域名黑名單(DNSBL)與垃圾訊息(Spam)檢查

dnsbl.cs

// 
// Copyright 2022 Orlys Ma
// 
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// to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
// 
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// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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namespace AntiSpam
{
    using DnsClient;
    using DnsClient.Protocol;

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Net;
    using System.Text.RegularExpressions;
    using System.Threading.Tasks;

    /// <summary>
    /// <see href="https://www.spamhaus.org/faq/section/DNSBL%20Usage">Spamhaus' DBL</see> 服務。
    /// </summary> 
    public class DomainSpamhausService
    {
        private readonly LookupClient _lookupClient = new LookupClient(/*上層伺服器不可為公用名稱伺服器, 例如 1.1.1.1 及 8.8.8.8 */);

        /// <summary>
        /// 域名是否存在於域名黑名單(DNSBL)中。
        /// </summary>
        /// <param name="domain"></param>
        /// <returns></returns>
        public async Task<bool> IsBlacklisted(string domain)
        {
            // Spamhaus DNSBL 服務
            var response = await _lookupClient
                .QueryAsync(domain + ".dbl.spamhaus.org", QueryType.A)
                .ConfigureAwait(false)
                ;

            var answer = response.Answers.OfType<ARecord>().FirstOrDefault();
            if (answer is null)
            {
                // 找不到視同通過黑名單。
                return false;
            }

            var address = answer.Address;

            return s_blacklistedAddresses.Contains(address);
        }

        private static HashSet<IPAddress> s_blacklistedAddresses = new(){
            IPAddress.Parse("127.0.1.2"),
            IPAddress.Parse("127.0.1.4"),
            IPAddress.Parse("127.0.1.5"),
            IPAddress.Parse("127.0.1.6"),
            IPAddress.Parse("127.0.1.102"),
            IPAddress.Parse("127.0.1.103"),
            IPAddress.Parse("127.0.1.104"),
            IPAddress.Parse("127.0.1.105"),
            IPAddress.Parse("127.0.1.106"),
        };
    }

    /// <summary>
    /// 反垃圾訊息服務
    /// </summary>
    public class AntiSpamService
    {
        private readonly DomainSpamhausService _dnsblService = new();
        private static readonly Regex s_urlPattern = new(
            @"(https?:\/\/)?(?<LINK>[\-a-z0-9@:%\/\._+~#=]+)",
                RegexOptions.Compiled | RegexOptions.IgnoreCase);

        /// <summary>
        /// 判斷文字內容是否為垃圾訊息。
        /// </summary>
        /// <param name="rawContent"></param>
        /// <returns></returns>
        public async Task<bool> IsSpam(string rawContent)
        {
            var m = s_urlPattern.Match(rawContent);

            // 不包含連結直接當作有效內容
            if (!m.Success)
                return false;

            // 取得連結
            var roughUri = m.Groups["LINK"].Value;

            // 從連結中取得 FQDN
            var uriBuilder = new UriBuilder(roughUri);
            var fqdn = uriBuilder.Host;

            // 驗證連結是否存在於 DNSBL
            return await _dnsblService
                .IsBlacklisted(fqdn)
                .ConfigureAwait(false);
        }
    }

}

使用範例

var content = """
    這是一封詐騙訊息，內含可以賺錢的連結。https://go.diryjyaz.com/30678887
    """
    ;

var antiSpamService = new AntiSpamService();
var isSpam = await antiSpamService.IsSpam(content);
Console.WriteLine(content);
Console.WriteLine("is spam? " + (isSpam ? "yes" : "no"));