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@@ -0,0 +1,78 @@
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+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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+// Use of this source code is governed by a BSD-style
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+// license that can be found in the LICENSE file.
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+
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+package resolver
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+
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+import (
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+ "fmt"
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+ "html"
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+ "net/http"
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+ "strconv"
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+ "sync"
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+ "sync/atomic"
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+ "time"
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+
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+ "tailscale.com/health"
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+)
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+
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+func init() {
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+ health.RegisterDebugHandler("dnsfwd", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
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+ n, _ := strconv.Atoi(r.FormValue("n"))
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+ if n <= 0 {
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+ n = 100
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+ } else if n > 10000 {
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+ n = 10000
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+ }
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+ fl, ok := fwdLogAtomic.Load().(*fwdLog)
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+ if !ok || n != len(fl.ent) {
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+ fl = &fwdLog{ent: make([]fwdLogEntry, n)}
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+ fwdLogAtomic.Store(fl)
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+ }
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+ fl.ServeHTTP(w, r)
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+ }))
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+}
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+
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+var fwdLogAtomic atomic.Value // of *fwdLog
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+
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+type fwdLog struct {
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+ mu sync.Mutex
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+ pos int // ent[pos] is next entry
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+ ent []fwdLogEntry
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+}
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+
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+type fwdLogEntry struct {
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+ Domain string
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+ Time time.Time
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+}
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+
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+func (fl *fwdLog) addName(name string) {
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+ if fl == nil {
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+ return
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+ }
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+ fl.mu.Lock()
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+ defer fl.mu.Unlock()
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+ if len(fl.ent) == 0 {
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+ return
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+ }
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+ fl.ent[fl.pos] = fwdLogEntry{Domain: name, Time: time.Now()}
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+ fl.pos++
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+ if fl.pos == len(fl.ent) {
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+ fl.pos = 0
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+ }
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+}
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+
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+func (fl *fwdLog) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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+ fl.mu.Lock()
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+ defer fl.mu.Unlock()
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+
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+ fmt.Fprintf(w, "<html><h1>DNS forwards</h1>")
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+ now := time.Now()
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+ for i := 0; i < len(fl.ent); i++ {
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+ ent := fl.ent[(i+fl.pos)%len(fl.ent)]
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+ if ent.Domain == "" {
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+ continue
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+ }
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+ fmt.Fprintf(w, "%v ago: %v<br>\n", now.Sub(ent.Time).Round(time.Second), html.EscapeString(ent.Domain))
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+ }
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+}
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Brad Fitzpatrick