Configuration: core directives

This page documents directives that belong in top-level global blocks:

{
    # global directives here, for example:
    console_log
}

Directives⁠#

Default ports⁠#

• default_http_port <port: integer | false>

  • This directive sets the default HTTP port when a host block does not specify a port. The value must be a positive integer ≤ 65535, or false to disable the default HTTP listener. Default: default_http_port 80

• default_https_port <port: integer | false>

  • This directive sets the default HTTPS port used for HTTP-to-HTTPS redirects and URL generation. The value must be a positive integer ≤ 65535, or false to disable the default HTTPS listener. Default: default_https_port 443

Configuration example:

{
    default_http_port 8080
    default_https_port 8443
}

Disable default HTTP listener (HTTPS only):

{
    default_http_port false
}

Disable both default listeners (only explicit ports work):

{
    default_http_port false
    default_https_port false
}

Runtime⁠#

• io_uring <bool>
  • This directive turns on io_uring for the server when it is available. Enabling io_uring could improve performance for static file serving involving large files or reverse proxying involving very high concurrency. If initialization fails, Ferron falls back to epoll and logs a warning. Default: disabled

Configuration example:

{
    runtime {
        io_uring
    }
}

Network and listeners⁠#

• listen <address: string>

  • This directive sets the bind addresses for TCP listeners. It accepts either IP addresses or full socket addresses. If you use a socket address, its port must match the HTTP port that Ferron starts. If you just need to change default HTTP ports, use default_http_port and default_https_port instead. Default: [::]:<http-port>

• send_buf <size: integer>

  • This directive sets the TCP send buffer size. It must resolve to a non-negative integer at runtime. Default: OS default

• recv_buf <size: integer>

  • This directive sets the TCP receive buffer size. It must resolve to a non-negative integer at runtime. Default: OS default

• backlog <size: integer>

  • This directive sets the maximum number of pending connections allowed on the listener socket. Default: -1 (Windows, *BSD, macOS), 4096 (others)

• multipath <bool>

  • This directive turns on Multipath TCP (MPTCP) for the listener. MPTCP allows a single TCP connection to use multiple network interfaces at the same time, improving throughput and resilience. When enabled, Ferron attempts to create an MPTCP socket. If the kernel lacks MPTCP support or MPTCP is off, Ferron logs a warning and falls back to standard TCP. Default: disabled

Configuration example:

{
    tcp {
        listen "127.0.0.1"
        send_buf 65536
        recv_buf 131072
        multipath
    }
}

Unix domain sockets (Unix only)⁠#

• unix <path: string> { ... }
  • This directive creates a Unix domain socket listener. You can repeat it to listen on multiple sockets. Each directive accepts a filesystem path and an optional block with backlog, mode, owner, and group subdirectives. The path can be absolute (/run/ferron.sock) or relative (ferron.sock, tmp/ferron.sock). Relative paths are resolved against the server’s current working directory and canonicalized when the target exists, otherwise kept as-is. Default: none

The unix directive also supports the subdirectives below:

Configuration example:

{
    unix "/run/ferron.sock" {
        backlog 128
        mode "0660"
        owner ferron
        group ferron
    }

    # Relative path (resolved against the server's working directory)
    # (for example, /var/log/ferron when using the default Docker image).
    unix "ferron.sock" {
        mode "0660"
    }

    # Multiple sockets (see above)
    unix "/run/ferron-api.sock" {
        mode "0660"
    }
}

PROXY protocol⁠#

• protocol_proxy [bool]
  • This directive turns on PROXY protocol v1/v2 parsing for incoming TCP connections. When enabled, Ferron reads the PROXY protocol header from HAProxy or similar load balancers before processing the HTTP request. The client and server addresses from the PROXY header replace the actual socket addresses while the connection is open. Default: protocol_proxy false

Reverse proxy connection limits⁠#

• concurrent_conns <limit: integer>
  • This directive sets the global maximum number of concurrent TCP connections maintained in the reverse proxy keep-alive connection pool. All hosts that use the proxy directive share the limit. Unix socket connections have no limit. Default: concurrent_conns 16384

Configuration example:

{
    concurrent_conns 10000
}

Admin API⁠#

The admin block configures the built-in administration endpoints. If the admin block is absent, Ferron disables the admin API entirely.

• listen <address: string> (admin-api)

  • This directive sets the socket address for the admin HTTP listener. Default: listen 127.0.0.1:8081

• auth_token <token: string> (admin-api)

  • This directive sets a bearer token for authenticating admin API requests. When set, clients must send Authorization: Bearer <token> header. The /health endpoint is always exempt from authentication (required by load balancers and orchestrators). Default: none (authentication disabled)

• health [bool] (admin-api)

  • This directive enables the GET /health endpoint. It returns 200 OK or 503 Service Unavailable during shutdown. Default: health true

• status [bool] (admin-api)

  • This directive enables the GET /status endpoint. It returns JSON with uptime, active connections, request count, and reload count. Default: status true

• config [bool] (admin-api)

  • This directive enables the GET /config endpoint. It returns the current effective configuration as sanitized JSON (sensitive fields redacted). Default: config true

• reload [bool] (admin-api)

  • This directive enables the POST /reload endpoint. It triggers a configuration reload equivalent to SIGHUP. Default: reload true

• reload_get [bool] (admin-api)

  • This directive enables the GET /reload endpoint. It returns the current reload status. Default: reload_get true

• runtime [bool] (admin-api)

  • This directive enables the GET /runtime endpoint. It returns runtime information such as thread count and io_uring status. Default: runtime true

Configuration example:

{
    admin {
        listen "127.0.0.1:8081"
        auth_token "my-secret-token"

        health true
        status true
        config true
        reload true
        reload_get true
        runtime true
    }
}

Observability⁠#

The observability block configures per-host event sinks for logging and metrics. Multiple observability directives for the same host accumulate event sinks.

• provider <name: string> (observability-consolelog, observability-logfile)
  • This directive sets the observability provider name. Required when observability is enabled through the block form. Supported providers: console (observability-consolelog), file (observability-logfile). Default: none

Configuration example:

example.com {
    observability {
        provider console
    }
}

provider console⁠#

The bundled console provider (observability-consolelog) takes no additional subdirectives and writes supported observability events to Ferron logs.

provider file⁠#

The bundled file provider (observability-logfile) writes observability events to specified log files.

Configuration example:

example.com {
    observability {
        provider file

        access_log /var/log/ferron/access.log
        error_log /var/log/ferron/error.log
        format text
        error_format json
    }
}

Observability aliases⁠#

Ferron has shorthand directives for common observability configurations. Ferron transforms these automatically into equivalent observability blocks.

log⁠#

The log directive is shorthand for configuring access logging with the file provider.

example.com {
    # These are equivalent:

    log /var/log/access.log {
        format text
    }

    observability {
        provider file
        access_log /var/log/access.log
        format text
    }
}

Examples:

example.com {
    # Enable access logging with default format
    log /var/log/access.log

    # Enable with custom format
    log /var/log/access.log {
        format json
    }

    # Enable with log rotation (100MB max, keep 5 rotated files)
    log /var/log/access.log {
        access_log_rotate_size 104857600
        access_log_rotate_keep 5
    }

    # Disable access logging
    log false
}

error_log⁠#

The error_log directive is shorthand for configuring error logging with the file provider.

example.com {
    # These are equivalent:

    error_log /var/log/error.log

    observability {
        provider file
        error_log /var/log/error.log
    }
}

Examples:

example.com {
    # Enable error logging
    error_log /var/log/error.log

    # Enable with log rotation (50MB max, keep 3 rotated files)
    error_log /var/log/error.log {
        error_log_rotate_size 52428800
        error_log_rotate_keep 3
    }

    # Enable with JSON application log formatting
    error_log /var/log/error.log {
        error_format json
    }

    # Disable error logging
    error_log false
}

console_log⁠#

The console_log directive is shorthand for configuring console-based observability.

example.com {
    # These are equivalent:

    console_log {
        format json
    }

    observability {
        provider console
        format json
    }
}

Admin API⁠#

The admin API is a built-in HTTP interface for server health checks, status monitoring, configuration inspection, and reload control. It is meant for local access and debugging purposes.

Security considerations⁠#

The admin API is a privileged control plane with full server configuration access and reload capability. It has no encryption and no authentication by default. You can enable bearer token authentication with the auth_token directive. Treat it with the same security posture as a root shell on your server.

Risks of binding to 0.0.0.0⁠#

Setting listen "0.0.0.0:<port>" makes the admin API completely open to any client that can reach the host. Omitting the bind address defaults to all interfaces and has the same effect. This can happen accidentally in containerized environments (for example, Docker with bridge networking) or misconfigured networks.

Consequences of an open admin API:

• Denial of service: Anyone can send POST /reload continuously, causing reload loops that degrade performance.
• Configuration leak: GET /config reveals the full server configuration, including hostnames, upstream addresses, and routing rules, with sensitive values redacted.
• Service disruption: Anyone can use reload with modified configuration to disable any endpoint or inject misconfigured directives.

Hardening recommendations⁠#

1. Always bind to localhost unless you have a specific, secure reason not to:

   {
       admin {
           listen "127.0.0.1:8081"
           health true
           status true
           config true
           reload true
       }
   }

2. Disable unnecessary endpoints. Only enable the endpoints you need:

   {
       admin {
           listen "127.0.0.1:8081"
           health true
           status false
           config false
           reload true
       }
   }

3. Use a reverse proxy for remote access. If you need to access the admin API from a remote machine, front it with an authenticating reverse proxy. Do not bind to 0.0.0.0:

   Remote user → reverse proxy (auth required) → 127.0.0.1:8081 (admin API)

4. Restrict network access at the infrastructure level. Use firewall rules, security groups, or VPC networking to make sure only trusted hosts can reach the admin port.

5. Monitor admin API access. Use your observability sinks to track requests to admin endpoints for anomaly detection.

6. Never expose the admin API to the public internet. If you need remote administration, use SSH tunneling:

   ssh -L 8081:127.0.0.1:8081 admin@your-server
   # Then access http://127.0.0.1:8081 locally

API reference⁠#

The admin API is a RESTful interface for server configuration and control. Below are the available endpoints:

GET /health⁠#

It returns 200 OK while the server runs, or 503 Service Unavailable when shutdown starts. Suitable for load balancer and orchestration health checks.

GET /status⁠#

Returns JSON with server metrics:

{
  "uptime_sec": 12345,
  "connections_active": 42,
  "requests_total": 100000,
  "reloads": 3,
  "observability_events_dropped": 0,
  "observability_event_queue_len": 0
}

GET /config⁠#

It returns the full effective server configuration as sanitized JSON. Ferron replaces sensitive directives (TLS keys, passwords, tokens) with "[redacted]". Useful for debugging and auditing.

GET /reload⁠#

Returns the current reload status as JSON:

{
  "last_reload_time": "2026-05-29T12:00:00Z",
  "last_reload_error": null,
  "active_generation": 42
}

POST /reload⁠#

It triggers a configuration reload, equivalent to sending SIGHUP to the daemon process.

Returns the reload status as JSON:

{
  "status": "reload_initiated",
  "error": null
}

GET /runtime⁠#

Returns the runtime status as JSON:

{
  "primary_threads": 8,
  "io_uring_supported": true,
  "io_uring_runtime_enabled": true
}

Best practices⁠#

ferron doctor reports the following best-practice checks for directives on this page.

Log rotation⁠#

• log without rotation: File-based access logging should include access_log_rotate_size (or an external log rotation policy) to prevent unbounded disk growth.
• error_log without rotation: File-based error logging should include error_log_rotate_size (or an external log rotation policy).

Default ports⁠#

• Both default ports disabled: Setting default_http_port false and default_https_port false means host blocks without explicit ports create no listeners. Make sure all host blocks specify explicit ports, or keep at least one default listener enabled.

PROXY protocol⁠#

• protocol_proxy enabled: PROXY protocol trusts addresses from clients. Enable it only on listeners reachable exclusively by trusted load balancers.

Admin API⁠#

• admin.listen on non-loopback address: The admin API has no encryption and no authentication by default. Use auth_token to enable bearer token auth. Bind to a loopback address or restrict access via network controls.
• admin without auth_token: The admin API has no authentication by default. Use auth_token to require a bearer token on all endpoints except /health when the listener is reachable from untrusted networks.

Location blocks⁠#

• No duplicate location block pathnames: Duplicate pathnames in location blocks cause the server to return an ambiguous response, so avoid them.