list[0].Kind != SourceLocal {
+		t.Fatalf("expected only the local source, got %+v", list)
+	}
+	if got := s.Selected(); got != LocalSourceID {
+		t.Fatalf("selected = %q, want %q", got, LocalSourceID)
+	}
+	if _, err := s.Save(Source{ID: LocalSourceID, Kind: SourceDocker, DockerHost: "tcp://x:2375"}); err == nil {
+		t.Fatal("editing the local source should be refused")
+	}
+	if err := s.Delete(LocalSourceID); err == nil {
+		t.Fatal("deleting the local source should be refused")
+	}
+}
+
+func TestSourcesValidation(t *testing.T) {
+	s, _ := newTestSources(t)
+
+	for _, tc := range []struct {
+		name string
+		src  Source
+		want string
+	}{
+		{"no docker address", Source{Kind: SourceDocker}, "docker address is required"},
+		{"address without scheme", Source{Kind: SourceDocker, DockerHost: "10.0.0.5:2375"}, "needs a scheme"},
+		{"ssh without host", Source{Kind: SourceSSH, SSH: &sshx.Config{User: "root"}}, "host is required"},
+		{"ssh without user", Source{Kind: SourceSSH, SSH: &sshx.Config{Host: "h"}}, "user is required"},
+		{"unknown kind", Source{Kind: "carrier-pigeon"}, "unknown source kind"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if _, err := s.Save(tc.src); err == nil || !strings.Contains(err.Error(), tc.want) {
+				t.Fatalf("error = %v, want it to mention %q", err, tc.want)
+			}
+		})
+	}
+}
+
+func TestSourcesSaveDefaults(t *testing.T) {
+	s, _ := newTestSources(t)
+
+	saved, err := s.Save(Source{Kind: SourceSSH, SSH: &sshx.Config{Host: "10.0.0.9", User: "root"}})
+	if err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	if saved.ID == "" {
+		t.Fatal("an id should have been generated")
+	}
+	if saved.Name != "10.0.0.9" {
+		t.Fatalf("name = %q, want the host as a fallback", saved.Name)
+	}
+	if saved.SSH.Port != 22 {
+		t.Fatalf("port = %d, want 22", saved.SSH.Port)
+	}
+
+	// A docker source drops any ssh configuration, and the other way round.
+	dock, err := s.Save(Source{Kind: SourceDocker, DockerHost: "tcp://10.0.0.5:2375", SSH: &sshx.Config{Host: "x", User: "y"}})
+	if err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	if dock.SSH != nil {
+		t.Fatalf("ssh configuration should be dropped for a docker source: %+v", dock.SSH)
+	}
+	if dock.Name != "tcp://10.0.0.5:2375" {
+		t.Fatalf("name = %q, want the address as a fallback", dock.Name)
+	}
+}
+
+func TestSourcesSecretsStayOffDiskUnlessAsked(t *testing.T) {
+	s, path := newTestSources(t)
+
+	kept, err := s.Save(Source{
+		Kind: SourceSSH,
+		SSH:  &sshx.Config{Host: "h1", User: "root", Auth: sshx.AuthPassword, Password: "in-memory"},
+	})
+	if err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	if kept.SSH.Password != "" {
+		t.Fatal("the returned form must be redacted")
+	}
+	if lst := s.List(); lst[1].SSH.Password != "" {
+		t.Fatal("List must not hand out credentials")
+	}
+	// Get is the dialling path, so it does see the password.
+	got, err := s.Get(kept.ID)
+	if err != nil || got.SSH.Password != "in-memory" {
+		t.Fatalf("Get password = %q (err %v), want the in-memory secret", got.SSH.Password, err)
+	}
+
+	remembered, err := s.Save(Source{
+		Kind: SourceSSH,
+		SSH:  &sshx.Config{Host: "h2", User: "root", Auth: sshx.AuthPassword, Password: "on-disk", SaveSecrets: true},
+	})
+	if err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+
+	b, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read file: %v", err)
+	}
+	if strings.Contains(string(b), "in-memory") {
+		t.Fatal("a secret the operator did not want persisted reached the disk")
+	}
+	if !strings.Contains(string(b), "on-disk") {
+		t.Fatal("a remembered secret should have been written")
+	}
+
+	// An update that omits the password keeps the one already held.
+	again, err := s.Save(Source{ID: remembered.ID, Kind: SourceSSH, SSH: &sshx.Config{Host: "h2", User: "admin", SaveSecrets: true}})
+	if err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	reloaded, err := s.Get(again.ID)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	if reloaded.SSH.Password != "on-disk" {
+		t.Fatalf("password = %q, want it carried forward", reloaded.SSH.Password)
+	}
+	if reloaded.SSH.User != "admin" {
+		t.Fatalf("user = %q, want the update to apply", reloaded.SSH.User)
+	}
+}
+
+func TestSourcesSelectionSurvivesRestart(t *testing.T) {
+	s, path := newTestSources(t)
+
+	saved, err := s.Save(Source{Name: "prod", Kind: SourceSSH, SSH: &sshx.Config{Host: "10.0.0.9", User: "root", SaveSecrets: true}})
+	if err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	if err := s.Select("nope"); !errors.Is(err, ErrNotFound) {
+		t.Fatalf("Select of an unknown id = %v, want ErrNotFound", err)
+	}
+	if err := s.Select(saved.ID); err != nil {
+		t.Fatalf("Select: %v", err)
+	}
+
+	reopened, err := NewSources(path, Source{Name: "this host"})
+	if err != nil {
+		t.Fatalf("NewSources: %v", err)
+	}
+	if got := reopened.Selected(); got != saved.ID {
+		t.Fatalf("selected after restart = %q, want %q", got, saved.ID)
+	}
+	if len(reopened.List()) != 2 {
+		t.Fatalf("sources after restart = %+v", reopened.List())
+	}
+
+	// Deleting the selected source falls back to the local one.
+	if err := reopened.Delete(saved.ID); err != nil {
+		t.Fatalf("Delete: %v", err)
+	}
+	if got := reopened.Selected(); got != LocalSourceID {
+		t.Fatalf("selected after delete = %q, want %q", got, LocalSourceID)
+	}
+	if _, err := reopened.Get(saved.ID); !errors.Is(err, ErrNotFound) {
+		t.Fatalf("Get after delete = %v, want ErrNotFound", err)
+	}
+}
+
+func TestSourcesUnknownSelectionIgnoredOnLoad(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "sources.json")
+	body := `{"selected":"gone","sources":[{"id":"gone-too","name":"x","kind":"docker","dockerHost":"tcp://h:2375"}]}`
+	if err := os.WriteFile(path, []byte(body), 0o600); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	s, err := NewSources(path, Source{Name: "this host"})
+	if err != nil {
+		t.Fatalf("NewSources: %v", err)
+	}
+	if got := s.Selected(); got != LocalSourceID {
+		t.Fatalf("selected = %q, want the local fallback", got)
+	}
+}

internal/webui/dist/index.html

@@ -6,7 +6,7 @@
     <meta name="color-scheme" content="dark light" />
     <link rel="icon" type="image/png" href="/favicon.png" />
     <title>DockMV</title>
-    <script type="module" crossorigin src="/assets/index-2w4y0Lpg.js"></script>
+    <script type="module" crossorigin src="/assets/index-qcSVszEj.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-CKzWD9Xt.css">
   </head>
   <body>

main.go

@@ -84,7 +84,7 @@ func serve(args []string) error {
 	token := fs.String("token", os.Getenv("DOCKMV_TOKEN"), "require this token on every request; \"auto\" generates one (default: $DOCKMV_TOKEN)")
 	dataDir := fs.String("data-dir", defaultDataDir(), "directory for connections and trusted host keys")
 	pkgDir := fs.String("package-dir", "", "directory for migration packages (default <data-dir>/packages)")
-	dockerHost := fs.String("docker-host", "", "source docker daemon (default: the DOCKER_HOST environment)")
+	dockerHost := fs.String("docker-host", "", "local source docker daemon (default: the DOCKER_HOST environment)")
 	verbose := fs.Bool("v", false, "verbose logging")
 	fs.Usage = func() {
 		fmt.Fprintln(os.Stderr, "Usage: dockmv serve [flags]\n\nFlags:")
@@ -93,6 +93,14 @@ func serve(args []string) error {
 	if err := fs.Parse(args); err != nil {
 		return err
 	}
+	// A --docker-host given on the command line is an instruction for this run,
+	// and takes precedence over the source remembered from the last one.
+	dockerHostSet := false
+	fs.Visit(func(f *flag.Flag) {
+		if f.Name == "docker-host" {
+			dockerHostSet = true
+		}
+	})
 
 	level := slog.LevelInfo
 	if *verbose {
@@ -118,6 +126,7 @@ func serve(args []string) error {
 		DataDir:       *dataDir,
 		PackageDir:    *pkgDir,
 		DockerHost:    *dockerHost,
+		DockerHostSet: dockerHostSet,
 		UI:            webui.FS(),
 		Logger:        logger,
 	}

web/src/App.tsx

@@ -1,10 +1,12 @@
 import { useCallback, useEffect, useMemo, useState } from 'react'
 import { api } from './api'
 import type {
-  Connection, Health, ItemSelection, JobSnapshot, Options, PackageInfo, Plan, SourceResponse, TargetInventory,
+  Connection, Health, ItemSelection, JobSnapshot, Options, PackageInfo, Plan, Source, SourceResponse,
+  SourceStatus, TargetInventory,
 } from './types'
 import { Notice } from './ui'
 import { Containers } from './Containers'
+import { SourcePanel } from './SourcePanel'
 import { Sidebar } from './Sidebar'
 import { Jobs } from './Jobs'
 import { Packages } from './Packages'
@@ -22,6 +24,9 @@ export default function App() {
   // Volume sizes come from a separate, slower endpoint so the container list
   // can render immediately; they are merged into the rows when they arrive.
   const [sizes, setSizes] = useState<Record<string, number>>({})
+  const [sources, setSources] = useState<Source[]>([])
+  const [selectedSource, setSelectedSource] = useState<string>('local')
+  const [sourceStatus, setSourceStatus] = useState<SourceStatus | null>(null)
   const [connections, setConnections] = useState<Connection[]>([])
   const [activeConn, setActiveConn] = useState<string>('')
   const [targetInv, setTargetInv] = useState<TargetInventory | null>(null)
@@ -57,6 +62,40 @@ export default function App() {
     }
   }, [])
 
+  const loadHealth = useCallback(async () => {
+    try {
+      const h = await api.health()
+      setHealth(h)
+      if (h.source) setSourceStatus(h.source)
+    } catch { /* the panel shows the source error on its own */ }
+  }, [])
+
+  const loadSources = useCallback(async () => {
+    try {
+      const r = await api.sources()
+      setSources(r.sources)
+      setSelectedSource(r.selected)
+      if (r.current) setSourceStatus(r.current)
+    } catch (e) {
+      setError(e instanceof Error ? e.message : String(e))
+    }
+  }, [])
+
+  // selectSource lets its error escape so the panel can turn an untrusted host
+  // key into a fingerprint prompt, the same way the target does.
+  const selectSource = useCallback(async (id: string) => {
+    const st = await api.selectSource(id)
+    setSelectedSource(id)
+    setSourceStatus(st)
+    // The selections describe containers on the host that was selected before,
+    // so they are dropped rather than carried over to a different inventory.
+    setSel({})
+    setSizes({})
+    setError('')
+    await loadSource()
+    await loadHealth()
+  }, [loadSource, loadHealth])
+
   const loadConnections = useCallback(async () => {
     try {
       const list = await api.connections()
@@ -80,12 +119,13 @@ export default function App() {
   }, [])
 
   useEffect(() => {
-    api.health().then(setHealth).catch(() => undefined)
+    loadHealth()
+    loadSources()
     loadSource()
     loadConnections()
     loadJobs()
     loadPackages()
-  }, [loadSource, loadConnections, loadJobs, loadPackages])
+  }, [loadHealth, loadSources, loadSource, loadConnections, loadJobs, loadPackages])
 
   // A slow poll keeps the job list current without holding a stream open for
   // every job; the detail view subscribes to its own live stream.
@@ -138,7 +178,9 @@ export default function App() {
         <div className="topbar-right">
           {health && (
             <span className="hostinfo">
-              source <b>{source?.inventory.host || health.dockerHost}</b>
+              source <b>{source?.inventory.host || sourceStatus?.name || health.dockerHost}</b>
+              {sourceStatus?.kind === 'ssh' && <> · over ssh</>}
+              {sourceStatus?.kind === 'docker' && <> · {sourceStatus.endpoint}</>}
               {health.dockerVersion && <> · docker {health.dockerVersion}</>}
             </span>
           )}
@@ -160,8 +202,10 @@ export default function App() {
       {health && !health.ok && (
         <div style={{ padding: '10px 16px' }}>
           <Notice kind="err">
-            Cannot reach the source Docker daemon at <span className="mono">{health.dockerHost}</span>
+            Cannot reach the source <b>{health.source?.name ?? 'docker daemon'}</b>
+            {health.dockerHost && <> at <span className="mono">{health.dockerHost}</span></>}
             {health.dockerError && <> — {health.dockerError}</>}
+            <div className="small">Pick another source in the panel on the right.</div>
           </Notice>
         </div>
       )}
@@ -192,6 +236,14 @@ export default function App() {
 
         {view === 'containers' && (
           <aside className="sidebar">
+            <SourcePanel
+              sources={sources}
+              selected={selectedSource}
+              status={sourceStatus}
+              selectSource={selectSource}
+              reload={loadSources}
+              onError={setError}
+            />
             <Sidebar
               source={source}
               plan={plan}

web/src/Sidebar.tsx

@@ -4,6 +4,7 @@ import type {
   Connection, HostKeyInfo, JobSnapshot, Options, Plan, PreviewResponse, SourceResponse, TargetInventory,
 } from './types'
 import { Check, Field, humanBytes, Modal, Notice } from './ui'
+import { HostKeyDialog, SshFields } from './SshFields'
 
 export function Sidebar({
   source, plan, includedCount, options, setOptions,
@@ -302,37 +303,7 @@ export function Sidebar({
       )}
 
       {hostKey && (
-        <Modal
+        <HostKeyDialog info={hostKey} onClose={() => setHostKey(null)} onTrust={trustHostKey} />
-          title="SSH host key"
-          onClose={() => setHostKey(null)}
-          footer={
-            <>
-              <button className="btn" onClick={() => setHostKey(null)}>cancel</button>
-              <button className="btn primary" onClick={trustHostKey}>
-                {hostKey.changed ? 'replace the stored key and trust' : 'trust this host'}
-              </button>
-            </>
-          }
-        >
-          <div className="stack">
-            {hostKey.changed && (
-              <Notice kind="err">
-                The key presented by this host is <b>different</b> from the one recorded earlier. This happens after a
-                reinstall — but it is also what a machine-in-the-middle looks like. Only continue if you know why it
-                changed.
-              </Notice>
-            )}
-            {hostKey.trusted && !hostKey.changed && <Notice kind="ok">This host key is already trusted.</Notice>}
-            <div className="small muted">
-              Compare this with the output of <span className="mono">ssh-keyscan -t {hostKey.keyType} {hostKey.host}</span>{' '}
-              run on the target itself, or with <span className="mono">ssh-keygen -lf /etc/ssh/ssh_host_*_key.pub</span>.
-            </div>
-            <div className="fingerprint">
-              {hostKey.keyType}<br />
-              {hostKey.fingerprint}
-            </div>
-          </div>
-        </Modal>
       )}
 
       {preview && <PreviewModal data={preview} onClose={() => setPreview(null)} />}
@@ -382,79 +353,8 @@ function ConnectionDialog({
       }
     >
       <div className="stack" style={{ gap: 12 }}>
-        <div className="row" style={{ gap: 12 }}>
         <Field label="label"><input type="text" value={c.name ?? ''} onChange={(e) => set('name', e.target.value)} /></Field>
-          <Field label="host"><input type="text" value={c.host ?? ''} onChange={(e) => set('host', e.target.value)} /></Field>
+        <SshFields value={c} set={set} where="target" />
-          <div style={{ width: 90 }}>
-            <Field label="port">
-              <input type="number" value={c.port ?? 22} onChange={(e) => set('port', Number(e.target.value))} />
-            </Field>
-          </div>
-        </div>
-
-        <div className="row" style={{ gap: 12 }}>
-          <Field label="user"><input type="text" value={c.user ?? ''} onChange={(e) => set('user', e.target.value)} /></Field>
-          <Field label="authentication">
-            <select value={c.auth ?? 'password'} onChange={(e) => set('auth', e.target.value as Connection['auth'])}>
-              <option value="password">password</option>
-              <option value="key">private key</option>
-              <option value="agent">ssh agent</option>
-            </select>
-          </Field>
-        </div>
-
-        {c.auth === 'password' && (
-          <Field label="password">
-            <input type="password" value={c.password ?? ''} onChange={(e) => set('password', e.target.value)} />
-          </Field>
-        )}
-
-        {c.auth === 'key' && (
-          <>
-            <Field label="private key path on this machine (leave empty to paste the key below)">
-              <input type="text" placeholder="/root/.ssh/id_ed25519" value={c.privateKeyPath ?? ''} onChange={(e) => set('privateKeyPath', e.target.value)} />
-            </Field>
-            <Field label="or paste the private key">
-              <textarea rows={5} value={c.privateKey ?? ''} onChange={(e) => set('privateKey', e.target.value)} placeholder="-----BEGIN OPENSSH PRIVATE KEY-----" />
-            </Field>
-            <Field label="passphrase (if the key is encrypted)">
-              <input type="password" value={c.passphrase ?? ''} onChange={(e) => set('passphrase', e.target.value)} />
-            </Field>
-          </>
-        )}
-
-        {c.auth === 'agent' && (
-          <div className="small muted">
-            Uses the agent at <span className="mono">$SSH_AUTH_SOCK</span> of the process running dockmv.
-          </div>
-        )}
-
-        <Check
-          checked={c.sudo ?? false}
-          onChange={(v) => set('sudo', v)}
-          label="run docker through sudo -n on the target"
-          title="Needed when the login user is not in the docker group. sudo must not ask for a password."
-        />
-
-        <Field label="docker command on the target (optional)">
-          <input type="text" placeholder="docker" value={c.dockerCmd ?? ''} onChange={(e) => set('dockerCmd', e.target.value)} />
-        </Field>
-
-        <Check
-          checked={c.saveSecrets ?? false}
-          onChange={(v) => set('saveSecrets', v)}
-          label="remember the password / key on disk"
-        />
-        {c.saveSecrets ? (
-          <Notice kind="warn">
-            Credentials are stored in plain text in the connections file, readable only by this user. Leave this off to
-            keep them in memory for this session only.
-          </Notice>
-        ) : (
-          <div className="small faint">
-            Credentials stay in memory and are lost when dockmv restarts.
-          </div>
-        )}
       </div>
     </Modal>
   )

web/src/SourcePanel.tsx

@@ -0,0 +1,283 @@
+import { useState } from 'react'
+import { api, ApiError } from './api'
+import type { Connection, HostKeyInfo, Source, SourceKind, SourceStatus } from './types'
+import { Field, Modal, Notice } from './ui'
+import { HostKeyDialog, SshFields } from './SshFields'
+
+/**
+ * SourcePanel picks the host containers are read from: the daemon dockmv runs
+ * next to, another daemon by address, or a remote host over SSH.
+ */
+export function SourcePanel({
+  sources, selected, status, selectSource, reload, onError,
+}: {
+  sources: Source[]
+  selected: string
+  status: SourceStatus | null
+  /** Switches the whole UI to another source; rejects so the caller can react. */
+  selectSource: (id: string) => Promise<void>
+  reload: () => Promise<void>
+  onError: (msg: string) => void
+}) {
+  const [editing, setEditing] = useState<Partial<Source> | null>(null)
+  const [hostKey, setHostKey] = useState<HostKeyInfo | null>(null)
+  const [busy, setBusy] = useState('')
+  const [trustFor, setTrustFor] = useState('')
+
+  const current = sources.find((s) => s.id === selected)
+  const isLocal = !current || current.kind === 'local'
+  const isSSH = current?.kind === 'ssh'
+
+  async function withBusy(what: string, id: string, fn: () => Promise<void>) {
+    setBusy(what)
+    try {
+      await fn()
+    } catch (e) {
+      // An untrusted host key is a decision for the operator, not an error.
+      if (e instanceof ApiError && e.needsTrust) {
+        setTrustFor(id)
+        await showHostKey(id)
+      } else {
+        onError(e instanceof Error ? e.message : String(e))
+      }
+    } finally {
+      setBusy('')
+    }
+  }
+
+  async function showHostKey(id: string) {
+    try {
+      setHostKey(await api.probeSource(id))
+    } catch (e) {
+      onError(e instanceof Error ? e.message : String(e))
+    }
+  }
+
+  async function trustHostKey() {
+    const id = trustFor || selected
+    if (!hostKey) return
+    await withBusy('trust', id, async () => {
+      await api.trustSource(id, hostKey.fingerprint)
+      setHostKey(null)
+      await selectSource(id)
+    })
+  }
+
+  return (
+    <div className="section">
+      <h3>source host</h3>
+      <div className="stack">
+        <div className="row">
+          <select
+            value={selected}
+            disabled={!!busy}
+            onChange={(e) => {
+              const id = e.target.value
+              void withBusy('select', id, () => selectSource(id))
+            }}
+          >
+            {sources.map((s) => (
+              <option key={s.id} value={s.id}>{sourceLabel(s)}</option>
+            ))}
+          </select>
+          <button
+            className="btn tiny"
+            onClick={() => setEditing({ kind: 'ssh', ssh: { port: 22, auth: 'password', saveSecrets: false, sudo: false } as Connection })}
+          >
+            new
+          </button>
+        </div>
+
+        <div className="row wrap" style={{ gap: 6 }}>
+          <button
+            className="btn tiny"
+            disabled={!!busy}
+            onClick={() => void withBusy('select', selected, () => selectSource(selected))}
+          >
+            {busy === 'select' ? 'connecting…' : 'reconnect'}
+          </button>
+          {!isLocal && <button className="btn tiny" onClick={() => setEditing(current)}>edit</button>}
+          {isSSH && <button className="btn tiny" onClick={() => { setTrustFor(selected); void showHostKey(selected) }}>host key</button>}
+          {!isLocal && (
+            <button
+              className="btn tiny danger"
+              onClick={() => {
+                if (!current || !confirm(`Delete source "${current.name}"?`)) return
+                void withBusy('del', current.id, async () => {
+                  await api.deleteSource(current.id)
+                  await reload()
+                  // Deleting the source in use drops back to the local daemon.
+                  if (selected === current.id) await selectSource('local')
+                })
+              }}
+            >
+              delete
+            </button>
+          )}
+        </div>
+
+        {status?.error && <Notice kind="err">{status.error}</Notice>}
+
+        {status && !status.error && (
+          <dl className="kv">
+            <dt>reached by</dt><dd className="mono">{status.endpoint}</dd>
+            {status.dockerVersion && <><dt>docker</dt><dd>{status.dockerVersion}</dd></>}
+          </dl>
+        )}
+
+        {isSSH && (
+          <div className="small faint">
+            Data is streamed through dockmv: source → this host → target. A local source moves it in one hop.
+          </div>
+        )}
+      </div>
+
+      {editing && (
+        <SourceDialog
+          initial={editing}
+          onClose={() => setEditing(null)}
+          onSaved={async (s) => {
+            setEditing(null)
+            // The list is refreshed first: selecting an id the dropdown does not
+            // know about yet would leave it blank.
+            await reload()
+            await withBusy('select', s.id, () => selectSource(s.id))
+          }}
+          onError={onError}
+        />
+      )}
+
+      {hostKey && (
+        <HostKeyDialog info={hostKey} onClose={() => setHostKey(null)} onTrust={trustHostKey} />
+      )}
+    </div>
+  )
+}
+
+function sourceLabel(s: Source): string {
+  switch (s.kind) {
+    case 'local':
+      return `${s.name} (local docker)`
+    case 'docker':
+      return `${s.name} (${s.dockerHost})`
+    default:
+      return `${s.name} (ssh ${s.ssh?.user}@${s.ssh?.host})`
+  }
+}
+
+function SourceDialog({
+  initial, onClose, onSaved, onError,
+}: {
+  initial: Partial<Source>
+  onClose: () => void
+  onSaved: (s: Source) => void | Promise<void>
+  onError: (m: string) => void
+}) {
+  const [s, setS] = useState<Partial<Source>>(initial)
+  const [saving, setSaving] = useState(false)
+  const kind: SourceKind = s.kind ?? 'ssh'
+  const ssh = (s.ssh ?? {}) as Partial<Connection>
+
+  function setSSH<K extends keyof Connection>(k: K, v: Connection[K]) {
+    setS((prev) => ({ ...prev, ssh: { ...(prev.ssh as Connection), [k]: v } as Connection }))
+  }
+
+  const valid = kind === 'ssh' ? !!ssh.host && !!ssh.user : !!s.dockerHost
+
+  return (
+    <Modal
+      title={initial.id ? `Edit ${initial.name}` : 'New source host'}
+      onClose={onClose}
+      footer={
+        <>
+          <button className="btn" onClick={onClose}>cancel</button>
+          <button
+            className="btn primary"
+            disabled={saving || !valid}
+            onClick={async () => {
+              setSaving(true)
+              try {
+                // The body is built field by field: the API rejects unknown ones.
+                await onSaved(await api.saveSource(payload(s, kind)))
+              } catch (e) {
+                onError(e instanceof Error ? e.message : String(e))
+              } finally {
+                setSaving(false)
+              }
+            }}
+          >
+            {saving ? 'saving…' : 'save'}
+          </button>
+        </>
+      }
+    >
+      <div className="stack" style={{ gap: 12 }}>
+        <div className="row" style={{ gap: 12 }}>
+          <Field label="label">
+            <input type="text" value={s.name ?? ''} onChange={(e) => setS((p) => ({ ...p, name: e.target.value }))} />
+          </Field>
+          <Field label="reached by">
+            <select value={kind} onChange={(e) => setS((p) => ({ ...p, kind: e.target.value as SourceKind }))}>
+              <option value="ssh">ssh — remote host, driven through its docker CLI</option>
+              <option value="docker">docker address — a daemon this host can reach</option>
+            </select>
+          </Field>
+        </div>
+
+        {kind === 'docker' ? (
+          <>
+            <Field label="docker address">
+              <input
+                type="text"
+                placeholder="tcp://10.0.0.5:2375"
+                value={s.dockerHost ?? ''}
+                onChange={(e) => setS((p) => ({ ...p, dockerHost: e.target.value }))}
+              />
+            </Field>
+            <div className="small muted">
+              Any address the docker CLI accepts: <span className="mono">tcp://host:2375</span>, or another socket with{' '}
+              <span className="mono">unix:///path/docker.sock</span>. A TLS-protected daemon uses the certificates from{' '}
+              <span className="mono">DOCKER_CERT_PATH</span> in dockmv's own environment.
+            </div>
+            <Notice kind="warn">
+              A plain <span className="mono">tcp://</span> daemon is unauthenticated: anyone who can reach that port is
+              root on that host. Prefer an ssh source unless the port is already protected.
+            </Notice>
+          </>
+        ) : (
+          <>
+            <SshFields value={ssh} set={setSSH} where="source host" />
+            <div className="small muted">
+              Needs <span className="mono">sshd</span> and a docker CLI of 18.09 or newer on that host — the API is
+              tunnelled through <span className="mono">docker system dial-stdio</span>. Nothing is installed.
+            </div>
+          </>
+        )}
+      </div>
+    </Modal>
+  )
+}
+
+/** payload keeps the request to the fields the server knows about. */
+function payload(s: Partial<Source>, kind: SourceKind): Partial<Source> {
+  const out: Partial<Source> = { id: s.id, name: s.name, kind }
+  if (kind === 'docker') {
+    out.dockerHost = s.dockerHost
+    return out
+  }
+  const c = (s.ssh ?? {}) as Partial<Connection>
+  out.ssh = {
+    host: c.host ?? '',
+    port: c.port ?? 22,
+    user: c.user ?? '',
+    auth: c.auth ?? 'password',
+    password: c.password,
+    privateKey: c.privateKey,
+    privateKeyPath: c.privateKeyPath,
+    passphrase: c.passphrase,
+    sudo: c.sudo ?? false,
+    dockerCmd: c.dockerCmd,
+    saveSecrets: c.saveSecrets ?? false,
+  } as Connection
+  return out
+}

web/src/SshFields.tsx

@@ -0,0 +1,138 @@
+import type { Connection, HostKeyInfo } from './types'
+import { Check, Field, Modal, Notice } from './ui'
+
+/**
+ * SshFields is the credential form shared by target connections and SSH
+ * sources: both are the same sshx.Config on the server, so they are edited the
+ * same way. `where` only changes the wording.
+ */
+export function SshFields({
+  value, set, where,
+}: {
+  value: Partial<Connection>
+  set: <K extends keyof Connection>(k: K, v: Connection[K]) => void
+  where: 'target' | 'source host'
+}) {
+  return (
+    <>
+      <div className="row" style={{ gap: 12 }}>
+        <Field label="host"><input type="text" value={value.host ?? ''} onChange={(e) => set('host', e.target.value)} /></Field>
+        <div style={{ width: 90 }}>
+          <Field label="port">
+            <input type="number" value={value.port ?? 22} onChange={(e) => set('port', Number(e.target.value))} />
+          </Field>
+        </div>
+      </div>
+
+      <div className="row" style={{ gap: 12 }}>
+        <Field label="user"><input type="text" value={value.user ?? ''} onChange={(e) => set('user', e.target.value)} /></Field>
+        <Field label="authentication">
+          <select value={value.auth ?? 'password'} onChange={(e) => set('auth', e.target.value as Connection['auth'])}>
+            <option value="password">password</option>
+            <option value="key">private key</option>
+            <option value="agent">ssh agent</option>
+          </select>
+        </Field>
+      </div>
+
+      {value.auth === 'password' && (
+        <Field label="password">
+          <input type="password" value={value.password ?? ''} onChange={(e) => set('password', e.target.value)} />
+        </Field>
+      )}
+
+      {value.auth === 'key' && (
+        <>
+          <Field label="private key path on this machine (leave empty to paste the key below)">
+            <input type="text" placeholder="/root/.ssh/id_ed25519" value={value.privateKeyPath ?? ''} onChange={(e) => set('privateKeyPath', e.target.value)} />
+          </Field>
+          <Field label="or paste the private key">
+            <textarea rows={5} value={value.privateKey ?? ''} onChange={(e) => set('privateKey', e.target.value)} placeholder="-----BEGIN OPENSSH PRIVATE KEY-----" />
+          </Field>
+          <Field label="passphrase (if the key is encrypted)">
+            <input type="password" value={value.passphrase ?? ''} onChange={(e) => set('passphrase', e.target.value)} />
+          </Field>
+        </>
+      )}
+
+      {value.auth === 'agent' && (
+        <div className="small muted">
+          Uses the agent at <span className="mono">$SSH_AUTH_SOCK</span> of the process running dockmv.
+        </div>
+      )}
+
+      <Check
+        checked={value.sudo ?? false}
+        onChange={(v) => set('sudo', v)}
+        label={`run docker through sudo -n on the ${where}`}
+        title="Needed when the login user is not in the docker group. sudo must not ask for a password."
+      />
+
+      <Field label={`docker command on the ${where} (optional)`}>
+        <input type="text" placeholder="docker" value={value.dockerCmd ?? ''} onChange={(e) => set('dockerCmd', e.target.value)} />
+      </Field>
+
+      <Check
+        checked={value.saveSecrets ?? false}
+        onChange={(v) => set('saveSecrets', v)}
+        label="remember the password / key on disk"
+      />
+      {value.saveSecrets ? (
+        <Notice kind="warn">
+          Credentials are stored in plain text in dockmv's data directory, readable only by this user. Leave this off to
+          keep them in memory for this session only.
+        </Notice>
+      ) : (
+        <div className="small faint">
+          Credentials stay in memory and are lost when dockmv restarts.
+        </div>
+      )}
+    </>
+  )
+}
+
+/**
+ * HostKeyDialog puts an unknown or changed SSH fingerprint in front of the
+ * operator. It is used for both source and target hosts.
+ */
+export function HostKeyDialog({
+  info, onClose, onTrust,
+}: {
+  info: HostKeyInfo
+  onClose: () => void
+  onTrust: () => void
+}) {
+  return (
+    <Modal
+      title="SSH host key"
+      onClose={onClose}
+      footer={
+        <>
+          <button className="btn" onClick={onClose}>cancel</button>
+          <button className="btn primary" onClick={onTrust}>
+            {info.changed ? 'replace the stored key and trust' : 'trust this host'}
+          </button>
+        </>
+      }
+    >
+      <div className="stack">
+        {info.changed && (
+          <Notice kind="err">
+            The key presented by this host is <b>different</b> from the one recorded earlier. This happens after a
+            reinstall — but it is also what a machine-in-the-middle looks like. Only continue if you know why it
+            changed.
+          </Notice>
+        )}
+        {info.trusted && !info.changed && <Notice kind="ok">This host key is already trusted.</Notice>}
+        <div className="small muted">
+          Compare this with the output of <span className="mono">ssh-keyscan -t {info.keyType} {info.host}</span>{' '}
+          run on the host itself, or with <span className="mono">ssh-keygen -lf /etc/ssh/ssh_host_*_key.pub</span>.
+        </div>
+        <div className="fingerprint">
+          {info.keyType}<br />
+          {info.fingerprint}
+        </div>
+      </div>
+    </Modal>
+  )
+}

web/src/api.ts

@@ -1,6 +1,6 @@
 import type {
   Connection, Health, HostKeyInfo, JobSnapshot, PackageInfo, Plan,
-  Preflight, PreviewResponse, SourceResponse, TargetInventory,
+  Preflight, PreviewResponse, Source, SourceResponse, SourcesResponse, SourceStatus, TargetInventory,
 } from './types'
 
 // The token, when the server requires one, arrives as a query parameter the
@@ -65,6 +65,14 @@ export const api = {
   source: () => request<SourceResponse>('/api/source'),
   volumeSizes: () => request<{ volumes: Record<string, number> }>('/api/source/sizes'),
 
+  sources: () => request<SourcesResponse>('/api/sources'),
+  saveSource: (s: Partial<Source>) => post<Source>('/api/sources', s),
+  deleteSource: (id: string) => request<void>(`/api/sources/${id}`, { method: 'DELETE' }),
+  selectSource: (id: string) => post<SourceStatus>(`/api/sources/${id}/select`),
+  probeSource: (id: string) => post<HostKeyInfo>(`/api/sources/${id}/probe`),
+  trustSource: (id: string, fingerprint: string) =>
+    post<{ trusted: boolean }>(`/api/sources/${id}/trust`, { fingerprint }),
+
   connections: () => request<Connection[]>('/api/connections'),
   saveConnection: (c: Partial<Connection>) => post<Connection>('/api/connections', c),
   deleteConnection: (id: string) => request<void>(`/api/connections/${id}`, { method: 'DELETE' }),

web/src/types.ts

@@ -120,6 +120,35 @@ export interface SourceResponse {
   options: Options
 }
 
+/** How a source daemon is reached. 'local' is the built-in, unremovable one. */
+export type SourceKind = 'local' | 'docker' | 'ssh'
+
+export interface Source {
+  id: string
+  name: string
+  kind: SourceKind
+  /** Daemon address for the 'docker' kind, e.g. tcp://10.0.0.5:2375. */
+  dockerHost?: string
+  /** Remote host for the 'ssh' kind; the same shape as a target connection. */
+  ssh?: Connection
+}
+
+export interface SourceStatus {
+  id: string
+  name: string
+  kind: SourceKind
+  endpoint: string
+  dockerVersion?: string
+  connected: boolean
+  error?: string
+}
+
+export interface SourcesResponse {
+  sources: Source[]
+  selected: string
+  current: SourceStatus | null
+}
+
 export type AuthMethod = 'password' | 'key' | 'agent'
 
 export interface Connection {
@@ -253,6 +282,7 @@ export interface Health {
   dockerHost: string
   dockerVersion?: string
   dockerError?: string
+  source?: SourceStatus
   packageDir: string
   dataDir: string
   knownHosts: string