Implement remote DNS

This commit implements remote DNS. It introduces two new dependencies:
ttlcache and dns.

Remote DNS implements a UDP listener DNS A record queries on port 53. It
replies with an unused IP address from an address pool, 198.18.0.0/15 by
default. When obtaining a new address from the pool, tun2socks needs to
memorize which name the address belongs to, so that when a client
connects to the address, it can instruct the proxy to connect to the
FQDN. To implement this IP to name mapping, the FakeIP module from clash
is used.

common/cache/lrucache.go

@@ -0,0 +1,223 @@
+package cache
+
+// Modified by https://github.com/die-net/lrucache
+
+import (
+	"container/list"
+	"sync"
+	"time"
+)
+
+// Option is part of Functional Options Pattern
+type Option func(*LruCache)
+
+// EvictCallback is used to get a callback when a cache entry is evicted
+type EvictCallback = func(key any, value any)
+
+// WithEvict set the evict callback
+func WithEvict(cb EvictCallback) Option {
+	return func(l *LruCache) {
+		l.onEvict = cb
+	}
+}
+
+// WithUpdateAgeOnGet update expires when Get element
+func WithUpdateAgeOnGet() Option {
+	return func(l *LruCache) {
+		l.updateAgeOnGet = true
+	}
+}
+
+// WithAge defined element max age (second)
+func WithAge(maxAge int64) Option {
+	return func(l *LruCache) {
+		l.maxAge = maxAge
+	}
+}
+
+// WithSize defined max length of LruCache
+func WithSize(maxSize int) Option {
+	return func(l *LruCache) {
+		l.maxSize = maxSize
+	}
+}
+
+// WithStale decide whether Stale return is enabled.
+// If this feature is enabled, element will not get Evicted according to `WithAge`.
+func WithStale(stale bool) Option {
+	return func(l *LruCache) {
+		l.staleReturn = stale
+	}
+}
+
+// LruCache is a thread-safe, in-memory lru-cache that evicts the
+// least recently used entries from memory when (if set) the entries are
+// older than maxAge (in seconds).  Use the New constructor to create one.
+type LruCache struct {
+	maxAge         int64
+	maxSize        int
+	mu             sync.Mutex
+	cache          map[any]*list.Element
+	lru            *list.List // Front is least-recent
+	updateAgeOnGet bool
+	staleReturn    bool
+	onEvict        EvictCallback
+}
+
+// New creates an LruCache
+func New(options ...Option) *LruCache {
+	lc := &LruCache{
+		lru:   list.New(),
+		cache: make(map[any]*list.Element),
+	}
+
+	for _, option := range options {
+		option(lc)
+	}
+
+	return lc
+}
+
+// Get returns the any representation of a cached response and a bool
+// set to true if the key was found.
+func (c *LruCache) Get(key any) (any, bool) {
+	entry := c.get(key)
+	if entry == nil {
+		return nil, false
+	}
+	value := entry.value
+
+	return value, true
+}
+
+// GetWithExpire returns the any representation of a cached response,
+// a time.Time Give expected expires,
+// and a bool set to true if the key was found.
+// This method will NOT check the maxAge of element and will NOT update the expires.
+func (c *LruCache) GetWithExpire(key any) (any, time.Time, bool) {
+	entry := c.get(key)
+	if entry == nil {
+		return nil, time.Time{}, false
+	}
+
+	return entry.value, time.Unix(entry.expires, 0), true
+}
+
+// Exist returns if key exist in cache but not put item to the head of linked list
+func (c *LruCache) Exist(key any) bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	_, ok := c.cache[key]
+	return ok
+}
+
+// Set stores the any representation of a response for a given key.
+func (c *LruCache) Set(key any, value any) {
+	expires := int64(0)
+	if c.maxAge > 0 {
+		expires = time.Now().Unix() + c.maxAge
+	}
+	c.SetWithExpire(key, value, time.Unix(expires, 0))
+}
+
+// SetWithExpire stores the any representation of a response for a given key and given expires.
+// The expires time will round to second.
+func (c *LruCache) SetWithExpire(key any, value any, expires time.Time) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if le, ok := c.cache[key]; ok {
+		c.lru.MoveToBack(le)
+		e := le.Value.(*entry)
+		e.value = value
+		e.expires = expires.Unix()
+	} else {
+		e := &entry{key: key, value: value, expires: expires.Unix()}
+		c.cache[key] = c.lru.PushBack(e)
+
+		if c.maxSize > 0 {
+			if len := c.lru.Len(); len > c.maxSize {
+				c.deleteElement(c.lru.Front())
+			}
+		}
+	}
+
+	c.maybeDeleteOldest()
+}
+
+// CloneTo clone and overwrite elements to another LruCache
+func (c *LruCache) CloneTo(n *LruCache) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
+	n.lru = list.New()
+	n.cache = make(map[any]*list.Element)
+
+	for e := c.lru.Front(); e != nil; e = e.Next() {
+		elm := e.Value.(*entry)
+		n.cache[elm.key] = n.lru.PushBack(elm)
+	}
+}
+
+func (c *LruCache) get(key any) *entry {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	le, ok := c.cache[key]
+	if !ok {
+		return nil
+	}
+
+	if !c.staleReturn && c.maxAge > 0 && le.Value.(*entry).expires <= time.Now().Unix() {
+		c.deleteElement(le)
+		c.maybeDeleteOldest()
+
+		return nil
+	}
+
+	c.lru.MoveToBack(le)
+	entry := le.Value.(*entry)
+	if c.maxAge > 0 && c.updateAgeOnGet {
+		entry.expires = time.Now().Unix() + c.maxAge
+	}
+	return entry
+}
+
+// Delete removes the value associated with a key.
+func (c *LruCache) Delete(key any) {
+	c.mu.Lock()
+
+	if le, ok := c.cache[key]; ok {
+		c.deleteElement(le)
+	}
+
+	c.mu.Unlock()
+}
+
+func (c *LruCache) maybeDeleteOldest() {
+	if !c.staleReturn && c.maxAge > 0 {
+		now := time.Now().Unix()
+		for le := c.lru.Front(); le != nil && le.Value.(*entry).expires <= now; le = c.lru.Front() {
+			c.deleteElement(le)
+		}
+	}
+}
+
+func (c *LruCache) deleteElement(le *list.Element) {
+	c.lru.Remove(le)
+	e := le.Value.(*entry)
+	delete(c.cache, e.key)
+	if c.onEvict != nil {
+		c.onEvict(e.key, e.value)
+	}
+}
+
+type entry struct {
+	key     any
+	value   any
+	expires int64
+}