RemoteImageProvider POC

AllowedDomainsRemoteImageUrlValidator.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Extensions.Options;

namespace SixLabors.ImageSharp.Web.Providers
{
    public class AllowedDomainsRemoteImageUrlValidator: IRemoteImageUrlValidator
    {
        private readonly List<string> _allowedDomains;
        
        public AllowedDomainsRemoteImageUrlValidator(IOptions<AllowedDomainsRemoteImageUrlValidatorOptions> options)
        {
            _allowedDomains = options.Value.AllowedDomains
                .Select(domain => domain.ToLower())
                .ToList();
        }
        
        public bool IsValidUrl(Uri url)
        {
            return _allowedDomains.Contains(url.Host);
        }
    }

}

AllowedDomainsRemoteImageUrlValidatorOptions.cs

using System.Collections.Generic;

namespace SixLabors.ImageSharp.Web.Providers
{
    public class AllowedDomainsRemoteImageUrlValidatorOptions
    {
        public ICollection<string> AllowedDomains { get; set; } = new List<string>();
    }
}

appsettings.json

{
  "ImageSharp.Web": {
    "Presets": {
      "preset1": "rmode=crop&width=232&height=164",
    },
    "AllowedDomains": [
      "example.com"
    ]
  }
}

IRemoteImageUrlValidator.cs

using System;

namespace SixLabors.ImageSharp.Web.Providers
{
    public interface IRemoteImageUrlValidator
    {
        bool IsValidUrl(Uri url);
    }
}

RemoteImageProvider.cs

using System;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using SixLabors.ImageSharp.Web.Providers;
using SixLabors.ImageSharp.Web.Resolvers;

namespace SixLabors.ImageSharp.Web.Providers
{
    public class RemoteImageProvider : IImageProvider
    {
        public ProcessingBehavior ProcessingBehavior => ProcessingBehavior.All;
        public Func<HttpContext, bool> Match { get; set; } = PathStartsWithRemote;

        public const string RemotePath = "/remote";

        private readonly IHttpClientFactory _clientFactory;
        private readonly IRemoteImageUrlValidator _urlValidator;

        public RemoteImageProvider(IHttpClientFactory clientFactory, IRemoteImageUrlValidator urlValidator)
        {
            _clientFactory = clientFactory;
            _urlValidator = urlValidator;
        }

        public bool IsValidRequest(HttpContext context)
        {
            var url = GetRemoteUrl(context);
            return Uri.IsWellFormedUriString(url, UriKind.Absolute) && _urlValidator.IsValidUrl(new Uri(url));
        }

        public Task<IImageResolver> GetAsync(HttpContext context)
        {
            var url = GetRemoteUrl(context);
            return Task.FromResult(new RemoteImageResolver(_clientFactory, url) as IImageResolver);
        }

        private static bool PathStartsWithRemote(HttpContext context)
        {
            return context.Request.Path.StartsWithSegments(RemotePath, StringComparison.OrdinalIgnoreCase);
        }

        private static string GetRemoteUrl(HttpContext context)
        {
            if (!context.Request.Path.HasValue || context.Request.Path.Value.Length <= RemotePath.Length)
            {
                return null;
            }
            var remoteUrl = context.Request.Path.Value?.Substring(RemotePath.Length + 1);
            return remoteUrl.Replace(" ", "%20");
        }
    }
}

RemoteImageResolver.cs

using System;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;
using SixLabors.ImageSharp.Web;
using SixLabors.ImageSharp.Web.Resolvers;

namespace SixLabors.ImageSharp.Web.Providers
{
    public class RemoteImageResolver: IImageResolver
    {
        private readonly IHttpClientFactory _clientFactory;
        private readonly string _url;
        
        public const string HttpClientName = "RemoteImageResolver";

        public RemoteImageResolver(IHttpClientFactory clientFactory, string url)
        {
            _clientFactory = clientFactory;
            _url = url;
        }
        
        public async Task<ImageMetadata> GetMetaDataAsync()
        {
            var client = _clientFactory.CreateClient(HttpClientName);
            
            var response = await client.SendAsync(new HttpRequestMessage(HttpMethod.Head, _url), HttpCompletionOption.ResponseHeadersRead);

            if (!response.Content.Headers.ContentLength.HasValue)
            {
                throw new Exception("Required header ContentLength is missing.");
            }
            
            if (!response.Content.Headers.LastModified.HasValue || !response.Content.Headers.ContentLength.HasValue)
            {
                throw new Exception("Required header LastModified is missing.");
            }

            return new ImageMetadata(response.Content.Headers.LastModified.Value.UtcDateTime, response.Content.Headers.ContentLength.Value);
        }   

        public async Task<Stream> OpenReadAsync()
        {
            var client = _clientFactory.CreateClient(HttpClientName);
            
            var response = await client.GetAsync(_url);
            
            return await response.Content.ReadAsStreamAsync();
        }
    }
}

Startup.cs

namespace PMP.Frontend.Core.Startup
{
    public class Startup
    {
        //...

        public void ConfigureServices(IServiceCollection services)
        {
            //...

            services.AddHttpClient(RemoteImageResolver.HttpClientName, client =>
            {
                client.Timeout = TimeSpan.FromSeconds(5);
                client.MaxResponseContentBufferSize = 8388608; //8mb
            });

            services
                .AddImageSharp()
                .ClearProviders()
                .AddProvider<RemoteImageProvider>()
                .SetRequestParser<PresetRequestParser>()
                .Configure<PresetRequestParserOptions>(_configuration.GetSection("ImageSharp.Web"))
                .Configure<AllowedDomainsRemoteImageUrlValidatorOptions>(_configuration.GetSection("ImageSharp.Web"));

           //...
        }
        
        //...
    }
}

This looks good Mark. How would you use it in the views/templates ?

The RemoteImageProvider handles everything under the /remote path.
You can pass it an url to an image, but with :// replaced by /:

<img src="/remote/https/example.com/images/image.png" />
<img src="/remote/https/example.com/images/image.png?preset=MyPreset" />
<img src="/remote/https/example.com/images/image.png?width=50&height=50&rmode=stretch" />