Only select dynamic ip addresses

internal/config/config.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"net/netip"
 	"os"
 )
 
@@ -13,9 +14,39 @@ type HealthchecksConfig struct {
 	PingKey     string `json:"ping_key"`
 }
 
+type DNSMasqConfig struct {
+	DHCPRangeStart netip.Addr
+	DHCPRangeEnd   netip.Addr
+	LeasesPath     string
+}
+
 type Config struct {
 	HealthchecksConfig `json:"healthchecks"`
-	LeasesPath         string `json:"leases_path"`
+	DNSMasqConfigFile  string `json:"dnsmasq_config"`
+	DNSMasqConfig
+}
+
+func (c *Config) parseDNSMasqConf() error {
+	dnsMasqRootContent, err := os.ReadFile(c.DNSMasqConfigFile)
+	if err != nil {
+		return fmt.Errorf("failed to read dnsmasq config: %w", err)
+	}
+
+	dnsmasqRawConf, err := parseDNSMasqOptions(string(dnsMasqRootContent))
+	if err != nil {
+		return fmt.Errorf("failed to parse dnsmasq config: %w", err)
+	}
+	c.LeasesPath = dnsmasqRawConf.leaseFile
+	c.DHCPRangeStart, err = netip.ParseAddr(dnsmasqRawConf.rangeStart)
+	if err != nil {
+		return fmt.Errorf("failed to parse dhcp range start: %w", err)
+	}
+	c.DHCPRangeEnd, err = netip.ParseAddr(dnsmasqRawConf.rangeEnd)
+	if err != nil {
+		return fmt.Errorf("failed to parse dhcp range end: %w", err)
+	}
+
+	return nil
 }
 
 func defaultConfig() *Config {
@@ -23,7 +54,7 @@ func defaultConfig() *Config {
 		HealthchecksConfig: HealthchecksConfig{
 			Enabled: false,
 		},
-		LeasesPath: "/var/lib/misc/dnsmasq.leases",
+		DNSMasqConfigFile: "etc/dnsmasq.conf",
 	}
 }
 
@@ -31,7 +62,8 @@ func New(path string) (*Config, error) {
 	fileContent, err := os.ReadFile(path)
 	if err != nil {
 		if errors.Is(err, os.ErrNotExist) {
-			return defaultConfig(), nil
+			conf := defaultConfig()
+			return conf, conf.parseDNSMasqConf()
 		}
 		return nil, fmt.Errorf("failed to read config file %w", err)
 	}
@@ -40,5 +72,5 @@ func New(path string) (*Config, error) {
 	if err := json.Unmarshal(fileContent, &c); err != nil {
 		return nil, fmt.Errorf("failed to parse config file: %w", err)
 	}
-	return &c, nil
+	return &c, c.parseDNSMasqConf()
 }

internal/config/dnsmasq.go

@@ -0,0 +1,96 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"path"
+	"strings"
+)
+
+const (
+	leaseFileKey = "dhcp-leasefile"
+	rangeKey     = "dhcp-range"
+	confFileKey  = "conf-file"
+	confDirKey   = "conf-dir"
+)
+
+func tryParseLineOption(line, key string) (string, bool) {
+	if !strings.HasPrefix(line, key) {
+		return "", false
+	}
+
+	_, val, found := strings.Cut(line, "=")
+	return val, found
+}
+
+type dnsmasqConfOptions struct {
+	leaseFile  string
+	rangeStart string
+	rangeEnd   string
+}
+
+func defaultDNSMasqOptions() map[string]string {
+	return map[string]string{
+		confDirKey:   "/etc/dnsmasq.d",
+		confFileKey:  "",
+		leaseFileKey: "/var/lib/misc/dnsmasq.leases",
+		rangeKey:     "",
+	}
+}
+
+func parseDNSMasqConfFile(content string, options map[string]string) {
+	for _, l := range strings.Split(content, "\n") {
+		for key := range options {
+			newVal, found := tryParseLineOption(l, key)
+			if found {
+				options[key] = newVal
+			}
+		}
+	}
+}
+
+func parseDNSMasqOptions(rootFileContent string) (dnsmasqConfOptions, error) {
+	rawOptions := defaultDNSMasqOptions()
+
+	// start with parsing the root file
+	parseDNSMasqConfFile(rootFileContent, rawOptions)
+
+	// get all additional conf files in the provided directory
+	confDir := rawOptions[confDirKey]
+	if confDir != "" {
+		entries, err := os.ReadDir(confDir)
+		if err != nil {
+			return dnsmasqConfOptions{}, fmt.Errorf("failed to get list of additional config files: %w", err)
+		}
+		for _, e := range entries {
+			if e.IsDir() || !strings.HasSuffix(e.Name(), ".conf") {
+				continue
+			}
+			fileContent, err := os.ReadFile(path.Join(confDir, e.Name()))
+			if err != nil {
+				return dnsmasqConfOptions{}, fmt.Errorf("failed to read sub config file: %w", err)
+			}
+			parseDNSMasqConfFile(string(fileContent), rawOptions)
+		}
+	}
+
+	// get any additional config file
+	if additionalFile := rawOptions[confFileKey]; additionalFile != "" {
+		filecontent, err := os.ReadFile(additionalFile)
+		if err != nil {
+			return dnsmasqConfOptions{}, fmt.Errorf("failed to read additional config file: %w", err)
+		}
+		parseDNSMasqConfFile(string(filecontent), rawOptions)
+	}
+
+	dhcpRange := strings.Split(rawOptions[rangeKey], ",")
+	if len(dhcpRange) < 2 {
+		return dnsmasqConfOptions{}, fmt.Errorf("invalid value for DHCP range: %s", rawOptions[rangeKey])
+	}
+
+	return dnsmasqConfOptions{
+		leaseFile:  rawOptions[leaseFileKey],
+		rangeStart: dhcpRange[0],
+		rangeEnd:   dhcpRange[1],
+	}, nil
+}

internal/dnsmasq/dnsmasq.go

@@ -2,6 +2,7 @@ package dnsmasq
 
 import (
 	"fmt"
+	"net/netip"
 	"os"
 	"strconv"
 	"strings"
@@ -11,7 +12,7 @@ import (
 type Lease struct {
 	ExpireDate time.Time
 	Mac        string
-	IP         string
+	IP         netip.Addr
 	Hostname   string
 	ClientID   string
 }
@@ -31,10 +32,15 @@ func parseLeaseLine(rawLine string) (*Lease, error) {
 		return nil, fmt.Errorf("unexpected unix timestamp value %s", lineValues[0])
 	}
 
+	ip, err := netip.ParseAddr(lineValues[2])
+	if err != nil {
+		return nil, fmt.Errorf("unexpected IP address: %s: %w", lineValues[2], err)
+	}
+
 	return &Lease{
 		ExpireDate: time.Unix(int64(expireTimeInt), 0),
 		Mac:        lineValues[1],
-		IP:         lineValues[2],
+		IP:         ip,
 		Hostname:   lineValues[3],
 		ClientID:   lineValues[4],
 	}, nil
@@ -59,3 +65,19 @@ func GetLeases(leasesPath string) ([]*Lease, error) {
 	}
 	return leases, nil
 }
+
+func GetDynamicLeases(leasesPath string, dhcpStart, dhcpEnd netip.Addr) ([]*Lease, error) {
+	allLeases, err := GetLeases(leasesPath)
+	if err != nil {
+		return nil, err
+	}
+
+	filteredLeases := []*Lease{}
+	for _, l := range allLeases {
+		if l.IP.Less(dhcpEnd) && dhcpStart.Less(l.IP) {
+			filteredLeases = append(filteredLeases, l)
+		}
+	}
+
+	return filteredLeases, nil
+}

main.go

@@ -31,7 +31,7 @@ func main() {
 		fmt.Fprintf(os.Stderr, "Failed to notify process start: %s\n", err)
 	}
 
-	leases, err := dnsmasq.GetLeases(conf.LeasesPath)
+	leases, err := dnsmasq.GetDynamicLeases(conf.LeasesPath, conf.DHCPRangeStart, conf.DHCPRangeEnd)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Failed to parse leases: %s\n", err)
 		if err := healthchecks.Failure(context.Background(), conf.HealthchecksConfig, healtchecksClt, check, "failed to parse lease file"); err != nil {