N.E.V.E.R. - Network Endpoint Validation with Endless Retries

N.E.V.E.R. (Network Endpoint Validation with Endless Retries) is a lightweight Go application that obsessively checks whether a TCP, HTTP, or ICMP target is reachable. It loops endlessly until the target responds — or until it’s killed.

Designed to run as a Kubernetes initContainer, N.E.V.E.R. ensures your service dependencies are fully up before anything else gets a chance to boot.

Features

• Continuously retries until the target responds — no backoff, no shame.
• Supports multiple concurrent targets, each with its own config.
• Configurable entirely via command-line arguments.
• Exits with 0 the moment everything is ready.

Whether you're waiting on a port, ping, or a 200 OK, N.E.V.E.R. backs down never.

Command-Line Flags

never accepts the following command-line flags:

Common Flags

Flag	Type	Default	Description
--default-interval	duration	2s	Default interval between checks. Can be overridden for each target.
--version	bool	false	Show version and exit.
--help, -h	bool	false	Show help.

Target Flags

never accepts "dynamic" flags that can be defined in the startup arguments. Use the --<TYPE>.<IDENTIFIER>.<PROPERTY>=<VALUE> format to define targets. Types are: http, icmp or tcp.

HTTP-Flags

• --http.<IDENTIFIER>.name = string The name of the target. If not specified, it uses the <IDENTIFIER> as the name.

• --http.<IDENTIFIER>.address = string The target's address. Resolvable: See Resolving Variables below.

  • --http.<IDENTIFIER>.interval = duration The interval between HTTP requests (e.g., 1s). Overwrites the global --default-interval.

• --http.<IDENTIFIER>.method = string The HTTP method to use (e.g., GET, POST). Defaults to GET.

• --http.<IDENTIFIER>.header = string A HTTP header in key=value format. Can be specified multiple times. Example: Authorization=Bearer token Resolvable: See Resolving Variables below.

• --http.<IDENTIFIER>.allow-duplicate-headers = bool Allow duplicate headers. Defaults to false.

• --http.<IDENTIFIER>.expected-status-codes = string A comma-separated list of expected HTTP status codes or ranges (e.g., 200,301-302). Defaults to 200.

• --http.<IDENTIFIER>.skip-tls-verify = bool Whether to skip TLS verification. Defaults to false.

• --http.<IDENTIFIER>.timeout = duration The timeout for the HTTP request (e.g., 5s). Defaults to 1s.

ICMP Flags

• --icmp.<IDENTIFIER>.name = string The name of the target. If not specified, it uses the <IDENTIFIER> as the name.

• --icmp.<IDENTIFIER>.address = string The target's address. Resolvable: See Resolving Variables below.

• --icmp.<IDENTIFIER>.interval = duration The interval between ICMP requests (e.g., 1s). Overwrites the global --default-interval.

• --icmp.<IDENTIFIER>.read-timeout = duration The read timeout for the ICMP connection (e.g., 1s). Defaults to 1s.

• --icmp.<IDENTIFIER>.write-timeout = duration The write timeout for the ICMP connection (e.g., 1s).Defaults to 1s.

TCP Flags

• --tcp.<IDENTIFIER>.name = string The name of the target. If not specified, it uses the <IDENTIFIER> as the name.

• --tcp.<IDENTIFIER>.address = string The target's address. Resolvable: See Resolving Variables below.

• --tcp.<IDENTIFIER>.interval = duration The interval between ICMP requests (e.g., 1s). Overwrites the global --default-interval.

Resolving variables

Each address field can be resolved using environment variables, files, JSON, YAML, and INI files.

• env: – Resolves environment variables. Example: env:PATH returns the value of the PATH environment variable.
• file: – Resolves values from a simple key-value file. Example: file:/config/app.txt//KeyName returns the value associated with KeyName in app.txt.
• json: – Resolves values from a JSON file. Supports dot notation for nested keys. Example: json:/config/app.json//database.host returns host field under database in app.json. It is also possible to indexing into arrays (e.g., json:/config/app.json//servers.0.host).
• yaml: – Resolves values from a YAML file. Supports dot notation for nested keys. Example: yaml:/config/app.yaml//server.port returns port under server in app.yaml.It is also possible to indexing into arrays (e.g., yaml:/config/app.yaml//servers.0.host).
• ini: – Resolves values from an INI file. Can specify a section and key, or just a key in the default section. Example: ini:/config/app.ini//Section.Key returns the value of Key under Section.
• No prefix – Returns the value as-is, unchanged.

HTTP headers values can also be resolved using the same mechanism, (from a environment variable --http.<IDENTIFIER>.header="header=env:SECRET_HEADER" or from a file --http.<IDENTIFIER>.header="header=file:PATH_TO_FILE").

Examples

Define an HTTP Target

never \
  --http.web.address=http://example.com:80 \
  --http.web.method=GET \
  --http.web.expected-status-codes=200,204 \
  --http.web.header="Authorization=Bearer token" \
  --http.web.header="Content-Type=application/json" \
  --http.web.skip-tls-verify=false \
  --default-interval=5s

Define Multiple Targets (HTTP and TCP) Running in Parallel

never \
  --http.web.address=http://example.com:80 \
  --tcp.db.address=tcp://localhost:5432 \
  --default-interval=10s

Notes

Proxy Settings: Proxy configurations (HTTP_PROXY, HTTPS_PROXY, NO_PROXY) are managed via environment variables.

Behavior Flowchart

TCP Check

Click here to see the flowchart

graph TD;
    classDef noFill fill:none;
    classDef violet stroke:#9775fa;
    classDef green stroke:#2f9e44;
    classDef error stroke:#fa5252;
    classDef transparent stroke:none,font-size:20px;

    subgraph MainFlow[ ]
        direction TB
        start((Start)) --> attemptConnect[Attempt to connect to <font color=orange>TARGET_ADDRESS</font>];
        class start violet;

        subgraph RetryLoop[Retry Loop]
            subgraph InnerLoop[ ]
                direction TB
                attemptConnect -->|Connection successful| targetReady[Target is ready];
                attemptConnect -->|Connection failed| waitRetry[Wait for retry <font color=orange>CHECK_INTERVAL</font>];
                waitRetry --> attemptConnect;
            end
        end

        targetReady --> processEnd((End));
        class processEnd violet;
        waitRetry --> processEnd;
    end

    programTerminated[Program terminated or canceled] --> processEnd;
    class programTerminated error;

    class start,attemptConnect,targetReady,waitRetry,processEnd,programTerminated,MainFlow,RetryLoop noFill;
    class MainFlow,RetryLoop transparent;

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Permissions

Only when using ICMP checks in Kubernetes, it's important to ensure that the container has the necessary permissions to send ICMP packets. It is necessary to add the CAP_NET_RAW capability to the container's security context.

Example:

- name: wait-for-host
  image: ghcr.io/containeroo/never:latest
  env:
    - name: TARGET_ADDRESS
      value: icmp://hostname.domain.com
  securityContext:
    readOnlyRootFilesystem: true
    allowPrivilegeEscalation: false
    capabilities:
      add: ["CAP_NET_RAW"]

For TCP and HTTP checks, the container does not require any additional permissions.

HTTP Check

Click here to see the flowchart

flowchart TD;
    direction TB
    classDef noFill fill:none;
    classDef violet stroke:#9775fa;
    classDef green stroke:#2f9e44;
    classDef error stroke:#fa5252;
    classDef decision stroke:#1971c2;
    classDef transparent stroke:none,font-size:20px;

    subgraph MainFlow[ ]
        direction TB
        processStart((Start)) --> createRequest[Create HTTP request for <font color=orange>TARGET_ADDRESS</font>];
        class start processStart;

        createRequest --> addHeaders[Add headers from <font color=orange>HTTP_HEADERS</font>];
        addHeaders --> addSkipTLS[Add skip TLS verify if <font color=orange>HTTP_SKIP_TLS_VERIFY</font> is set];
        addSkipTLS --> sendRequest[Send HTTP request];

        subgraph RetryLoop[Retry Loop]
            subgraph InnerLoop[ ]
                direction TB
                sendRequest --> checkTimeout{Answers within <font color=orange>DIAL_TIMEOUT</font>?};
                class checkTimeout decision;
                checkTimeout -->|Yes| checkStatusCode[Check response status code <font color=orange>HTTP_EXPECTED_STATUS_CODES</font>];
                checkStatusCode --> statusMatch{Matches?};
                class statusMatch decision;

                statusMatch -->|Yes| targetReady[Target is ready];
                class targetReady success;

                statusMatch -->|No| targetNotReady[Target is not ready];
                class targetNotReady error;
                targetNotReady --> waitRetry[Wait for retry <font color=orange>CHECK_INTERVAL</font>];
                waitRetry --> sendRequest;
            end
        end

    targetReady --> processEnd((End));
    class processEnd violet;
    end

    programTerminated[Program terminated or canceled] --> processEnd;
    class programTerminated error;

class processStart,createRequest,addHeaders,addSkipTLS,sendRequest,checkTimeout,checkStatusCode,statusMatch,targetReady,targetNotReady,waitRetry,programTerminated,processEnd,MainFlow,RetryLoop noFill;
class MainFlow,RetryLoop transparent;

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ICMP Check

Click here to see the flowchart

flowchart TD;
    direction TB
    classDef noFill fill:none;
    classDef violet stroke:#9775fa;
    classDef green stroke:#2f9e44;
    classDef error stroke:#fa5252;
    classDef decision stroke:#1971c2;
    classDef transparent stroke:none,font-size:20px;

    subgraph MainFlow[ ]
        direction TB
        processStart((Start)) --> createRequest[Create ICMP request for <font color=orange>TARGET_ADDRESS</font>];
        class start processStart;

        createRequest --> sendRequest[Send ICMP request];

        subgraph RetryLoop[Retry Loop]
            subgraph InnerLoop[ ]
                direction TB
                sendRequest --> checkTimeout{Answers within timeouts <font color=orange>DIAL_TIMEOUT</font>/<font color=orange>ICMP_READ_TIMEOUT</font>?};

                checkTimeout -->|Connection successful| targetReady[Target is ready];
                checkTimeout -->|Connection failed| waitRetry[Wait for retry <font color=orange>CHECK_INTERVAL</font>];
                waitRetry --> sendRequest;

            end
        end

    targetReady --> processEnd((End));
    class processEnd violet;
    end

    programTerminated[Program terminated or canceled] --> processEnd;
    class programTerminated error;

class processStart,createRequest,sendRequest,checkTimeout,targetReady,waitRetry,processEnd,MainFlow,RetryLoop noFill;
class MainFlow,RetryLoop transparent;

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Kubernetes initContainer Configuration

Configure your Kubernetes deployment to use this init container:

initContainers:
  - name: wait-for-vm
    image: ghcr.io/containeroo/never:latest
    args:
      - --icmp.vm.address=hostname.domain.tld
    securityContext: # icmp requires CAP_NET_RAW
      readOnlyRootFilesystem: true
      allowPrivilegeEscalation: false
      capabilities:
        add: ["CAP_NET_RAW"]
  - name: wait-for-it
    image: ghcr.io/containeroo/never:latest
    args:
      - --target.postgres.address=postgres.default.svc.cluster.local:9000/healthz # use healthz endpoint to check if postgres is ready
      - --target.postgres.method=POST
      - --target.postgres.header=Authorization=env:BEARER_TOKEN
      - --target.postgres.expected-status-codes=200,202
      - --target.redis.name=redis
      - --target.redis.address=redis.default.svc.cluster.local:6437
      - --tcp.vaultkey.address=valkey.default.svc.cluster.local:6379
      - --tcp.vaultkey.interval=5s
      - --tcp.vaultkey.timeout=5s
    envFrom:
      - secretRef:
          name: bearer-token

License

This project is licensed under the Apache License. See the LICENSE file for details.