463 lines
21 KiB
C#
463 lines
21 KiB
C#
using System.Security.Cryptography;
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using System.Text;
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namespace WpywMail.Native;
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/// <summary>
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/// DKIM 验签（RFC 6376）。**独立按 RFC 实现，不复用签名端代码** ——
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/// 2026-09-13 那次 DKIM 顺序 bug 的教训就是「自己验自己」会一起错。
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/// </summary>
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public static class DkimVerifier
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{
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public sealed record Result(string Outcome, string Domain, string Selector, string Detail);
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public static async Task<IReadOnlyList<Result>> VerifyAllAsync(byte[] raw, IDnsLookup dns, CancellationToken token)
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{
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var text = Encoding.Latin1.GetString(raw); // 1 字节 ↔ 1 字符，索引即字节偏移
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var headers = ParseHeaders(text);
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var results = new List<Result>();
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foreach (var header in headers.Where(h => h.Name.Equals("DKIM-Signature", StringComparison.OrdinalIgnoreCase)))
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{
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try { results.Add(await VerifyOneAsync(text, headers, header, dns, token)); }
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catch (Exception ex) when (ex is not OperationCanceledException)
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{
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results.Add(new Result("temperror", "", "", ex.Message));
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}
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}
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return results;
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}
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private sealed record Header(string Name, string Raw, string Value)
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{
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public bool Used { get; set; }
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}
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private static async Task<Result> VerifyOneAsync(string text, List<Header> headers, Header signature, IDnsLookup dns, CancellationToken token)
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{
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var tags = ParseTags(signature.Value);
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var domain = tags.GetValueOrDefault("d", "");
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var selector = tags.GetValueOrDefault("s", "");
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if (tags.GetValueOrDefault("v", "1") != "1") return new Result("fail", domain, selector, "v= 不是 1");
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if (domain.Length == 0 || selector.Length == 0) return new Result("fail", domain, selector, "缺 d= 或 s=");
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var algorithm = tags.GetValueOrDefault("a", "rsa-sha256").ToLowerInvariant();
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var hashName = algorithm switch
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{
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"rsa-sha256" => HashAlgorithmName.SHA256,
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"rsa-sha1" => HashAlgorithmName.SHA1,
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"ed25519-sha256" => HashAlgorithmName.SHA256,
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_ => default,
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};
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if (hashName == default) return new Result("fail", domain, selector, $"不支持的算法 a={algorithm}");
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if (algorithm.StartsWith("ed25519", StringComparison.Ordinal))
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return new Result("neutral", domain, selector, "ed25519 本实现不支持（极少见）");
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var canon = tags.GetValueOrDefault("c", "simple/simple").ToLowerInvariant().Split('/');
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var headerCanon = canon[0] is "relaxed" ? "relaxed" : "simple";
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var bodyCanon = canon.Length > 1 && canon[1] == "relaxed" ? "relaxed" : "simple";
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var body = ExtractBody(text);
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var canonicalBody = CanonicalizeBody(body, bodyCanon);
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if (tags.TryGetValue("l", out var lengthText) && int.TryParse(lengthText, out var limit) && limit >= 0 && limit < canonicalBody.Length)
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canonicalBody = canonicalBody[..limit];
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var expectedBodyHash = tags.GetValueOrDefault("bh", "");
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var actualBodyHash = Convert.ToBase64String(Hash(hashName, Encoding.Latin1.GetBytes(canonicalBody)));
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if (!string.Equals(expectedBodyHash, actualBodyHash, StringComparison.Ordinal))
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return new Result("fail", domain, selector, $"正文哈希不符（bh={Short(expectedBodyHash)} 实际={Short(actualBodyHash)}）");
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// 按 h= 的先后顺序取头，重名时从**下往上**取（RFC 6376 §5.4.2）
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var signedNames = tags.GetValueOrDefault("h", "").Split(':', StringSplitOptions.RemoveEmptyEntries);
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if (signedNames.Length == 0) return new Result("fail", domain, selector, "h= 为空");
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var builder = new StringBuilder();
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foreach (var name in signedNames)
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{
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var picked = headers.LastOrDefault(h => !h.Used && !ReferenceEquals(h, signature)
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&& h.Name.Equals(name.Trim(), StringComparison.OrdinalIgnoreCase));
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if (picked is not null) picked.Used = true;
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builder.Append(CanonicalizeHeader(picked?.Name ?? name.Trim(), picked?.Raw ?? "", headerCanon));
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}
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builder.Append(CanonicalizeHeader(signature.Name, StripSignatureValue(signature.Raw), headerCanon, trimCrLf: true));
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var signatureBytes = Convert.FromBase64String(tags.GetValueOrDefault("b", "").Trim());
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var data = Encoding.Latin1.GetBytes(builder.ToString());
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var keyText = await LookupKeyAsync(domain, selector, dns, token);
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if (keyText is null) return new Result("temperror", domain, selector, $"取不到公钥 {selector}._domainkey.{domain}");
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if (keyText.Length == 0) return new Result("fail", domain, selector, "公钥记录里 p= 为空（已吊销）");
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try
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{
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using var rsa = RSA.Create();
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rsa.ImportSubjectPublicKeyInfo(Convert.FromBase64String(keyText), out _);
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var ok = rsa.VerifyData(data, signatureBytes, hashName, RSASignaturePadding.Pkcs1);
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return ok
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? new Result("pass", domain, selector, $"a={algorithm} c={string.Join('/', canon)}")
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: new Result("fail", domain, selector, "签名不符（报文可能被改过）");
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}
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catch (FormatException)
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{
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return new Result("fail", domain, selector, "公钥不是合法的 SPKI base64");
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}
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catch (CryptographicException ex)
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{
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return new Result("fail", domain, selector, $"验签异常：{ex.Message}");
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}
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}
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private static async Task<string?> LookupKeyAsync(string domain, string selector, IDnsLookup dns, CancellationToken token)
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{
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var name = $"{selector}._domainkey.{domain}";
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var records = await dns.TxtAsync(name, token);
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foreach (var record in records)
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{
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var tags = ParseTags(record);
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if (!tags.ContainsKey("p")) continue;
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return tags["p"];
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}
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return null;
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}
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private static string ExtractBody(string text)
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{
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var crlf = text.IndexOf("\r\n\r\n", StringComparison.Ordinal);
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var lf = text.IndexOf("\n\n", StringComparison.Ordinal);
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var index = crlf >= 0 && (lf < 0 || crlf <= lf) ? crlf + 4 : lf >= 0 ? lf + 2 : -1;
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return index < 0 ? "" : text[index..];
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}
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private static List<Header> ParseHeaders(string text)
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{
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var list = new List<Header>();
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var block = text;
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var crlf = text.IndexOf("\r\n\r\n", StringComparison.Ordinal);
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var lf = text.IndexOf("\n\n", StringComparison.Ordinal);
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if (crlf >= 0 && (lf < 0 || crlf <= lf)) block = text[..crlf];
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else if (lf >= 0) block = text[..lf];
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var lines = block.Split('\n');
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StringBuilder? current = null;
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foreach (var rawLine in lines)
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{
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var line = rawLine.TrimEnd('\r');
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if (line.Length == 0) continue;
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if ((line[0] == ' ' || line[0] == '\t') && current is not null)
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{
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current.Append("\r\n").Append(line);
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continue;
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}
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if (current is not null) list.Add(Finish(current.ToString()));
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current = new StringBuilder(line);
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}
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if (current is not null) list.Add(Finish(current.ToString()));
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return list;
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static Header Finish(string raw)
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{
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var colon = raw.IndexOf(':');
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if (colon < 0) return new Header(raw.Trim(), raw, "");
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var name = raw[..colon].Trim();
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var value = raw[(colon + 1)..].Trim();
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return new Header(name, raw, value);
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}
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}
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/// <summary>
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/// 把 b= 的值抹掉（验签输入里的 DKIM-Signature 头不能带签名本身）。
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///
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/// ⚠ 必须**按标签边界**找 b=：直接 IndexOf("b=") 会被前面的 `bh=` 的 base64 内容误伤
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/// （base64 以 `b=` 结尾完全合法），一位之差就整封验不过 —— 自检里正是这一条抓出来的。
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/// </summary>
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private static string StripSignatureValue(string raw)
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{
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var index = TagValueStart(raw, "b");
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if (index < 0) return raw;
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var end = raw.IndexOf(';', index);
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var head = raw[..index];
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var tail = end < 0 ? "" : raw[end..];
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return head + tail;
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}
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/// <summary>返回名为 name 的标签「值」的起始下标（-1 表示没有）。标签必须出现在 `;` 之后或开头。</summary>
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private static int TagValueStart(string raw, string name)
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{
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var position = raw.IndexOf(':');
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position = position < 0 ? 0 : position + 1;
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while (position < raw.Length)
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{
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while (position < raw.Length && (raw[position] is ' ' or '\t' or '\r' or '\n' or ';')) position++;
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if (position >= raw.Length) return -1;
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var equals = raw.IndexOf('=', position);
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if (equals < 0) return -1;
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var tag = raw[position..equals].Trim();
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var semicolon = raw.IndexOf(';', equals);
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if (tag.Equals(name, StringComparison.OrdinalIgnoreCase)) return equals + 1;
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if (semicolon < 0) return -1;
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position = semicolon + 1;
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}
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return -1;
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}
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private static string CanonicalizeHeader(string name, string raw, string mode, bool trimCrLf = false)
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{
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string result;
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if (mode == "relaxed")
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{
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var colon = raw.IndexOf(':');
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var value = colon < 0 ? "" : raw[(colon + 1)..];
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var unfolded = value.Replace("\r\n", " ");
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var collapsed = Collapse(unfolded).TrimEnd(' ', '\t');
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result = name.ToLowerInvariant().Trim() + ":" + collapsed + "\r\n";
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}
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else
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{
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var text = raw.Length > 0 ? raw : name + ":";
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result = text + "\r\n";
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}
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return trimCrLf ? result[..^2] : result;
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}
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private static string CanonicalizeBody(string body, string mode)
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{
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if (body.Length == 0) return "";
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var lines = new List<string>();
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var start = 0;
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while (start <= body.Length)
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{
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var end = body.IndexOf('\n', start);
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var hasTerminator = end >= 0;
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var line = hasTerminator ? body[start..end].TrimEnd('\r') : body[start..];
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lines.Add(line);
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if (!hasTerminator) break;
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start = end + 1;
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}
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// 去掉末尾的空行（保留最后一行的行尾）
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while (lines.Count > 0 && lines[^1].Length == 0) lines.RemoveAt(lines.Count - 1);
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if (lines.Count == 0) return "";
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var builder = new StringBuilder();
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for (var i = 0; i < lines.Count; i++)
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{
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var line = lines[i];
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if (mode == "relaxed")
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{
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line = Collapse(line).TrimEnd(' ', '\t');
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}
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builder.Append(line).Append("\r\n");
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}
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return builder.ToString();
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}
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private static string Collapse(string value)
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{
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var builder = new StringBuilder(value.Length);
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var space = false;
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foreach (var c in value)
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{
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if (c is ' ' or '\t')
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{
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space = true;
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continue;
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}
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if (space && builder.Length > 0) builder.Append(' ');
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space = false;
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builder.Append(c);
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}
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return builder.ToString();
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}
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private static byte[] Hash(HashAlgorithmName name, byte[] data) =>
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name == HashAlgorithmName.SHA1 ? SHA1.HashData(data) : SHA256.HashData(data);
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private static string Short(string value) => value.Length <= 10 ? value : value[..10] + "…";
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/// <summary>解析 `k=v; k=v` 形式的标签（DKIM 签名头与 DNS 公钥记录共用）。</summary>
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public static Dictionary<string, string> ParseTags(string text)
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{
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var tags = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
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foreach (var part in text.Split(';'))
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{
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var equals = part.IndexOf('=');
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if (equals <= 0) continue;
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var key = part[..equals].Trim().ToLowerInvariant();
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var value = part[(equals + 1)..];
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// 值里的换行/空白都要去掉（b=、p= 常被折成多行）
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value = new string(value.Where(c => c is not (' ' or '\t' or '\r' or '\n')).ToArray());
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if (key.Length > 0) tags[key] = value;
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}
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return tags;
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}
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}
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/// <summary>DMARC 求值（RFC 7489 的常用子集）。</summary>
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public static class Dmarc
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{
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public sealed record Result(string Outcome, string Policy, string Domain, string Detail);
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/// <summary>公共后缀的常用子集（判断「组织域」用；没列到的按最后两段算）。</summary>
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private static readonly HashSet<string> MultiPartSuffixes = new(StringComparer.OrdinalIgnoreCase)
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{
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"co.uk", "org.uk", "me.uk", "ac.uk", "gov.uk", "co.jp", "ne.jp", "or.jp",
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"com.cn", "net.cn", "org.cn", "gov.cn", "edu.cn", "com.hk", "com.tw", "com.au",
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"com.br", "com.sg", "co.kr", "com.mx", "co.in", "com.tr",
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};
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/// <summary>组织域（relaxed 对齐用）：`mail.example.co.uk` → `example.co.uk`。</summary>
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public static string OrganizationalDomain(string domain)
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{
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var parts = (domain ?? "").Trim().TrimEnd('.').ToLowerInvariant().Split('.', StringSplitOptions.RemoveEmptyEntries);
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if (parts.Length <= 2) return string.Join('.', parts);
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var lastTwo = string.Join('.', parts[^2..]);
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return MultiPartSuffixes.Contains(lastTwo) && parts.Length >= 3
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? string.Join('.', parts[^3..])
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: lastTwo;
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}
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public static async Task<Result> EvaluateAsync(string fromDomain, (string Outcome, string Domain) spf,
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IReadOnlyList<(string Outcome, string Domain)> dkims, IDnsLookup dns, CancellationToken token)
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{
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fromDomain = (fromDomain ?? "").Trim().TrimEnd('.').ToLowerInvariant();
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if (fromDomain.Length == 0) return new Result("none", "none", "", "报文没有可用的 From 域名");
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var records = await dns.TxtAsync("_dmarc." + fromDomain, token);
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var record = records.FirstOrDefault(r => r.TrimStart().StartsWith("v=DMARC1", StringComparison.OrdinalIgnoreCase));
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if (record is null) return new Result("none", "none", fromDomain, $"_dmarc.{fromDomain} 没有 DMARC 记录");
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var tags = DkimVerifier.ParseTags(record);
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var policy = tags.GetValueOrDefault("p", "").ToLowerInvariant();
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if (policy is not ("none" or "quarantine" or "reject"))
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return new Result("none", "none", fromDomain, $"DMARC 记录里的 p= 无效：{policy}");
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var strictSpf = tags.GetValueOrDefault("aspf", "r").Equals("s", StringComparison.OrdinalIgnoreCase);
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var strictDkim = tags.GetValueOrDefault("adkim", "r").Equals("s", StringComparison.OrdinalIgnoreCase);
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bool Aligned(string candidate, bool strict)
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{
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if (candidate.Length == 0) return false;
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return strict
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? candidate.Equals(fromDomain, StringComparison.OrdinalIgnoreCase)
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: OrganizationalDomain(candidate) == OrganizationalDomain(fromDomain);
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}
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if (spf.Outcome == "pass" && Aligned(spf.Domain, strictSpf))
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return new Result("pass", policy, fromDomain, $"SPF 对齐通过（{spf.Domain}，aspf={(strictSpf ? "s" : "r")}）");
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foreach (var dkim in dkims.Where(d => d.Outcome == "pass"))
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if (Aligned(dkim.Domain, strictDkim))
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return new Result("pass", policy, fromDomain, $"DKIM 对齐通过（{dkim.Domain}，adkim={(strictDkim ? "s" : "r")}）");
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// 没有任何对齐的通过项 → 失败（temperror 时按 RFC 也不该直接判失败，这里如实标注）
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var temperror = dkims.Any(d => d.Outcome == "temperror") || spf.Outcome == "temperror";
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var detail = $"SPF={spf.Outcome}({spf.Domain}) DKIM=[{string.Join(",", dkims.Select(d => $"{d.Outcome}:{d.Domain}"))}] 与 From 域 {fromDomain} 无对齐";
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return new Result(temperror ? "temperror" : "fail", policy, fromDomain, detail);
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}
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}
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/// <summary>把三件事串起来：SPF → DKIM → DMARC，产出可写进报文的结论与是否判为垃圾。</summary>
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public static class InboundAuth
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{
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public static async Task<InboundAuthVerdict> CheckAsync(byte[] raw, string clientIp, string helo, string mailFrom,
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AppConfig config, CancellationToken token, IDnsLookup? dns = null)
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{
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dns ??= new UdpDnsLookup(config);
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var cfg = config.InboundAuth;
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var fromDomain = "";
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try
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{
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var parsed = Mime.Parse(raw);
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// 注意：服务端 Mime.Parse 的 From 是**字符串**（不是地址数组），要先用 Mime.Addresses 拆
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fromDomain = Spf.DomainOf(Mime.Addresses(parsed.From).FirstOrDefault() ?? parsed.From);
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}
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catch { /* 报文解析失败就按无 From 处理 */ }
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var spf = await Spf.EvaluateAsync(clientIp, helo, mailFrom, dns, cfg, token);
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var dkimResults = new List<(string Outcome, string Domain)>();
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var dkimDetail = "";
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if (cfg.VerifyDkim)
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{
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var verified = await DkimVerifier.VerifyAllAsync(raw, dns, token);
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foreach (var v in verified) dkimResults.Add((v.Outcome, v.Domain));
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var best = verified.OrderBy(v => v.Outcome == "pass" ? 0 : v.Outcome == "temperror" ? 1 : 2).FirstOrDefault();
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if (best is not null) dkimDetail = $"d={best.Domain} s={best.Selector} {best.Detail}";
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}
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var dmarc = await Dmarc.EvaluateAsync(fromDomain, (spf.Outcome, spf.Domain), dkimResults, dns, token);
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// ── 打分（0 = 干净；阈值默认 3）
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var score = 0;
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var reasons = new List<string>();
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switch (dmarc.Outcome)
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{
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case "fail":
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score += 4;
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reasons.Add($"DMARC 失败（p={dmarc.Policy}）：{dmarc.Detail}");
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if (dmarc.Policy == "reject") score += 2;
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break;
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case "none":
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if (spf.Outcome == "fail") { score += 2; reasons.Add("SPF 硬失败且该域没有 DMARC 记录"); }
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else if (spf.Outcome == "softfail") { score += 1; reasons.Add("SPF 软失败"); }
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else if (spf.Outcome == "permerror") { score += 1; reasons.Add("SPF 记录有错（permerror）"); }
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var anyDkim = dkimResults.Count > 0;
|
||
if (anyDkim && !dkimResults.Any(d => d.Outcome == "pass")) { score += 1; reasons.Add("带了 DKIM 签名但验不过"); }
|
||
break;
|
||
}
|
||
if (spf.Outcome is "none" && dkimResults.Count == 0 && dmarc.Outcome == "none")
|
||
reasons.Add("既没有 SPF 也没有 DKIM（小发件人常见，仅作提示）");
|
||
|
||
var hardFail = dmarc.Outcome == "fail" || score >= Math.Max(1, cfg.SpamScoreThreshold);
|
||
var spam = hardFail && cfg.SpamFolderOnFail;
|
||
var reject = dmarc.Outcome == "fail" && dmarc.Policy == "reject" && cfg.RejectOnDmarcReject;
|
||
|
||
var verdict = new InboundAuthVerdict
|
||
{
|
||
Spf = spf.Outcome,
|
||
SpfDomain = spf.Domain,
|
||
SpfDetail = spf.Detail,
|
||
Dkim = dkimResults.Count == 0 ? "none"
|
||
: dkimResults.Any(d => d.Outcome == "pass") ? "pass"
|
||
: dkimResults.Any(d => d.Outcome == "temperror") ? "temperror" : "fail",
|
||
DkimDomain = dkimResults.FirstOrDefault().Domain ?? "",
|
||
DkimDetail = dkimDetail,
|
||
Dmarc = dmarc.Outcome,
|
||
DmarcDomain = dmarc.Domain,
|
||
DmarcPolicy = dmarc.Policy,
|
||
Score = score,
|
||
Reasons = reasons.ToArray(),
|
||
Spam = spam,
|
||
Reject = reject,
|
||
};
|
||
return cfg.AddAuthenticationResults ? verdict with { HeaderBlock = BuildHeader(verdict, config) } : verdict;
|
||
}
|
||
|
||
private static string BuildHeader(InboundAuthVerdict v, AppConfig config)
|
||
{
|
||
var builder = new StringBuilder();
|
||
builder.Append("Authentication-Results: ").Append(config.Hostname).Append(";\r\n");
|
||
builder.Append("\tspf=").Append(v.Spf);
|
||
if (v.SpfDomain.Length > 0) builder.Append(" smtp.mailfrom=").Append(v.SpfDomain);
|
||
builder.Append(";\r\n");
|
||
builder.Append("\tdkim=").Append(v.Dkim);
|
||
if (v.DkimDomain.Length > 0) builder.Append(" header.d=").Append(v.DkimDomain);
|
||
builder.Append(";\r\n");
|
||
builder.Append("\tdmarc=").Append(v.Dmarc);
|
||
if (v.DmarcDomain.Length > 0) builder.Append(" header.from=").Append(v.DmarcDomain);
|
||
if (v.DmarcPolicy != "none") builder.Append(" policy=").Append(v.DmarcPolicy);
|
||
builder.Append("\r\n");
|
||
builder.Append("X-Spam-Score: ").Append(v.Score).Append("\r\n");
|
||
if (v.Reasons.Length > 0)
|
||
builder.Append("X-Spam-Reason: ").Append(string.Join(" / ", v.Reasons)).Append("\r\n");
|
||
return builder.ToString();
|
||
}
|
||
|
||
/// <summary>把校验收到的头前置到报文最前面（不碰原有字节，避免破坏对方 DKIM 签名）。</summary>
|
||
public static byte[] PrependHeaders(byte[] raw, string headerBlock)
|
||
{
|
||
if (string.IsNullOrEmpty(headerBlock)) return raw;
|
||
var prefix = Encoding.Latin1.GetBytes(headerBlock);
|
||
var output = new byte[prefix.Length + raw.Length];
|
||
Buffer.BlockCopy(prefix, 0, output, 0, prefix.Length);
|
||
Buffer.BlockCopy(raw, 0, output, prefix.Length, raw.Length);
|
||
return output;
|
||
}
|
||
}
|