diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0413fb05e21..f49e0e22243 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,7 +15,7 @@ and this project adheres to
 
 ### Added
 
-- Linux interfaces' configuration file "source" directive support ([#3257]).
+- `source` directives support in `/etc/network/interfaces` on Linux ([#3257]).
 - RFC 9000 support in DNS-over-QUIC.
 - Completely disabling statistics by setting the statistics interval to zero
   ([#2141]).
diff --git a/HACKING.md b/HACKING.md
index 4c2ee095fb2..101a77579b3 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -98,8 +98,9 @@ attributes to make it work in Markdown renderers that strip "id". -->
  *  Constructors should validate their arguments and return meaningful errors.
     As a corollary, avoid lazy initialization.
 
- *  Define `MarshalFoo` methods on non-pointer receivers, as pointer receivers
-    [can have surprising results][staticcheck-911].
+ *  Prefer to define methods on pointer receievers, unless the type is small or
+    a non-pointer receiever is required, for example `MarshalFoo` methods (see
+    [staticcheck-911]).
 
  *  Don't mix horizontal and vertical placement of arguments in function and
     method calls.  That is, either this:
diff --git a/internal/aghnet/net_linux.go b/internal/aghnet/net_linux.go
index 5f410f87704..bf0c65d0004 100644
--- a/internal/aghnet/net_linux.go
+++ b/internal/aghnet/net_linux.go
@@ -20,16 +20,25 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+// recurrentChecker is used to check all the files which may include references
+// for other ones.
 type recurrentChecker struct {
-	checker  func(r io.Reader, ifaceName string) (patterns []string, has bool, err error)
+	// checker is the function to check if r's stream contains the desired
+	// attribute.  It must return all the patterns for files which should
+	// also be checked and each of them should be valid for filepath.Glob
+	// function.
+	checker func(r io.Reader, desired string) (patterns []string, has bool, err error)
+	// initPath is the path of the first member in the sequence of checked
+	// files.
 	initPath string
 }
 
-// maxConfigFileSize is the maximum assumed length of the interfaces
-// configuration file.
-const maxConfigFileSize = 1024 * 1024
+// maxCheckedFileSize is the maximum length of the file that recurrentChecker
+// may check.
+const maxCheckedFileSize = 1024 * 1024
 
-func (rc recurrentChecker) checkFile(sourcePath, ifaceName string) (
+// checkFile tries to open and to check single file located on the sourcePath.
+func (rc *recurrentChecker) checkFile(sourcePath, desired string) (
 	subsources []string,
 	has bool,
 	err error,
@@ -40,17 +49,15 @@ func (rc recurrentChecker) checkFile(sourcePath, ifaceName string) (
 		return nil, false, err
 	}
 
-	err = nil
-
 	defer func() { err = errors.WithDeferred(err, f.Close()) }()
 
-	var fileReader io.Reader
-	fileReader, err = aghio.LimitReader(f, maxConfigFileSize)
+	var r io.Reader
+	r, err = aghio.LimitReader(f, maxCheckedFileSize)
 	if err != nil {
 		return nil, false, err
 	}
 
-	subsources, has, err = rc.checker(fileReader, ifaceName)
+	subsources, has, err = rc.checker(r, desired)
 	if err != nil {
 		return nil, false, err
 	}
@@ -62,7 +69,8 @@ func (rc recurrentChecker) checkFile(sourcePath, ifaceName string) (
 	return subsources, has, nil
 }
 
-func (rc recurrentChecker) handlePatterns(sourcesSet *aghstrings.Set, patterns []string) (
+// handlePatterns parses the patterns and takes care of duplicates.
+func (rc *recurrentChecker) handlePatterns(sourcesSet *aghstrings.Set, patterns []string) (
 	subsources []string,
 	err error,
 ) {
@@ -87,7 +95,8 @@ func (rc recurrentChecker) handlePatterns(sourcesSet *aghstrings.Set, patterns [
 	return subsources, nil
 }
 
-func (rc recurrentChecker) check(ifaceName string) (has bool, err error) {
+// check walks through all the files searching for the desired attribute.
+func (rc *recurrentChecker) check(desired string) (has bool, err error) {
 	var i int
 	sources := []string{rc.initPath}
 
@@ -103,7 +112,7 @@ func (rc recurrentChecker) check(ifaceName string) (has bool, err error) {
 	// The slice of sources is separate from the set of sources to keep the
 	// order in which the files are walked.
 	for sourcesSet := aghstrings.NewSet(rc.initPath); i < len(sources); i++ {
-		patterns, has, err = rc.checkFile(sources[i], ifaceName)
+		patterns, has, err = rc.checkFile(sources[i], desired)
 		if err != nil {
 			if errors.Is(err, os.ErrNotExist) {
 				continue
@@ -133,7 +142,7 @@ func ifaceHasStaticIP(ifaceName string) (has bool, err error) {
 	// /etc/network/interfaces doesn't, it will return true.  Perhaps this
 	// is not the most desirable behavior.
 
-	for _, rc := range []recurrentChecker{{
+	for _, rc := range []*recurrentChecker{{
 		checker:  dhcpcdStaticConfig,
 		initPath: "/etc/dhcpcd.conf",
 	}, {
diff --git a/internal/aghnet/net_linux_test.go b/internal/aghnet/net_linux_test.go
index 9897abc2705..9dd9a8665e2 100644
--- a/internal/aghnet/net_linux_test.go
+++ b/internal/aghnet/net_linux_test.go
@@ -13,7 +13,7 @@ import (
 )
 
 func TestRecurrentChecker(t *testing.T) {
-	c := recurrentChecker{
+	c := &recurrentChecker{
 		checker:  ifacesStaticConfig,
 		initPath: "./testdata/include-subsources",
 	}
@@ -101,7 +101,7 @@ func TestIfacesStaticConfig(t *testing.T) {
 		want:         true,
 		wantPatterns: []string{},
 	}, {
-		name: "return_subsources",
+		name: "return_patterns",
 		data: []byte(`source hello` + nl +
 			`source world` + nl +
 			`#iface enp0s3 inet static` + nl,
@@ -109,7 +109,9 @@ func TestIfacesStaticConfig(t *testing.T) {
 		want:         false,
 		wantPatterns: []string{"hello", "world"},
 	}, {
-		name: "ignore_subsources",
+		// This one tests if the first found valid interface prevents
+		// checking files under the `source` directive.
+		name: "ignore_patterns",
 		data: []byte(`source hello` + nl +
 			`source world` + nl +
 			`iface enp0s3 inet static` + nl,
diff --git a/internal/aghnet/testdata/include-subsources b/internal/aghnet/testdata/include-subsources
index efa415c5e10..1663e423c85 100644
--- a/internal/aghnet/testdata/include-subsources
+++ b/internal/aghnet/testdata/include-subsources
@@ -1,8 +1,5 @@
-# Including the subsource.
-source ./testdata/ifaces
-
-# Including subsource wildcard.
-source ./testdata/*
+# The "testdata" part is added here because the test is actually run from the
+# parent directory.  Real interface files usually contain only absolute paths.
 
-# ./testdata is the path to the current directory relative to the *_test.go.
-# Real interfaces configuration files are usually contain only absolute paths.
\ No newline at end of file
+source ./testdata/ifaces
+source ./testdata/*
\ No newline at end of file