diff --git a/xCAT-server/share/xcat/netboot/imgutils/imgutils.pm b/xCAT-server/share/xcat/netboot/imgutils/imgutils.pm
index 0907db83b..ea01a75ff 100644
--- a/xCAT-server/share/xcat/netboot/imgutils/imgutils.pm
+++ b/xCAT-server/share/xcat/netboot/imgutils/imgutils.pm
@@ -12,6 +12,42 @@ use File::Path;
 use Cwd qw(realpath);
 use xCAT::SvrUtils;
 
+sub el_major_version {
+    my $version = shift;
+
+    return if !defined($version);
+
+    my ($os_family, $os_major) = xCAT::SvrUtils::parseosver($version);
+    return
+      if !defined($os_family)
+      || $os_family !~ /^(?:rhels?|rhelc|rhelhpc|centos(?:-stream)?|rocky|alma(?:linux)?|ol)$/;
+
+    return $os_major if length($os_major);
+    return;
+}
+
+sub rpm_installroot_command {
+    my ( $osver, $rootimg_dir, $non_interactive, $dnf_available ) = @_;
+    $non_interactive ||= "";
+    $dnf_available = -x "/usr/bin/dnf" unless defined($dnf_available);
+    my $majorrel = el_major_version($osver);
+    my $pkgmgr = "yum";
+
+    # EL8 and newer are dnf-native. Keep yum as the fallback for legacy
+    # systems and minimal environments that still provide only yum.
+    if (defined($majorrel) && $majorrel > 7 && $dnf_available) {
+        $pkgmgr = "dnf";
+    }
+
+    my $cmd = "$pkgmgr $non_interactive -c /tmp/genimage.$$.yum.conf --installroot=$rootimg_dir/ --disablerepo=* ";
+    if (defined($majorrel) && $majorrel > 7) {
+        $cmd .= "--releasever=" . $majorrel . " ";
+        $cmd .= "--setopt=module_platform_id=platform:el" . $majorrel . " ";
+    }
+
+    return $cmd;
+}
+
 sub varsubinline{
     my $line=shift;
     my $refvardict=shift;
diff --git a/xCAT-server/share/xcat/netboot/rh/genimage b/xCAT-server/share/xcat/netboot/rh/genimage
index 4a41286ce..20424cd82 100755
--- a/xCAT-server/share/xcat/netboot/rh/genimage
+++ b/xCAT-server/share/xcat/netboot/rh/genimage
@@ -92,38 +92,6 @@ sub majversion {
     return $majorrel;
 }
 
-sub el_major_version {
-    my $version = shift;
-
-    if (defined($version)
-        && $version =~ /^(?:rhels?|centos|rocky|alma(?:linux)?|ol)\D*(\d+)/)
-    {
-        return $1;
-    }
-
-    return;
-}
-
-sub rpm_installroot_command {
-    my $non_interactive = shift || "";
-    my $majorrel = el_major_version($osver);
-    my $pkgmgr = "yum";
-
-    # EL8 and newer are dnf-native. Keep yum as the fallback for legacy
-    # systems and minimal environments that still provide only yum.
-    if (defined($majorrel) && $majorrel > 7 && -x "/usr/bin/dnf") {
-        $pkgmgr = "dnf";
-    }
-
-    my $cmd = "$pkgmgr $non_interactive -c /tmp/genimage.$$.yum.conf --installroot=$rootimg_dir/ --disablerepo=* ";
-    if (defined($majorrel) && $majorrel > 7) {
-        $cmd .= "--releasever=" . $majorrel . " ";
-        $cmd .= "--setopt=module_platform_id=platform:el" . $majorrel . " ";
-    }
-
-    return $cmd;
-}
-
 sub mount_chroot {
     my $rootimage_dir = shift;
 
@@ -396,7 +364,9 @@ if($onlyinitrd){
     my $non_interactive;
     if (!$prompt) { $non_interactive = "-y"; }
 
-    my $yumcmd = rpm_installroot_command($non_interactive);
+    my $yumcmd = imgutils::rpm_installroot_command(
+        $osver, $rootimg_dir, $non_interactive
+    );
 
     foreach (0 .. $repnum) {
         $yumcmd .= "--enablerepo=$osver-$arch-$_ ";
@@ -562,7 +532,9 @@ if($onlyinitrd){
         }
         close($yumconfig);
         $index--;
-        my $yumcmd_base = rpm_installroot_command($non_interactive);
+        my $yumcmd_base = imgutils::rpm_installroot_command(
+            $osver, $rootimg_dir, $non_interactive
+        );
 
         #yum/rpm/zypper has defect on calculating diskspace usage when installing rpm on a NFS mounted installroot
         if (isNFSdir("$rootimg_dir")) {
diff --git a/xCAT-test/unit/rh_genimage_package_manager.t b/xCAT-test/unit/rh_genimage_package_manager.t
index b419a3351..177668cf0 100644
--- a/xCAT-test/unit/rh_genimage_package_manager.t
+++ b/xCAT-test/unit/rh_genimage_package_manager.t
@@ -3,19 +3,82 @@ use strict;
 use warnings;
 
 use FindBin;
-use lib "$FindBin::Bin/../lib";
+use lib "$FindBin::Bin/../../perl-xCAT";
+use lib "$FindBin::Bin/../../xCAT-server/share/xcat/netboot/imgutils";
+use lib "$FindBin::Bin/../../xCAT-server/lib/perl";
 use Test::More;
 
-use XCAT::Test::File qw(slurp_repo_file);
+use imgutils;
 
-my $genimage = slurp_repo_file('xCAT-server/share/xcat/netboot/rh/genimage');
+my @version_cases = (
+    [ 'rhel8',                 8,     'RHEL' ],
+    [ 'rhels8.10',            8,     'RHEL Server' ],
+    [ 'rhelc5',               5,     'RHEL Client' ],
+    [ 'rhelhpc7.2',           7,     'RHEL ComputeNode' ],
+    [ 'rhels7.5-alternate',    7,     'RHEL alternate-media suffix' ],
+    [ 'centos7.9',             7,     'CentOS Linux' ],
+    [ 'centos-stream9',        9,     'CentOS Stream' ],
+    [ 'rocky9.6',              9,     'Rocky Linux' ],
+    [ 'alma8.10',              8,     'AlmaLinux short name' ],
+    [ 'almalinux9.4',          9,     'AlmaLinux long name' ],
+    [ 'ol8.4.0',               8,     'Oracle Linux' ],
+    [ 'rhels10.0',             10,    'two-digit RHEL Server' ],
+    [ 'fedora42',              undef, 'reachable unsupported Fedora' ],
+    [ 'SL7.9',                 undef, 'reachable unsupported Scientific Linux' ],
+    [ 'RHEL9',                 undef, 'case-mismatched distribution' ],
+    [ 'rhels',                 undef, 'missing major version' ],
+    [ '',                      undef, 'empty version' ],
+    [ undef,                   undef, 'undefined version' ],
+);
 
-like( $genimage, qr/sub el_major_version/, 'RH genimage has an EL major-version helper' );
-like( $genimage, qr/sub rpm_installroot_command/, 'RH genimage builds RPM installroot commands through one helper' );
-like( $genimage, qr/-x "\/usr\/bin\/dnf".*?\$pkgmgr = "dnf"/s, 'EL8+ genimage prefers dnf when it is available' );
-like( $genimage, qr/--releasever=.*?--setopt=module_platform_id=platform:el/s, 'EL8+ installroot commands keep releasever and module platform options' );
-like( $genimage, qr/my \$yumcmd = rpm_installroot_command\(\$non_interactive\);/, 'base package pass uses shared installroot command builder' );
-like( $genimage, qr/my \$yumcmd_base = rpm_installroot_command\(\$non_interactive\);/, 'otherpkgs pass uses shared installroot command builder' );
-unlike( $genimage, qr/my \$yumcmd(?:_base)? = "yum /, 'RH genimage no longer hardcodes yum in installroot command builders' );
+for my $case (@version_cases) {
+    my ( $version, $expected, $description ) = @{$case};
+    is(
+        imgutils::el_major_version($version),
+        $expected,
+        "$description resolves to the expected EL major version"
+    );
+}
+
+sub installroot_command_for {
+    my ( $version, $dnf_available, $non_interactive ) = @_;
+
+    return imgutils::rpm_installroot_command(
+        $version, '/var/tmp/root-image', $non_interactive, $dnf_available
+    );
+}
+
+my $el7_command = installroot_command_for( 'rhels7.9', 1, '-y' );
+like( $el7_command, qr/^yum -y /, 'EL7 keeps yum as its package manager' );
+unlike( $el7_command, qr/--releasever=/, 'EL7 omits releasever' );
+unlike( $el7_command, qr/--setopt=module_platform_id=/, 'EL7 omits module platform configuration' );
+
+my $el8_command = installroot_command_for( 'rhels8.10', 1, '-y' );
+like( $el8_command, qr/^dnf -y /, 'EL8 uses dnf when it is available' );
+like( $el8_command, qr/--releasever=8 /, 'EL8 sets releasever at the package-manager boundary' );
+like( $el8_command, qr/--setopt=module_platform_id=platform:el8 /, 'EL8 sets its module platform' );
+
+my $el9_dnf_command = installroot_command_for( 'centos-stream9', 1, '-y' );
+like( $el9_dnf_command, qr/^dnf -y /, 'EL9 uses dnf when it is available' );
+like( $el9_dnf_command, qr/--installroot=\/var\/tmp\/root-image\//, 'EL9 preserves the requested installroot' );
+like( $el9_dnf_command, qr/--releasever=9 /, 'EL9 sets releasever from the parsed major version' );
+like( $el9_dnf_command, qr/--setopt=module_platform_id=platform:el9 /, 'EL9 sets its module platform' );
+
+my $el9_yum_command = installroot_command_for( 'centos-stream9', 0, '-y' );
+like( $el9_yum_command, qr/^yum -y /, 'EL9 falls back to yum when dnf is unavailable' );
+like( $el9_yum_command, qr/--releasever=9 /, 'the EL9 yum fallback keeps releasever' );
+like( $el9_yum_command, qr/--setopt=module_platform_id=platform:el9 /, 'the EL9 yum fallback keeps its module platform' );
+
+my $el10_command = installroot_command_for( 'rhels10.0', 1, '-y' );
+like( $el10_command, qr/--releasever=10 /, 'EL10 preserves its two-digit releasever' );
+like( $el10_command, qr/--setopt=module_platform_id=platform:el10 /, 'EL10 preserves its two-digit module platform' );
+
+my $unsupported_command = installroot_command_for( 'fedora42', 1, '-y' );
+like( $unsupported_command, qr/^yum -y /, 'unsupported distributions keep the yum fallback' );
+unlike( $unsupported_command, qr/--releasever=/, 'unsupported distributions omit releasever' );
+unlike( $unsupported_command, qr/--setopt=module_platform_id=/, 'unsupported distributions omit module platform configuration' );
+
+my $interactive_command = installroot_command_for( 'rhels8.10', 1, undef );
+like( $interactive_command, qr/^dnf  -c /, 'interactive mode omits the non-interactive package-manager option' );
 
 done_testing();