ipn/ipnlocal, feature/ssh: move SSH code out of LocalBackend to feature

This makes tsnet apps not depend on x/crypto/ssh and locks that in with a test.

It also paves the wave for tsnet apps to opt-in to SSH support via a
blank feature import in the future.

Updates #12614

Change-Id: Ica85628f89c8f015413b074f5001b82b27c953a9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/c2n.go

@@ -0,0 +1,109 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build (linux && !android) || (darwin && !ios) || freebsd || openbsd || plan9
+
+package tailssh
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"os/exec"
+	"runtime"
+	"slices"
+
+	"go4.org/mem"
+	"tailscale.com/ipn/ipnlocal"
+	"tailscale.com/tailcfg"
+	"tailscale.com/util/lineiter"
+)
+
+func handleC2NSSHUsernames(b *ipnlocal.LocalBackend, w http.ResponseWriter, r *http.Request) {
+	var req tailcfg.C2NSSHUsernamesRequest
+	if r.Method == "POST" {
+		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+	}
+	res, err := getSSHUsernames(b, &req)
+	if err != nil {
+		http.Error(w, err.Error(), 500)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(res)
+}
+
+// getSSHUsernames discovers and returns the list of usernames that are
+// potential Tailscale SSH user targets.
+func getSSHUsernames(b *ipnlocal.LocalBackend, req *tailcfg.C2NSSHUsernamesRequest) (*tailcfg.C2NSSHUsernamesResponse, error) {
+	res := new(tailcfg.C2NSSHUsernamesResponse)
+	if b == nil || !b.ShouldRunSSH() {
+		return res, nil
+	}
+
+	max := 10
+	if req != nil && req.Max != 0 {
+		max = req.Max
+	}
+
+	add := func(u string) {
+		if req != nil && req.Exclude[u] {
+			return
+		}
+		switch u {
+		case "nobody", "daemon", "sync":
+			return
+		}
+		if slices.Contains(res.Usernames, u) {
+			return
+		}
+		if len(res.Usernames) > max {
+			// Enough for a hint.
+			return
+		}
+		res.Usernames = append(res.Usernames, u)
+	}
+
+	if opUser := b.OperatorUserName(); opUser != "" {
+		add(opUser)
+	}
+
+	// Check popular usernames and see if they exist with a real shell.
+	switch runtime.GOOS {
+	case "darwin":
+		out, err := exec.Command("dscl", ".", "list", "/Users").Output()
+		if err != nil {
+			return nil, err
+		}
+		for line := range lineiter.Bytes(out) {
+			line = bytes.TrimSpace(line)
+			if len(line) == 0 || line[0] == '_' {
+				continue
+			}
+			add(string(line))
+		}
+	default:
+		for lr := range lineiter.File("/etc/passwd") {
+			line, err := lr.Value()
+			if err != nil {
+				break
+			}
+			line = bytes.TrimSpace(line)
+			if len(line) == 0 || line[0] == '#' || line[0] == '_' {
+				continue
+			}
+			if mem.HasSuffix(mem.B(line), mem.S("/nologin")) ||
+				mem.HasSuffix(mem.B(line), mem.S("/false")) {
+				continue
+			}
+			before, _, ok := bytes.Cut(line, []byte{':'})
+			if ok {
+				add(string(before))
+			}
+		}
+	}
+	return res, nil
+}