docs: add long overdue readme

README.md

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+# prometheus-dnssec-exporter
+
+A Prometheus exporter that exposes metrics on DNSSEC-signed DNS zones.
+One of its core features is that it compares a zone's `CDS` record set
+to the `DS` record set in the parent zone.
+
+## Installation
+
+### From Source
+
+```bash
+git clone https://gitlab.com/s3lph/prometheus-dnssec-exporter
+cd prometheus-dnssec-exporter
+make prometheus-dnssec-exporter
+```
+
+### Debian Package
+
+There is an automatically built Debian package in my repository [https://repo.s3lph.me/]:
+
+```bash
+wget -O /etc/apt/trusted.gpg.d/repo.s3lph.me.gpg https://repo.s3lph.me/debian/repo.s3lph.me.gpg
+echo "deb https://repo.s3lph.me/debian stable main" > /etc/apt/sources.list.d/repo.s3lph.me.list
+apt update
+apt install prometheus-dnssec-exporter
+```
+
+If you do not want to add my repository to your system, you can also download the deb package from the repository: https://repo.s3lph.me/debian/pool/main/p/prometheus-dnssec-exporter/
+
+Or you can build the package yourself: https://gitlab.com/s3lph/custom-packages/-/blob/main/prometheus-dnssec-exporter/build.sh
+
+## Usage
+
+### Configuration
+
+The DNSSEC exporter requires a configuration file.  When using the Debian package, this file is located at `/etc/prometheus/dnssec-exporter/config.yaml`
+
+```yaml
+---
+
+## dnssec exporter configuration
+#
+#dns:
+#  # The resolver to use.  Must be DNSSEC validating, and
+#  # must not strip DNSSEC responses.
+#  resolver: 1.1.1.1:53
+#  # List of zones to resolve.
+#  zones:
+#    - example.org.
+#    - example.com.
+
+## TLS and Basic Auth can be configured here as well, see for details:
+## https://github.com/prometheus/exporter-toolkit/blob/master/web/tls_config.go#L36
+#
+#basic_auth_users:
+#  user1: pass1
+#  user2: pass2
+#tls_server_config:
+#  cert_file: server.crt
+#  key_file: server.key
+```
+
+You should at least provide the resolver to use (the DNSSEC exporter only works with a DNSSEC-validating resolver!) and the zones you want to collect metrics on:
+
+```yaml
+---
+dns:
+  resolver: 9.9.9.9:53
+  zones:
+    - example.org.
+	- example.com.
+```
+
+### Running
+
+You can start the exporter using the following command:
+
+```bash
+prometheus-dnssec-exporter --config=path/to/dnssec-exporter/config.yaml --web.listen-address=:9142
+```
+
+The Debian package provides a systemd service unit that does the job for you:
+
+```bash
+systemctl enable --now prometheus-dnssec-exporter
+```
+
+## Metrics
+
+The following metrics are exposed at the `/metrics` HTTP endpoint:
+
+```prom
+# HELP dnssec_cds_count Number of CDS records present in the zone
+# TYPE dnssec_cds_count gauge
+dnssec_cds_count{parent="ch.",tld="ch.",zone="example.org."} 1
+dnssec_cds_count{parent="me.",tld="me.",zone="example.com."} 1
+dnssec_cds_count{parent="example.com.",tld="me.",zone="subdomain.example.com."} 1
+# HELP dnssec_cds_ds_match 1 if the CDS and DS records match, 0 otherwise
+# TYPE dnssec_cds_ds_match gauge
+dnssec_cds_ds_match{parent="ch.",tld="ch.",zone="example.org."} 0
+dnssec_cds_ds_match{parent="me.",tld="me.",zone="example.com."} 1
+dnssec_cds_ds_match{parent="example.com.",tld="me.",zone="subdomain.example.com."} 1
+# HELP dnssec_cds_rcode RCode of the CDS record answer
+# TYPE dnssec_cds_rcode gauge
+dnssec_cds_rcode{parent="ch.",tld="ch.",zone="example.org."} 0
+dnssec_cds_rcode{parent="me.",tld="me.",zone="example.com."} 0
+dnssec_cds_rcode{parent="example.com.",tld="me.",zone="subdomain.example.com."} 0
+# HELP dnssec_ds_count Number of DS record is present in the parent zone
+# TYPE dnssec_ds_count gauge
+dnssec_ds_count{parent="ch.",tld="ch.",zone="example.org."} 1
+dnssec_ds_count{parent="me.",tld="me.",zone="example.com."} 1
+dnssec_ds_count{parent="example.com.",tld="me.",zone="subdomain.example.com."} 1
+# HELP dnssec_ds_rcode RCode of the DS record answer
+# TYPE dnssec_ds_rcode gauge
+dnssec_ds_rcode{parent="ch.",tld="ch.",zone="example.org."} 0
+dnssec_ds_rcode{parent="me.",tld="me.",zone="example.com."} 0
+dnssec_ds_rcode{parent="example.com.",tld="me.",zone="subdomain.example.com."} 0
+# HELP dnssec_signature_ok 1 if the DNSSEC signature is present and valid, 0 otherwise
+# TYPE dnssec_signature_ok gauge
+dnssec_signature_ok{parent="ch.",tld="ch.",zone="example.org."} 1
+dnssec_signature_ok{parent="me.",tld="me.",zone="example.com."} 1
+dnssec_signature_ok{parent="example.com.",tld="me.",zone="subdomain.example.com."} 1
+# HELP dnssec_signature_rcode RCode of the DNS query
+# TYPE dnssec_signature_rcode gauge
+dnssec_signature_rcode{parent="ch.",tld="ch.",zone="example.org."} 0
+dnssec_signature_rcode{parent="me.",tld="me.",zone="example.com."} 0
+dnssec_signature_rcode{parent="example.com.",tld="me.",zone="subdomain.example.com."} 0
+```
+
+The two metrics that are probably the most important are:
+
+- `dnssec_signature_ok`: If this is 0, the chain of trust to your zone is broken.
+- `dnssec_cds_ds_match`: If this is 0, it's most likely a KSK rollover is in progress.  If your registry does not support CDS submission, this is the sign that you need to replace the `DS` records in the parent zone.