解决 asp.net core swagger nginx 代理转发后，添加中间路径导致swagger页面无法访问的问题 Swashbuckle

通过代理转发后，webapi的swagger无法访问，本质原因是代理后url路径发生变化导致swagger无法正常定位资源。

一般而言代理转发如果发布到网址的根路径下，不会发生这种问题，但是如果添加了中间路径，则会出现此类问题，如：

http://some-site.com/swagger => http://some-site.com/middle-path/swagger

官方文档配置：

https://docs.microsoft.com/en-us/aspnet/core/tutorials/getting-started-with-swashbuckle

https://docs.microsoft.com/en-us/aspnet/core/host-and-deploy/linux-nginx?view=aspnetcore-3.1#use-a-reverse-proxy-server

c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");

解决方法：

1. 使用url相对路径
2. 通过PreSerializeFilters设置根地址，并通过nginx的X-Forwarded-Prefix指定api名称

nginx配置：

server {
        listen       5011;
        server_name  _;

        location /api1/ {
            proxy_pass         http://192.168.56.1:5000/;
            proxy_http_version 1.1;
            proxy_set_header   Upgrade $http_upgrade;
            proxy_set_header   Connection keep-alive;
            proxy_set_header   Host $http_host; # 使用http_host而非host以满足带有端口号的情况
            proxy_cache_bypass $http_upgrade;
            proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header   X-Forwarded-Proto $scheme;
            proxy_set_header   X-Forwarded-Prefix api1; # api1用于传递路由名称
        }
    }

Swashbuckle 5.x 以前通过PreSerializeFilters设置BasePath的方案，5.x以后，设置OpenApiServer.Url属性

代码如下：

app.UseSwagger(c =>
{
        c.PreSerializeFilters.Add((swagger, httpReq) =>
        {
                //根据访问地址，设置swagger服务路径
                swagger.Servers = new List<OpenApiServer> { new OpenApiServer { Url = $"{httpReq.Scheme}://{httpReq.Host.Value}/{httpReq.Headers["X-Forwarded-Prefix"]}" } };
        });
});

app.UseSwaggerUI(c =>
{
        //使用相对路径
        c.SwaggerEndpoint("v1/swagger.json", "My API V1");
});

集成nginx别忘了设置中间件

public static IServiceCollection AddNginx(this IServiceCollection services)
{
        services.Configure<ForwardedHeadersOptions>(options =>
        {
                options.ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto;
                options.KnownNetworks.Clear();
                options.KnownProxies.Clear();
        });
        return services;
}

github demo： https://github.com/wswind/swagger-proxy

官方文档： https://github.com/domaindrivendev/Swashbuckle.AspNetCore#modify-swagger-with-request-context

相关issue:

https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/1617

https://github.com/microsoft/service-fabric-issues/issues/327

https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/662

https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/427

https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/380

https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/1298#issuecomment-620269062

https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/1253

---- update 20220304 ----

除了上述方法外，也可以设置PathBase，设置PathBase对于MVC资源也生效，是处理这种转发带中间路径情况的通用方案。

微软的eShop Demo采用的是这种方式，不过是用过配置文件来设置的PathBase，代码如下

//https://github.com/dotnet-architecture/eShopOnContainers/blob/dev/src/Services/Ordering/Ordering.API/Startup.cs
var pathBase = Configuration["PATH_BASE"];
if (!string.IsNullOrEmpty(pathBase))
{
    loggerFactory.CreateLogger<Startup>().LogDebug("Using PATH BASE '{pathBase}'", pathBase);
    app.UsePathBase(pathBase);
}

app.UseSwagger()
    .UseSwaggerUI(c =>
    {
        c.SwaggerEndpoint($"{ (!string.IsNullOrEmpty(pathBase) ? pathBase : string.Empty) }/swagger/v1/swagger.json", "Ordering.API V1");
        c.OAuthClientId("orderingswaggerui");
        c.OAuthAppName("Ordering Swagger UI");
    });

你使用PathBase的方式和X-Forwarded-Prefix结合一下，在设置nginx转发中间件的同时，基于X-Forwarded-Prefix设置一下PathBase，如

public static IApplicationBuilder UseForwardedPrefix(this IApplicationBuilder app)
{
        app.Use(async (ctx, next) =>
        {
                //https://github.com/dotnet/aspnetcore/issues/23263#issuecomment-767394044
                if (ctx.Request.Headers.TryGetValue("X-Forwarded-Prefix", out var prefixVals))
                {
                        if (!StringValues.IsNullOrEmpty(prefixVals))
                        {
                                string prefix = prefixVals.Last();
                                ctx.Request.PathBase = $"/{prefix}";
                        }
                }
                
                await next();
        });
        return app;
}

最后再基于PathBase来设置Swagger UI路径。