Every outbound federation request goes through one guarded client

Infrastructure/Http adds the client the roadmap's S3 and S4 ask for:
- the connect callback resolves the name itself and refuses loopback,
  private, link-local, CGNAT, documentation, multicast, ULA, NAT64, 6to4,
  Teredo and IPv4-mapped/compatible forms, then connects to the vetted
  address, so DNS rebinding cannot swap it afterwards;
- redirects are followed by hand, at most three, each one re-checked;
- bodies are capped at 1 MB after decompression, only JSON media types are
  read, every request has a 15 s budget, and a refused URL is not asked
  again for five minutes.

Actor and WebFinger fetches and inbox deliveries all use it. Test networks
can switch on Federation:AllowPrivateNetworks/AllowPlainHttp; startup
refuses both in Production.

PrivaPub.Tests (xUnit v3) starts with the address table and the fetcher's
limits against an in-process peer; build.yml and deploy.yml run it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Infrastructure/Http/FederationHttp.cs

@@ -0,0 +1,164 @@
+using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Extensions.Options;
+
+using System.Net;
+using System.Text.Json;
+
+namespace PrivaPub.Infrastructure.Http
+{
+	public sealed class FetchedJson : IDisposable
+	{
+		public Uri FinalUri { get; init; }
+		public JsonDocument Document { get; init; }
+		public JsonElement Root => Document.RootElement;
+
+		public void Dispose() => Document?.Dispose();
+	}
+
+	public interface IFederationHttp
+	{
+		bool IsAllowed(Uri target);
+		Task<FetchedJson> GetJson(string url, string accept, Action<HttpRequestMessage> sign, CancellationToken token);
+		Task<HttpResponseMessage> Send(HttpRequestMessage request, CancellationToken token);
+	}
+
+	public class FederationHttp : IFederationHttp
+	{
+		public const string ClientName = "Federation";
+		public const int MaxResponseBytes = 1024 * 1024;
+		public static readonly TimeSpan RequestTimeout = TimeSpan.FromSeconds(15);
+
+		const int MaxRedirects = 3;
+		static readonly TimeSpan NegativeCacheLifetime = TimeSpan.FromMinutes(5);
+		static readonly string[] JsonMediaTypes =
+		{
+			"application/activity+json",
+			"application/ld+json",
+			"application/jrd+json",
+			"application/json"
+		};
+
+		readonly IHttpClientFactory _httpClientFactory;
+		readonly IMemoryCache _cache;
+		readonly IOptionsMonitor<FederationOptions> _options;
+		readonly ILogger<FederationHttp> _logger;
+
+		public FederationHttp(IHttpClientFactory httpClientFactory, IMemoryCache cache, IOptionsMonitor<FederationOptions> options,
+			ILogger<FederationHttp> logger)
+		{
+			_httpClientFactory = httpClientFactory;
+			_cache = cache;
+			_options = options;
+			_logger = logger;
+		}
+
+		public bool IsAllowed(Uri target)
+		{
+			if (target is not { IsAbsoluteUri: true } || !string.IsNullOrEmpty(target.UserInfo))
+				return false;
+			var options = _options.CurrentValue;
+			if (target.Scheme != Uri.UriSchemeHttps && !(options.AllowPlainHttp && target.Scheme == Uri.UriSchemeHttp))
+				return false;
+			if (options.AllowPrivateNetworks)
+				return true;
+			return target.HostNameType == UriHostNameType.Dns
+				&& target.Host.Contains('.')
+				&& !target.Host.EndsWith(".localhost", StringComparison.OrdinalIgnoreCase)
+				&& !target.Host.EndsWith(".local", StringComparison.OrdinalIgnoreCase)
+				&& !target.Host.EndsWith(".internal", StringComparison.OrdinalIgnoreCase);
+		}
+
+		public async Task<FetchedJson> GetJson(string url, string accept, Action<HttpRequestMessage> sign, CancellationToken token)
+		{
+			if (!Uri.TryCreate(url, UriKind.Absolute, out var target) || !IsAllowed(target))
+				return default;
+			var negativeKey = NegativeKey(target);
+			if (_cache.TryGetValue(negativeKey, out _))
+				return default;
+
+			using var timeout = CancellationTokenSource.CreateLinkedTokenSource(token);
+			timeout.CancelAfter(RequestTimeout);
+			try
+			{
+				for (var hop = 0; hop <= MaxRedirects; hop++)
+				{
+					using var request = new HttpRequestMessage(HttpMethod.Get, target);
+					request.Headers.Accept.ParseAdd(accept);
+					sign?.Invoke(request);
+
+					using var response = await _httpClientFactory.CreateClient(ClientName)
+						.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, timeout.Token);
+
+					if (IsRedirect(response.StatusCode))
+					{
+						var location = response.Headers.Location;
+						var next = location == default ? default : location.IsAbsoluteUri ? location : new Uri(target, location);
+						if (!IsAllowed(next))
+							return Refuse(negativeKey, url, "a redirect to a disallowed location");
+						target = next;
+						continue;
+					}
+
+					if (!response.IsSuccessStatusCode)
+						return Refuse(negativeKey, url, $"status {(int)response.StatusCode}");
+
+					var mediaType = response.Content.Headers.ContentType?.MediaType;
+					if (mediaType == default || !JsonMediaTypes.Contains(mediaType, StringComparer.OrdinalIgnoreCase))
+						return Refuse(negativeKey, url, $"content type '{mediaType}'");
+					if (response.Content.Headers.ContentLength > MaxResponseBytes)
+						return Refuse(negativeKey, url, "a body over the size limit");
+
+					var body = await ReadBounded(response.Content, MaxResponseBytes, timeout.Token);
+					if (body == default)
+						return Refuse(negativeKey, url, "a body over the size limit");
+
+					return new FetchedJson { FinalUri = target, Document = JsonDocument.Parse(body) };
+				}
+				return Refuse(negativeKey, url, "too many redirects");
+			}
+			catch (OperationCanceledException) when (!token.IsCancellationRequested)
+			{
+				return Refuse(negativeKey, url, "a timeout");
+			}
+			catch (Exception ex) when (ex is HttpRequestException or JsonException or BlockedDestinationException)
+			{
+				return Refuse(negativeKey, url, ex.Message);
+			}
+		}
+
+		public async Task<HttpResponseMessage> Send(HttpRequestMessage request, CancellationToken token)
+		{
+			if (!IsAllowed(request.RequestUri))
+				throw new BlockedDestinationException(request.RequestUri?.Host);
+			return await _httpClientFactory.CreateClient(ClientName).SendAsync(request, HttpCompletionOption.ResponseHeadersRead, token);
+		}
+
+		public static async Task<byte[]> ReadBounded(HttpContent content, int limit, CancellationToken token)
+		{
+			await using var stream = await content.ReadAsStreamAsync(token);
+			using var buffer = new MemoryStream();
+			var chunk = new byte[16 * 1024];
+			int read;
+			while ((read = await stream.ReadAsync(chunk, token)) > 0)
+			{
+				if (buffer.Length + read > limit)
+					return default;
+				buffer.Write(chunk, 0, read);
+			}
+			return buffer.ToArray();
+		}
+
+		static bool IsRedirect(HttpStatusCode status) =>
+			status is HttpStatusCode.MovedPermanently or HttpStatusCode.Found or HttpStatusCode.SeeOther
+				or HttpStatusCode.TemporaryRedirect or HttpStatusCode.PermanentRedirect;
+
+		static string NegativeKey(Uri target) => "federation-http:refused:" + target.AbsoluteUri;
+
+		FetchedJson Refuse(string negativeKey, string url, string reason)
+		{
+			_cache.Set(negativeKey, true, NegativeCacheLifetime);
+			_logger.LogInformation("GET {Url} refused: {Reason}", url, reason);
+			return default;
+		}
+	}
+}