I am currently working on a application which will be running in a kubernetes pod. It is supposed to connect to a postgressql pod, that is ran within the same cluster.

but I can for some reason not deduce what the connection string should be

I for now defined the postgressql deployment as such:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: postgres
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
        - name: postgres
          image: postgres:10.4
          imagePullPolicy: "IfNotPresent"
          ports:
            - containerPort: 5432
          volumeMounts:
            - mountPath: /var/lib/postgresql/data
              name: postgredb
      volumes:
        - name: postgredb
          persistentVolumeClaim:
            claimName: postgres-pv-claim   

---
apiVersion: v1
kind: Service
metadata:
  name: postgres-service
  labels:
    app: postgres
spec:
  ports:
   - port: 5432
     targetPort: 5432 
  selector:
   app: postgres

but for a connection string

            x.UseNpgsql("Host=postgres-service:5432;Database=postgres;Username=postgres;Password=postgres"));

Which does not seem to work?

Something as simple as

using System;
using System.Net.NetworkInformation;

    namespace pingMe
    {
        class Program
        {
            static void Main(string[] args)
            {
                Console.WriteLine("Hello World!");
                Ping ping = new Ping();
                PingReply pingresult = ping.Send("postgres-service.default.svc.cluster.local");
                if (pingresult.Status.ToString() == "Success")
                {
                    Console.WriteLine("I can reach");
                }
            }
        }
    }

resolve into this:

within the cluster triggers an error

    System.Net.NetworkInformation.PingException: An exception occurred during a Ping request.
 ---> System.Net.Internals.SocketExceptionFactory+ExtendedSocketException (00000005, 0xFFFDFFFF): Name or service not known
   at System.Net.Dns.InternalGetHostByName(String hostName)
   at System.Net.Dns.GetHostAddresses(String hostNameOrAddress)
   at System.Net.NetworkInformation.Ping.GetAddressAndSend(String hostNameOrAddress, Int32 timeout, Byte[] buffer, PingOptions options)
   --- End of inner exception stack trace ---
   at System.Net.NetworkInformation.Ping.GetAddressAndSend(String hostNameOrAddress, Int32 timeout, Byte[] buffer, PingOptions options)
   at System.Net.NetworkInformation.Ping.Send(String hostNameOrAddress, Int32 timeout, Byte[] buffer, PingOptions options)
   at System.Net.NetworkInformation.Ping.Send(String hostNameOrAddress)
   at API.Startup.Configure(IApplicationBuilder app, IWebHostEnvironment env, SchemaContext schemaContext) in /src/API/Startup.cs:line 42
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.AspNetCore.Hosting.ConfigureBuilder.Invoke(Object instance, IApplicationBuilder builder)
   at Microsoft.AspNetCore.Hosting.ConfigureBuilder.<>c__DisplayClass4_0.b__0(IApplicationBuilder builder)
   at Microsoft.AspNetCore.Hosting.GenericWebHostBuilder.<>c__DisplayClass13_0.b__2(IApplicationBuilder app)
   at Microsoft.AspNetCore.Mvc.Filters.MiddlewareFilterBuilderStartupFilter.<>c__DisplayClass0_0.g__MiddlewareFilterBuilder|0(IApplicationBuilder builder)
   at Microsoft.AspNetCore.HostFilteringStartupFilter.<>c__DisplayClass0_0.b__0(IApplicationBuilder app)
   at Microsoft.AspNetCore.Hosting.GenericWebHostService.StartAsync(CancellationToken cancellationToken)
Unhandled exception. System.Net.NetworkInformation.PingException: An exception occurred during a Ping request.

Kubernetes service

kubectl get svc postgres-service
NAME               TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)    AGE
postgres-service   ClusterIP   10.106.91.9   <none>        5432/TCP   74m

Dockerfile:

#See https://aka.ms/containerfastmode to understand how Visual Studio uses this Dockerfile to build your images for faster debugging.

FROM mcr.microsoft.com/dotnet/core/runtime:3.1-buster-slim AS base
WORKDIR /app

FROM mcr.microsoft.com/dotnet/core/sdk:3.1-buster AS build
WORKDIR /src
COPY ["pingMe/pingMe.csproj", "pingMe/"]
RUN dotnet restore "pingMe/pingMe.csproj"
COPY . .
WORKDIR "/src/pingMe"
RUN dotnet build "pingMe.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "pingMe.csproj" -c Release -o /app/publish

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "pingMe.dll"]

Local pod:

kind: Deployment
apiVersion: extensions/v1beta1
metadata:
  name: local-deployment
spec:
  replicas: 1
  revisionHistoryLimit: 3
  template:
    metadata:
      labels:
        app: local-pod
    spec:
      containers:
      - name: local-deployment
        image: api:dev5
        imagePullPolicy: Never
        ports:
        - containerPort: 80
        livenessProbe:
          httpGet:
            path: /WeatherForecast
            port: 80
          initialDelaySeconds: 3
          periodSeconds: 3