feat: trust forwarded client addresses only from configured proxies (rate limit, audit, SSE) [CI]

Docs: access.md covers the missing node-side Backend identity and the hash-first install flow.

backend/src/MoShiro.Backend.Api/Common/ClientAddress.cs

@@ -0,0 +1,80 @@
+using System.Net;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.DependencyInjection;
+using MoShiro.Backend.Application;
+
+namespace MoShiro.Backend.Api;
+
+/// <summary>
+/// 请求的真实来源地址。后端前面通常有反向代理（Cloudflare、frp、Caddy），套接字上看到的是最近一跳；
+/// 只有配置了 <see cref="BackendOptions.TrustedProxies"/>，并且连进来的正是其中之一，才相信转发头，
+/// 且从右往左跳过所有可信代理，取第一个不可信的地址（客户端自己伪造的在最左边，不会被采信）。
+/// 没配置就一律用套接字地址：宁可限流和审计里是代理的地址，也不让客户端自己报地址。
+/// </summary>
+public sealed class ClientAddressResolver
+{
+    private readonly IPNetwork[] _trusted;
+    private readonly string _header;
+
+    public ClientAddressResolver(BackendOptions options)
+    {
+        _header = string.IsNullOrWhiteSpace(options.TrustedProxyHeader) ? "X-Forwarded-For" : options.TrustedProxyHeader.Trim();
+        _trusted = [.. (options.TrustedProxies ?? "").Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries).Select(ParseNetwork)];
+    }
+
+    private static IPNetwork ParseNetwork(string text)
+    {
+        if (IPNetwork.TryParse(text, out IPNetwork network))
+        {
+            return network;
+        }
+
+        if (IPAddress.TryParse(text, out IPAddress? single))
+        {
+            return new IPNetwork(single, single.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork ? 32 : 128);
+        }
+
+        throw new InvalidOperationException($"TrustedProxies: '{text}' is not an IP address or CIDR range");
+    }
+
+    private bool IsTrusted(IPAddress address)
+    {
+        IPAddress normalized = address.IsIPv4MappedToIPv6 ? address.MapToIPv4() : address;
+        return _trusted.Any(n => n.Contains(normalized));
+    }
+
+    public string Resolve(HttpContext http)
+    {
+        IPAddress? remote = http.Connection.RemoteIpAddress;
+        if (remote is null)
+        {
+            return "unknown";
+        }
+
+        if (_trusted.Length == 0 || !IsTrusted(remote))
+        {
+            return remote.ToString();
+        }
+
+        string[] chain = [.. http.Request.Headers[_header].SelectMany(v => (v ?? "").Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))];
+        for (int i = chain.Length - 1; i >= 0; i--)
+        {
+            if (!IPAddress.TryParse(chain[i], out IPAddress? hop))
+            {
+                break; // 不是地址就不信这条头
+            }
+
+            if (!IsTrusted(hop))
+            {
+                return hop.ToString();
+            }
+        }
+
+        return remote.ToString();
+    }
+}
+
+public static class ClientAddressExtensions
+{
+    public static string ClientAddress(this HttpContext http) => http.RequestServices.GetRequiredService<ClientAddressResolver>().Resolve(http);
+}

backend/src/MoShiro.Backend.Api/Common/Guards.cs

@@ -109,7 +109,7 @@ public static class CapabilityGuard
             {
                 http.Items.TryGetValue(AuditEndpoints.DetailItem, out object? detail);
                 return http.RequestServices.GetRequiredService<AuditService>()
-                    .RecordAsync(principal, action, target, detail as string, http.Connection.RemoteIpAddress?.ToString(), http.Response.StatusCode < 400).AsTask();
+                    .RecordAsync(principal, action, target, detail as string, http.ClientAddress(), http.Response.StatusCode < 400).AsTask();
             });
         }