5 files changed, 103 insertions(+), 0 deletions(-)

A LICENCE
A README.md
A go.mod
A go.sum
A main.go

@@ 1,15 @@
+ISC License
+
+Copyright (c) 2021, Oliver Leaver-Smith
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

@@ 1,1 @@
+# DNS URL Shortener

@@ 1,8 @@
+module dns-short
+
+go 1.13
+
+require (
+	github.com/gorilla/mux v1.8.0
+	github.com/miekg/dns v1.1.40
+)

@@ 1,20 @@
+github.com/gorilla/mux v1.8.0 h1:i40aqfkR1h2SlN9hojwV5ZA91wcXFOvkdNIeFDP5koI=
+github.com/gorilla/mux v1.8.0/go.mod h1:DVbg23sWSpFRCP0SfiEN6jmj59UnW/n46BH5rLB71So=
+github.com/miekg/dns v1.1.40 h1:pyyPFfGMnciYUk/mXpKkVmeMQjfXqt3FAJ2hy7tPiLA=
+github.com/miekg/dns v1.1.40/go.mod h1:KNUDUusw/aVsxyTYZM1oqvCicbwhgbNgztCETuNZ7xM=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550 h1:ObdrDkeb4kJdCP557AjRjq69pTHfNouLtWZG7j9rPN8=
+golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
+golang.org/x/mod v0.1.1-0.20191105210325-c90efee705ee/go.mod h1:QqPTAvyqsEbceGzBzNggFXnrqF1CaUcvgkdR5Ot7KZg=
+golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20190923162816-aa69164e4478 h1:l5EDrHhldLYb3ZRHDUhXF7Om7MvYXnkV9/iQNo1lX6g=
+golang.org/x/net v0.0.0-20190923162816-aa69164e4478/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20190924154521-2837fb4f24fe h1:6fAMxZRR6sl1Uq8U61gxU+kPTs2tR8uOySCbBP7BN/M=
+golang.org/x/sys v0.0.0-20190924154521-2837fb4f24fe/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/tools v0.0.0-20191216052735-49a3e744a425/go.mod h1:TB2adYChydJhpapKDTa4BR/hXlZSLoq2Wpct/0txZ28=
+golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

@@ 1,59 @@
+package main
+
+import (
+	"context"
+	"log"
+	"net"
+	"net/http"
+	"net/url"
+	"os"
+	"time"
+
+	"github.com/gorilla/mux"
+)
+
+const timeout = 1 * time.Second
+
+var attemptForward = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	baseDomain := os.Getenv("BASE_DOMAIN")
+	vars := mux.Vars(r)
+	slug := vars["shortcode"]
+	domain := slug + "." + baseDomain
+	ctx, cancel := context.WithTimeout(context.TODO(), timeout)
+	defer cancel()
+	var res net.Resolver
+	txtRecords, err := res.LookupTXT(ctx, domain)
+	if err != nil {
+		log.Printf("No TXT records found: %s", err)
+		w.Write([]byte("No entry found for that short code"))
+		return
+	}
+	redirect := txtRecords[0]
+	if len(txtRecords) > 0 {
+		isUrl := IsUrl(redirect)
+		if !isUrl {
+			log.Printf("Provided forward (%s) is not a URL", redirect)
+			w.Write([]byte("No entry found for that short code"))
+			return
+		}
+		log.Printf("Matching entry found for %s, redirecting to %s", slug, redirect)
+		http.Redirect(w, r, redirect, 302)
+	} else {
+		log.Printf("No entry found for %s", slug)
+		w.Write([]byte("No entry found for that short code"))
+	}
+
+})
+
+func IsUrl(str string) bool {
+	// We do this because url.ParseRequestURI now says
+	// that a relative URL like /cabbage.html is legit
+	u, err := url.Parse(str)
+	return err == nil && u.Scheme != "" && u.Host != ""
+}
+
+func main() {
+	r := mux.NewRouter()
+	r.Handle("/{shortcode}", attemptForward).Methods("GET")
+	http.ListenAndServe(":"+os.Getenv("PORT"), r)
+}