# Package Contracts: dns-helper

**Feature Branch**: `001-safety-reliability-refactor`  
**Date**: 2026-03-03  

## Package: `resolver`

Handles DNS name resolution against a user-specified DNS server.

### Interface

```go
// Resolver performs DNS lookups against a specific server.
type Resolver interface {
    // LookupIP resolves a hostname to a list of IPv4 addresses using the
    // specified DNS server. Follows CNAME chains up to a maximum depth.
    // Returns deduplicated IPs. Returns an error if the hostname cannot
    // be resolved (NXDOMAIN, timeout, server unreachable, etc.).
    LookupIP(hostname, server string) ([]string, error)
}
```

### Behavior Contract

- Queries are sent over UDP to `<server>:53`.
- The `hostname` is automatically converted to FQDN (trailing dot appended if missing).
- An A query is sent first. If A records are returned, they are collected.
- If only CNAME records are returned (no A records in the answer), the CNAME target is resolved recursively.
- CNAME chain depth is limited to 10 levels. Exceeding this returns an error.
- Results are deduplicated before return.
- Timeout: 5 seconds per query.
- On NXDOMAIN: returns `error` with descriptive message.
- On server unreachable/timeout: returns `error` with descriptive message.
- Response ID must match query ID; mismatches are treated as errors.
- Truncated responses (TC flag set) are treated as errors (no TCP fallback for simplicity).

---

## Package: `hostfile`

Manages reading, parsing, modifying, and atomically writing the hosts file.

### Interface

```go
// Manager handles hosts file operations.
type Manager interface {
    // Read reads and parses the hosts file at the given path.
    // Returns an error if the file cannot be read or the managed block is corrupt.
    Read(path string) (*HostsFile, error)

    // Write atomically writes the hosts file content to the given path.
    // Creates a backup before writing. Uses temp-file-and-rename pattern.
    // backupDir is the directory for the backup file (typically the exe directory).
    Write(hf *HostsFile, backupDir string) error
}

// HostsFile represents the parsed content of a hosts file.
type HostsFile struct {
    Path            string
    OriginalContent []string
    PrefixContent   []string
    ManagedContent  []string
    PostfixContent  []string
    HasManagedBlock bool
}
```

### Sub-package: Managed Block Operations

```go
// AddEntries adds DNS entries to the managed content, removing any
// existing entries for the same hostnames first. Returns updated content.
func AddEntries(existing []string, entries []DNSEntry) []string

// RemoveByHostname removes all managed entries matching any of the
// specified hostnames. Returns updated content.
func RemoveByHostname(existing []string, hostnames []string) []string

// RemoveAll returns an empty slice (clears all managed entries).
func RemoveAll() []string
```

### Behavior Contract — Read

- Opens the file at `path` and reads all lines.
- Scans for start marker (`# DNSHelper <<-> START CONFIG`) and end marker (`# DNSHelper <->> END CONFIG`).
- If neither marker found: `HasManagedBlock = false`, all content goes to `PrefixContent`.
- If both markers found in correct order: splits content into Prefix, Managed, Postfix.
- **Corruption detection** (returns error):
  - Start marker without end marker.
  - End marker without start marker.
  - End marker before start marker.
  - Duplicate start or end markers.
- Uses sentinel values (not zero-index) for marker detection.

### Behavior Contract — Write

1. Creates backup: copies current hosts file to `<backupDir>/hosts.bak.<YYYYMMDD>-<4hex>`.
2. Assembles full file content: prefix + (managed block if non-empty) + postfix.
3. Compares assembled content with original — skips write if identical.
4. Creates temp file in same directory as hosts file: `.dns-helper-tmp-*`.
5. Writes content to temp file.
6. Sets temp file permissions to match original hosts file permissions.
7. Calls `Sync()` on temp file.
8. Closes temp file.
9. Calls `os.Rename(temp, hostsPath)`.
10. On rename success: deletes backup, returns nil.
11. On rename failure (including Windows retry 2-3x with 200ms delay): cleans up temp file, returns error. Backup is cleaned up after confirming original is intact.

---

## Package: `platform`

Provides platform-specific hosts file location.

### Interface

```go
// GetHostsFilePath returns the absolute path to the system hosts file.
// Returns an error if the file does not exist or cannot be accessed.
func GetHostsFilePath() (string, error)
```

### Platform Implementations

| Platform | Path | Detection |
|----------|------|-----------|
| Windows | `%SystemRoot%\System32\drivers\etc\hosts` (fallback: `C:\Windows\...`) | `os.LookupEnv("SystemRoot")` |
| Linux | `/etc/hosts` | Hardcoded |
| macOS | `/etc/hosts` | Hardcoded |

### Behavior Contract

- Returns error to caller on file-not-found or access error.
- **Must NOT** call `os.Exit()`, `log.Fatal()`, or any process-terminating function.
- Error messages include the path that was checked.

---

## Package: `lockfile`

Serializes concurrent access to the hosts file.

### Interface

```go
// Lock represents an acquired lock.
type Lock interface {
    // Release deletes the lock file. Safe to call multiple times.
    Release() error
}

// Acquire attempts to acquire a lock file in the specified directory.
// Retries for up to maxWait on contention. Breaks stale locks older
// than staleTimeout. Returns an error if the lock cannot be acquired.
func Acquire(dir string) (Lock, error)
```

### Behavior Contract

- Lock file path: `<dir>/.dns-helper.lock`.
- Created atomically via `os.OpenFile` with `O_CREATE|O_EXCL`.
- PID written to lock file for debugging (not used for staleness).
- Staleness: lock file with `ModTime` older than 2 minutes is considered stale and removed.
- Retry: up to 2 seconds total, 200ms between attempts.
- Release: `os.Remove(lockPath)`. Idempotent (no error if already deleted).
- Lock acquired **after** DNS resolution, released **after** file write (or on error).
