Configure proxy headers .net core.

ConfigureForwardedHeadersOptions.cs

namespace Philo.Start.Forwarding
{
    using System;
    using System.Linq;
    using System.Net;
    using System.Text.RegularExpressions;
    using Microsoft.AspNetCore.Builder;
    using Microsoft.AspNetCore.HttpOverrides;
    using Microsoft.Extensions.Configuration;
    using Microsoft.Extensions.Options;

    public class ConfigureForwardedHeadersOptions : IConfigureNamedOptions<ForwardedHeadersOptions>
    {
        private const string ProxyNetworksConfigName = "ProxyNetworks";
        private static Regex pattern = new Regex(@"^(?<prefix>(?:[0-9]{1,3}\.){3}[0-9]{1,3})\/(?<prefixLength>(?:[0-9]|[1-2][0-9]|3[0-2]))?$", RegexOptions.Compiled | RegexOptions.Singleline);
        private readonly IConfiguration configuration;

        public ConfigureForwardedHeadersOptions(IConfiguration configuration)
        {
            this.configuration = configuration;
        }

        public void Configure(ForwardedHeadersOptions options)
        {
            Configure(Options.DefaultName, options);
        }

        public void Configure(string name, ForwardedHeadersOptions options)
        {
            configuration.GetSection(nameof(ForwardedHeadersOptions)).Bind(options);
            var proxyNetworks = configuration.GetValue<string>(ProxyNetworksConfigName) ?? string.Empty;

            if (!string.IsNullOrWhiteSpace(proxyNetworks))
            {
                foreach (var cidr in proxyNetworks.Split(',', StringSplitOptions.RemoveEmptyEntries)
                    .Select(s => s.Trim()))
                {
                    var match = pattern.Match(cidr);
                    if (match.Success && match.Groups["prefix"].Success && match.Groups["prefixLength"].Success &&
                        int.TryParse(match.Groups["prefixLength"].Value, out var prefixLength))
                    {
                        var prefix = match.Groups["prefix"].Value;
                        options.KnownNetworks.Add(new IPNetwork(IPAddress.Parse(prefix), prefixLength));
                    }
                }

                if ((options.KnownNetworks?.Any() ?? false) || (options.KnownProxies?.Any() ?? false))
                {
                    options.ForwardLimit = null;
                }
            }
        }
    }
}

StartupExtensions.cs

namespace Philo.Start.Forwarding
{
    using Microsoft.AspNetCore.Builder;
    using Microsoft.Extensions.DependencyInjection;
    using Microsoft.Extensions.Options;

    public static class ForwardedHeadersOptionsServiceCollectionExtensions
    {
        public static IServiceCollection ConfigureForwardedHeaders(this IServiceCollection services)
        {
            return
                services.AddTransient<IConfigureOptions<ForwardedHeadersOptions>, ConfigureForwardedHeadersOptions>();
        }
    }
}