<!-- SPDX-License-Identifier: Apache-2.0 -->
# nginx-ipng-stats-plugin — Configuration Reference

This document enumerates every directive and `listen` parameter introduced by `ngx_http_ipng_stats_module`, the nginx contexts in which
each is legal, the allowed values, and the default (NFR-7.2). For an end-to-end walkthrough read [`user-guide.md`](user-guide.md); for
the reasoning behind the design read [`design.md`](design.md).

## `listen` parameters

These extend the stock nginx `listen` directive. They are parsed by the module and stripped from `cf->args` before the original handler
is invoked, so they compose with every standard `listen` parameter (`ssl`, `http2`, `default_server`, `reuseport`, etc.).

### `device=<ifname>`

**Context:** `listen` directive (wherever `listen` itself is legal — typically inside `server { ... }`).

**Value:** a Linux interface name, e.g. `gre-mg1`, `eth0`. Maximum `IFNAMSIZ - 1` characters (15 on current kernels).

**Default:** not set (plain listen).

**Effect:** the resulting listening socket has `SO_BINDTODEVICE` applied at init-module time, making the kernel accept only connections
whose ingress interface is `<ifname>`. Combined with a wildcard listen address (`80`, `[::]:80`) this is the mechanism by which the
plugin attributes traffic to a specific ingress interface.

The `setsockopt(SO_BINDTODEVICE)` call runs in the nginx master process while it still holds its initial privileges — workers never
call it, and no additional Linux capability is required beyond what stock nginx already has (NFR-6.1).

See FR-1.1, FR-1.5, FR-1.6.

### `ipng_source_tag=<tag>`

**Context:** `listen` directive.

**Value:** a short opaque string identifying the traffic source. No length limit is enforced, but keep it ≤ 32 characters
for readable metric output.

**Default:** when `ipng_source_tag=` is absent but `device=X` is set, the tag defaults to the interface name `X` (FR-1.4). When both
are absent, the tag defaults to the value of `ipng_stats_default_source` at the enclosing `http` level.

**Effect:** every counter recorded on this listener carries `source_tag=<tag>` as a Prometheus label and as the outer key in the JSON
output. Scrape consumers can use this tag to filter the response to only the traffic they delivered. To obtain the VIP address in
nginx config (e.g. in `log_format` or `map`), use nginx's built-in `$server_addr` variable.

See FR-1.2, FR-1.3, FR-1.4.

## `http`-level directives

All plugin-wide settings live in the `http { ... }` block. They cannot be overridden in inner contexts.

### `ipng_stats_zone <name>:<size>`

**Context:** `http`.

**Value:** `<name>` is a string identifier for the shared-memory zone; `<size>` is an nginx size spec with `k` or `m` suffix.

**Default:** none — the directive is mandatory if the module is loaded.

**Effect:** allocates a shared-memory zone of `<size>` bytes to hold the counter hash table. The `<name>` must be stable across
`nginx -s reload` — renaming it forces a fresh segment, which is the one situation where counters reset without a master restart.

**Sizing guidance:** the dominant factor in zone size is `~60 keys per (source, vip)` (one per observed status code). A host serving
50 VIPs behind 4 source interfaces uses `4 × 50 × 60 ≈ 12000` keys, each a few hundred bytes. The default-sized `4m` zone comfortably fits that.
If the zone fills, the module drops new keys and increments `nginx_ipng_zone_full_events_total` — resize and reload.

See FR-5.1, NFR-3.1.

### `ipng_stats_flush_interval <duration>`

**Context:** `http`.

**Value:** an nginx duration string, e.g. `500ms`, `1s`, `2s`.

**Default:** `1s`.

**Minimum:** `100ms`.

**Effect:** sets the cadence of the per-worker flush timer that moves private counter deltas into the shared-memory zone. Lower values
reduce the window of data loss if a worker crashes; higher values reduce the number of atomic adds on the shared zone. The default
is sized so that a scrape interval of 5–15 s sees effectively no lag.

See FR-4.2, FR-5.2.

### `ipng_stats_default_source <tag>`

**Context:** `http`.

**Value:** a short string; see `ipng_source_tag=` above for conventions.

**Default:** `direct`.

**Effect:** sets the tag applied to listening sockets that have neither `device=` nor `ipng_source_tag=`. A host serving a mix of device-attributed
and direct web traffic will see direct traffic under this tag in the scrape output. Rename it to `public`, `localnet`, or anything else
that reads better for your deployment.

See FR-1.3, FR-5.3.

### `ipng_stats_buckets <ms> <ms> <ms> ...`

**Context:** `http`.

**Value:** two or more positive integers, strictly increasing, representing histogram bucket upper bounds in milliseconds.

**Default:** `1 5 10 25 50 100 250 500 1000 2500 5000 10000`, plus an implicit `+Inf` bucket.

**Effect:** overrides the default histogram bucket boundaries for both `request_duration` and `upstream_response_time` histograms. The
same set applies to every `(source, vip)` key in the module (v0.1 does not support per-key override; see
[`design.md`](design.md#decisions-deferred-post-v01)).

See FR-2.3, FR-5.4.

### `ipng_stats on | off`

**Context:** `http`, `server`, `location`.

**Value:** boolean (`on` or `off`).

**Default:** `on` at the `http` level when the module is loaded.

**Effect:** opts a context into or out of counting. Cost of a disabled context is one branch in the log-phase handler. A location
serving the `ipng_stats` scrape handler is automatically excluded from counting regardless of this directive — scraping the scrape
endpoint does not inflate its own counters.

See FR-5.5.

### `ipng_stats_logtail <format_name> udp://<host>:<port> [buffer=<size>] [flush=<duration>] [if=<$variable>]`

**Context:** `http`.

**Value:** `<format_name>` is the name of an existing `log_format` defined earlier in the same `http` block. The destination MUST be a
`udp://host:port` URI. `buffer=<size>` is an optional nginx size spec (default `64k`, minimum `1k`). `flush=<duration>` is an optional
nginx duration string (default `1s`, minimum `100ms`). `if=<$variable>` is an optional condition variable — when set, the log line is
only emitted if the variable evaluates to a non-empty value other than `"0"`.

**Default:** not set — the directive is optional. When absent, no global logtail output is written.

**Effect:** registers a global log-phase writer that fires unconditionally for every request (unless suppressed by `if=`), regardless
of `server` or `location` context. The named `log_format` is looked up from nginx's log module at configuration time; nginx's standard
variable-expansion machinery renders each line, so any variable usable in a regular `log_format` — including `$ipng_source_tag` and
`$server_addr` — is available here.

Each worker maintains a private in-memory write buffer of `buffer=<size>` bytes. Each buffer flush is transmitted as a single
`sendto()` call on a per-worker `SOCK_DGRAM` socket that is opened at worker init and closed at worker exit. The address is resolved
once at configuration time — there is no DNS lookup at flush time. The buffer is flushed when:

- the buffer is full (immediate flush, no lines are dropped);
- the `flush=<duration>` timer fires (periodic flush); or
- the worker exits during a graceful reload or shutdown (final flush).

This covers all request traffic with a single directive at the `http` level, eliminating the need to repeat `access_log` in every
`server` block. It is particularly useful when the format includes `$ipng_source_tag` and `$server_addr`, giving per-device attribution
in every log line at no extra configuration cost.

File-based access logging is intentionally not supported by this directive — use nginx's built-in `access_log` directive for that.

```nginx
log_format ipng_stats_logtail '$host\t$remote_addr\t$request_method\t$request_uri\t'
                              '$status\t$body_bytes_sent\t'
                              '$ipng_source_tag\t$server_addr\t$scheme';
ipng_stats_logtail ipng_stats_logtail udp://127.0.0.1:9514 buffer=16k flush=1s;
```

#### Conditional logging with `if=`

The `if=$variable` parameter suppresses log lines for requests where the variable is empty, not found, or `"0"`. This uses the same
semantics as nginx's built-in `access_log ... if=` and works well with `map` blocks:

```nginx
# Suppress health checks from the logtail stream:
map $request_uri $logtail_enabled {
    ~^/\.well-known/ipng/healthz  0;
    default                       1;
}

ipng_stats_logtail ipng_stats_logtail udp://127.0.0.1:9514 if=$logtail_enabled;
```

The `map` compiles to a hash table at configuration time; at request time it costs a single hash probe, evaluated lazily only when
the variable is read. The condition is checked before the log format is rendered, so filtered requests skip the format rendering
entirely.

**Constraints and behavior:**

- `host` MUST be a literal IPv4 address. Hostnames and IPv6 addresses are not supported in v0.1.
- Each flush emits a single UDP datagram. At the default `buffer=64k` size, datagram payloads comfortably fit within the ~64 KB
  loopback MTU. Operators using very large buffers on non-loopback paths should be aware of path MTU limits.
- If no receiver is listening, the kernel silently discards the datagram. The worker receives no error and is not blocked. This is
  intentional: the logtail is a fire-and-forget analytics transport — zero disk I/O and no backpressure are the point.
- There is no acknowledgment, no retry, and no sequence number. Datagrams lost in transit or because the receiver is down are
  permanently lost.

**Receiver side:** any UDP server works. Two minimal examples:

```bash
# Quick inspection with netcat:
nc -u -l 127.0.0.1 9514

# Production Go receiver snippet:
conn, _ := net.ListenPacket("udp", ":9514")
buf := make([]byte, 65536)
for {
    n, _, _ := conn.ReadFrom(buf)
    process(buf[:n])
}
```

See FR-8.1, FR-8.2, FR-8.3, FR-8.4.

### `ipng_stats;` (scrape handler)

**Context:** `location`.

**Value:** no argument. Placed on its own line inside a `location` block.

**Default:** not set.

**Effect:** turns the enclosing location into the module's scrape handler. No other content handler (`proxy_pass`, `root`, `return`,
`fastcgi_pass`, ...) may be combined with `ipng_stats;` in the same location. The handler honors:

- `Accept:` header — `application/json` for JSON, anything else for Prometheus text.
- `?source_tag=<tag>` — filter output to only counters whose `source_tag` dimension equals the tag. Exact match, case-sensitive.
- `?vip=<address>` — filter output to only counters whose `vip` dimension equals the canonicalized address.

Filters MAY be combined; their effect is the intersection.

**Security:** the module does not ship authentication. Place an `allow`/`deny` ACL in the same `location` block (or its enclosing
`server`) to control access (NFR-6.2).

See FR-3.1, FR-3.2, FR-3.3, FR-3.4, FR-3.5.

## Metric names

For Prometheus, the module exports under the `nginx_ipng_` prefix.

| metric | type | labels | meaning |
| --- | --- | --- | --- |
| `nginx_ipng_requests_total` | counter | `source_tag`, `vip`, `code` | Request count per `(source, vip, status_code)`. |
| `nginx_ipng_bytes_in_total` | counter | `source_tag`, `vip`, `code` | Request bytes received (request line + headers + body). |
| `nginx_ipng_bytes_out_total` | counter | `source_tag`, `vip`, `code` | Response bytes sent (status line + headers + body). |
| `nginx_ipng_request_duration_seconds_bucket` | histogram bucket | `source_tag`, `vip`, `le` | Request duration histogram (Prometheus shape). |
| `nginx_ipng_request_duration_seconds_sum` | histogram sum | `source_tag`, `vip` | Sum of observed durations in seconds. |
| `nginx_ipng_request_duration_seconds_count` | histogram count | `source_tag`, `vip` | Count of observations. |
| `nginx_ipng_upstream_response_seconds_bucket` | histogram bucket | `source_tag`, `vip`, `le` | Upstream response time histogram. |
| `nginx_ipng_upstream_response_seconds_sum` | histogram sum | `source_tag`, `vip` | |
| `nginx_ipng_upstream_response_seconds_count` | histogram count | `source_tag`, `vip` | |
| `nginx_ipng_rate_1s` | gauge | `source_tag`, `vip` | EWMA requests/sec, 1-second decay. |
| `nginx_ipng_rate_10s` | gauge | `source_tag`, `vip` | EWMA requests/sec, 10-second decay. |
| `nginx_ipng_rate_60s` | gauge | `source_tag`, `vip` | EWMA requests/sec, 60-second decay. |
| `nginx_ipng_zone_bytes_used` | gauge | — | Shared-memory zone bytes currently allocated. |
| `nginx_ipng_zone_bytes_total` | gauge | — | Shared-memory zone capacity in bytes. |
| `nginx_ipng_zone_full_events_total` | counter | — | Number of key insertions dropped because the zone was full. |
| `nginx_ipng_flushes_total` | counter | `worker` | Number of per-worker flush ticks executed. |
| `nginx_ipng_flush_duration_seconds` | histogram | `worker` | Histogram of flush durations. |
| `nginx_ipng_scrape_duration_seconds` | histogram | — | Histogram of scrape handler runtimes. |

See FR-2.*, FR-3.7.

## JSON output shape

```json
{
  "schema": 1,
  "by_source": {
    "mg1": {
      "vips": {
        "192.0.2.10": {
          "rate_1s": 42.3,
          "rate_10s": 40.1,
          "rate_60s": 39.8,
          "codes": {
            "200": { "requests": 12345, "bytes_in": 9876543, "bytes_out": 54321098 },
            "404": { "requests": 17,    "bytes_in": 2048,    "bytes_out": 9216 }
          },
          "request_duration_ms": {
            "buckets": { "1": 10, "5": 40, "10": 120, "25": 350, "50": 870, "100": 2100,
                         "250": 3400, "500": 4000, "1000": 4100, "2500": 4120,
                         "5000": 4123, "10000": 4124, "+Inf": 4124 },
            "sum_ms": 87654,
            "count": 4124
          },
          "upstream_response_ms": { "...": "..." }
        }
      }
    }
  },
  "meta": {
    "zone_bytes_used": 131072,
    "zone_bytes_total": 4194304,
    "zone_full_events": 0
  }
}
```

The top-level `schema` field is versioned — breaking changes bump it, additive changes don't. Consumers SHOULD check `schema`
before parsing.

See FR-3.6.

## Context summary

| knob | `http` | `server` | `location` | `listen` |
| --- | --- | --- | --- | --- |
| `ipng_stats_zone` | ✅ | — | — | — |
| `ipng_stats_flush_interval` | ✅ | — | — | — |
| `ipng_stats_default_source` | ✅ | — | — | — |
| `ipng_stats_buckets` | ✅ | — | — | — |
| `ipng_stats_logtail` | ✅ | — | — | — |
| `ipng_stats on\|off` | ✅ | ✅ | ✅ | — |
| `ipng_stats;` (handler) | — | — | ✅ | — |
| `device=<ifname>` | — | — | — | ✅ |
| `ipng_source_tag=<tag>` | — | — | — | ✅ |