diff --git a/net/net-detect.pl b/net/net-detect.pl
new file mode 100644
index 000000000..c87ded6d6
--- /dev/null
+++ b/net/net-detect.pl
@@ -0,0 +1,32 @@
+# net-detect.pl
+# Helper functions for choosing the network config backend
+
+sub net_has_network_manager_config
+{
+my ($dir) = @_;
+$dir ||= "/etc/NetworkManager/system-connections";
+my @files = glob("$dir/*.nmconnection");
+return -d $dir && scalar(@files);
+}
+
+sub net_has_netplan_config
+{
+my ($dir) = @_;
+$dir ||= "/etc/netplan";
+return &has_command("netplan") &&
+       -d $dir;
+}
+
+sub net_auto_backend
+{
+my ($os_type, $netplan_dir, $nm_conn_dir) = @_;
+return "netplan"
+	if ($os_type eq "debian-linux" &&
+	    &net_has_netplan_config($netplan_dir));
+return "nm"
+	if (($os_type eq "redhat-linux" || $os_type eq "debian-linux") &&
+	    &net_has_network_manager_config($nm_conn_dir));
+return undef;
+}
+
+1;
diff --git a/net/net-lib.pl b/net/net-lib.pl
index 9023a2d3d..318dcfd4d 100755
--- a/net/net-lib.pl
+++ b/net/net-lib.pl
@@ -6,6 +6,9 @@ use WebminCore;
 &init_config();
 %access = &get_module_acl();
 $access{'ipnodes'} = $access{'hosts'};
+do "net-detect.pl";
+
+$auto_net_mode = &net_auto_backend($gconfig{'os_type'});
 
 if (-r "$module_root_directory/$gconfig{'os_type'}-$gconfig{'os_version'}-lib.pl") {
 	do "$gconfig{'os_type'}-$gconfig{'os_version'}-lib.pl";
@@ -23,20 +26,16 @@ elsif ($gconfig{'os_type'} eq 'slackware-linux' &&
 	do "$gconfig{'os_type'}-9.1-ALL-lib.pl";
 	$net_mode = $gconfig{'os_type'}."/9.1";
 	}
-elsif ($gconfig{'os_type'} eq 'redhat-linux' &&
-       -d "/etc/NetworkManager/system-connections" &&
-       glob("/etc/NetworkManager/system-connections/*.nmconnection")) {
-	# Special case for systems with network manager
-	do 'nm-lib.pl';
-	$net_mode = "nm";
-	}
-elsif ($gconfig{'os_type'} eq 'debian-linux' && 
-       &has_command("netplan") &&
-       -d "/etc/netplan") {
+elsif ($auto_net_mode eq "netplan") {
 	# Special case for newer Ubuntu versions
 	do "netplan-lib.pl";
 	$net_mode = "netplan";
 	}
+elsif ($auto_net_mode eq "nm") {
+	# Special case for systems with network manager
+	do 'nm-lib.pl';
+	$net_mode = "nm";
+	}
 else {
 	do "$gconfig{'os_type'}-lib.pl";
 	$net_mode = $gconfig{'os_type'};
diff --git a/net/t/run-tests.t b/net/t/run-tests.t
index 8012538ac..21f936e3e 100644
--- a/net/t/run-tests.t
+++ b/net/t/run-tests.t
@@ -4,6 +4,7 @@ use warnings;
 use Test::More;
 use Cwd qw(abs_path);
 use File::Basename qw(dirname);
+use File::Path qw(make_path);
 use File::Temp qw(tempdir);
 
 my $root = abs_path(dirname(__FILE__)."/../..") or die "rootdir: $!";
@@ -58,6 +59,7 @@ close($fh) || die "close $file: $!";
 sub lock_file { return 1; }
 sub unlock_file { return 1; }
 sub error { die join("", @_), "\n"; }
+sub unflush_file_lines { delete($file_cache{$_[0]}); }
 sub has_command { return $_[0] eq "netplan" ? "/usr/sbin/netplan" : undef; }
 sub execute_command_logged
 {
@@ -68,6 +70,13 @@ $$stdout = $out if (ref($stdout));
 $$stderr = $out if (ref($stderr) && $stderr ne $stdout);
 return $command_status{$cmd} || 0;
 }
+sub backquote_logged
+{
+my ($cmd) = @_;
+push(@commands, $cmd);
+$? = $command_status{$cmd} || 0;
+return $command_output{$cmd} || "";
+}
 sub check_ipaddress { return $_[0] =~ /^\d+\.\d+\.\d+\.\d+$/; }
 sub check_ip6address { return $_[0] =~ /:/; }
 sub check_ipaddress_any { return &check_ipaddress($_[0]) || &check_ip6address($_[0]); }
@@ -83,6 +92,29 @@ return -1;
 }
 
 unshift(@INC, "$root/net", $root);
+do "$root/net/net-detect.pl" || die "net-detect.pl: $@ $!";
+
+my $detect_root = tempdir(CLEANUP => 1);
+my $detect_netplan = "$detect_root/netplan";
+my $detect_no_netplan = "$detect_root/no-netplan";
+my $detect_nm = "$detect_root/NetworkManager/system-connections";
+my $detect_nm_empty = "$detect_root/NetworkManager-empty/system-connections";
+make_path($detect_netplan, $detect_nm, $detect_nm_empty);
+write_text("$detect_nm/eth0.nmconnection", "");
+
+is(main::net_auto_backend("debian-linux", $detect_netplan, $detect_nm_empty),
+   "netplan", "Debian uses Netplan when the config directory exists");
+is(main::net_auto_backend("debian-linux", $detect_no_netplan, $detect_nm),
+   "nm", "Debian uses NetworkManager when only nmconnection files exist");
+is(main::net_auto_backend("redhat-linux", $detect_no_netplan, $detect_nm),
+   "nm", "Red Hat still uses NetworkManager when nmconnection files exist");
+write_text("$detect_netplan/50-cloud-init.yaml", "");
+is(main::net_auto_backend("debian-linux", $detect_netplan, $detect_nm),
+   "netplan", "Debian prefers Netplan over NetworkManager when YAML exists");
+unlink("$detect_netplan/50-cloud-init.yaml");
+is(main::net_auto_backend("debian-linux", $detect_no_netplan, $detect_nm_empty),
+   undef, "Debian falls back when no Netplan or NetworkManager config exists");
+
 do "$root/net/netplan-lib.pl" || die "netplan-lib.pl: $@ $!";
 
 {
@@ -167,4 +199,37 @@ is_deeply(\@commands,
 	    "(cd / && /usr/sbin/netplan apply)" ],
 	  "apply_network validates before applying");
 
+do "$root/net/nm-lib.pl" || die "nm-lib.pl: $@ $!";
+my $nmfile = "$tmp/eth0.nmconnection";
+write_text($nmfile, <<'NM');
+[connection]
+id=eth0
+uuid=11111111-2222-3333-4444-555555555555
+type=ethernet
+interface-name=eth0
+
+[ipv4]
+method=auto
+
+[ipv6]
+method=disabled
+NM
+my $nmcfg = main::read_nm_config($nmfile);
+my $nmiface = {
+	'name' => 'eth0',
+	'fullname' => 'eth0',
+	'file' => $nmfile,
+	'cfg' => $nmcfg,
+	'edit' => 1,
+	'up' => 1,
+	'dhcp' => 1,
+	'address6' => [ ],
+	'netmask6' => [ ],
+	'nameserver' => [ "2001:4860:4860::8888" ],
+	};
+@commands = ( );
+main::save_interface($nmiface, [ $nmiface ]);
+like(join("\n", @commands), qr/ipv6\.dns/,
+     "NetworkManager save_interface writes IPv6 nameservers");
+
 done_testing();