Add ARP collector for Linux (#540)

* Implement commonalities and linux support for ARP collection

* Add ARP collector to fixtures and run as part of e2e tests

* Bubble up scanner errors

* Use single return values where it makes sense

* Add missing annotation

* Move arp_common into arp_linux

* Add license header to arp_linux.go

* Address initial feedback

* Use strings.Fields instead of strings.Split

* Deal with scanner.Err() rather than throwing away errors

* Check for scan errors in-line before interacting with the entries map

* Don't interact with potentially empty text from scan

* Check for scan errors outside the scan loop

* Add comment about moving procfs parsing

* Add more direct comment

* Update initialism style to match go style guide

* Put function args on the same line

* Add TODO in front of comment about procfs extraction

* Guard against strings.Fields returning an empty slice

* Be more defensive about ARP table format and use upcase more broadly

* Enable the ARP collector by default

* Add ARP collector to the README

* Remove 'entry'

6 files changed, 117 insertions, 1 deletions

diff --git a/README.md b/README.md
index d139cef..645925f 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,7 @@ Which collectors are used is controlled by the `--collectors.enabled` flag.
 
 Name     | Description | OS
 ---------|-------------|----
+arp | Exposes ARP statistics from `/proc/net/arp`. | Linux
 conntrack | Shows conntrack statistics (does nothing if no `/proc/sys/net/netfilter/` present). | Linux
 cpu | Exposes CPU statistics | Darwin, Dragonfly, FreeBSD
 diskstats | Exposes disk I/O statistics from `/proc/diskstats`. | Linux
diff --git a/collector/arp_linux.go b/collector/arp_linux.go
new file mode 100644
index 0000000..71c1c5a
--- /dev/null
+++ b/collector/arp_linux.go
@@ -0,0 +1,102 @@
+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !noarp
+
+package collector
+
+import (
+        "bufio"
+        "fmt"
+        "io"
+        "os"
+        "strings"
+
+        "github.com/prometheus/client_golang/prometheus"
+)
+
+type arpCollector struct {
+        entries *prometheus.Desc
+}
+
+func init() {
+        Factories["arp"] = NewARPCollector
+}
+
+// NewARPCollector returns a new Collector exposing ARP stats.
+func NewARPCollector() (Collector, error) {
+        return &arpCollector{
+                entries: prometheus.NewDesc(
+                        prometheus.BuildFQName(Namespace, "arp", "entries"),
+                        "ARP entries by device",
+                        []string{"device"}, nil,
+                ),
+        }, nil
+}
+
+func getARPEntries() (map[string]uint32, error) {
+        file, err := os.Open(procFilePath("net/arp"))
+        if err != nil {
+                return nil, err
+        }
+        defer file.Close()
+
+        entries, err := parseARPEntries(file)
+        if err != nil {
+                return nil, err
+        }
+
+        return entries, nil
+}
+
+// TODO: This should get extracted to the github.com/prometheus/procfs package
+// to support more complete parsing of /proc/net/arp. Instead of adding
+// more fields to this function's return values it should get moved and
+// changed to support each field.
+func parseARPEntries(data io.Reader) (map[string]uint32, error) {
+        scanner := bufio.NewScanner(data)
+        entries := make(map[string]uint32)
+
+        for scanner.Scan() {
+                columns := strings.Fields(scanner.Text())
+
+                if len(columns) < 6 {
+                        return nil, fmt.Errorf("unexpected ARP table format")
+                }
+
+                if columns[0] != "IP" {
+                        deviceIndex := len(columns) - 1
+                        entries[columns[deviceIndex]]++
+                }
+        }
+
+        if err := scanner.Err(); err != nil {
+                return nil, fmt.Errorf("failed to parse ARP info: %s", err)
+        }
+
+        return entries, nil
+}
+
+func (c *arpCollector) Update(ch chan<- prometheus.Metric) error {
+        entries, err := getARPEntries()
+        if err != nil {
+                return fmt.Errorf("could not get ARP entries: %s", err)
+        }
+
+        for device, entryCount := range entries {
+                ch <- prometheus.MustNewConstMetric(
+                        c.entries, prometheus.GaugeValue, float64(entryCount), device)
+        }
+
+        return nil
+}
diff --git a/collector/fixtures/e2e-output.txt b/collector/fixtures/e2e-output.txt
index 06c27ae..1828299 100644
--- a/collector/fixtures/e2e-output.txt
+++ b/collector/fixtures/e2e-output.txt
@@ -71,6 +71,10 @@ http_response_size_bytes{handler="prometheus",quantile="0.9"} NaN
 http_response_size_bytes{handler="prometheus",quantile="0.99"} NaN
 http_response_size_bytes_sum{handler="prometheus"} 0
 http_response_size_bytes_count{handler="prometheus"} 0
+# HELP node_arp_entries ARP entries by device
+# TYPE node_arp_entries gauge
+node_arp_entries{device="eth0"} 3
+node_arp_entries{device="eth1"} 3
 # HELP node_bonding_active Number of active slaves per bonding interface.
 # TYPE node_bonding_active gauge
 node_bonding_active{master="bond0"} 0
@@ -2106,6 +2110,7 @@ node_procs_running 2
 # TYPE node_scrape_collector_duration_seconds gauge
 # HELP node_scrape_collector_success node_exporter: Whether a collector succeeded.
 # TYPE node_scrape_collector_success gauge
+node_scrape_collector_success{collector="arp"} 1
 node_scrape_collector_success{collector="bonding"} 1
 node_scrape_collector_success{collector="buddyinfo"} 1
 node_scrape_collector_success{collector="conntrack"} 1
diff --git a/collector/fixtures/proc/net/arp b/collector/fixtures/proc/net/arp
new file mode 100644
index 0000000..84c67f8
--- /dev/null
+++ b/collector/fixtures/proc/net/arp
@@ -0,0 +1,7 @@
+IP address       HW type     Flags       HW address            Mask     Device
+192.168.1.1    0x1         0x2         cc:aa:dd:ee:aa:bb     *        eth0
+192.168.1.2    0x1         0x2         bb:cc:dd:ee:ff:aa     *        eth0
+192.168.1.3    0x1         0x2         aa:bb:cc:dd:ee:ff     *        eth0
+192.168.1.4    0x1         0x2         dd:ee:ff:aa:bb:cc     *        eth1
+192.168.1.5    0x1         0x2         ee:ff:aa:bb:cc:dd     *        eth1
+192.168.1.6    0x1         0x2         ff:aa:bb:cc:dd:ee     *        eth1
diff --git a/end-to-end-test.sh b/end-to-end-test.sh
index e074556..f47fbb5 100755
--- a/end-to-end-test.sh
+++ b/end-to-end-test.sh
@@ -3,6 +3,7 @@
 set -euf -o pipefail
 
 collectors=$(cat << COLLECTORS
+  arp
   buddyinfo
   conntrack
   diskstats
diff --git a/node_exporter.go b/node_exporter.go
index 4a7f88a..f6daa2d 100644
--- a/node_exporter.go
+++ b/node_exporter.go
@@ -32,7 +32,7 @@ import (
 )
 
 const (
-        defaultCollectors = "conntrack,cpu,diskstats,entropy,edac,exec,filefd,filesystem,hwmon,infiniband,loadavg,mdadm,meminfo,netdev,netstat,sockstat,stat,textfile,time,uname,vmstat,wifi,zfs"
+        defaultCollectors = "arp,conntrack,cpu,diskstats,entropy,edac,exec,filefd,filesystem,hwmon,infiniband,loadavg,mdadm,meminfo,netdev,netstat,sockstat,stat,textfile,time,uname,vmstat,wifi,zfs"
 )
 
 var (