Add proof of concept

.gitignore

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+.env

Dockerfile

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+FROM golang:1.10
+WORKDIR $GOPATH/src/git.k-space.ee/pinecrypt/goredns
+COPY main.go .
+RUN go get -d -v ./...
+RUN go install -v ./...
+CMD ["goredns"]

docker-compose.yml

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+version: '3.7'
+
+services:
+  goredns:
+    network_mode: host
+    build:
+      context: .
+    env_file: .env
+

main.go

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+package main
+
+import (
+    "context"
+    "fmt"
+    "log"
+    "os"
+    "strconv"
+    "strings"
+    "time"
+
+    "github.com/miekg/dns"
+    "go.mongodb.org/mongo-driver/bson"
+    "go.mongodb.org/mongo-driver/mongo"
+    "go.mongodb.org/mongo-driver/mongo/options"
+)
+
+type InventoryItem struct {
+    Ip []string
+}
+
+func wrapper(coll *mongo.Collection) func(dns.ResponseWriter, *dns.Msg) {
+    return func(w dns.ResponseWriter, r *dns.Msg) {
+        m := new(dns.Msg)
+        m.SetReply(r)
+        m.Compress = false
+        switch r.Opcode {
+        case dns.OpcodeQuery:
+            for _, q := range m.Question {
+                switch q.Qtype {
+                case dns.TypeA:
+                    log.Printf("Query for %s\n", q.Name[:len(q.Name)-1])
+                    cur, err := coll.Find(context.TODO(), bson.M{"hostname":q.Name[:len(q.Name)-1]})
+                    if err != nil {
+                        log.Fatal(err)
+                    }
+
+                    for cur.Next(context.TODO()) {
+                        var elem InventoryItem
+                        err := cur.Decode(&elem)
+                        if err != nil {
+                            log.Fatal(err)
+                        }
+
+                        for _, ip := range elem.Ip {
+                            tp := "A"
+                            if (strings.Contains(ip, ":")) {
+                                tp = "AAAA"
+                            }
+                            rr, err := dns.NewRR(fmt.Sprintf("%s %s %s",
+                              q.Name, tp, ip))
+                            if err == nil {
+                                m.Answer = append(m.Answer, rr)
+                            }
+                        }
+                    }
+                }
+            }
+        }
+        w.WriteMsg(m)
+    }
+}
+
+func main() {
+    client, err := mongo.NewClient(options.Client().ApplyURI(os.Getenv("MONGO_URI")))
+    if err != nil {
+        log.Fatal(err)
+    }
+    ctx, _ := context.WithTimeout(context.Background(), 10*time.Second)
+    err = client.Connect(ctx)
+    if err != nil {
+        log.Fatal(err)
+    }
+
+    coll := client.Database("kspace_accounting").Collection("inventory")
+    defer client.Disconnect(ctx)
+    dns.HandleFunc(".", wrapper(coll))
+    port := 5354
+    server := &dns.Server{Addr: ":" + strconv.Itoa(port), Net: "udp"}
+    log.Printf("Starting at %d\n", port)
+    err2 := server.ListenAndServe()
+    defer server.Shutdown()
+    if err2 != nil {
+        log.Fatalf("Failed to start server: %s\n ", err2.Error())
+    }
+}