mdns-termux/README.md

# mdns-termux

**mDNS + Tailscale hostname resolution for Termux (Android).**

Termux runs on Android's Bionic libc, which does **not** resolve `.local` names
and lacks `getent`. This repo ships small, dependency-light helpers that fix
that — without root, and without a custom system resolver.

Works on any network: LAN via multicast, off-LAN via Tailscale MagicDNS.

---

## What's in here

**Unified CLI:** `mdns` with five subcommands (`resolve`, `browse`, `publish`,
`reverse`, `cache`). Backed by `zeroconf` for RFC-complete behavior — TTL
cache, IPv4 + IPv6, service discovery, probing on publish, goodbye packets
on exit.

| Script | Role |
|--------|------|
| `mdns`            | Unified CLI (zeroconf-backed). Preferred entry point. |
| `mdns-resolve`    | Standalone stdlib fallback: works with **zero** dependencies. |
| `mdns-publish.py` | Compat shim over `mdns publish` for existing aliases. |
| `ssh-mdns-proxy`  | SSH `ProxyCommand`: mDNS → Tailscale DNS → cached IP. |
| `ssh-fallback`    | SSH `ProxyCommand`: Tailscale → LAN, for flaky Tailscale. |
| `resolve`         | Bash wrapper: **`mdns` → stdlib → Tailscale → cache**, usable in any command. |

Python library under `lib/mdns_tools/` (each module < 100 lines):
`resolve.py`, `browse.py`, `publish.py`, `reverse.py`, `cache.py`,
`_stdlib_resolve.py` (dep-free A-record resolver).

## Install

```bash
git clone https://devops.ingeniumcodex.com/andresgarcia0313/mdns-termux.git
cd mdns-termux
bash install.sh          # or --dry-run first
```

Then follow the printed reminders (append `bashrc.snippet`, set your
`TAILSCALE_DOMAIN`, optionally merge `ssh_config.example`).

## Usage

### `mdns` CLI

```bash
mdns resolve <host> [--family v4|v6] [--timeout 1.5] [--stale] [--no-cache]
mdns reverse <ip>                            # PTR: IP -> hostname
mdns browse [--type _svc._tcp.local.] [--json] [--list-types]
mdns browse --watch [--type ...] [--no-resolve]   # live monitor (avahi-browse -a)
mdns publish [--config PATH] [--hostname NAME]     # services from JSON
mdns publish --address <name> <ip>                 # bare A record (avahi-publish -a)
mdns cache list | clear [host]
```

Examples:

```bash
mdns browse --list-types                     # every service type on the LAN
mdns browse --type _ssh._tcp.local.          # every SSH host on the LAN
mdns resolve lenovo-ideapad                  # -> 192.168.1.106  (LAN, fast stdlib path)
mdns resolve lenovo-ideapad --family v6      # -> fe80::...      (via zeroconf)
mdns reverse 192.168.1.17                    # -> dell-latitude3400.local
mdns publish                                 # advertise services from ~/.config/mdns/services.json
mdns publish --address printer 192.168.1.50  # publish printer.local -> that IP
mdns browse --watch                          # live +/=/- events, all types
mdns cache list                              # inspect TTL-aware cache
```

### `resolve` (bash) — for any command

```bash
resolve lenovo                       # -> 100.69.236.16
ping   $(resolve hp62a)
curl   "http://$(resolve dell):8080"
kubectl --server="https://$(resolve master):6443" get nodes
```

Environment knobs:

- `TAILSCALE_DOMAIN` — defaults to `tailb0bb74.ts.net`. Set to your own tailnet.
- `MDNS_TIMEOUT` — seconds waiting for multicast (default `2`).
- `MDNS_CACHE_PATH` — override cache location (default `~/.cache/mdns/hosts.json`).

### Shell helpers (from `config/bashrc.snippet`)

```bash
alias r='resolve'
pingr <host>              # ping -c 3 to resolved IP
curlr <host> [flags]      # curl http://<resolved-ip>
sshr  <host> [cmd...]     # ssh via resolved IP with correct HostKeyAlias
whor  <ip>                # PTR reverse: IP -> hostname
mdns-types                # list all service types advertised on LAN
mdns-ssh / mdns-smb / mdns-http   # browse specific service types
mdns-cache / mdns-forget          # inspect / clear the resolver cache
```

### SSH by hostname (via `ProxyCommand`)

Drop `config/ssh_config.example` into `~/.ssh/config`. Then:

```bash
ssh lenovo                # resolves via mDNS then Tailscale, connects
ssh lenovo.local          # forces mDNS-first path
```

### Publishing this device on the LAN

Edit `~/.config/mdns/services.json` (the installer seeds a template), then:

```bash
mdns publish                                          # foreground
nohup mdns publish >>~/.cache/mdns.log 2>&1 & disown  # background
```

The publisher registers `<hostname>.local` (`_workstation._tcp`) plus every
service listed in the config. On Ctrl+C it sends proper mDNS goodbye packets.
Example `services.json`:

```json
{
  "hostname": "movil",
  "services": [
    {"type": "_ssh._tcp", "port": 8022, "name": "Termux SSH"},
    {"type": "_smb._tcp", "port": 4450, "name": "Termux SMB"}
  ]
}
```

## Behavior by network

| Scenario           | Path                                                 | Typical latency |
|--------------------|------------------------------------------------------|-----------------|
| On home LAN        | stdlib multicast → zeroconf (IPv6 or miss)           | < 1 s (IPv4)    |
| Mobile data / away | multicast times out → Tailscale MagicDNS             | < 2 s           |
| No Tailscale       | all fail → TTL-aware cache → legacy flat cache       | instant         |

Cache honors real mDNS TTL (default 30 – 3600 s) and stores IPv4 + IPv6
separately. `mdns resolve --stale <host>` accepts expired entries as a
last-resort fallback.

## RFC coverage (what's implemented)

| Feature                          | Status | Notes                                    |
|----------------------------------|:------:|------------------------------------------|
| A / AAAA hostname resolution     |   ✓    | stdlib for A, zeroconf for AAAA          |
| PTR reverse lookup               |   ✓    | stdlib multicast, IPv4 + IPv6            |
| DNS-SD service browsing          |   ✓    | `mdns browse`, all types or one          |
| Live service monitor             |   ✓    | `mdns browse --watch` (like avahi-browse)|
| Publisher (hostname + services)  |   ✓    | via `zeroconf`, with probing + goodbye   |
| Bare hostname->IP publish        |   ✓    | `mdns publish --address` (avahi-publish -a)|
| Name-conflict handling           |   ✓    | RFC 6762 probe + auto-rename             |
| Multicast group membership       |   ✓    | `IP_ADD_MEMBERSHIP` on all sockets       |
| Retries with backoff             |   ✓    | 2 attempts, split timeout                |
| TTL-honoring cache               |   ✓    | JSON at `~/.cache/mdns/hosts.json`       |
| Legacy unicast queries           |   —    | Not implemented (rarely needed)          |
| Known-answer suppression         |   —    | zeroconf handles internally              |

## Interoperability (Avahi / Bonjour)

Because publishing goes through `zeroconf` (a full RFC 6762 / 6763
implementation), anything you advertise is visible to standard mDNS stacks —
**no special client needed on the other end**.

Verified on a real LAN:

- **Termux → Avahi (Linux):** with `mdns publish` running on the phone, a Linux
  box with `nss-mdns` resolved `movil.local` and pinged it, with no extra config:
  ```
  $ getent hosts movil.local
  192.168.1.112   movil.local
  $ ping movil.local            # 64 bytes ... time=3.84 ms
  ```
- **Avahi → Termux:** `mdns browse` on the phone sees services published by
  Avahi/Bonjour hosts (SMB, SSH, `_workstation`, Chromecast, AirPlay, …).

So a Mac (Bonjour), a Linux box (Avahi), or a Windows machine (Bonjour Print
Services) will all discover and resolve what this tool publishes, and vice
versa.

## Gotchas found the hard way

- **`getent` doesn't exist on Termux.** Scripts that shell out to it silently
  fail. This repo uses `python -c "import socket; socket.gethostbyname(...)"`
  as the portable fallback on Bionic.
- **Shebang matters.** `#!/data/data/com.termux/files/usr/bin/bash` is required
  for `declare -A`. A stray backslash (`#\!`) makes the kernel fall through to
  `sh`, which rejects bash-only syntax with `Syntax error: "(" unexpected`.
- **mDNS off-LAN is a no-op.** Multicast (224.0.0.251) does not cross networks.
  Over mobile data, only Tailscale resolves.
- **`zeroconf` is not `pkg`-managed.** After restoring Termux from a `tar`
  backup, re-run `pip install zeroconf` or `mdns-publish.py` will silently die
  with `ModuleNotFoundError`.
- **Silent failures are the enemy.** The `.bashrc` snippet deliberately logs
  to `~/.cache/mdns.log` instead of `/dev/null`. Keep it that way.

## Tailscale on Termux

Tailscale runs as the **Android app**, not as a Termux CLI — `tailscale up`
inside a shell won't work. The app provides MagicDNS for
`*.<your-tailnet>.ts.net`, which Bionic-libc resolves fine (it's just a
regular DNS lookup).

## Related

- [claude-code-termux](https://devops.ingeniumcodex.com/andresgarcia0313/claude-code-termux)
  — Claude Code CLI setup for Termux. Uses this repo for SSH/host resolution.

## License

MIT — see `LICENSE`.