TLS certificate check

Simple service for remote health check of TLS-certificates. Designed to be a data source for Prometheus monitoring system.

Could be useful for monitoring infrastructure of numerous services with short-living TLS-certificates, e.g. obtained from LetsEncrypt.

Build

cabal sandbox init
cabal install

Usage

In order to monitor TLS-certificates at first one need to create config file with description of endpoints. Simple example could be found in file sample.yaml.

There are three sections in config file to be filled:

• general
• certificates:
• keys

- General

Here one defines monitoring service port for prometheus server, path to collect metrics and interval between check procedures in seconds.

- Certificates

In this section endpoints to monitor TLS-certificate expiration time are described. One should define hostname and port for each service to monitor. Service labels will be constructed as <host>:<port>.

For each item in section certificates monitor get number of days remaining to certificate expiration.

- Keys

Besides of certificate expiration, monitor could also check public key signature in order to detect key change. In this section endpoints are defined in a similar way, but there is additional field key - public key signature. Public key signature for each endpoint could be obtained with Openssl:

#!/bin/sh

HOST="127.0.0.1"
PORT=11011

openssl s_client -connect $HOST:$PORT | openssl x509 -noout -modulus | openssl md5

Result interpretation

Monitoring service expose interface for Prometheus server on the defined port and path. Results look as follows:

# TYPE crt_mon_exp gauge
crt_mon_exp{host="www.google.com:443"} 74.0
crt_mon_exp{host="www.yandex.ru:443"} 714.0
# TYPE crt_mon_key_valid gauge
crt_mon_key_valid{host="127.0.0.1:11011"} -1000.0

Here metric crt_mon_exp gives certificate expiration time in days and crt_mon_key_valid indicates if endpoint public key remains the same (value: 1) or been changed (value: 0).

All metric values could be distinguished by host label.

Errors

Any possible error must be also encoded as a float number in order to be processed with Prometheus. To separate values and errors the latter are negative.

Here is mapping table for general errors:

Error description	Value
endpoint connection error	-1000
certificate retrieval error	-2000
public key retrieval error	-3000

Certificate expiration time errors:

Certificate error	Value
certificate not valid yet	-100
certificate expired	-200

Modifications

All described mappings could be easily changed. One just need to modify following functions in file TLSProcessing.hs and recompile the project:

• errToMetric
• daysToMetric
• keyValidToMetric