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# nginx-logtail User Guide
## Overview
nginx-logtail is a four-component system for real-time traffic analysis across a cluster of nginx
machines. It answers questions like:
- Which client prefix is causing the most HTTP 429s right now?
- Which website is getting the most 503s over the last 24 hours?
- Which nginx machine is the busiest?
- Is there a DDoS in progress, and from where?
Components:
| Binary | Runs on | Role |
|---------------|------------------|----------------------------------------------------|
| `collector` | each nginx host | Tails log files, aggregates in memory, serves gRPC |
| `aggregator` | central host | Merges all collectors, serves unified gRPC |
| `frontend` | central host | HTTP dashboard with drilldown UI |
| `cli` | operator laptop | Shell queries against collector or aggregator |
---
## nginx Configuration
Add the `logtail` log format to your `nginx.conf` and apply it to each `server` block:
```nginx
http {
log_format logtail '$host\t$remote_addr\t$msec\t$request_method\t$request_uri\t$status\t$body_bytes_sent\t$request_time\t$is_tor\t$asn';
server {
access_log /var/log/nginx/access.log logtail;
# or per-vhost:
access_log /var/log/nginx/www.example.com.access.log logtail;
}
}
```
The format is tab-separated with fixed field positions. Query strings are stripped from the URI
by the collector at ingest time — only the path is tracked.
`$is_tor` must be set to `1` when the client IP is a TOR exit node and `0` otherwise (typically
populated by a custom nginx variable or a Lua script that checks the IP against a TOR exit list).
The field is optional for backward compatibility — log lines without it are accepted and treated
as `is_tor=0`.
`$asn` must be set to the client's AS number as a decimal integer (e.g. from MaxMind GeoIP2's
`$geoip2_data_autonomous_system_number`). The field is optional — log lines without it default
to `asn=0`.
---
## Building
```bash
git clone https://git.ipng.ch/ipng/nginx-logtail
cd nginx-logtail
go build ./cmd/collector/
go build ./cmd/aggregator/
go build ./cmd/frontend/
go build ./cmd/cli/
```
Requires Go 1.21+. No CGO, no external runtime dependencies.
---
## Collector
Runs on each nginx machine. Tails log files, maintains in-memory top-K counters across six time
windows, and exposes a gRPC interface for the aggregator (and directly for the CLI).
### Flags
| Flag | Default | Description |
|-------------------|--------------|-----------------------------------------------------------|
| `--listen` | `:9090` | gRPC listen address |
| `--prom-listen` | `:9100` | Prometheus metrics address; empty string to disable |
| `--logs` | — | Comma-separated log file paths or glob patterns |
| `--logs-file` | — | File containing one log path/glob per line |
| `--source` | hostname | Name for this collector in query responses |
| `--v4prefix` | `24` | IPv4 prefix length for client bucketing (e.g. /24 → /23) |
| `--v6prefix` | `48` | IPv6 prefix length for client bucketing |
| `--scan-interval` | `10s` | How often to rescan glob patterns for new/removed files |
At least one of `--logs` or `--logs-file` is required.
### Examples
```bash
# Single file
./collector --logs /var/log/nginx/access.log
# Multiple files via glob (one inotify instance regardless of count)
./collector --logs "/var/log/nginx/*/access.log"
# Many files via a config file
./collector --logs-file /etc/nginx-logtail/logs.conf
# Custom prefix lengths and listen address
./collector \
--logs "/var/log/nginx/*.log" \
--listen :9091 \
--source nginx3.prod \
--v4prefix 24 \
--v6prefix 48
```
### logs-file format
One path or glob pattern per line. Lines starting with `#` are ignored.
```
# /etc/nginx-logtail/logs.conf
/var/log/nginx/access.log
/var/log/nginx/*/access.log
/var/log/nginx/api.example.com.access.log
```
### Log rotation
The collector handles logrotate automatically. On `RENAME`/`REMOVE` events it drains the old file
descriptor to EOF (so no lines are lost), then retries opening the original path with backoff until
the new file appears. No restart or SIGHUP required.
### Prometheus metrics
The collector exposes a Prometheus-compatible `/metrics` endpoint on `--prom-listen` (default
`:9100`). Set `--prom-listen ""` to disable it entirely.
Three metrics are exported:
**`nginx_http_requests_total`** — counter, labeled `{host, method, status}`:
```
nginx_http_requests_total{host="example.com",method="GET",status="200"} 18432
nginx_http_requests_total{host="example.com",method="POST",status="201"} 304
nginx_http_requests_total{host="api.example.com",method="GET",status="429"} 57
```
**`nginx_http_response_body_bytes`** — histogram, labeled `{host}`. Observes the
`$body_bytes_sent` value for every request. Bucket upper bounds (bytes):
`256, 1024, 4096, 16384, 65536, 262144, 1048576, +Inf`.
**`nginx_http_request_duration_seconds`** — histogram, labeled `{host}`. Observes the
`$request_time` value for every request. Bucket upper bounds (seconds):
`0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10, +Inf`.
Body and request-time histograms use only the `host` label (not method/status) to keep
cardinality bounded — the label sets stay proportional to the number of virtual hosts, not
the number of unique method × status combinations.
The counter map is capped at 100 000 distinct `{host, method, status}` tuples. Entries beyond
the cap are silently dropped for the current scrape interval, so memory is bounded regardless
of traffic patterns.
**Prometheus scrape config:**
```yaml
scrape_configs:
- job_name: nginx_logtail
static_configs:
- targets:
- nginx1:9100
- nginx2:9100
- nginx3:9100
```
Or with service discovery — the collector has no special requirements beyond a reachable
TCP port.
**Example queries:**
```promql
# Request rate per host over last 5 minutes
rate(nginx_http_requests_total[5m])
# 5xx error rate fraction per host
sum by (host) (rate(nginx_http_requests_total{status=~"5.."}[5m]))
/
sum by (host) (rate(nginx_http_requests_total[5m]))
# 95th percentile response time per host
histogram_quantile(0.95,
sum by (host, le) (rate(nginx_http_request_duration_seconds_bucket[5m]))
)
# Median response body size per host
histogram_quantile(0.50,
sum by (host, le) (rate(nginx_http_response_body_bytes_bucket[5m]))
)
```
### Memory usage
The collector is designed to stay well under 1 GB:
| Structure | Max entries | Approx size |
|-----------------------------|-------------|-------------|
| Live map (current minute) | 100 000 | ~19 MB |
| Fine ring (60 × 1-min) | 60 × 50 000 | ~558 MB |
| Coarse ring (288 × 5-min) | 288 × 5 000 | ~268 MB |
| **Total** | | **~845 MB** |
When the live map reaches 100 000 distinct 6-tuples, new keys are dropped for the rest of that
minute. Existing keys continue to accumulate counts. The cap resets at each minute rotation.
### Time windows
Data is served from two tiered ring buffers:
| Window | Source ring | Resolution |
|--------|-------------|------------|
| 1 min | fine | 1 minute |
| 5 min | fine | 1 minute |
| 15 min | fine | 1 minute |
| 60 min | fine | 1 minute |
| 6 h | coarse | 5 minutes |
| 24 h | coarse | 5 minutes |
History is lost on restart — the collector resumes tailing immediately but all ring buffers start
empty. The fine ring fills in 1 hour; the coarse ring fills in 24 hours.
### Systemd unit example
```ini
[Unit]
Description=nginx-logtail collector
After=network.target
[Service]
ExecStart=/usr/local/bin/collector \
--logs-file /etc/nginx-logtail/logs.conf \
--listen :9090 \
--source %H
Restart=on-failure
RestartSec=5
[Install]
WantedBy=multi-user.target
```
---
## Aggregator
Runs on a central machine. Subscribes to the `StreamSnapshots` push stream from every configured
collector, merges their snapshots into a unified in-memory cache, and serves the same gRPC
interface as the collector. The frontend and CLI query the aggregator exactly as they would query
a single collector.
### Flags
| Flag | Default | Description |
|----------------|-----------|--------------------------------------------------------|
| `--listen` | `:9091` | gRPC listen address |
| `--collectors` | — | Comma-separated `host:port` addresses of collectors |
| `--source` | hostname | Name for this aggregator in query responses |
`--collectors` is required; the aggregator exits immediately if it is not set.
### Example
```bash
./aggregator \
--collectors nginx1:9090,nginx2:9090,nginx3:9090 \
--listen :9091 \
--source agg-prod
```
### Fault tolerance
The aggregator reconnects to each collector independently with exponential backoff (100 ms →
doubles → cap 30 s). After 3 consecutive failures to a collector it marks that collector
**degraded**: its last-known contribution is subtracted from the merged view so stale counts
do not accumulate. When the collector recovers and sends a new snapshot, it is automatically
reintegrated. The remaining collectors continue serving queries throughout.
### Memory
The aggregator's merged cache uses the same tiered ring-buffer structure as the collector
(60 × 1-min fine, 288 × 5-min coarse) but holds at most top-50 000 entries per fine bucket
and top-5 000 per coarse bucket across all collectors combined. Memory footprint is roughly
the same as one collector (~845 MB worst case).
### Systemd unit example
```ini
[Unit]
Description=nginx-logtail aggregator
After=network.target
[Service]
ExecStart=/usr/local/bin/aggregator \
--collectors nginx1:9090,nginx2:9090,nginx3:9090 \
--listen :9091 \
--source %H
Restart=on-failure
RestartSec=5
[Install]
WantedBy=multi-user.target
```
---
## Frontend
HTTP dashboard. Connects to the aggregator (or directly to a single collector for debugging).
Zero JavaScript — server-rendered HTML with inline SVG sparklines.
### Flags
| Flag | Default | Description |
|-------------|-------------------|--------------------------------------------------|
| `--listen` | `:8080` | HTTP listen address |
| `--target` | `localhost:9091` | Default gRPC endpoint (aggregator or collector) |
| `--n` | `25` | Default number of table rows |
| `--refresh` | `30` | Auto-refresh interval in seconds; `0` to disable |
### Usage
Navigate to `http://your-host:8080`. The dashboard shows a ranked table of the top entries for
the selected dimension and time window.
**Window tabs** — switch between `1m / 5m / 15m / 60m / 6h / 24h`. Only the window changes;
all active filters are preserved.
**Dimension tabs** — switch between grouping by `website / asn / prefix / status / uri`.
**Drilldown** — click any table row to add that value as a filter and advance to the next
dimension in the hierarchy:
```
website → client prefix → request URI → HTTP status → ASN → website (cycles)
```
Example: click `example.com` in the website view to see which client prefixes are hitting it;
click a prefix there to see which URIs it is requesting; and so on.
**Breadcrumb strip** — shows all active filters above the table. Click `×` next to any token
to remove just that filter, keeping the others.
**Sparkline** — inline SVG trend chart showing total request count per time bucket for the
current filter state. Useful for spotting sudden spikes or sustained DDoS ramps.
**Filter expression box** — a text input above the table accepts a mini filter language that
lets you type expressions directly without editing the URL:
```
status>=400
status>=400 AND website~=gouda.*
status>=400 AND website~=gouda.* AND uri~="^/api/"
website=example.com AND prefix=1.2.3.0/24
```
Supported fields and operators:
| Field | Operators | Example |
|-----------|---------------------|-----------------------------------|
| `status` | `=` `!=` `>` `>=` `<` `<=` | `status>=400` |
| `website` | `=` `~=` | `website~=gouda.*` |
| `uri` | `=` `~=` | `uri~=^/api/` |
| `prefix` | `=` | `prefix=1.2.3.0/24` |
| `is_tor` | `=` `!=` | `is_tor=1`, `is_tor!=0` |
| `asn` | `=` `!=` `>` `>=` `<` `<=` | `asn=8298`, `asn>=1000` |
`is_tor=1` and `is_tor!=0` are equivalent (TOR traffic only). `is_tor=0` and `is_tor!=1` are
equivalent (non-TOR traffic only).
`asn` accepts the same comparison expressions as `status`. Use `asn=8298` to match a single AS,
`asn>=64512` to match the private-use ASN range, or `asn!=0` to exclude unresolved entries.
`~=` means RE2 regex match. Values with spaces or quotes may be wrapped in double or single
quotes: `uri~="^/search\?q="`.
The box pre-fills with the current active filter (including filters set by drilldown clicks),
so you can see and extend what is applied. Submitting redirects to a clean URL with the
individual filter params; `× clear` removes all filters at once.
On a parse error the page re-renders with the error shown below the input and the current
data and filters unchanged.
**Status expressions** — the `f_status` URL param (and `status` in the expression box) accepts
comparison expressions: `200`, `!=200`, `>=400`, `<500`, etc.
**Regex filters** — `f_website_re` and `f_uri_re` URL params (and `~=` in the expression box)
accept RE2 regular expressions. The breadcrumb strip shows them as `website~=gouda.*` and
`uri~=^/api/` with the usual `×` remove link.
**URL sharing** — all filter state is in the URL query string (`w`, `by`, `f_website`,
`f_prefix`, `f_uri`, `f_status`, `f_website_re`, `f_uri_re`, `f_is_tor`, `f_asn`, `n`). Copy
the URL to share an exact view with another operator, or bookmark a recurring query.
**JSON output** — append `&raw=1` to any URL to receive the TopN result as JSON instead of
HTML. Useful for scripting without the CLI binary:
```bash
# All 429s by prefix
curl -s 'http://frontend:8080/?f_status=429&by=prefix&w=1m&raw=1' | jq '.entries[0]'
# All errors (>=400) on gouda hosts
curl -s 'http://frontend:8080/?f_status=%3E%3D400&f_website_re=gouda.*&by=uri&w=5m&raw=1'
```
**Target override** — append `?target=host:port` to point the frontend at a different gRPC
endpoint for that request (useful for comparing a single collector against the aggregator):
```bash
http://frontend:8080/?target=nginx3:9090&w=5m
```
**Source picker** — when the frontend is pointed at an aggregator, a `source:` tab row appears
below the dimension tabs listing each individual collector alongside an **all** tab (the default
merged view). Clicking a collector tab switches the frontend to query that collector directly for
the current request, letting you answer "which nginx machine is the busiest?" without leaving the
dashboard. The picker is hidden when querying a collector directly (it has no sub-sources to list).
---
## CLI
A shell companion for one-off queries and debugging. Works with any `LogtailService` endpoint —
collector or aggregator. Accepts multiple targets, fans out concurrently, and labels each result.
Default output is a human-readable table; add `--json` for machine-readable NDJSON.
### Subcommands
```
logtail-cli topn [flags] ranked label → count table
logtail-cli trend [flags] per-bucket time series
logtail-cli stream [flags] live snapshot feed (runs until Ctrl-C)
logtail-cli targets [flags] list targets known to the queried endpoint
```
### Shared flags (all subcommands)
| Flag | Default | Description |
|---------------|------------------|----------------------------------------------------------|
| `--target` | `localhost:9090` | Comma-separated `host:port` list; queries fan out to all |
| `--json` | false | Emit newline-delimited JSON instead of a table |
| `--website` | — | Filter to this website |
| `--prefix` | — | Filter to this client prefix |
| `--uri` | — | Filter to this request URI |
| `--status` | — | Filter: HTTP status expression (`200`, `!=200`, `>=400`, `<500`, …) |
| `--website-re`| — | Filter: RE2 regex against website |
| `--uri-re` | — | Filter: RE2 regex against request URI |
| `--is-tor` | — | Filter: `1` or `!=0` = TOR only; `0` or `!=1` = non-TOR only |
| `--asn` | — | Filter: ASN expression (`12345`, `!=65000`, `>=1000`, `<64512`, …) |
### `topn` flags
| Flag | Default | Description |
|---------------|------------|----------------------------------------------------------|
| `--n` | `10` | Number of entries |
| `--window` | `5m` | `1m` `5m` `15m` `60m` `6h` `24h` |
| `--group-by` | `website` | `website` `prefix` `uri` `status` `asn` |
### `trend` flags
| Flag | Default | Description |
|---------------|------------|----------------------------------------------------------|
| `--window` | `5m` | `1m` `5m` `15m` `60m` `6h` `24h` |
### Output format
**Table** (default — single target, no header):
```
RANK COUNT LABEL
1 18 432 example.com
2 4 211 other.com
```
**Multi-target** — each target gets a labeled section:
```
=== col-1 (nginx1:9090) ===
RANK COUNT LABEL
1 10 000 example.com
=== agg-prod (agg:9091) ===
RANK COUNT LABEL
1 18 432 example.com
```
**JSON** (`--json`) — a single JSON array with one object per target, suitable for `jq`:
```json
[{"source":"agg-prod","target":"agg:9091","entries":[{"label":"example.com","count":18432},...]}]
```
**`stream` JSON** — one object per snapshot received (NDJSON), runs until interrupted:
```json
{"ts":1773516180,"source":"col-1","target":"nginx1:9090","total_entries":823,"top_label":"example.com","top_count":10000}
```
### `targets` subcommand
Lists the targets (collectors) known to the queried endpoint. When querying an aggregator, returns
all configured collectors with their display names and addresses. When querying a collector,
returns the collector itself (address shown as `(self)`).
```bash
# List collectors behind the aggregator
logtail-cli targets --target agg:9091
# Machine-readable output
logtail-cli targets --target agg:9091 --json
```
Table output example:
```
nginx1.prod nginx1:9090
nginx2.prod nginx2:9090
nginx3.prod (self)
```
JSON output (`--json`) — one object per target:
```json
{"query_target":"agg:9091","name":"nginx1.prod","addr":"nginx1:9090"}
```
### Examples
```bash
# Top 20 client prefixes sending 429s right now
logtail-cli topn --target agg:9091 --window 1m --group-by prefix --status 429 --n 20
# Same query, pipe to jq for scripting
logtail-cli topn --target agg:9091 --window 1m --group-by prefix --status 429 --n 20 \
--json | jq '.[0].entries[0]'
# Which website has the most errors (4xx or 5xx) over the last 24h?
logtail-cli topn --target agg:9091 --window 24h --group-by website --status '>=400'
# Which client prefixes are NOT getting 200s? (anything non-success)
logtail-cli topn --target agg:9091 --window 5m --group-by prefix --status '!=200'
# Drill: top URIs on one website over the last 60 minutes
logtail-cli topn --target agg:9091 --window 60m --group-by uri --website api.example.com
# Filter by website regex: all gouda hosts
logtail-cli topn --target agg:9091 --window 5m --website-re 'gouda.*'
# Filter by URI regex: all /api/ paths
logtail-cli topn --target agg:9091 --window 5m --group-by uri --uri-re '^/api/'
# Show only TOR traffic — which websites are TOR clients hitting?
logtail-cli topn --target agg:9091 --window 5m --is-tor 1
# Show non-TOR traffic only — exclude exit nodes from the view
logtail-cli topn --target agg:9091 --window 5m --is-tor 0
# Top ASNs by request count over the last 5 minutes
logtail-cli topn --target agg:9091 --window 5m --group-by asn
# Which ASNs are generating the most 429s?
logtail-cli topn --target agg:9091 --window 5m --group-by asn --status 429
# Filter to traffic from a specific ASN
logtail-cli topn --target agg:9091 --window 5m --asn 8298
# Filter to traffic from private-use / unallocated ASNs
logtail-cli topn --target agg:9091 --window 5m --group-by prefix --asn '>=64512'
# Exclude unresolved entries (ASN 0) and show top source ASNs
logtail-cli topn --target agg:9091 --window 5m --group-by asn --asn '!=0'
# Compare two collectors side by side in one command
logtail-cli topn --target nginx1:9090,nginx2:9090 --window 5m
# Query both a collector and the aggregator at once
logtail-cli topn --target nginx3:9090,agg:9091 --window 5m --group-by prefix
# Trend of total traffic over 6h (for a quick sparkline in the terminal)
logtail-cli trend --target agg:9091 --window 6h --json | jq '.[0].points | [.[].count]'
# Watch live merged snapshots from the aggregator
logtail-cli stream --target agg:9091
# Watch two collectors simultaneously; each snapshot is labeled by source
logtail-cli stream --target nginx1:9090,nginx2:9090
```
The `stream` subcommand reconnects automatically after errors (5 s backoff) and runs until
interrupted with Ctrl-C. The `topn` and `trend` subcommands exit immediately after one response.
---
## Operational notes
**No persistence.** All data is in-memory. A collector restart loses ring buffer history but
resumes tailing the log file from the current position immediately.
**No TLS.** Designed for trusted internal networks. If you need encryption in transit, put a
TLS-terminating proxy (e.g. stunnel, nginx stream) in front of the gRPC port.
**inotify limits.** The collector uses a single inotify instance regardless of how many files it
tails. If you tail files across many different directories, check
`/proc/sys/fs/inotify/max_user_watches` (default 8192); increase it if needed:
```bash
echo 65536 | sudo tee /proc/sys/fs/inotify/max_user_watches
```
**High-cardinality attacks.** If a DDoS sends traffic from thousands of unique /24 prefixes with
unique URIs, the live map will hit its 100 000 entry cap and drop new keys for the rest of that
minute. The top-K entries already tracked continue accumulating counts. This is by design — the
cap prevents memory exhaustion under attack conditions.
**Clock skew.** Trend sparklines are based on the collector's local clock. If collectors have
significant clock skew, trend buckets from different collectors may not align precisely in the
aggregator. NTP sync is recommended.
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