Cloudflare IApplicationBuilder forward header and proxy security

CloudflareForwardHeaders.cs

private static string[] GetStrings(string url)
{
	return new WebClient().DownloadString(url).Split('\n').Select(s => s.Trim()).ToArray();
}

private static string[] GetCloudflareIP()
{
	try
	{
		return GetStrings("https://www.cloudflare.com/ips-v4").Union(GetStrings("https://www.cloudflare.com/ips-v6")).ToArray();
	}
	catch
	{
		return @"173.245.48.0/20
103.21.244.0/22
103.22.200.0/22
103.31.4.0/22
141.101.64.0/18
108.162.192.0/18
190.93.240.0/20
188.114.96.0/20
197.234.240.0/22
198.41.128.0/17
162.158.0.0/15
104.16.0.0/12
172.64.0.0/13
131.0.72.0/22
2400:cb00::/32
2606:4700::/32
2803:f800::/32
2405:b500::/32
2405:8100::/32
2a06:98c0::/29
2c0f:f248::/32
".Split('\n').Select(s => s.Trim()).ToArray();
	}
}

/// <summary>
/// Add cloudflare forward header options
/// </summary>
/// <param name="builder">Application builder</param>
public static void UseCloudflareForwardHeaderOptions(this IApplicationBuilder builder)
{
	ForwardedHeadersOptions options = new ForwardedHeadersOptions
	{
		ForwardedForHeaderName = "CF_CONNECTING_IP",
		ForwardedHeaders = ForwardedHeaders.All
	};
	try
	{
		ICollection<string> urls = builder.ServerFeatures.Get<IServerAddressesFeature>().Addresses;
		if (urls != null && urls.Count != 0)
		{
			string[] cloudFlareIP = GetCloudflareIP();
			foreach (string line in cloudFlareIP)
			{
				if (IPAddressRange.TryParse(line, out IPAddressRange range))
				{
					options.KnownNetworks.Add(new IPNetwork(range.Begin, range.GetPrefixLength()));
				}
			}
		}
	}
	catch
	{
	}
	builder.UseForwardedHeaders(options);
}

Note that you need to add https://www.nuget.org/packages/IPAddressRange/