Why is there an intermittent error (in production only) when parsing from another HttpClient site in ASP.NET Core?

Question

ASP.NET

Web application on ASP.NET Core 3.1. On production Nginx + Kestrel + CentOS.
I am fetching data through the public web application API using HttpClient.
When developing in the studio, it works without errors at all. And in production, the following error periodically falls out:

System.AggregateException: One or more errors occurred. (Name or service not known)
 ---> System.Net.Http.HttpRequestException: Name or service not known
 ---> System.Net.Sockets.SocketException (0xFFFDFFFF): Name or service not known

spoiler

System.AggregateException: One or more errors occurred. (Name or service not known)
 ---> System.Net.Http.HttpRequestException: Name or service not known
 ---> System.Net.Sockets.SocketException (0xFFFDFFFF): Name or service not known
   at System.Net.Http.ConnectHelper.ConnectAsync(String host, Int32 port, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.ConnectHelper.ConnectAsync(String host, Int32 port, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean allowHttp2, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.CreateHttp11ConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.GetHttpConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.DiagnosticsHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at UMCPsyho.Services.Admin.AdminService.EmploymentPars(String iin) in C:\Users\1\source\repos\UMCPsyho\UMCPsyho\Services\Admin\AdminService.cs:line 475
   at UMCPsyho.Controllers.AdminController.IINParsing(String iin) in C:\Users\1\source\repos\UMCPsyho\UMCPsyho\Controllers\AdminController.cs:line 99
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.TaskOfIActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Localization.RequestLocalizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

Parsing code:

spoiler

EmploymentPars model = new EmploymentPars();
        string url = "https://api.site.net/api/public/loadNameBy/" + iin;
        string fio = "";
        HttpClient client = new HttpClient();
        client.BaseAddress = new Uri(url);
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        HttpResponseMessage response = client.GetAsync(url).Result;
        if (response.IsSuccessStatusCode)
        {
            var result = response.Content.ReadAsStringAsync().Result;
            var x = JsonConvert.DeserializeObject<HashMap>(result);
            if (x.name != null)
            {
                //--------------
                string[] fio2 = x.name.Split();
                model.iin = iin;
                model.fam = fio2[0];
                model.im = fio2[1];
                string otc = "";
                try
                {
                    otc = fio2[2];
                }
                catch
                {

                }
                model.otc = otc;
                //----------------
            }
            else
            {
                model.iin = "";
                model.fam = "";
                model.im = "";
                model.otc = "";
            }
        }

Answer 1

The DNS name is not recognized by the client, or simply repeat in case of an exception, or contact by IP if there are several sites sitting there, fill in the Host header accordingly.