internal/server: serve prometheus metrics

This commit makes it possible to serve prometheus metrics on a configurable
bind address. In a default setup, this is exposed at 0.0.0.0:9100.

Signed-off-by: David Bond <davidsbond93@gmail.com>

Author: davidsbond

dev.toml

@@ -10,3 +10,6 @@ bind = "127.0.0.1:4000"
 
 [transport.tcp]
 bind = "127.0.0.1:4000"
+
+[metrics]
+bind = "127.0.0.1:9100"

go.sum

@@ -61,12 +61,16 @@ github.com/google/martian/v3 v3.3.2/go.mod h1:oBOf6HBosgwRXnUGWUB05QECsc6uvmMiJ3
 github.com/google/uuid v1.1.2/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2s0bqwp9tc8=
 github.com/inconshreveable/mousetrap v1.1.0/go.mod h1:vpF70FUmC8bwa3OWnCshd2FqLfsEA9PFc4w1p2J65bw=
+github.com/klauspost/compress v1.18.0 h1:c/Cqfb0r+Yi+JtIEq73FWXVkRonBlf0CRNYc8Zttxdo=
+github.com/klauspost/compress v1.18.0/go.mod h1:2Pp+KzxcywXVXMr50+X0Q/Lsb43OQHYWRCY2AiWywWQ=
 github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
+github.com/kylelemons/godebug v1.1.0 h1:RPNrshWIDI6G2gRW9EHilWtl7Z6Sb1BR0xunSBf0SNc=
+github.com/kylelemons/godebug v1.1.0/go.mod h1:9/0rRGxNHcop5bhtWyNeEfOS8JIWk580+fNqagV/RAw=
 github.com/miekg/dns v1.1.69 h1:Kb7Y/1Jo+SG+a2GtfoFUfDkG//csdRPwRLkCsxDG9Sc=
 github.com/miekg/dns v1.1.69/go.mod h1:7OyjD9nEba5OkqQ/hB4fy3PIoxafSZJtducccIelz3g=
 github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq1c1nUAm88MOHcQC9l5mIlSMApZMrHA=

internal/server/config.go

@@ -14,6 +14,8 @@ type (
 		DNS DNSConfig `toml:"dns"`
 		// Individual transports used by the DNS server.
 		Transport TransportConfig `toml:"transport"`
+		// Prometheus metric configuration.
+		Metrics *MetricsConfig `toml:"metrics"`
 	}
 
 	// The DNSConfig type contains fields for configuring specific DNS behavior.
@@ -85,6 +87,12 @@ type (
 		// The path to the key file.
 		Key string `toml:"key"`
 	}
+
+	// The MetricsConfig type contains fields for configuring the Prometheus metrics endpoint.
+	MetricsConfig struct {
+		// The bind address of the metrics HTTP listener.
+		Bind string `toml:"bind"`
+	}
 )
 
 // DefaultConfig returns a Config type containing default working values for the DNS server. By default, it will
@@ -106,6 +114,9 @@ func DefaultConfig() Config {
 				Bind: "0.0.0.0:53",
 			},
 		},
+		Metrics: &MetricsConfig{
+			Bind: "0.0.0.0:9100",
+		},
 	}
 }
 
@@ -124,10 +135,15 @@ func (c *Config) Validate() error {
 	return errors.Join(
 		c.DNS.validate(),
 		c.Transport.validate(),
+		c.Metrics.validate(),
 	)
 }
 
 func (c *DNSConfig) validate() error {
+	if c == nil {
+		return errors.New("no DNS configuration specified")
+	}
+
 	if len(c.Upstreams) == 0 {
 		return errors.New("no dns upstreams specified")
 	}
@@ -144,6 +160,10 @@ func (c *DNSConfig) validate() error {
 }
 
 func (t *TransportConfig) validate() error {
+	if t == nil {
+		return errors.New("no transport configuration specified")
+	}
+
 	// We never want a server that does nothing at all.
 	if t.UDP == nil && t.TCP == nil && t.DOT == nil && t.DOH == nil {
 		return errors.New("at least one transport must be specified")
@@ -183,3 +203,15 @@ func (t *TransportConfig) validate() error {
 
 	return nil
 }
+
+func (c *MetricsConfig) validate() error {
+	if c == nil {
+		return nil
+	}
+
+	if c.Bind == "" {
+		return errors.New("metrics bind address must be specified")
+	}
+
+	return nil
+}

internal/server/config_test.go

@@ -49,6 +49,9 @@ func TestLoadConfig(t *testing.T) {
 						},
 					},
 				},
+				Metrics: &server.MetricsConfig{
+					Bind: "127.0.0.1:9100",
+				},
 			},
 		},
 		{

internal/server/server.go

@@ -12,13 +12,20 @@ import (
 
 	"github.com/miekg/dns"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"golang.org/x/sync/errgroup"
 
 	"github.com/davidsbond/dns/internal/cache"
 	"github.com/davidsbond/dns/internal/handler"
 	"github.com/davidsbond/dns/internal/list"
 )
 
+func init() {
+	handler.RegisterMetrics(prometheus.DefaultRegisterer)
+	cache.RegisterMetrics(prometheus.DefaultRegisterer)
+	list.RegisterMetrics(prometheus.DefaultRegisterer)
+}
+
 // Run the DNS server.
 func Run(ctx context.Context, config Config) error {
 	if err := config.Validate(); err != nil {
@@ -112,9 +119,14 @@ func Run(ctx context.Context, config Config) error {
 		})
 	}
 
-	handler.RegisterMetrics(prometheus.DefaultRegisterer)
-	cache.RegisterMetrics(prometheus.DefaultRegisterer)
-	list.RegisterMetrics(prometheus.DefaultRegisterer)
+	if config.Metrics != nil {
+		group.Go(func() error {
+			return runHTTPServer(ctx, logger, &http.Server{
+				Addr:    config.Metrics.Bind,
+				Handler: promhttp.Handler(),
+			})
+		})
+	}
 
 	return group.Wait()
 }

internal/server/server_test.go

@@ -55,6 +55,9 @@ func TestRun(t *testing.T) {
 				DeferTLS: true,
 			},
 		},
+		Metrics: &server.MetricsConfig{
+			Bind: "127.0.0.1:9100",
+		},
 	}
 
 	group, ctx := errgroup.WithContext(ctx)

internal/server/testdata/full.toml

@@ -24,3 +24,6 @@ bind = "127.0.0.1:443"
 [transport.doh.tls]
 cert = "path/to/cert.pem"
 key = "path/to/key.pem"
+
+[metrics]
+bind = "127.0.0.1:9100"

licenses/github.com/prometheus/client_golang/internal/github.com/golang/gddo/httputil/LICENSE

@@ -0,0 +1,27 @@
+Copyright (c) 2013 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.