lib/dns: add method to get the record in HostsFile by name and/or value

The Get method return the first resource record that match with domain
name and/or value.
The value parameter is optional, if its empty, then only the first record
that match with domain name that will be returned.

If no record matched, it will return nil.

While at it, reformat the hosts_file comments with next Go 1.19 format.

2 files changed, 71 insertions, 21 deletions

diff --git a/lib/dns/hosts_file.go b/lib/dns/hosts_file.go
index c24548bd..079d4438 100644
--- a/lib/dns/hosts_file.go
+++ b/lib/dns/hosts_file.go
@@ -1,3 +1,7 @@
+// Copyright 2018, Shulhan <ms@kilabit.info>. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package dns
 
 import (
@@ -7,6 +11,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 
 	libio "github.com/shuLhan/share/lib/io"
 )
@@ -18,9 +23,7 @@ const (
 	defaultTTL       = 604800 // 7 days
 )
 
-//
 // HostsFile represent content of single hosts file.
-//
 type HostsFile struct {
 	out     *os.File
 	Path    string `json:"-"`
@@ -28,9 +31,7 @@ type HostsFile struct {
 	Records []*ResourceRecord `json:"-"`
 }
 
-//
 // NewHostsFile create and store the host records in file defined by "path".
-//
 func NewHostsFile(path string, records []*ResourceRecord) (
 	hfile *HostsFile, err error,
 ) {
@@ -45,9 +46,7 @@ func NewHostsFile(path string, records []*ResourceRecord) (
 	return hfile, err
 }
 
-//
 // GetSystemHosts return path to system hosts file.
-//
 func GetSystemHosts() string {
 	if runtime.GOOS == "windows" {
 		return HostsFileWindows
@@ -55,12 +54,10 @@ func GetSystemHosts() string {
 	return HostsFilePOSIX
 }
 
-//
 // LoadHostsDir load all of hosts formatted files inside a directory.
 // On success, it will return map of filename and the content of hosts file as
 // list of Message.
 // On fail, it will return partial loadeded hosts files and an error.
-//
 func LoadHostsDir(dir string) (hostsFiles map[string]*HostsFile, err error) {
 	if len(dir) == 0 {
 		return nil, nil
@@ -112,10 +109,8 @@ func LoadHostsDir(dir string) (hostsFiles map[string]*HostsFile, err error) {
 	return hostsFiles, nil
 }
 
-//
 // ParseHostsFile parse the content of hosts file as packed DNS message.
 // If path is empty, it will load from the system hosts file.
-//
 func ParseHostsFile(path string) (hfile *HostsFile, err error) {
 	if len(path) == 0 {
 		path = GetSystemHosts()
@@ -135,7 +130,6 @@ func ParseHostsFile(path string) (hfile *HostsFile, err error) {
 	return hfile, nil
 }
 
-//
 // Fields of the entry are separated by any number of blanks and/or tab
 // characters.
 // Text from a "#" character until the end of the line is a comment, and is
@@ -148,7 +142,6 @@ func ParseHostsFile(path string) (hfile *HostsFile, err error) {
 // shorter hostnames, or generic hostnames (for example, localhost). [1]
 //
 // [1] man 5 hosts
-//
 func parse(reader *libio.Reader) (listRR []*ResourceRecord) {
 	var (
 		seps  = []byte{'\t', '\v', ' '}
@@ -215,9 +208,7 @@ func parse(reader *libio.Reader) (listRR []*ResourceRecord) {
 	return listRR
 }
 
-//
 // AppendAndSaveRecord append new record and save it to hosts file.
-//
 func (hfile *HostsFile) AppendAndSaveRecord(rr *ResourceRecord) (err error) {
 	f, err := os.OpenFile(
 		hfile.Path,
@@ -247,16 +238,48 @@ func (hfile *HostsFile) AppendAndSaveRecord(rr *ResourceRecord) (err error) {
 	return err
 }
 
-//
 // Delete the hosts file from the storage.
-//
 func (hfile *HostsFile) Delete() (err error) {
 	return os.RemoveAll(hfile.Path)
 }
 
+// Get the first resource record that match with domain name and/or value.
+// The value parameter is optional, if its empty, then only the first record
+// that match with domain name that will be returned.
 //
+// If no record matched, it will return nil.
+func (hfile *HostsFile) Get(dname, value string) (rr *ResourceRecord) {
+	var (
+		rrValue string
+		ok      bool
+	)
+
+	dname = strings.ToLower(dname)
+
+	if len(value) != 0 {
+		value = strings.ToLower(value)
+	}
+
+	for _, rr = range hfile.Records {
+		if rr.Name != dname {
+			continue
+		}
+		if len(value) == 0 {
+			return rr
+		}
+		rrValue, ok = rr.Value.(string)
+		if !ok {
+			continue
+		}
+		if rrValue != value {
+			continue
+		}
+		return rr
+	}
+	return nil
+}
+
 // Names return all hosts domain names.
-//
 func (hfile *HostsFile) Names() (names []string) {
 	names = make([]string, 0, len(hfile.Records))
 
@@ -267,10 +290,8 @@ func (hfile *HostsFile) Names() (names []string) {
 	return names
 }
 
-//
 // RemoveRecord remove single record from hosts file by domain name.
 // It will return true if record found and removed.
-//
 func (hfile *HostsFile) RemoveRecord(dname string) bool {
 	for x := 0; x < len(hfile.Records); x++ {
 		if hfile.Records[x].Name != dname {
@@ -284,9 +305,7 @@ func (hfile *HostsFile) RemoveRecord(dname string) bool {
 	return false
 }
 
-//
 // Save the hosts records into the file defined by field "Path".
-//
 func (hfile *HostsFile) Save() (err error) {
 	if hfile.out == nil {
 		hfile.out, err = os.OpenFile(
diff --git a/lib/dns/hosts_file_example_test.go b/lib/dns/hosts_file_example_test.go
new file mode 100644
index 00000000..7b08f5e9
--- /dev/null
+++ b/lib/dns/hosts_file_example_test.go
@@ -0,0 +1,31 @@
+// Copyright 2022, Shulhan <ms@kilabit.info>. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dns
+
+import "fmt"
+
+func ExampleHostsFile_Get() {
+	var (
+		hostsFile = &HostsFile{
+			Records: []*ResourceRecord{{
+				Name:  "my.local",
+				Type:  RecordTypeA,
+				Value: "127.0.0.1",
+			}, {
+				Name:  "my.local",
+				Type:  RecordTypeA,
+				Value: "127.0.0.2",
+			}},
+		}
+	)
+
+	fmt.Println(hostsFile.Get("my.local", ""))
+	fmt.Println(hostsFile.Get("my.local", "127.0.0.2"))
+	fmt.Println(hostsFile.Get("my.my", ""))
+	//Output:
+	//{Name:my.local Type:1 Class:0 TTL:0 rdlen:0}
+	//{Name:my.local Type:1 Class:0 TTL:0 rdlen:0}
+	//<nil>
+}