add DNS checker and run against suspicious hosts

external/cmd/signup/main.go

@@ -3,11 +3,15 @@ package main
 import (
 	"bytes"
 	"database/sql"
+	"errors"
 	"fmt"
 	"io"
 	"log"
+	"net"
 	"os"
 	"path"
+	"regexp"
+	"slices"
 	"strings"
 	"time"
 
@@ -110,6 +114,46 @@ func (c *character) Say(msg string) string {
 		strings.TrimSpace(msg))
 }
 
+// TODO: move this into an admin-editable world-unreadable file somewhere
+var suspiciousHosts = []string{
+	"mx1.cock.li",
+	"mx2.cock.li",
+}
+
+var ErrNoSuchDomain = errors.New("no host found for email address")
+var ErrNoSuchMailserver = errors.New("no mail server found for email address")
+
+// DigMX does some grubbing around to attempt to find valid email hosts, and
+// then runs then through [net.LookupMX] and returns their mailserver domains.
+// may return [ErrNoSuchDomain] or [ErrNoSuchMailserver].
+func DigMX(raw string) (domains []string, err error) {
+	re := regexp.MustCompile(`@[A-Za-z0-9_-]+(\.[A-Za-z0-9_-]+)+\b`) // good enough
+	candidates := re.FindAllString(raw, -1)
+
+	// the error checking tries to be very generous: if anything comes up
+	// positive we will throw no errors and just assume the rest was a fluke.
+	ok := false
+	for _, host := range candidates {
+		records, e := net.LookupMX(host[1:])
+		if e != nil {
+			err = ErrNoSuchDomain
+		}
+		if len(records) == 0 {
+			err = ErrNoSuchMailserver
+		} else {
+			ok = true
+			for _, record := range records {
+				domains = append(domains, record.Host)
+			}
+		}
+	}
+
+	if ok {
+		return domains, nil
+	}
+	return
+}
+
 func main() {
 	logFile := path.Join(logDir, fmt.Sprintf("%d", time.Now().Unix()))
 	logF, err := os.Create(logFile)
@@ -201,9 +245,23 @@ func _main(l *log.Logger, db *sql.DB) error {
 			`),
 			"i'm sorry, before going further could you share an email with me?",
 			newCharacter("wire guy", "a lil homonculus made of discarded computer cables"),
-			func(s *scene) { su.Email = string(s.Input.Bytes()) },
+			func(s *scene) {
+				su.Email = string(s.Input.Bytes())
+				if records, err := DigMX(su.Email); err != nil {
+					for _, record := range records {
+						if slices.Contains(suspiciousHosts, record) {
+							su.Notes = append(su.Notes, models.SignupNote{
+								Author:   "dns",
+								Content:  fmt.Sprintf("email address has suspicious host %s", record),
+								SignupID: su.ID,
+							})
+						}
+					}
+				}
+			},
 			func(s *scene, tv *tview.TextView, msg string) {
 				// TODO could check and see if it's email shaped and admonish if not
+				// NOTE(nbsp): DigMX call can see if email is invalid but this isn't used yet
 				trimmed := strings.TrimSpace(msg)
 				fmt.Fprintln(tv, s.Host.Say(fmt.Sprintf("I heard '%s'. Is that right? if so, /nod", trimmed)))
 			}),