dnshelper/README.md

# ekekDNSHelper

A cross-platform CLI tool that resolves hostnames against DNS servers and writes the resulting IP-to-hostname mappings into the local hosts file. It manages a clearly delimited block within the hosts file so it can cleanly add and remove only its own entries.

By default, ekdns performs **smart resolution**: it discovers the authoritative nameserver for each hostname through a parallel NS fan-out across local and public resolvers, then queries that NS directly for the freshest answer. Internal hostnames (without zone delegation) fall back to a parallel A-record query across all resolvers.

Built as a single static binary with no runtime dependencies.

**Copyright (c) 2024 Emberkom LLC**

## Features

- Smart default resolution: parallel NS fan-out → authoritative query → parallel A fallback (no `-server` flag needed for most use cases)
- `-server local`: query only locally configured DNS resolvers (for internal hostnames)
- `-server gateway`: query the default gateway as a DNS server
- `-server <ip>[:<port>]`: query a specific DNS server (existing explicit mode)
- Split-horizon conflict detection: warns when the authoritative answer differs from what local resolvers say
- CNAME chain following (up to 10 hops) with loop detection
- Context-aware error messages with private TLD hints (`.local`, `.corp`, `.internal`, etc.)
- Per-query timeout control via `-timeout`
- Verbose per-stage resolution trace via `-verbose`
- Managed block markers ensure only ekdns's entries are ever modified
- Automatic backup of the hosts file before every write
- File-based lock prevents concurrent modifications
- Cross-platform: Windows, macOS, and Linux

## Requirements

- **Go 1.20** toolchain (this project must not use Go 1.21+ features)
- Administrator / root privileges (required to modify the system hosts file)

## Installation

### Build from source

```powershell
# Clone the repository
git clone <repo-url>
cd ekdns

# Build for the current platform
.\build.ps1

# Or build optimized release binaries for all platforms
.\build.ps1 -Release -CrossCompile
```

The default build output is `build\ekdns.exe` (Windows). Cross-compiled binaries are named `build\ekdns-<os>-<arch>[.exe]`.

### Release packages

```powershell
# Build release binaries and create distributable archives
.\build.ps1 -Release -CrossCompile -Package
```

Archives are written to `releases\ekdns-<version>-<os>-<arch>.zip` (Windows) or `.tar.gz` (macOS/Linux).

## Usage

> **Note:** ekdns must be run with elevated privileges (Run as Administrator on Windows, `sudo` on macOS/Linux) because it modifies the system hosts file.

### Add entries

Resolve hostnames using smart default resolution and add to hosts file:

```sh
ekdns add -host www.example.com
```

Specify a resolution mode:

```sh
# Use only local DNS resolvers (no public fallback)
ekdns add -host internal.corp -server local

# Use the default gateway as DNS
ekdns add -host www.example.com -server gateway

# Use a specific DNS server
ekdns add -host www.example.com -server 8.8.8.8

# Use a specific DNS server on a non-standard port
ekdns add -host www.example.com -server 10.0.0.53:5353
```

Add multiple hostnames at once:

```sh
ekdns add -host "host1.example.com,host2.example.com"
```

Use timeout and verbose options:

```sh
ekdns add -host www.example.com -timeout 5 -verbose
```

### Delete entries

Remove a specific hostname:

```sh
ekdns delete -host hostname.example.com
```

Remove multiple hostnames:

```sh
ekdns delete -host "host1.example.com,host2.example.com"
```

Remove **all** managed entries:

```sh
ekdns delete all
```

Short aliases: `a` for `add`, `d` / `del` for `delete`

### Command reference

| Command | Flag | Required | Default | Description |
|---------|------|----------|---------|-------------|
| `add` | `-host` | Yes | — | Comma-separated hostnames to resolve and add |
| `add` | `-server` | No | smart default | Resolution mode: omit, `local`, `gateway`, `<ip>`, or `<ip>:<port>` |
| `add` | `-timeout` | No | 3 | Per-query DNS timeout in seconds |
| `add` | `-verbose` | No | false | Emit per-stage resolution trace to stderr |
| `delete` | `-host` | Yes (unless `all`) | — | Comma-separated hostnames to remove |
| `delete all` | — | — | — | Removes every entry ekdns has written |

### Exit codes

| Code | Meaning |
|------|---------|
| 0 | Success |
| 1 | Error or partial failure (e.g. one hostname failed to resolve) |

## Resolution Modes

### Smart Default (no `-server` flag)

When `-server` is omitted, ekdns performs multi-stage authoritative resolution:

1. **Discover** local DNS resolvers and default gateway from OS network configuration.
2. **Build resolver pool**: local resolvers + hardcoded bootstrap set (1.1.1.1, 8.8.8.8, etc.), deduplicated.
3. **Stage 1 — NS Fan-out**: Query NS records for every label level of the hostname across all resolvers simultaneously (e.g., `www.example.com` and `example.com` × all resolvers in parallel).
4. **Select authority**: The most-specific zone with NS records wins.
5. **Stage 2 — Authoritative Query**: Resolve the NS hostname to an IP, then send a non-recursive A query directly to that nameserver.
6. **Stage 2.5 — Split-Horizon Check**: Concurrently with Stage 2, query local resolvers for the same hostname. If local and authoritative answers differ, report a conflict error — neither IP is written.
7. **CNAME Following**: If Stage 2 returns a CNAME, restart from Stage 1 for the target (max 10 hops).
8. **Stage 3 — Parallel A Fallback**: If no NS records were found at any level (internal hosts without zone delegation), send A queries to all resolvers simultaneously and take the first successful response.

```sh
# Example: smart default resolution with verbose trace
ekdns add -host www.example.com -verbose
```

Verbose output:
```
[dns] Resolver pool: [10.26.1.1, 1.1.1.1, 8.8.8.8, 1.0.0.1, 8.8.4.4, 9.9.9.9, 208.67.222.222]
[dns] Stage 1: NS fan-out for www.example.com (2 levels × 7 resolvers = 14 queries)
[dns]   example.com NS: ns1.example.com., ns2.example.com. (via 8.8.8.8, 1.1.1.1)
[dns]   www.example.com NS: (none)
[dns] Selected authority: example.com → ns1.example.com., ns2.example.com.
[dns] Stage 2: Querying ns1.example.com. (93.184.216.34) for www.example.com A (RD=0)
[dns] Result: 93.184.216.34
```

### Local Mode (`-server local`)

Queries only the locally configured DNS resolvers (from OS network configuration) in priority order. No fallback to public resolvers.

```sh
ekdns add -host internal.client.local -server local
```

Fails with an error if all local resolvers are unreachable or no local resolvers are found.

### Gateway Mode (`-server gateway`)

Discovers the default gateway IP and uses it as the DNS server. No fallback.

```sh
ekdns add -host www.example.com -server gateway
```

### Explicit Server (`-server <ip>[:<port>]`)

Queries the specified DNS server directly. Port defaults to 53 if omitted.

```sh
ekdns add -host www.example.com -server 8.8.8.8
ekdns add -host www.example.com -server 10.0.0.53:5353
```

## Error Messages

### Private TLD hint

When a hostname with a private TLD (`.local`, `.corp`, `.internal`, `.lan`, `.home`, `.private`) cannot be resolved:

```
Error: failed to resolve myservice.client.local: ...
  Hint: the hostname uses a private TLD (.local). Try specifying an internal DNS server:
        ekdns add -host myservice.client.local -server <internal-dns-ip>
```

### Split-horizon conflict

When the authoritative answer differs from the local resolver's answer:

```
Error: conflicting DNS answers for app.acme.com
  Authoritative (ns1.acme.com): 203.0.113.50
  Local resolver (10.26.1.1):   10.0.5.100
  The hostname resolves to different IPs depending on the DNS source.
  Use -server local to trust your internal DNS, or -server <ip> to choose explicitly.
```

### CNAME loop

```
Error: CNAME chain depth exceeded for www.example.com (max 10 hops): probable CNAME loop or misconfigured zone
```

## How it works

ekdns wraps its entries between two marker lines in the hosts file:

```
# ekDNSHelper <<-> START CONFIG
192.0.2.1	hostname.example.com
# ekDNSHelper <->> END CONFIG
```

Only lines within this managed block are ever modified or removed. The rest of the hosts file is preserved exactly as-is.

Before every write, a backup of the hosts file is saved to the directory containing the ekdns executable.

### Hosts file locations

| Platform | Path |
|----------|------|
| Windows | `%SystemRoot%\System32\drivers\etc\hosts` |
| macOS | `/etc/hosts` |
| Linux | `/etc/hosts` |

### Network discovery (per platform)

When using smart default, local, or gateway modes, ekdns discovers your network configuration via OS utilities:

| Platform | Method |
|----------|--------|
| Windows | `netsh interface ipv4 show route` + `netsh interface ipv4 show dnsservers` |
| Linux | `ip route show default` + `/etc/resolv.conf` (with `resolvectl` fallback for systemd-resolved) |
| macOS | `route -n get default` + `scutil --dns` |

## Project structure

```
main.go              CLI entry point (add / delete commands, flag parsing)
build.ps1            Build & packaging script
go.mod               Module definition (Go 1.20, golang.org/x/net)
hostfile/
  hostfile.go        Hosts file parsing, DNSEntry type, add/remove helpers
  managed.go         Managed block read/write logic
  filesystem.go      FileSystem interface + OSFileSystem implementation
  hostfile_test.go
  managed_test.go
lockfile/
  lockfile.go        File-based mutex to prevent concurrent modifications
  lockfile_test.go
platform/
  platform.go             Package doc — GetHostsFilePath()
  platform_windows.go     Windows: GetHostsFilePath()
  platform_darwin.go      macOS: GetHostsFilePath()
  platform_linux.go       Linux: GetHostsFilePath()
  network.go              NetworkInfo type, NetworkDiscoverer interface, FakeNetworkDiscoverer
  network_windows.go      Windows: WindowsNetworkDiscoverer (netsh)
  network_darwin.go       macOS: DarwinNetworkDiscoverer (route + scutil)
  network_linux.go        Linux: LinuxNetworkDiscoverer (ip route + resolv.conf)
  discoverer_windows.go   Windows: NewNetworkDiscoverer() constructor
  discoverer_darwin.go    macOS: NewNetworkDiscoverer() constructor
  discoverer_linux.go     Linux: NewNetworkDiscoverer() constructor
  platform_test.go
  network_test.go
resolver/
  resolver.go        Existing single-server A-record lookup (used internally)
  parse.go           ServerMode, QueryConfig, ParseServerFlag(), ExtractLabelLevels()
  parse_test.go
  transport.go       Shared UDP query transport (UDPQuery)
  transport_test.go
  pool.go            BuildResolverPool() — constructs resolver list per mode
  pool_test.go
  authority.go       ParallelNSFanOut(), SelectAuthoritativeNS(), QueryAuthoritative()
  authority_test.go
  fallback.go        ParallelAFallback() — Stage 3 parallel A fallback
  fallback_test.go
  splithorizon.go    CheckSplitHorizon() — Stage 2.5 cross-check
  splithorizon_test.go
  modes.go           Resolve() dispatcher and mode implementations
  modes_test.go
  resolver_test.go
```

## Development

### Running tests

```sh
go1.20 test ./...
```

### Running tests with coverage

```sh
go1.20 test -cover ./...
```

### Vetting code

```sh
go1.20 vet ./...
```

## Dependencies

- [`golang.org/x/net`](https://pkg.go.dev/golang.org/x/net) — DNS message parsing (`golang.org/x/net/dns/dnsmessage`)

No other third-party dependencies. Standard library first.

## License

Copyright (c) 2024 Emberkom LLC. All rights reserved.