Category Archives: Firewall Netfilter Iptables

Beiträge zum Einsatz von Iptables

Samba 4, shares, wsdd and Windows 10 – how to list Linux Samba servers in the Win 10 Explorer

Posted on by

These days I relatively often need to work with Windows 10 at home (home-office, corona virus, …). Normally, I isolate my own Win 10 instance in a VMware virtual machine on my Linux PC – and reduce any network connections of this VM to selected external servers. Under normal conditions all ports on the Linux host are closed for the virtual machine [VM]. But on a few temporary occasions I want to the Win 10 system to access a specific Samba exchange directory on a KVM virtualized Linux instance on the same host.

Off topic: You see that I never present directories of my Linux host directly to a Win 10 guest via Samba. Instead I transfer files via an exchange directory on an intermediate VM whose Samba service is configured to disallow access of the Win system on shares presented to the host. A primitive, but effective form of separation. The only inconvenient consequence is that synchronization becomes a two-fold process on the host and the Linux VM. But we have Linux tools for this, so the effort is limited. )

Of course we want to use the SMB protocol in a modern version, i.e. version 3.x (SMB3), over TCP/IP for this purpose (port 445). In addition we need some mechanism to detect and browse SMB servers on the Windows system. In the old days NetBIOS was used for the latter. On the Linux side we had the nmbd-daemon for it – and we could set up a special Samba server as a WINS server.

During the last year Microsoft has – via updates and new builds of Windows 10 – followed a consistent politics of deactivating the use of SMB V1.0 systematically. This, however, led to problems – not only between Windows PCs, but also between Win 10 instances and Samba 4 servers. This article addresses one of these problems: the missing list of available Samba servers in the Windows Explorer.

There are many contributions on the Internet describing this problem and some even say that you only can solve it by restoring SMB V1 capabilities in Win 10 again. In this article I want to recommend two different solutions:

  • Ignore the problem of Samba server detection and use your Samba shares on Win 10 with the SMB3 protocol as network drives.
  • If you absolutely want to see and list your Samba servers in the Windows Explorer of a Win 10 client, use the “Web-Service-Discovery” service via a WSDD-daemon provided by a Python script of Steffen Christgau.

I myself got on the right track of solving the named problem by an article of a guy called “Stilez”. His article is the first one listed under the section “Links” below. I recommend strongly to read it; it is Stilez who deserves all credit in pointing out both the problem and the solution. I just applied his insight to my own situation with virtualized Samba servers based on Opensuse Leap 15.1.

SMB V1.0 should be avoided – but NetBIOS needs it to exchange information about SMB servers

SMB, especially version SMB V1.0, is well known for security problems. Even MS has understood this – especially after the Wannacry disaster. See e.g. the links in the section “Links” => “Warnings of SMBV1” at the end of this article. MS has deactivated SMB V1 in the background via some updates of Win 8 and Win 10.

One of the resulting problem is that we do not see Samba servers in the Windows Explorer of a Win 10 system any longer. In the section “Network” of the Windows Explorer you normally should see a list of servers which are members of a Workgroup and offer shares.

Two years ago we would use NetBIOS’s discovery protocol and a WINS server to get this information. Unfortunately, the NetBIOS service detection ability depends on SMB1 features. The stupid thing is that we for a long while now had and have a relatively secure SMB2/3, but NetBIOS discovery only worked with SMB V1 enabled on the Windows client. Deactivating SMB V1 means deactivating NetBIOS at the
same time – and if you watch your Firewall logs for incoming packets from the Win 10 clients you will notice that exactly such a thing happened on Win 10 clients.

This actually means that you can have a full featured Samba/NetBIOS setup on the Linux side, that you may have opened the right ports on the firewalls for your Samba/WINS server and client systems, but that you will nevertheless not get any list of available Samba servers in Win 10’s Explorer. 🙁

Having understood this leads to the key question for our problem:

By what did MS replace the detection features of NetBIOS in combination with SMB-services?

Settings on the MS Win side – which alone will not help

When you google a bit you may find many hints regarding settings by which you activate network “discovery” functionalities via two Windows services. See

https://www.wintips.org/fix-windows-10-network-computers-not-showing/
https://winaero.com/blog/network-computers-not-visible-windows-10-version-1803/

You can follow these recommendations. If you want to see your own PC and other Windows systems in the Explorer’s list of network resources you must have activated them (see below). However, in my Win 10 client the recommended settings were already activated – with the exception of SMB V1, which I did and do not wish to reactivate again. The “discovery” settings may help you with other older Windows systems, but they do not enable a listing of Samba 4 servers without additional measures on Win 10.

There is another category of hints which in my opinion are contra-productive regarding security. See https://devanswers.co/network-error-problem-windows-cannot-access-hostname-samba/
Why activate an insecure setting? Especially, as such a setting does not really help us with our special problem? 🙁

A last set of hints concerns the settings on the Samba server, itself. I find it especially nice when such recommendations come from Microsoft :-). See: http://woshub.com/cannot-access-smb-network-shares-windows-10-1709/

[global]
server min protocol = SMB2_10
client max protocol = SMB3
client min protocol = SMB2_10
encrypt passwords = true
restrict anonymous = 2

Thanks to MS we now understand that we should not use SMB V1 …. But, actually, these hints are again insufficient regarding the Explorer problem …

What you could do – but should NOT do

Once you have understood that NetBIOS and SMB V1 still have an intimate relation (at least on a Windows systems) you may get the idea that there might exist some option to reactivate SMBV1 again on the Win 10 system. This is indeed possible. See here:
https://community.nethserver.org/t/windows-10-not-showing-servers-shares-in-network-browser/14263/4
https://www.wintips.org/fix-windows-10-network-computers-not-showing/

If you follow the advice of the authors and in addition re-open the standard ports for NetBIOS (UDP) 137, 138, (TCP) 139 on your firewalls between the Win 10 machine and your Samba servers you will – almost at once – get up the list of your accessible Samba servers in the Network section of the Win 10 Explorer. (Maybe you have to restart the smb and nmb services on your Linux machines).

But: You should not do this! SMB V1 should definitely become history!

Fortunately, a re-activation of SMB V1
on a Win 10 system is NOT required to mount Samba shares and it is neither required to get a list of available Samba servers in the Win 10 Explorer.

What you should do: Win 10 service settings

There are two service settings which are required to see other servers (and your own Win10 PC itself) in the list of network hosts presented by the Windows explorer:
Start services.msc ( press the Windows key + R => Enter “services.msc” in the dialog. Or: start services.msc it via the Control Panel => System and Security => Services)

  • Look for “Function Discovery Provider Host” => Set : Startup Type => Automatic
  • Look for “Function Discovery Resource Publication” => Set : Startup Type => Automatic (Delayed Start) !!

I noticed that on my VMware Win 10 guests the second setting appeared to be crucial to get the Win 10 PC itself listed among the network servers.

What you should do: Use the SMBV3 protocol!

As you as a Linux user meanwhile have probably replaced all your virtualized Win 7 guests, you should use the following settings in the [global] section of the configuration file “/etc/samba/smb.conf” of your Samba servers:

[global]
…
“protocol = SMB3”.
…

This is what Win 10 supports; you need SMB2_10 with some builds of Win 8 (???), only. Remember also that port 445 must be open on a firewall between the Win 10 client and your Samba server.

For Linux requirements to use SMB3 see
https://wiki.samba.org: SMB3 kernel status
For “SMB Direct” (RDMA) you normally need a kernel version > 4.16. On Opensuse Leap 15.1 most of the required kernel features have been backported. In Win 10 SMB Direct is normally activated; you find it in the “Window-Features” settings (https://www.windowscentral.com/how-manage-optional-features-windows-10)

Not seeing Samba servers in the Explorer does not mean that mounting a Samba share as a network drive does not work

Not seeing the Samba servers in the Win 10 Explorer – because the NetBIOS detection is defunct – does not mean that you cannot work with a Samba share on a Win 10 system. You can just “mount” it on Windows as a “network drive“:

Open a Windows Explorer, choose “This PC” on the left side, then click “Map network drive” in the upper area of the window and follow the instructions:
You choose a free drive letter and provide the Samba server name and its share in the usual MS form as “\\SERVERNAME\SHARE”.
Afterwards, you must activate the option “Connect using different credentials” in the dialog on the Win 10 side, if your Win 10 user for security reasons has a different UID and Password on the Samba server than on Win 10. Needless to say that this is a setting I strongly recommend – and of course we do not allow any direct anonymous or guest access to our Samba server without credentials delivered from a Windows machine (at least not without any central authentication systems).
So, you eventually must provide a valid Samba user name on your Samba server and the password – and there you happily go and use your resources on the Samba share from your Win 10 client.

I assumed of course that you have allowed access from the Win 10 host and the user by respective settings of “hosts allow” and “valid users” for the share in your Samba configuration.
Note: You need not mark the option for reconnecting the share in the Windows dialog for network drives if you only use the Samba exchange shares temporarily.

On an Opensuse system this works perfectly with the protocol settings for SMB3 on the server. So, you can use your shares even without seeing the samba
server in the Explorer: You just have to know what your shares are named and on which Samba servers they are located. No problem for a Linux admin.

In my opinion this approach is the most secure one among all “peer to peer”-approaches which have to work without a central network wide authentication service. It only requires to open port 445 for the time of a Samba session to a specific Samba server. Otherwise you do not provide any information for free to the Win 10 system and its “users”. (Well, an open question is what MS really does with the provided Samba credentials. But that is another story ….)

What you should do: Use the WSDD service on your Samba server

If you allow for some information sharing between your virtualized Win 10 and other KVM based virtual Samba machines in your LAN – and are not afraid of Microsoft or Antivirus companies on the Windows system to collect respective information – then there is a working option to get a stable list of the available Samba servers in the Windows Explorer – without the use of SMB V1.0.

Windows 10 implements web service detection via multiple mechanisms; among them: Multicast messages over ports 3702 (UDP), TCP 5357 and 1900 (UDP). For a detection of Samba services you “only” need ports 3702 (UDP) and 5357 (TCP). The general service detection port 1900 can remain closed in the firewalls between your Win 10 instances and your Linux world for our specific purpose. See
https://www.speedguide.net/port.php?port=5357
https://www.speedguide.net/port.php?port=3702
https://techcommunity.microsoft.com/t5/ask-the-performance-team/ws2008-the-wsd-port-monitor/ba-p/372760
https://en.wikipedia.org/wiki/Simple Service Discovery Protocol

The mechanism using ports 3702 and 5351 is called “Web Service Discovery” and was introduced by MS to cover the detection of printers and other devices in networks. In combination with SMB2 and SMB3 it is the preferred service to detect Samba services.

OK, do we have something like a counter-part available on a Linux system? Obviously, such a service is not (yet?) included in Samba 4 – at least not in the 4.9 version on my system with Opensuse Leap 15.1. The fact that WSD is not (yet?) a part of Samba may have some good reasons. See link.
One can understand the reservations and hesitation to include it, as WSD also serves other purposes than just the detection of SMB services.

Fortunately, a guy named Steffen Christgau, has written an (interesting) Python 3 script, which offers you the basic WSD functionality. See https://github.com/christgau/wsdd.

You can use the script in form of a daemon process on a Linux system – hence we speak of WSDD.

Using YaST I quickly found out that a WSDD RPM package is actually included in my “Opensuse Leap 15.1 Update” repository. People with other Linux distros may download the present WSDD version from GitHub.

On Opensuse it comes with an associated systemd service-file which you find in the directory “/usr/lib/systemd/system”. 

[Unit]
Description=Web Services Dynamic Discovery host daemon
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
AmbientCapabilities=CAP_SYS_CHROOT
PermissionsStartOnly=true
Environment= WSDD_ARGS=-p
ExecStartPre=/usr/lib/wsdd/wsdd-init.sh
EnvironmentFile=-/run/sysconfig/wsdd
ExecStart=/usr/sbin/wsdd --shortlog -c /run/wsdd $WSDD_ARGS
ExecStartPost=/usr/bin/rm /run/
sysconfig/wsdd
User=wsdd
Group=wsdd

[Install]
WantedBy=multi-user.target

Reading the documentation you find out that the daemon runs chrooted – which is a reasonable security measure.
Opensuse even provides an elementary configuration file in “/etc/sysconfig/wsdd“.

I used the parameter

WSDD_WORKGROUP=”MYWORKGROUP”

there to announce the right Workgroup for my (virtualized) Samba server.

So, I had everything ready to start WSDD by “rcwsdd start” (or by “systemctl start wsdd.service”) on my Samba server.

On the local firewall of the SMB server I opened

  • port 445 (TCP) for SMB(3) In/Out for the server and from/to the Win-10-Client,
  • port 3702 (UDP) for incoming packets to the server and outgoing packets from the server to the Multicast address 239.255.255.250,
  • port 5357 (TCP) In/Out for the server and from/to the Win 10 client.

And: I closed all NetBIOS ports (UDP 137, 138 / TCP 139) and eventually stopped the “nmbd”-service on the Samba server! (UDP 137, 138 / TCP 139)

Within a second or so, my Samba 4 server appeared in the Windows 10 Explorer!

Further hints:
As the 3702 port is used with the UDP protocol it should be regarded as potentially dangerous. See: https://blogs.akamai.com/sitr/2019/09/new-ddos-vector-observed-in-the-wild-wsd-attacks-hitting-35gbps.html
The port 1900 which appeared in the firewall logs does not seem to be important. I blocked it.

So far, so good. However, when I refreshed the list in the Win 10 Explorer my SAMBA server disappeared again. 🙁

What you should do: Take special care about the network interface to which the WSDD service gets attached to

It took me a while to find out that the origin of the last problem had to do with the fact that my virtualized server and my Win 10 client both had multiple network interfaces on virtualized bridges. There are no loops in the configuration, but it occurred that multiple broadcasts packets arrive via different paths at the Samba server and were answered – and thus multiple return messages appeared at the Win 10 client during a refresh – which Win 10 did not like (see the discussion in the following link.
https://github.com/christgau/wsdd/issues/8

As soon as I restricted the answer of the Samba server to exactly one of the interfaces on my virtual bridge via the the parameter “WSDD_INTERFACES” in the “/etc/sysconfig/wsdd”-configuration file everything went fine. Refreshes now lead to an immediate update including the Samba server.

So, be a little careful, when you have some complicated bridge structures associated with your virtualized VMware or KVM guests. The WSDD service should be limited to exactly one interface of the Samba server.

Note: As we do not need NetBIOS any longer – block ports 137, 138 (UDP) and 139 (TCP) in your firewalls! It will make you feel better instantaneously.

Conclusion

The “end” of SMB V1 on Win 10 is a reasonable step. However, it undermines the visibility of Samba servers in the Windows Explorers. The reason is that NetBIOS requires SMB1.0 features on Windows. NetBIOS is/was therefore consistently deactivated on Win 10, too. The service detection on the network is replaced by the WSD service which was originally introduced for printer detection (and possibly other devices). Activating it on the Win 10 system may help with the detection of other Windows (8 and 10) systems on the network, but not with Samba 4 servers. Samba servers presently only serve NetBIOS requests of Win clients
to allow for server and share detection. Therefore, without additional measures, they are not displayed in the Windows Explorer of a regular Win 10 client.

This does, however, not restrict the usage of Samba shares on the Win 10 client via the SMB3 protocol. They can be used as “network drives” – just as before. Not distributing name and device information on a network has its advantages regarding security.

If you absolutely must see your Samba servers in the Win 10 Explorer install and configure the WSDD package of Steffen Christgau. You can use it as a systemd service. You should restrict the interfaces WSDD gets attached to – especially if your Samba servers are attached to virtual network bridges (Linux bridges or VMware bridges).

So:

  • Disable SMBV1 in Windows 10 if an update has not yet done it for you!
  • Set the protocol in the Samba servers to SMBV3!
  • Try to work with “networks drives” on your Win 10 guests, only!
  • Install, configure and use WSDD, if you really need to see your Samba servers in the Windows Explorer.
  • Open the port 445 (TCP, IN/OUT between the Win 10 client and the server), 3072 (UDP, OUT from the server and the Win 10 client to 239.255.255.250, IN to the server from the Win 10 client / IN to the Win 10 client from the server; rules details depending on the firewall location), port 5357 (TCP; In/OUT between the Samba server and the Win 10 client) on your firewalls between the Samba server and the Win 10 system.
  • Close the NetBIOS ports in your firewalls!
  • You should also take care of stopping multicast messages leaving perimeter firewalls; normally packets to multicast addresses should not be routed, but blocking them explicitly for certain interfaces is no harm, either.

Of course you must repeat the WSDD and firewall setup for all your Samba servers. But as a Linux admin you have your tools for distributing common configuration files or copying virtualization setups.

Links

The real story
!!!! https://www.ixsystems.com/community/resources/how-to-kill-off-smb1-netbios-wins-and-still-have-windows-network-neighbourhood-better-than-ever.106/ !!!

https://forums.linuxmint.com/viewtopic.php?p=1799875

https://devanswers.co/discover-ubuntu-machines-samba-shares-windows-10-network/

https://bugs.launchpad.net/ubuntu/ source/ samba/ +bug/ 1831441

https://forums.opensuse.org/ showthread.php/ 540083-Samba-Network-Device-Type-for-Windows-10

https://kofler.info/zugriff-auf-netzwerkverzeichnisse-mit-nautilus/

WSDD and its problems
https://github.com/christgau/wsdd
https://github.com/christgau/wsdd/issues/8
https://forums.opensuse.org/ showthread.php/ 540083-Samba-Network-Device-Type-for-Windows-10

Warnings of SMB V1
https://docs.microsoft.com/de-de/windows-server/storage/file-server/troubleshoot/detect-enable-and-disable-smbv1-v2-v3
https://blog.malwarebytes.com/101/2018/12/how-threat-actors-are-using-smb-vulnerabilities/
https://securityboulevard.com/2018/12/whats-the-problem-with-smb-1-and-should-you-worry-about-smb-2-and-3/
https://techcommunity.microsoft.com/t5/storage-at-microsoft/stop-using-smb1/ba-p/425858
https://www.cubespotter.de/cubespotter/wannacry-nsa-exploits-und-das-maerchen-von-smbv1/

Problems with Win 10 and shares
https://social.technet.microsoft.com/ Forums/ en-US: cannot-connect-to-cifs-smb-samba-network-shares-amp-shared-folders-in-windows-10-after-update?forum=win10itpronetworking

RDMA and SMB Direct
https://searchstorage.techtarget.com/ definition/ Remote-Direct-Memory-Access

Other settings in the SMB/Samba environment of minor relevance
http://woshub.com/cannot-access-smb-network-shares-windows-10-1709/
https://superuser.com/questions/1466968/unable-to-connect-to-a-linux-samba-server-via-hostname-on-windows-10
https://superuser.com/questions/1522896/windows-10-cannot-connect-to-linux-samba-shares-except-from-smb1-cifs
https://www.reddit.com/ r/ techsupport/ comments/ 3yevip/ windows 10 cant see samba shares/
https://devanswers.co/network-error-problem-windows-cannot-access-hostname-samba/

 

ufw auf Strato-vServern mit Debian 8 – fehlende iptables Log-Meldungen im systemd-Journal – rsyslogd

Posted on by

Gestern hatte ich das Vergnügen, ein Debian-Server-System auf einer aktuellen vServer-Plattform bei Strato einzurichten. Ich bereite entsprechende Arbeiten in der Regel vor, indem ich elementare Konfigurationsschritte – im Besonderen solche, die sicherheitsrelevant sind – vorab auf einem ähnlichen KVM-Gast-System in unserem Hausnetz simuliere und teste.

Diese Art von vorbereitenden Tests hat jedoch ihre Grenzen; nicht alles ist vergleichbar. Gestern bin ich mal wieder auf einen Unterschied im Zusammenhang mit iptables, ufw und den zugehörigen LOG-Meldungen unter systemd gestoßen. Letztere fehlten nämlich im systemd-Journal des Strato-vServers völlig.

Da fragt man sich schon, wie man denn unter solchen Voraussetzungen das gehostete System bzgl. von Angriffsmustern monitoren soll. Ich finde, diese Frage ist so relevant, dass sie sich auch andere Strato-Kunden besser vor dem Mieten eines vServers beantworten sollten. Deshalb dieser Post. Die gute Nachricht ist: Es gibt unabhängig von den Ursachen für das Fehlen der LOG-Meldungen einen Workaround.

Die schlechte Nachricht ist: Die Ursache der fehlenden Kernel-Meldungen im systemd-Journal ist unklar; zumindest mir. Auf einem KVM-Host funktioniert alles wie erwartet. Unterschiede zu gehosteten Servern sind meist auf einen anderen Ansatz in der Virtualisierung zurückzuführen (Stichwort: Container-Technologie vs. Hypervisor für Full/Para-Virtualisierung).

In diesem Falle erscheint mir das aber als Erklärungsansatz nicht plausibel und hinreichend. Ich gehe nachfolgend auf die Gründe etwas genauer ein. Zudem ist bei Debian 8 (leider) eben auch systemd in den Logging-Prozess involviert. Defizite von “systemd” in der Interaktion mit bestimmten Virtualisierungsumgebungen halte ich für durchaus möglich. Der Irrwitz, dass ein Programm beim Systemstart die Umgebung analysieren und für jeden Fall die richtige Antwort ziehen muss, hat halt seinen Preis …

ufw, netfilter/iptables und das Logging-Problem

Ich bin eigentlich ein Freund von Firewall-Builder (FWB). Für Debian-Systeme verwende ich aber auch “ufw“, um initial die wichtigsten Paketfilter-Regeln, also iptables-Anweisungen, bequem und zeitsparend aufzusetzen. Die drehen sich zunächst um den SSH-Zugang von außen und die Erlaubnis, dass der gehostete Server DNS-Server, NTP-Server und bestimmte Update-Server kontaktieren darf. Auch “pings” und “traceroute” vom Server nach außen erlaube ich. Alles andere wird von mir anfänglich rigoros geblockt. Später wird dann für die angestrebten Services des Servers gezielt nachgearbeitet. (Off topic: Viele Dienste, die mein Kunde benötigt, tunnele ich auf dem Server über eine SSH-Verbindung; ein direkter SSH-Zugang des Users root wird sowieso unterbunden und der SSH-Port verschoben.)

Anfänglich ist hinsichtlich eines minimalen Regelsatzes gar nicht viel zu tun. Im Anschluss an das Etablieren der ersten Paketfilter-Regeln möchte ich gerne die Arbeit von “netfilter” testen und das zugehörige Logging mitverfolgen. Typischerweise lasse ich dann “nmap” von außen auf das gehostete System los. Für einen Test des Serverzugriffs auf externe DNS-Dienste und Zugriffe auf Update-Server tut es dagegen “apt-get”. In beiden Fällen verfolge ich per SSH auf einem (Remote-) Terminal den Strom der Meldungen der (ufw-)”Firewall”.

Das erhoffte Verfolgen der iptables-Log-Meldungen schlug auf dem Strato-vServer mit installiertem Debian 8 aber fehl.

Debian 8.x nutzt wie gesagt systemd. Ufw schreibt die iptables-Log-Daten mit eigenen Zusätzen in das Log-System des Servers – bei einem systemd-basierten Systemen also in das dortige binäre Journal. Das systemd-Journal fängt im Normalfall neben System-Meldungen und Meldungen aus dem Userspace auch Kernel-Messages auf. Da systemd den gesamten Mix aus Messages in ein
binäres Datenformat in einer Datei überführt, muss man das Kommando “journalctl” mit geeigneten Filtern bzw. der Option “-f -nxxx” benutzen, um Log-Einträge auswerten bzw. direkt am Schirm mitverfolgen zu können.

Gesagt, getan. Leider tauchen auf einem Strato-vServer im Journal von “systemd” generell nur sehr wenig Informationen auf; hinsichtlich der Paketfilter-LOG-Meldungen findet man dort jedoch leider gar nichts.

Das iptables-Target “LOG” mündet auf einem mit “rsyslogd” ausgestatteten Log- und Warnsystem dagegen in Meldungen in der Datei “/var/log/kern.log” – schließlich handelt es sich ja um Kernel-Meldungen.

Die aus meiner Sicht schon immer kritikwürdige Idee, alle systemrelevanten Meldungen an einer Stelle in einem Binärformat zu sammeln, wird uns auf einem Strato vServer nun offenbar zum Verhängnis: Nur mit systemd können wir kleine und große externe Zugriffsversuche auf einen vServer offenbar nicht überwachen!

Ich bin übrigens nicht der Einzige, der dieses Problem hatte; siehe:
http://linux.debian.user.german.narkive.com/8AbyxJTP/keine-eintrage-von-dmesg-im-journal-systemd
Erstaunlich ist dennoch, dass man ansonsten im Internet fast nichts zu dieser Thematik findet.

Ob das Problem nun etwas mit systemd-Defiziten oder einer speziellen Konfiguration der systemd-Interaktion mit der Virtualisierungsumgebung bei Strato zu tun hat, muss man natürlich ein wenig austesten.

Firewall-Logging, Virtualisierung und Container

Tatsächlich erweist sich das Verhalten von Debian 8 mit “ufw” auf einem KVM-Gastsystem als gänzlich anders. Hier ein Auszug der ufw-Meldungen von einem KVM-Gast mit Debian 8, die mittels des Befehls

“journalctl -f -n20”

zur Anzeige gebracht wurden: 

Apr 05 16:05:52 deb11 kernel: [UFW BLOCK] IN=eth0 OUT= MAC=52:54:00:d5:4a:9b:52:54:00:fc:27:c5:08:00 SRC=192.168.10.1 DST=192.168.10.11 LEN=44 TOS=0x00 PREC=0x00 TTL=54 ID=25556 PROTO=TCP SPT=64358 DPT=110 WINDOW=1024 RES=0x00 SYN URGP=0 
Apr 05 16:05:52 deb11 kernel: [UFW BLOCK] IN=eth0 OUT= MAC=52:54:00:d5:4a:9b:52:54:00:fc:27:c5:08:00 SRC=192.168.10.1 DST=192.168.10.11 LEN=44 TOS=0x00 PREC=0x00 TTL=59 ID=47868 PROTO=TCP SPT=64358 DPT=135 WINDOW=1024 RES=0x00 SYN URGP=0 
Apr 05 16:05:52 deb11 kernel: [UFW BLOCK] IN=eth0 OUT= MAC=52:54:00:d5:4a:9b:52:54:00:fc:27:c5:08:00 SRC=192.168.10.1 DST=192.168.10.11 LEN=44 TOS=0x00 PREC=0x00 TTL=45 ID=41401 PROTO=TCP SPT=64358 DPT=53 WINDOW=1024 RES=0x00 SYN URGP=0 
Apr 05 16:05:52 deb11 kernel: [UFW ALLOW] IN=eth0 OUT= MAC=52:54:00:d5:4a:9b:52:54:00:fc:27:c5:08:00 SRC=192.168.10.1 DST=192.168.10.11 LEN=44 TOS=0x00 PREC=0x00 TTL=42 ID=10106 PROTO=TCP SPT=64358 DPT=22 WINDOW=1024 RES=0x00 SYN URGP=0 
Apr 05 16:05:52 deb11 kernel: [UFW BLOCK] IN=eth0 OUT= MAC=52:54:00:d5:4a:9b:52:54:00:fc:27:c5:08:00 SRC=192.168.10.1 DST=192.168.10.11 LEN=44 TOS=0x00 PREC=0x00 TTL=54 ID=65381 PROTO=TCP SPT=64358 DPT=113 WINDOW=1024 RES=0x00 SYN URGP=0 
Apr 05 16:05:52 deb11 kernel: [UFW BLOCK] IN=eth0 OUT= MAC=52:54:00:d5:4a:9b:52:54:00:fc:27:c5:08:00 SRC=192.168.10.1 DST=192.168.10.11 LEN=44 TOS=0x00 PREC=0x00 TTL=47 ID=1758 PROTO=TCP SPT=64358 DPT=587 WINDOW=1024 RES=0x00 SYN URGP=0 
Apr 05 16:05:52 deb11 kernel: [UFW BLOCK] IN=eth0 OUT= MAC=52:54:00:d5:4a:9b:52:54:00:fc:27:c5:08:00 SRC=192.168.10.1 DST=192.168.10.11 LEN=44 TOS=0x00 PREC=0x00 TTL=47 ID=22236 PROTO=TCP SPT=64358 DPT=443 WINDOW=1024 RES=0x00 SYN URGP=0 
Apr 05 16:05:52 deb11 kernel: [UFW BLOCK] IN=eth0 OUT= MAC=52:54:00:d5:4a:9b:52:54:00:fc:27:c5:08:00 SRC=192.168.10.1 DST=192.168.10.11 LEN=44 TOS=0x00 PREC=0x00 TTL=55 ID=60478 PROTO=TCP SPT=64358 DPT=23 WINDOW=1024 RES=0x00 SYN URGP=0 
Apr 
05 16:05:52 deb11 kernel: [UFW BLOCK] IN=eth0 OUT=

 
Offensichtlich führt im obigen Fall das System 192.168.10.1 einen Portscan auf dem betroffenen KVM-Host mit der IP 192.168.10.11 durch.

Ähnliche Meldungen erhält man bei einem Portscan auf einem vServer aber – wie gesagt – nicht.

Wie könnte man das erklären?
Ein naheliegender Erklärungsansatz wäre etwa folgender:
Das Logging von Kernel-Messages klappt auf einem KVM-Gast, also unter dem QEMU-Hypervisor (sog. Typ 2 Hypervisor), der über “virtio” auf dem Host nur partiell Paravirtualisierung und keine Container-Technologie einsetzt, natürlich problemfrei. Das Betriebssystem des KVM-Gastes und dessen Kernel arbeiten ja weitgehend autonom und greifen nur über Vermittlungsschichten auf den Kernel des Hosts und dessen HW-Unterstützung zu. Es besteht von Haus aus kein Problem bzgl. des Loggings von Kernel-Meldungen – sie beziehen sich immer auf den Kernel des Gastsystems.

Dagegen setzt Strato Container-Technologie ein – genauer VZ-Container unter Virtuozzo; selbiges basiert auf OpenVZ. Zu Grundeigenschaften siehe :
https://openvz.org/Main_Page und https://openvz.org/Features

Es handelt sich bei Strato wohl um Version 4.7 oder eine frühe 6er Version von “Virtuozzo Containers”. Dafür gibt es Indizien (u.a, dass sich Docker nicht installieren lässt); um einen genauen Nachweis habe ich mich aber (noch) nicht gekümmert. Ist auch egal.

In einer Container-Lösung wird jedenfalls die Kapazität und Funktionalität des Host-Kernels zwischen den Containern, die keinen eigenen Kernel besitzen, geteilt (schlanker “Single-Kernel-Approach”). Der Zugriff auf Netze erfolgt über eine entsprechende Netzwerk- und Schnittstellen-Virtualisierung. Typischerweise werden virtuelle venet- oder veth-NICs eingesetzt; je nachdem, auf welcher Ebene OSI-Stacks man arbeiten will. (veth-NICs setze ich selbst vielfach auch in komplexeren KVM/Qemu-Umgebungen bei der Netzwerkvirtualisierung ein.)

Die notwendige Separation der Container und ihrer Netzwerk-Kommunikation gegeneinander und gegenüber dem Host muss vom Host-Kernel bzw. dessen Modulen auf der Basis von Konfigurationsvorgaben für unpriviligierte Container (in ihren separaten Namespaces und bei modernen Ansätzen ggf. in cGroups) gewährleistet werden. Man wird den Container-Systemen jedenfalls nicht erlauben, alles einzusehen, was auf dem für alle zuständigen Host-Kernel abläuft. Dies bedeutet u.a., dass Containersysteme nicht beliebige Kernel-Module (z.B. für Packettracking unter Wireshark) laden dürfen.

Wer “iptables” im Zusammenhang mit Virtualisierungshosts aber ein wenig genauer kennt, kann sich vorstellen, dass man eine Host-Firewall natürlich immer so konfigurieren kann, dass die einzelnen virtuellen Netzwerkschnittstellen der (Container-) Gäste gegeneinander geblockt werden, aber dass generelle Forward-Regeln für physikalische Interfaces des Hosts nicht in Konflikt mit speziellen Filter-Regeln für ein spezifisches (virtuelles) Gast-Interface geraten müssen.

OpenVZ kann man deshalb sehr wohl so einrichten, dass der Admin eines Container-Systems seine eigenen iptables-Regeln für seine gastspezifischen NICs definieren kann. Siehe hierzu z.B.:
https://openvz.org/Setting_up_an_iptables_firewall.

Wesentliche Teile der verschiedenen netfilter-Module – im Besonderen für die Schicht 3 – stehen also auch Gästen zur Verfügung. Voraussetzung ist in einer Container-Architektur natürlich, dass grundlegende “netfilter”-Module auf dem OpenVZ-Host selbst geladen wurden.

Aber: Es wäre fahrlässig, wenn ein Container-Host alle netzwerkspezifischen Kernel-Meldungen (darunter iptables-Meldungen) auch für die Einsichtnahme durch die root-User der Container preisgeben würde. Das würde u.a ein
Ausspionieren der virtuellen Netzwerkumgebung und darauf aufbauend bestimmte Angriffsszenarien ermöglichen. Wenn wir überhaupt etwas im Container sehen, dann höchstens Meldungen zu selbst gesetzten Paketfilterregeln für die Container-spezifische NIC.

Zwischenfazit:

  • Wir dürfen uns in einer Container-Umgebung u.a. nicht darüber wundern, dass man bestimmte Kernel-Module vom Container aus erst gar nicht laden darf und z.B. lsmod eine vernünftige Antwort schuldig bleibt.
  • Wir dürfen uns nicht wundern, dass bestimmte sysctl-Befehle, die im Container abgesetzt werden, ggf. ignoriert werden.
  • Wir dürfen uns in einer Container-Umgebung nicht wundern, wenn man bestimmte Teile des systemd-Logs auf einem Container – und damit auf einem Strato-V-Server – nicht ggf. zu Gesicht bekommt. (Im Gegensatz zu einem KVM-Gast).

Der erste Punkt ist u.a. für den Betrieb der ufw relevant; s.u..

Bzgl. des zweiten Punktes ist zu beachten, dass OpenVZ, genauer der OpenVZ-Kernel, (network-) “Namespaces” nutzt. (“Namespaces” werden natürlich aber auch von aktuellen Linux-Kerneln unterstützt. Zu “Namespaces” siehe etwa
https://jvns.ca/blog/2016/10/10/what-even-is-a-container/
https://de.slideshare.net/jpetazzo/anatomy-of-a-container-namespaces-cgroups-some-filesystem-magic-linuxcon
https://openvz.org/WP/What_are_containers.

Deshalb lassen sich bestimmte Einstellung unterhalb von “/proc/sys” durchaus auch vom Container aus anpassen. Was in der jeweiligen OpenVZ-Umgebung erlaubt ist und was nicht, muss man ggf. durch Probieren herausfinden.

Den dritten Punkt werden wir in den nächsten Abschnitten für vServer kritisch hinterfragen.

Erste Konsequenzen für den Einsatz von “ufw”

Auch durch Probieren wird man herausfinden, dass “ufw” auch auf einem mit Debian 8 betriebenen vServer von Strato läuft – und man eigene iptables-Regeln problemfrei an den OpenVZ-Kernel weiterreichen kann.

Achtung:

Nach der Installation von ufw auf dem vServer NICHT unmittelbar “ufw enable” absetzen!! Zuerst den Port, auf dem man SSH betreibt, freischalten. Also etwa durch “ufw allow 22”, wenn man den SSH-Standardport benutzt. Ihr wollt euch ja nicht durch Anschalten der Firewall selbst aussperren!

Wie man schnell (hier durch einen Blick in das systemd-Journal feststellt), ist der Start von ufw auf einem vServer – auch im Rahmen eines Systemstarts – mit ein paar Fehlermeldungen verbunden. 

Apr 05 14:21:03 xxx.stratoserver.net ufw[147]: Starting firewall: ufw...modprobe: ERROR: ../libkmod/libkmod.c:557 kmod_search_moddep() could not ope
Apr 05 14:21:03 xxx.stratoserver.net ufw[147]: modprobe: ERROR: ../libkmod/libkmod.c:557 kmod_search_moddep() could not open moddep file '/lib/modul
Apr 05 14:21:03 xxx.stratoserver.net ufw[147]: modprobe: ERROR: ../libkmod/libkmod.c:557 kmod_search_moddep() could not open moddep file '/lib/modul
Apr 05 14:21:03 xxx.stratoserver.net systemd-journal[114]: Permanent journal is using 24.0M (max allowed 4.0G, trying to leave 4.0G free of 499.5G a
Apr 05 14:21:03 xxx.stratoserver.net systemd-journal[114]: Time spent on flushing to /var is 1.770ms for 8 entries.
Apr 05 14:21:03 xxx.stratoserver.net ufw[147]: sysctl: permission denied on key 'net.ipv4.tcp_sack'
Apr 05 14:21:03 xxx.stratoserver.net ufw[147]: Setting kernel variables (/etc/ufw/sysctl.conf)...done.

r
 
Diese Meldungen rühren u.a. daher, dass ufw mit Hilfe von modprobe versucht, bestimmte “conntrack”-Sub-Module zu laden. Zudem versucht ufw per sysctl Kernel-Parameter abzuändern. Vorgegeben sind diese Schritte in den Dateien “/etc/default/ufw” und “/etc/ufw/sysctl.conf“.

Die genannten Fehler blockieren den Start aktueller ufw-Versionen aber nicht; wer sich dennoch an den Meldungen stört, kann die über Modifikationen der genannten Dateien, nämlich durch Auskommentieren der fehlerträchtigen Statements, verhindern. Siehe auch
https://www.hosteurope.de/faq/server/virtual-server/besonderheiten-firewall-virtual-server/; im Unterschied zu den dortigen Tipps aber beachten, dass auf dem vServer nur einer der sysctl-Befehle problematisch ist.)

Übrigens: Über das Starten von ufw bei einem Reboot des vServers muss man sich nach einem Absetzen von

systemctl enable ufw

keine Gedanken mehr machen. Debian 8 beinhaltet für ufw einen passenden LSB-Service, der beim Hochfahren ausgeführt wird.

Monitoring von ufw-iptables-Meldungen auf dem vServer mit Hilfe von dmesg

Nachdem man in systemd-Journal nichts findet: Gibt es andere Möglichkeiten, die LOG-Messages von ufw-/iptables zu verfolgen?

Da es sich um Kernel-Messages handelt, liegt ein versuchsweiser Blick auf den “dmesg”-Output nahe. Und tatsächlich – auf meinem vServer: 

    
root@xxx:~ # dmesg
[1240949.664984] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=45.55.2.201 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=244 ID=54321 PROTO=TCP SPT=40788 DPT=3306 WINDOW=65535 RES=0x00 SYN URGP=0 
[1240954.051018] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=93.174.93.136 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=250 ID=4710 PROTO=TCP SPT=43745 DPT=3128 WINDOW=1024 RES=0x00 SYN URGP=0 
[1240986.448807] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=45.55.1.72 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=244 ID=54321 PROTO=TCP SPT=46822 DPT=1900 WINDOW=65535 RES=0x00 SYN URGP=0 
[1241002.495868] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=88.100.184.82 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=50 ID=13380 PROTO=TCP SPT=35180 DPT=23 WINDOW=44554 RES=0x00 SYN URGP=0 
[1241015.141452] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.163.144.224 DST=xxx LEN=445 TOS=0x00 PREC=0x00 TTL=57 ID=32820 DF PROTO=UDP SPT=5180 DPT=5046 LEN=425 
[1241132.233004] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=197.44.69.222 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=239 ID=44476 PROTO=TCP SPT=53226 DPT=1433 WINDOW=1024 RES=0x00 SYN URGP=0 
[1241145.520318] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=208.100.26.228 DST=xxx LEN=40 TOS=0x08 PREC=0x00 TTL=242 ID=51210 PROTO=TCP SPT=47975 DPT=15672 WINDOW=1024 RES=0x00 SYN URGP=0 
[1241185.158299] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.163.144.224 DST=xxx LEN=443 TOS=0x00 PREC=0x00 TTL=57 ID=56812 DF PROTO=UDP SPT=5400 DPT=4000 LEN=423 
[1241297.661764] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=186.45.130.20 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=241 ID=12134 PROTO=TCP SPT=63715 DPT=23 WINDOW=14600 RES=0x00 SYN URGP=0 
[1241350.742715] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.163.144.224 DST=xxx LEN=446 TOS=0x00 PREC=0x00 TTL=57 ID=14769 DF PROTO=UDP SPT=5320 DPT=5172 LEN=426 
[1241353.098569] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=5.53.113.195 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=51 ID=48667 PROTO=TCP SPT=46919 DPT=23 WINDOW=39535 RES=0x00 SYN URGP=0 
[1241377.620483] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=46.152.41.83 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=240 ID=40038 PROTO=TCP SPT=12600 DPT=23 WINDOW=14600 RES=0x00 SYN URGP=0 
[1241386.187457] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=122.114.187.140 DST=xxx LEN=40 TOS=0x00 
PREC=0x00 TTL=238 ID=8477 PROTO=TCP SPT=46170 DPT=23 WINDOW=1024 RES=0x00 SYN URGP=0 
[1241437.193431] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=218.91.210.142 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=48 ID=4557 PROTO=TCP SPT=45821 DPT=23 WINDOW=27541 RES=0x00 SYN URGP=0 
[1241512.054090] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.163.144.224 DST=xxx LEN=443 TOS=0x00 PREC=0x00 TTL=57 ID=37720 DF PROTO=UDP SPT=5179 DPT=1028 LEN=423 
[1241553.246515] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=49.4.143.59 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=104 ID=256 PROTO=TCP SPT=6000 DPT=135 WINDOW=16384 RES=0x00 SYN URGP=0 
[1241632.706391] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=39.71.216.3 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=48 ID=4841 PROTO=TCP SPT=57398 DPT=22 WINDOW=55725 RES=0x00 SYN URGP=0 
[1241643.559480] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=196.202.5.43 DST=xxx LEN=44 TOS=0x00 PREC=0x00 TTL=48 ID=53691 PROTO=TCP SPT=34866 DPT=23 WINDOW=33710 RES=0x00 SYN URGP=0 
[1241674.241572] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.163.144.224 DST=xxx LEN=446 TOS=0x00 PREC=0x00 TTL=57 ID=60393 DF PROTO=UDP SPT=5288 DPT=1029 LEN=426 
[1241683.411659] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=124.167.232.138 DST=xxx LEN=60 TOS=0x00 PREC=0x00 TTL=47 ID=40536 DF PROTO=TCP SPT=57844 DPT=22 WINDOW=14600 RES=0x00 SYN URGP=0 
[1241686.407572] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=124.167.232.138 DST=xxx LEN=60 TOS=0x00 PREC=0x00 TTL=47 ID=40537 DF PROTO=TCP SPT=57844 DPT=22 WINDOW=14600 RES=0x00 SYN URGP=0

 
Ja, die Welt ist schlecht – und wir tun offenbar gut daran, den Zugang zum Server zu blocken bzw. die Logs auch mal auszuwerten und später Blacklists einzusetzen. “fail2ban” zu konfigurieren schadet nebenbei auch nichts.

Wer periodische Updates des Outputs von dmesg ähnlich zu “tail -f” verfolgen will, probiere mal das Kommando

watch -n 0,2 “dmesg | tail -n $((LINES-6))”

in einem Terminal aus. (Ggf. das Terminalfenster etwas vergößern. Auf englischsprachigen Systemen “0.2” statt wie hier “0,2” ! Auf neueren Kerneln als dem der aktuellen vServer gibt es übrigens auch die dmesg-Option “-w”).

Aber das eigentlich Feststellenswerte ist ja, dass wir überhaupt was sehen!

Natürlich ist das, was man unter OpenVZ unter dmesg zu Gesicht bekommt, eingeschränkt (s. etwa https://bugs.openvz.org/browse/OVZ-5328).
Aber:
Der OpenVZ-Kernel liefert dem Container zulässige, relevante Informationen in den lokalen Message-Ringpuffer, die dort von root eingesehen werden können. Darunter auch die ersehnten iptables-Meldungen!

Nun stellt sich die große Frage, wie systemd mit diesen Kernelmeldungen interagiert und warum das, was unter dmesg erschient, nicht ins Journal der OpenVZ-Container-Umgebung eingestellt wird.

Das Problematische an systemd ist, wie immer, dass man das ohne seitenweises Lesen in systemd Blogs etc. und/oder gar Codestudium vermutlich nicht beantworten kann. Logisch erscheint mir das Ganze jedenfalls nicht. OpenVZ sorgt offenbar für eine Reduktion der Kernelinformationen auf das, was root im Container sehen sollte. iptables-Meldungen zur lokalen NIC des Containers werden dabei nicht ausgespart. Sie sollten daher eigentlich auch im systemd-Journal erscheinen!

Monitoring mittels rsyslogd ? !

Durch den dmesg-Test ermutigt, fragte ich mich, was wohl passieren würde, wenn rsyslog auf dem vServer-Debian-System installiert und aktiviert wäre. Das ist insofern interessant, als systemd ja externe Linux System-Logging-Services über Schnittstellen bedient und trotzdem sein eigenes Journal weiter versorgt. Man loggt dann im Normalfall sozusagen zweimal …

Eigentlich würde man nun erwarten, dass die Meldungen von dmesg auch in den verschiedenen Dateien, die der rsyslog-Dämon bedient, nicht auftauchen sollten. Weil systemd ja schon den
Transfer in die eigene Binärdatei verweigert. Also machen wir mal die Probe:

root@xxx:~# apt-get rsyslog
root@xxx:~# systemctl start rsyslog
root@xxx:~# systemctl enable rsyslog

Wenn nun doch etwas passieren sollte, so müssten iptables-Meldungen als Einträge unter “/var/log/kern.log” und/oder in der von ufw vorgesehenen Datei “/var/log/ufw.log” auftauchen.

Und tatsächlich finden wir (wider Erwarten) nach einer Weile in “kern.log” iptables-LOG-Meldungen: 

  
Apr  6 13:19:57 xxx kernel: [1245761.082097] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=118.163.90.134 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=48 ID=14764 PROTO=TCP SPT=33202 DPT=23 WINDOW=44186 RES=0x00 SYN URGP=0 
Apr  6 13:21:03 xxx kernel: [1245827.018789] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=85.90.163.248 DST=xxx LEN=44 TOS=0x00 PREC=0x00 TTL=52 ID=358 PROTO=TCP SPT=21965 DPT=23 WINDOW=12453 RES=0x00 SYN URGP=0 
Apr  6 13:21:16 xxx kernel: [1245840.027789] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.163.144.224 DST=xxx LEN=445 TOS=0x00 PREC=0x00 TTL=57 ID=61654 DF PROTO=UDP SPT=5201 DPT=4011 LEN=425 
Apr  6 13:22:26 xxx kernel: [1245910.326051] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=91.98.36.115 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=46 ID=3739 PROTO=TCP SPT=31242 DPT=23 WINDOW=22306 RES=0x00 SYN URGP=0 
Apr  6 13:23:13 xxx kernel: [1245957.077099] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=117.193.182.117 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=48 ID=29137 PROTO=TCP SPT=12209 DPT=22 WINDOW=52845 RES=0x00 SYN URGP=0 
Apr  6 13:23:54 xxx kernel: [1245997.536218] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=45.55.1.114 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=244 ID=54321 PROTO=TCP SPT=43185 DPT=8123 WINDOW=65535 RES=0x00 SYN URGP=0 
Apr  6 13:23:56 xxx kernel: [1246000.108495] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=192.151.169.29 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=51 ID=46742 PROTO=TCP SPT=54014 DPT=23 WINDOW=49390 RES=0x00 SYN URGP=0 
Apr  6 13:24:05 xxx kernel: [1246008.982092] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.163.144.224 DST=xxx LEN=445 TOS=0x00 PREC=0x00 TTL=57 ID=19866 DF PROTO=UDP SPT=5391 DPT=4012 LEN=425 
Apr  6 13:24:08 xxx kernel: [1246011.450635] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=163.172.204.214 DST=xxx LEN=447 TOS=0x00 PREC=0x00 TTL=58 ID=25411 DF PROTO=UDP SPT=5440 DPT=5060 LEN=427 
Apr  6 13:24:18 xxx kernel: [1246022.133966] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=181.193.99.26 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=48 ID=47683 PROTO=TCP SPT=36520 DPT=23 WINDOW=41856 RES=0x00 SYN URGP=0 
Apr  6 13:24:29 xxx kernel: [1246032.599018] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.248.166.146 DST=xxx LEN=48 TOS=0x00 PREC=0x00 TTL=123 ID=26103 PROTO=TCP SPT=11206 DPT=2083 WINDOW=65535 RES=0x00 SYN URGP=0 
Apr  6 13:24:50 xxx kernel: [1246053.507827] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=71.165.26.106 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=50 ID=381 PROTO=TCP SPT=29725 DPT=23 WINDOW=3178 RES=0x00 SYN URGP=0 
Apr  6 13:25:44 xxx kernel: [1246108.065452] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=139.162.118.251 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=246 ID=54321 PROTO=TCP SPT=56219 DPT=6379 WINDOW=65535 RES=0x00 SYN URGP=0 
Apr  6 13:26:16 xxx kernel: [1246139.839711] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=174.16.249.159 DST=xxx LEN=40 TOS=0x00 PREC=0x00 TTL=240 ID=54734 PROTO=TCP SPT=43074 DPT=23 WINDOW=14600 RES=0x00 SYN URGP=0 
Apr  6 13:26:28 xxx kernel: [1246151.805797] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=117.1.220.212 DST=xxx LEN=44 TOS=0x00 PREC=0x00 TTL=242 ID=2085 PROTO=TCP SPT=14216 DPT=5358 WINDOW=14600 RES=0x00 SYN URGP=0 
Apr  6 13:26:50 xxx kernel: [1246174.013725] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=123.151.42.61 DST=xxx LEN=135 TOS=0x00 PREC=0x00 TTL=47 ID=32696 PROTO=UDP SPT=9019 DPT=1701 LEN=115 
Apr  6 13:27:01 xxx kernel: [1246184.993843] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.163.144.224 DST=xxx LEN=445 TOS=0x00 
PREC=0x00 TTL=57 ID=44551 DF PROTO=UDP SPT=5365 DPT=4013 LEN=425 
Apr  6 13:27:27 xxx kernel: [1246211.300971] [UFW BLOCK] IN=venet0 OUT= MAC= SRC=89.120.177.89 DST=xxx LEN=44 TOS=0x00 PREC=0x00 TTL=55 ID=64605 PROTO=TCP SPT=59589 DPT=23 WINDOW=64213 RES=0x00 SYN URGP=0

 
Die Welt ist inzwischen nicht besser geworden, aber nun können wir das Schlechte wenigstens mal verfolgen. (Dieselben Einträge erscheinen übrigens auch in ufw.log).

Tja, nicht alles im Leben mit systemd ist offenbar nachvollziehbar ….

Fazit

Das erwartete native Zusammenspiel zwischen einem Strato OpenVZ vServer und systemd unter Debian 8 funktioniert bzgl. der Protokollierung der LOG-Target-Meldungen von iptables nicht. Kernel-Meldungen zu iptables-Rules für die Container-NIC erschienen nicht im Journal von systemd.

Workarounds bestehen darin

  • den Output von dmesg kontinuierlich per Skript abzufragen und in eine Datei umzulenken.
  • rsyslog zu installieren und die Arbeit dem Zusammenspiel von systemd mit dem rsyslog-Dämon zu überlassen. Man protokolliert dann doppelt, aber man erhält wenigstens dauerhafte Log-Protokolle, die jeder sicherheitsbewußte Admin zur vorsorglichen Gefahrenabwehr benötigt.

Da die gewünschten Kernel-Meldungen bei gleicher Debian- und Systemd-Version in einem KVM-Gast erscheinen, ist das Problem mit dem systemd-Journal entweder

  • auf einen Fehler oder ein Sicherheitsfeature der OpenVZ-Umgebung,
  • oder auf ein seltsames (gewolltes oder fehlerhaftes) Zusammenspiel des OpenVZ-Kernels mit systemd in den Container-Umgebungen,
  • oder schlicht auf ein fehlerhaftes, bislang nicht erkanntes/bedachtes Verhalten von systemd in einem OpenVZ-Container

zurückzuführen.

Für letzteres spricht die Tatsache, dass der OpenVZ-Kernel iptables-Meldungen zur Container-NIC unter dmesg offenbart und dass systemd die Meldungen, die unter dmesg auftauchen wohl korrekt an weitere System-Logging-Services wie rsyslog weiterleitet.

Eine entsprechende Anfrage bei Strato, die hoffentlich Virtuozzo einschalten, läuft.

Das Fehlen von iptables-Log-Protokolle ist im Sinne der ISO 27000 (Strato hat da ein Zertifikat!) zudem als Sicherheitsproblem einzustufen, das Kunden kommentarlos zugemutet wird und von diesen Kunden selbst gelöst werden muss.

Weiterführende Links

Fehlende Einträge im systemd-Journal
http://linux.debian.user.german.narkive.com/8AbyxJTP/keine-eintrage-von-dmesg-im-journal-systemd

Container, Virtuozzo, OpenVZ und iptables, ufw
http://forum.openvirtuozzo.org/index.php?t=msg&goto=37264&&srch=container
https://openvz.org/Setting_up_an_iptables_firewall
http://askubuntu.com/questions/399624/ubuntu-server-12-04-and-ufw-failure-on-startup-and-several-module-not-found-err
https://www.hosteurope.de/faq/server/virtual-server/besonderheiten-firewall-virtual-server/
https://superuser.com/questions/659236/permission-denied-when-setting-values-in-sysctl-on-ubuntu-12-04
https://help.
ubuntu.com/community/UFW
https://help.virtuozzo.com/customer/en/portal/articles/2509437?_ga=1.206607316.635252319.1491384754
ab S. 317 in folgender Referenz
http://www.odin.com/fileadmin/parallels/documents/hosting-cloud-enablement/pvc/Production_Documents/VzLinuxUG_03132013.pdf
https://bugs.openvz.org/browse/OVZ-5328

Namespaces
http://www.netdevconf.org/1.1/proceedings/slides/rosen-namespaces-cgroups-lxc.pdf

LXC vs. OpenVZ
https://www.janoszen.com/2013/01/22/lxc-vs-openvz/
https://en.wikipedia.org/wiki/LXC
https://openvz.org/Comparison

OpenVZ integrates KVM/Qemu
http://openvz.livejournal.com/tag/criu
https://www.heise.de/ix/meldung/Virtualisierungsplattform-OpenVZ-wird-eigenstaendige-Distribution-3278115.html
https://openvz.org/Virtuozzo
https://openvz.org/QEMU
https://www.heise.de/ix/meldung/Virtualisierungsplattform-OpenVZ-wird-eigenstaendige-Distribution-3278115.html
https://openvz.org/FAQ
https://virtuozzo.com/virtual-machines-in-virtuozzo-7/
https://openvz.org/WP/What_are_containers#Networking

Virtualisierungsangebote in D – Vergleich
https://timreeves.de/trip-content/uploads/dokumente/Internet-Mietserver-Typen_im-Vergleich.pdf

Watch dmesg Output
http://unix.stackexchange.com/questions/95842/how-can-i-see-dmesg-output-as-it-changes

Linux bridges – can iptables be used against MiM attacks based on ARP spoofing ? – III

Posted on by

This small series of blog contributions was written to understand a little better how to use iptables in the context of Linux bridges as a countermeasure against some of the effects of a man-in-the-middle [MiM] attack based on ARP spoofing. The attacking system as well as the attacked systems are in our scenarios attached to Linux bridge ports. My objective was to block redirected TCP/IP packets from and to the attacking system.

In the first post
Linux bridges – can iptables be used against MiM attacks based on ARP spoofing ? – I
we had discussed how we have to set up iptables rules for ports of a single Linux bridge and their associated IP-addresses to get the desired blocking. We found that a certain order of DENY and ACCEPT rules is required.

In the second post
Linux bridges – can iptables be used against MiM attacks based on ARP spoofing ? – II
we investigated how iptables reacts to the existence of multiple and linked Linux bridges on one and the same host.

We defined “border ports” as ports that connect a Linux bridge to other bridges, to external network segments or to the virtualization host itself – but not to guests directly attached to the bridge via tap or veth-devices. “Border ports” may be passed by packets traveling to their destination across multiple bridges. We then extended our previous considerations on iptables rules to such “border ports” and found a general recipe for the order of the required DENY and ACCEPT rules for the ports of the multiple bridges.

In the present post we shall test the required rules for the bridge setup presented in the 2nd post. We consider some examples of attack variants with respect to the 2 bridges and ICMP packets. However, the tests would also work for any TCP based service. The reason is that the central DENY rules are very general and compiled without reference to any specific service type.

I assume that you have had a look at the screenshots of the logical rules displayed in a FWbuilder interface in the 2nd article (II).

Setup and iptables rules created by FWbuilder

See the following drawing for the setup of our test scenario:

bridge3

The general DENY rules and the ICMP-related ACCEPT rules displayed in the last article are compiled by FWbuilder to create the following script commands: 

    # Rule 2 (vk63)
    # 
    echo "Rule 2 (vk63)"
    # 
    # virbr6 guest port
    $IPTABLES -N Out_RULE_2
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vk63 !  -d 192.168.50.13   -j Out_RULE_2  
    $IPTABLES -A Out_RULE_2  -j LOG  --log-level info --log-prefix "RULE 2 -- DENY "
    $IPTABLES -A Out_RULE_2  -j DROP
    # 
    # Rule 3 (vk64)
    # 
    echo "Rule 3 (vk64)"
    # 
    # virbr6 guest port
    $IPTABLES -N Out_RULE_3
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vk64 !  -d 192.168.50.14   -j Out_RULE_3 
    $IPTABLES -A Out_RULE_3  -j LOG  --log-level info --log-prefix "RULE 3 -- DENY "  
    $IPTABLES -A Out_RULE_3  -j DROP
    # 
    # Rule 4 (vk65)
    # 
    echo "Rule 4 (vk65)"
    # 
    # virbr6 guest port
    $IPTABLES -N Out_RULE_4
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vk65 !  -d 192.168.50.15   -j Out_RULE_4  
    $IPTABLES -A Out_RULE_4  -j LOG  --log-level info --log-prefix "RULE 4 -- DENY "    
    $IPTABLES -A Out_RULE_4  -j DROP
    # 
    # Rule 5 (vk42)
    # 
    echo "Rule 5 (vk42)"
    # 
    # virbr4 guest port
    $IPTABLES -N Out_RULE_5
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vk42 !  -d 192.168.50.12   -j Out_RULE_5  
    $IPTABLES -A Out_RULE_5  -j LOG  --log-level info --log-prefix "RULE 5 -- DENY "
    $IPTABLES -A Out_RULE_5  -j DROP
    # 
    # Rule 6 (vk63)
    # 
    echo "Rule 6 (vk63)"
    # 
    # virbr6 guest port
    $IPTABLES -N Out_RULE_6
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vk63  -s 192.168.50.13   -j Out_RULE_6  
    $IPTABLES -A Out_RULE_6  -j LOG  --log-level info --log-prefix "RULE 6 -- DENY "
    $IPTABLES -A Out_RULE_6  -j DROP
    # 
    # Rule 7 (vk64)
    # 
    echo "Rule 7 (vk64)"
    # 
    # virbr6 guest port
    $IPTABLES -N Out_RULE_7
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vk64  -s 192.168.50.14   -j Out_RULE_7  
    $IPTABLES -A Out_RULE_7  -j LOG  --log-level info --log-prefix "RULE 7 -- DENY "
    $IPTABLES -A Out_RULE_7  -j DROP
    # 
    # Rule 8 (vk65)
    # 
    echo "Rule 8 (vk65)"
    # 
    # virbr6 guest port
    $IPTABLES -N Out_RULE_8
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vk65  -s 192.168.50.15   -j Out_RULE_8  
    $IPTABLES -A Out_RULE_8  -j LOG  --log-level info --log-prefix "RULE 8 -- DENY "
    $IPTABLES -A Out_RULE_8  -j DROP
    # 
    # Rule 9 (vk42)
    # 
    echo "Rule 9 (vk42)"
    # 
    # virbr4 guest port
    $IPTABLES -N Out_RULE_9
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vk42  -s 192.168.50.12   -j Out_RULE_9  
    $IPTABLES -A Out_RULE_9  -j LOG  --log-level info --log-prefix "RULE 9 -- DENY "
    $IPTABLES -A Out_RULE_9  -j DROP
    # 
    # Rule 10 (vk63)
    # 
    echo "Rule 10 (vk63)"
    # 
    # virbr6 guest port
    $IPTABLES -N In_RULE_10
    $IPTABLES -A INPUT -m physdev --physdev-in vk63 !  -s 192.168.50.13   -j In_RULE_10
    $IPTABLES -A FORWARD -m physdev --physdev-in vk63 !  -s 192.168.50.13   -j In_RULE_10  
    $IPTABLES -A In_RULE_10  -j LOG  --log-level info --log-prefix "RULE 10 -- DENY "
    $IPTABLES -A In_RULE_10  -j DROP
    # 
    # Rule 11 (vk64)
    # 
    echo "Rule 11 (vk64)"
    # 
    # virbr6 guest port
    $IPTABLES -N In_RULE_11
    $IPTABLES -A INPUT -m physdev --physdev-in vk64 !  -s 192.168.50.14   -j In_RULE_11
    $IPTABLES -A FORWARD -m physdev --physdev-in vk64 !  -s 192.168.50.14   -j In_RULE_11  
    $IPTABLES -A In_RULE_11  -j LOG  --log-level info --log-prefix "RULE 11 -- DENY "
    $IPTABLES -A In_RULE_11  -j DROP
    # 
    # Rule 12 (vk65)
    # 
    echo "Rule 12 (vk65)"
    # 
    # virbr6 guest port
    $IPTABLES -N In_RULE_12
    $IPTABLES -A INPUT -m physdev --physdev-in vk65 !  -s 192.168.50.15   -j In_RULE_12
    $IPTABLES -A FORWARD -m physdev --physdev-in vk65 !  -s 192.168.50.15   -j In_RULE_12  
    $IPTABLES -A In_RULE_12  -j LOG  --log-level info --log-prefix "RULE 12 -- DENY "
    $IPTABLES -A In_RULE_12  -j DROP
    # 
    # Rule 13 (vk42)
    # 
    echo "Rule 13 (vk42)"
    # 
    # virbr4 guest port
    $IPTABLES -N In_RULE_13
    $IPTABLES -A INPUT -m physdev --physdev-in vk42 !  -s 192.168.50.12   -j In_RULE_13
    $IPTABLES -A FORWARD -m physdev --physdev-in vk42 !  -s 192.168.50.12   -j In_RULE_13   
    $IPTABLES -A In_RULE_13  -j LOG  --log-level info --log-prefix "RULE 13 -- DENY "
    $IPTABLES -A In_RULE_13  -j DROP
    # 
    # Rule 15 (vethb2)
    # 
    echo "Rule 15 (vethb2)"
    # 
    # br6 border out
    $IPTABLES -N Cid7404X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vethb2  -j Cid7404X2034.0
    $IPTABLES -A Cid7404X2034.0  -s 192.168.50.13   -j RETURN
    $IPTABLES -A Cid7404X2034.0  -s 192.168.50.14   -j RETURN
    $IPTABLES -A Cid7404X2034.0  -s 192.168.50.15   -j RETURN
    $IPTABLES -N Out_RULE_15_3
    $IPTABLES -A Cid7404X2034.0  -j Out_RULE_15_3
    $IPTABLES -A Out_RULE_15_3  -j LOG  --log-level info --log-prefix "RULE 15 -- DENY "  
    $IPTABLES -A Out_RULE_15_3  -j DROP
    # 
    # Rule 16 (vethb2)
    # 
    echo "Rule 16 (vethb2)"
    # 
    # br6 border out
    $IPTABLES -N Cid7478X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vethb2  -j Cid7478X2034.0  
    $IPTABLES -A Cid7478X2034.0  -d 192.168.50.1   -j RETURN
    $IPTABLES -A Cid7478X2034.0  -d 192.168.0.37   -j RETURN
    $IPTABLES -A Cid7478X2034.0  -d 192.168.50.12   -j RETURN
    $IPTABLES -N Out_RULE_16_3
    $IPTABLES -A Cid7478X2034.0  -j Out_RULE_16_3
    $IPTABLES -A Out_RULE_16_3  -j LOG  --log-level info --log-prefix "RULE 16 -- DENY "
    $IPTABLES -A Out_RULE_16_3  -j DROP
    # 
    # Rule 17 (vethb1)
    # 
    echo "Rule 17 (vethb1)"
    # 
    # br4 border out
    $IPTABLES -N Cid8637X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vethb1  -j Cid8637X2034.0  
    $IPTABLES -A Cid8637X2034.0  -s 192.168.50.1   -j RETURN
    $IPTABLES -A Cid8637X2034.0  -s 192.168.0.37   -j RETURN
    $IPTABLES -A Cid8637X2034.0  -s 192.168.50.12   -j RETURN
    $IPTABLES -N Out_RULE_17_3
    $IPTABLES -A Cid8637X2034.0  -j Out_RULE_17_3
    $IPTABLES -A Out_RULE_17_3  -j LOG  --log-level info --log-prefix "RULE 17 -- DENY "
    $IPTABLES -A Out_RULE_17_3  -j DROP
    # 
    # Rule 18 (vethb1)
    # 
    echo "Rule 18 (vethb1)"
    # 
    # br4 border out
    $IPTABLES -N Cid8753X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vethb1  -j Cid8753X2034.0  
    $IPTABLES -A Cid8753X2034.0  -d 192.168.50.13   -j RETURN
    $IPTABLES -A Cid8753X2034.0  -d 192.168.50.14   -j RETURN
    $IPTABLES -A Cid8753X2034.0  -d 192.168.50.15   -j RETURN
    $IPTABLES -N Out_RULE_18_3
    $IPTABLES -A Cid8753X2034.0  -j Out_RULE_18_3
    $IPTABLES -A Out_RULE_18_3  -j LOG  --log-level info --log-prefix "RULE 18 -- DENY "
    $IPTABLES -A Out_RULE_18_3  -j DROP
    # 
    # Rule 19 (vmh1)
    # 
    echo "Rule 19 (vmh1)"
    # 
    # br4 border out
    $IPTABLES -N Cid8374X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vmh1  -j Cid8374X2034.0  
    $IPTABLES -A Cid8374X2034.0  -s 192.168.50.12   -j RETURN
    $IPTABLES -A Cid8374X2034.0  -s 192.168.50.13   -j RETURN
    $IPTABLES -A Cid8374X2034.0  -s 192.168.50.14   -j RETURN
    $IPTABLES -A Cid8374X2034.0  -s 192.168.50.15   -j RETURN
    $IPTABLES -N Out_RULE_19_3
    $IPTABLES -A Cid8374X2034.0  -j Out_RULE_19_3
    $IPTABLES -A Out_RULE_19_3  -j LOG  --log-level info --log-prefix "RULE 19 -- DENY "
    $IPTABLES -A Out_RULE_19_3  -j DROP
    # 
    # Rule 20 (vmh1)
    # 
    echo "Rule 20 (vmh1)"
    # 
    # br4 border out
    $IPTABLES -N Cid8530X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-is-bridged --physdev-out vmh1  -j Cid8530X2034.0  
    $IPTABLES -A Cid8530X2034.0  -d 192.168.50.1   -j RETURN
    $IPTABLES -A Cid8530X2034.0  -d 192.168.0.37   -j RETURN
    $IPTABLES -N Out_RULE_20_3
    $IPTABLES -A Cid8530X2034.0  -j Out_RULE_20_3
    $IPTABLES -A Out_RULE_20_3  -j LOG  --log-level info --log-prefix "RULE 20 -- DENY "
    $IPTABLES -A Out_RULE_20_3  -j DROP
    # 
    # Rule 22 (vethb2)
    # 
    echo "Rule 22 (vethb2)"
    # 
    # br6 border IN
    $IPTABLES -N Cid7917X2034.0
    $IPTABLES -A INPUT -m physdev --physdev-in vethb2  -j Cid7917X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb2  -j Cid7917X2034.0
    $IPTABLES -A Cid7917X2034.0  -d 192.168.50.13   -j RETURN
    $IPTABLES -A Cid7917X2034.0  -d 192.168.50.14   -j RETURN
    $IPTABLES -A Cid7917X2034.0  -d 192.168.50.15   -j RETURN
    $IPTABLES -N In_RULE_22_3
    $IPTABLES -A Cid7917X2034.0  -j In_RULE_22_3
    $IPTABLES -A In_RULE_22_3  -j LOG  --log-level info --log-prefix "RULE 22 -- DENY "
    $IPTABLES -A In_RULE_22_3  -j DROP
    # 
    # Rule 23 (vethb2)
    # 
    echo "Rule 23 (vethb2)"
    # 
    # br6 border IN
    $IPTABLES -N Cid8013X2034.0
    $IPTABLES -A INPUT -m physdev --physdev-in vethb2  -j Cid8013X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb2  -j Cid8013X2034.0
    $IPTABLES -A Cid8013X2034.0  -s 192.168.50.1   -j RETURN
    $IPTABLES -A Cid8013X2034.0  -s 192.168.0.37   -j RETURN
    $IPTABLES -A Cid8013X2034.0  -s 192.168.50.12   -j RETURN
    $IPTABLES -N In_RULE_23_3
    $IPTABLES -A Cid8013X2034.0  -j In_RULE_23_3
    $IPTABLES -A In_RULE_23_3  -j LOG  --log-level info --log-prefix "RULE 23 -- DENY "
    $IPTABLES -A In_RULE_23_3  -j DROP
    # 
    # Rule 24 (vethb1)
    # 
    echo "Rule 24 (vethb1)"
    # 
    # br4 border IN
    $IPTABLES -N Cid7639X2034.0
    $IPTABLES -A INPUT -m physdev --physdev-in vethb1  -j Cid7639X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb1  -j Cid7639X2034.0
    $IPTABLES -A Cid7639X2034.0  -s 192.168.50.13   -j RETURN
    $IPTABLES -A Cid7639X2034.0  -s 192.168.50.14   -j RETURN
    $IPTABLES -A Cid7639X2034.0  -s 192.168.50.15   -j RETURN
    $IPTABLES -N In_RULE_24_3
    $IPTABLES -A Cid7639X2034.0  -j In_RULE_24_3
    $IPTABLES -A In_RULE_24_3  -j LOG  --log-level info --log-prefix "RULE 24 -- DENY "  
    $IPTABLES -A In_RULE_24_3  -j DROP
    # 
    # Rule 25 (vethb1)
    # 
    echo "Rule 25 (vethb1)"
    # 
    # br4 border IN
    $IPTABLES -N Cid7736X2034.0
    $IPTABLES -A INPUT -m physdev --physdev-in vethb1  -j Cid7736X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb1  -j Cid7736X2034.0
    $IPTABLES -A Cid7736X2034.0  -d 192.168.50.1   -j RETURN
    $IPTABLES -A Cid7736X2034.0  -d 192.168.0.37   -j RETURN
    $IPTABLES -A Cid7736X2034.0  -d 192.168.50.12   -j RETURN
    $IPTABLES -N In_RULE_25_3
    $IPTABLES -A Cid7736X2034.0  -j In_RULE_25_3
    $IPTABLES -A In_RULE_25_3  -j LOG  --log-level info --log-prefix "RULE 25 -- DENY "  
    $IPTABLES -A In_RULE_25_3  -j DROP
    # 
    # Rule 26 (vmh1)
    # 
    echo "Rule 26 (vmh1)"
    # 
    # br4 border IN
    $IPTABLES -N Cid8881X2034.0
    $IPTABLES -A INPUT -m physdev --physdev-in vmh1  -j Cid8881X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vmh1  -j Cid8881X2034.0
    $IPTABLES -A Cid8881X2034.0  -s 192.168.50.1   -j RETURN
    $IPTABLES -A Cid8881X2034.0  -s 192.168.0.37   -j RETURN
    $IPTABLES -N In_RULE_26_3
    $IPTABLES -A Cid8881X2034.0  -j In_RULE_26_3
    $IPTABLES -A In_RULE_26_3  -j LOG  --log-level info --log-prefix "RULE 26 -- DENY "  
    $IPTABLES -A In_RULE_26_3  -j DROP
    # 
    # Rule 27 (vmh1)
    # 
    echo "Rule 27 (vmh1)"
    # 
    # br4 border IN
    $IPTABLES -N Cid9010X2034.0
    $IPTABLES -A INPUT -m physdev --physdev-in vmh1  -j Cid9010X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vmh1  -j Cid9010X2034.0
    $IPTABLES -A Cid9010X2034.0  -d 192.168.50.12   -j RETURN
    $IPTABLES -A Cid9010X2034.0  -d 192.168.50.13   -j RETURN
    $IPTABLES -A Cid9010X2034.0  -d 192.168.50.14   -j RETURN
    $IPTABLES -A Cid9010X2034.0  -d 192.168.50.15   -j RETURN
    $IPTABLES -N In_RULE_27_3
    $IPTABLES -A Cid9010X2034.0  -j In_RULE_27_3
    $IPTABLES -A In_RULE_27_3  -j LOG  --log-level info --log-prefix "RULE 27 -- DENY "  
    $IPTABLES -A In_RULE_27_3  -j DROP
    # 
    # Rule 29 (vmh2)
    # 
    echo "Rule 29 (vmh2)"
    # 
    # host OUT
    $IPTABLES -N Cid10297X2034.0
    $IPTABLES -A OUTPUT -o vmh2   -s 192.168.0.19   -j Cid10297X2034.0
    $IPTABLES -A OUTPUT -o vmh2   -s 192.168.50.1   -j Cid10297X2034.0
    $IPTABLES -A Cid10297X2034.0  -d 192.168.50.12   -j RETURN
    $IPTABLES -A Cid10297X2034.0  -d 192.168.50.13   -j RETURN
    $IPTABLES -A Cid10297X2034.0  -d 192.168.50.14   -j RETURN
    $IPTABLES -A Cid10297X2034.0  -d 192.168.50.15   -j RETURN
    $IPTABLES -N Out_RULE_29_3
    $IPTABLES -A Cid10297X2034.0  -j Out_RULE_29_3
    $IPTABLES -A Out_RULE_29_3  -j LOG  --log-level info --log-prefix "RULE 29 -- DENY "  
    $IPTABLES -A Out_RULE_29_3  -j DROP
    # 
    # Rule 30 (vmh2)
    # 
    echo "Rule 30 (vmh2)"
    # 
    # host IN
    $IPTABLES -N Cid10437X2034.0
    $IPTABLES -A INPUT -i vmh2   -d 192.168.0.19   -j Cid10437X2034.0
    $IPTABLES -A INPUT -i vmh2   -d 192.168.50.1   -j Cid10437X2034.0
    $IPTABLES -A FORWARD -i vmh2   -d 192.168.0.37   -j Cid10437X2034.0
    $IPTABLES -A Cid10437X2034.0  -s 192.168.50.12   -j RETURN
    $IPTABLES -A Cid10437X2034.0  -s 192.168.50.13   -j RETURN
    $IPTABLES -A Cid10437X2034.0  -s 192.168.50.14   -j RETURN
    $IPTABLES -A Cid10437X2034.0  -s 192.168.50.15   -j RETURN
    $IPTABLES -N In_RULE_30_3
    $IPTABLES -A Cid10437X2034.0  -j In_RULE_30_3
    $IPTABLES -A In_RULE_30_3  -j LOG  --log-level info --log-prefix "RULE 30 -- DENY "  
    $IPTABLES -A In_RULE_30_3  -j DROP
    # 
    # Rule 32 (vk63)
    # 
    echo "Rule 32 (vk63)"
    # 
    # br6 IN
    $IPTABLES -N In_RULE_32
    $IPTABLES -A INPUT -m physdev --physdev-in vk63 -p icmp  -m icmp  -s 192.168.50.13   --icmp-type any  -m state --state NEW  -j In_RULE_32  
    $IPTABLES -A FORWARD -m physdev --physdev-in vk63 -p icmp  -m icmp  -s 192.168.50.13   --icmp-type any  -m state --state NEW  -j In_RULE_32  
    $IPTABLES -A In_RULE_32  -j LOG  --log-level info --log-prefix "RULE 32 -- ACCEPT "
    $IPTABLES -A In_RULE_32  -j ACCEPT
    # 
    # Rule 33 (vk64)
    # 
    echo "Rule 33 (vk64)"
    # 
    # br6 IN
    $IPTABLES -N In_RULE_33
    $IPTABLES -A INPUT -m physdev --physdev-in vk64 -p icmp  -m icmp  -s 192.168.50.14   --icmp-type any  -m state --state NEW  -j In_RULE_33  
    $IPTABLES -A FORWARD -m physdev --physdev-in vk64 -p icmp  -m icmp  -s 192.168.50.14   --icmp-type any  -m state --state NEW  -j In_RULE_33  
    $IPTABLES -A In_RULE_33  -j LOG  --log-level info --log-prefix "RULE 33 -- ACCEPT "
    $IPTABLES -A In_RULE_33  -j ACCEPT
    # 
    # Rule 34 (vk64)
    # 
    echo "Rule 34 (vk64)"
    # 
    # br6 IN HTTP
    $IPTABLES -N In_RULE_34
    $IPTABLES -A FORWARD -m physdev --physdev-in vk64 -p tcp -m tcp  -m multiport  -s 192.168.50.14   -d 192.168.0.37   --dports 80,443  -m state --state NEW  -j In_RULE_34   
    $IPTABLES -A In_RULE_34  -j LOG  --log-level info --log-prefix "RULE 34 -- ACCEPT "
    $IPTABLES -A In_RULE_34  -j ACCEPT
    # 
    # Rule 35 (vk65)
    # 
    echo "Rule 35 (vk65)"
    # 
    # br6 IN
    $IPTABLES -N In_RULE_35
    $IPTABLES -A INPUT -m physdev --physdev-in vk65 -p icmp  -m icmp  -s 192.168.50.15   --icmp-type any  -m state --state NEW  -j In_RULE_35
    $IPTABLES -A FORWARD -m physdev --physdev-in vk65 -p icmp  -m icmp  -s 192.168.50.15   --icmp-type any  -m state --state NEW  -j In_RULE_35
    $IPTABLES -A In_RULE_35  -j LOG  --log-level info --log-prefix "RULE 35 -- ACCEPT "
    $IPTABLES -A In_RULE_35  -j ACCEPT
    # 
    # Rule 36 (vethb2)
    # 
    echo "Rule 36 (vethb2)"
    # 
    # br6 border IN
    $IPTABLES -N Cid9698X2034.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb2 -p icmp  -m icmp  -s 192.168.50.1   --icmp-type any  -m state --state NEW  -j Cid9698X2034.0  
    $IPTABLES -N In_RULE_36
    $IPTABLES -A Cid9698X2034.0  -d 192.168.50.13   -j In_RULE_36
    $IPTABLES -A Cid9698X2034.0  -d 192.168.50.14   -j In_RULE_36
    $IPTABLES -A Cid9698X2034.0  -d 192.168.50.15   -j In_RULE_36
    $IPTABLES -N Cid9698X2034.1
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb2 -p icmp  -m icmp  -s 192.168.50.1   --icmp-type any  -m state --state NEW  -j Cid9698X2034.1  
    $IPTABLES -A Cid9698X2034.1  -d 192.168.50.13   -j In_RULE_36
    $IPTABLES -A Cid9698X2034.1  -d 192.168.50.14   -j In_RULE_36
    $IPTABLES -A Cid9698X2034.1  -d 192.168.50.15   -j In_RULE_36
    $IPTABLES -N Cid9698X2034.2
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb2 -p icmp  -m icmp  --icmp-type any  -m state --state NEW  -j Cid9698X2034.  2
    $IPTABLES -N Cid9698X2034.3
    $IPTABLES -A Cid9698X2034.2  -s 192.168.0.37   -j Cid9698X2034.3
    $IPTABLES -A Cid9698X2034.2  -s 192.168.50.12   -j Cid9698X2034.3
    $IPTABLES -A Cid9698X2034.3  -d 192.168.50.13   -j In_RULE_36
    $IPTABLES -A Cid9698X2034.3  -d 192.168.50.14   -j In_RULE_36
    $IPTABLES -A Cid9698X2034.3  -d 192.168.50.15   -j In_RULE_36
    $IPTABLES -A In_RULE_36  -j LOG  --log-level info --log-prefix "RULE 36 -- ACCEPT "
    $IPTABLES -A In_RULE_36  -j ACCEPT
    # 
    # Rule 38 (vk42)
    # 
    echo "Rule 38 (vk42)"
    # 
    # br4 IN
    $IPTABLES -N In_RULE_38
    $IPTABLES -A INPUT -m physdev --physdev-in vk42 -p icmp  -m icmp  -s 192.168.50.12   --icmp-type any  -m state --state NEW  -j In_RULE_38
    $IPTABLES -A FORWARD -m physdev --physdev-in vk42 -p icmp  -m icmp  -s 192.168.50.12   --icmp-type any  -m state --state NEW  -j In_RULE_38  
    $IPTABLES -A In_RULE_38  -j LOG  --log-level info --log-prefix "RULE 38 -- ACCEPT "
    $IPTABLES -A In_RULE_38  -j ACCEPT
    # 
    # Rule 39 (vethb1)
    # 
    echo "Rule 39 (vethb1)"
    # 
    # br4 border IN
    $IPTABLES -N Cid15566X9203.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb1 -p icmp  -m icmp  -d 192.168.50.1   --icmp-type any  -m state --state NEW  -j Cid15566X9203.0  
    $IPTABLES -N In_RULE_39
    $IPTABLES -A Cid15566X9203.0  -s 192.168.50.13   -j In_RULE_39
    $IPTABLES -A Cid15566X9203.0  -s 192.168.50.14   -j In_RULE_39
    $IPTABLES -A Cid15566X9203.0  -s 192.168.50.15   -j In_RULE_39
    $IPTABLES -N Cid15566X9203.1
    $IPTABLES -A INPUT -m physdev --physdev-in vethb1 -p icmp  -m icmp  -d 192.168.50.1   --icmp-type any  -m state --state NEW  -j Cid15566X9203.1  
    $IPTABLES -A Cid15566X9203.1  -s 192.168.50.13   -j In_RULE_39
    $IPTABLES -A Cid15566X9203.1  -s 192.168.50.14   -j In_RULE_39
    $IPTABLES -A Cid15566X9203.1  -s 192.168.50.15   -j In_RULE_39
    $IPTABLES -N Cid15566X9203.2
    $IPTABLES -A FORWARD -m physdev --physdev-in vethb1 -p icmp  -m icmp  --icmp-type any  -m state --state NEW  -j Cid15566X9203.2  
    $IPTABLES -N Cid15566X9203.3
    $IPTABLES -A Cid15566X9203.2  -d 192.168.0.37   -j Cid15566X9203.3
    $IPTABLES -A Cid15566X9203.2  -d 192.168.50.12   -j Cid15566X9203.3
    $IPTABLES -A Cid15566X9203.3  -s 192.168.50.13   -j In_RULE_39
    $IPTABLES -A Cid15566X9203.3  -s 192.168.50.14   -j In_RULE_39
    $IPTABLES -A Cid15566X9203.3  -s 192.168.50.15   -j In_RULE_39
    $IPTABLES -A In_RULE_39  -j LOG  --log-level info --log-prefix "RULE 39 -- ACCEPT "
    $IPTABLES -A In_RULE_39  -j ACCEPT
    # 
    # Rule 40 (vmh1)
    # 
    echo "Rule 40 (vmh1)"
    # 
    # br4 border IN
    $IPTABLES -N Cid16232X9203.0
    $IPTABLES -A FORWARD -m physdev --physdev-in vmh1 -p icmp  -m icmp  -s 192.168.50.1   --icmp-type any  -m state --state NEW  -j Cid16232X9203.0  
    $IPTABLES -N In_RULE_40
    $IPTABLES -A Cid16232X9203.0  -d 192.168.50.12   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.0  -d 192.168.50.13   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.0  -d 192.168.50.14   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.0  -d 192.168.50.15   -j In_RULE_40
    $IPTABLES -N Cid16232X9203.1
    $IPTABLES -A FORWARD -m physdev --physdev-in vmh1 -p icmp  -m icmp  -s 192.168.50.1   --icmp-type any  -m state --state NEW  -j Cid16232X9203.1   
    $IPTABLES -A Cid16232X9203.1  -d 192.168.50.12   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.1  -d 192.168.50.13   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.1  -d 192.168.50.14   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.1  -d 192.168.50.15   -j In_RULE_40
    $IPTABLES -N Cid16232X9203.2
    $IPTABLES -A FORWARD -m physdev --physdev-in vmh1 -p icmp  -m icmp  -s 192.168.0.37   --icmp-type any  -m state --state NEW  -j Cid16232X9203.2  
    $IPTABLES -A Cid16232X9203.2  -d 192.168.50.12   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.2  -d 192.168.50.13   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.2  -d 192.168.50.14   -j In_RULE_40
    $IPTABLES -A Cid16232X9203.2  -d 192.168.50.15   -j In_RULE_40
    $IPTABLES -A In_RULE_40  -j LOG  --log-level info --log-prefix "RULE 40 -- ACCEPT "
    $IPTABLES -A In_RULE_40  -j ACCEPT
    # 
    # Rule 42 (vmh2)
    # 
    echo "Rule 42 (vmh2)"
    # 
    # external are IN
    $IPTABLES -N Cid16691X6788.0
    $IPTABLES -A INPUT -i vmh2  -p icmp  -m icmp  -d 192.168.50.1   --icmp-type any  -m state --state NEW  -j Cid16691X6788.0  
    $IPTABLES -N In_RULE_42
    $IPTABLES -A Cid16691X6788.0  -s 192.168.50.12   -j In_RULE_42
    $IPTABLES -A Cid16691X6788.0  -s 192.168.50.13   -j In_RULE_42
    $IPTABLES -A Cid16691X6788.0  -s 192.168.50.14   -j In_RULE_42
    $IPTABLES -A Cid16691X6788.0  -s 192.168.50.15   -j In_RULE_42
    $IPTABLES -N Cid16691X6788.1
    $IPTABLES -A FORWARD -i vmh2  -p icmp  -m icmp  -d 192.168.0.37   --icmp-type any  -m state --state NEW  -j Cid16691X6788.1  
    $IPTABLES -A Cid16691X6788.1  -s 192.168.50.12   -j In_RULE_42
    $IPTABLES -A Cid16691X6788.1  -s 192.168.50.13   -j In_RULE_42
    $IPTABLES -A Cid16691X6788.1  -s 192.168.50.14   -j In_RULE_42
    $IPTABLES -A Cid16691X6788.1  -s 192.168.50.15   -j In_RULE_42
    $IPTABLES -A In_RULE_42  -j LOG  --log-level info --log-prefix "RULE 42 -- ACCEPT "
    $IPTABLES -A In_RULE_42  -j ACCEPT
    # 
    # Rule 43 (vmh2)
    # 
    echo "Rule 43 (vmh2)"
    # 
    # host border OUT
    $IPTABLES -N Cid16236X6788.0
    $IPTABLES -A OUTPUT -o vmh2  -p icmp  -m icmp  -s 192.168.50.1   --icmp-type any  -m state --state NEW  -j Cid16236X6788.0  
    $IPTABLES -N Out_RULE_43
    $IPTABLES -A Cid16236X6788.0  -d 192.168.50.12   -j Out_RULE_43
    $IPTABLES -A Cid16236X6788.0  -d 192.168.50.13   -j Out_RULE_43
    $IPTABLES -A Cid16236X6788.0  -d 192.168.50.14   -j Out_RULE_43
    $IPTABLES -A Cid16236X6788.0  -d 192.168.50.15   -j Out_RULE_43
    $IPTABLES -N Cid16236X6788.1
    $IPTABLES -A FORWARD -o vmh2  -p icmp  -m icmp  -s 192.168.0.37   --icmp-type any  -m state --state NEW  -j Cid16236X6788.1  
    $IPTABLES -A Cid16236X6788.1  -d 192.168.50.12   -j Out_RULE_43
    $IPTABLES -A Cid16236X6788.1  -d 192.168.50.13   -j Out_RULE_43
    $IPTABLES -A Cid16236X6788.1  -d 192.168.50.14   -j Out_RULE_43
    $IPTABLES -A Cid16236X6788.1  -d 192.168.50.15   -j Out_RULE_43
    $IPTABLES -A Out_RULE_43  -j LOG  --log-level info --log-prefix "RULE 43 -- ACCEPT "
    $IPTABLES -A Out_RULE_43  -j ACCEPT
    # 
    # Rule 45 (br0)
    # 
    echo "Rule 45 (br0)"
    # 
    # external
    $IPTABLES -A OUTPUT -o br0   -m state --state NEW  -j ACCEPT
    # 
    # Rule 46 (br0)
    # 
    echo "Rule 46 (br0)"
    # 
    # external
    $IPTABLES -A FORWARD -i br0   -s 192.168.0.10   -d 192.168.0.255   -m state --state NEW  -j ACCEPT
    $IPTABLES -A INPUT -i br0   -s 192.168.0.10   -d 192.168.0.255   -m state --state NEW  -j ACCEPT
    $IPTABLES -A INPUT -i br0   -s 192.168.0.10   -m state --state NEW  -j ACCEPT
    # 
    # Rule 47 (global)
    # 
    echo "Rule 47 (global)"
    # 
    $IPTABLES -N RULE_47
    $IPTABLES -A OUTPUT  -j RULE_47
    $IPTABLES -A INPUT  -j RULE_47
    $IPTABLES -A FORWARD  -j RULE_47
    $IPTABLES -A RULE_47  -j LOG  --log-level info --log-prefix "RULE 47 -- DENY "
    $IPTABLES -A RULE_47  -j DROP

The variable “$IPTABLES” identifies the local iptables command. As already discussed in the last articles we arranged our (virtual) guest systems, the virtualization host and external systems in 3 defined host groups in FWbuilder (see the last post):

  • br6_grp: kali3, kali4, kali5,
  • br4_grp: kali2,
  • ext_grp: the host and some external web server “lamp”.

Remember that rules for bridge-ports are investigated separately and independently as a packet moves from one bridge to another. Note that the host and further systems attached to “virbr4” via a veth device “vmh2” are recognized as members of a distinct logical host area for which iptables rules again are reinvestigated separatly by the kernel during packet transport. Therefore we need ACCEPT rules to allow for incoming and outgoing packets at the host’s interface “vmh2”.

Examples of spoofing scenarios

With 2 bridges in place we can define already a variety of ARP spoofing scenarios with a subsequent MiM-attack. We only test some selected, but typical scenarios. Note again that we cannot prevent the act of spoofing itself with iptables – however, we can prevent that redirected packets arrive at the MiM system.

Example 1: kali2 of virbr4 attacks the communication between kali3 and kali5 within virbr6

Which rule do we expect to prevent this? Actually as kali2 tries to redirect the intended communication from bridge virbr6 into bridge virbr4 we would already expect a DENY rule at the border port “vethb2” to stop redirected packets. In our rules list this would be rule 16.

So let us see. We start ARP spoofing on kali2: 

root@kali2: ~# echo 1 > /proc/sys/net/ipv4/ip_forward
root@kali2: ~# iptables -A OUTPUT -p icmp --icmp-type redirect -j REJECT
root@kali2: ~# arpspoof -i eth3 -t 192.168.50.13 192.168.50.14 & 2> /dev/null  
root@kali2: ~# arpspoof -i eth3 -t 192.168.50.14 192.168.50.13 & 2> /dev/null

eth3 is the relevant Ethernet interface to net 192.168.50.0/24 on guest kali2.
After some time we get the following ARP information on e.g. kali3:

bridges_1

Consequently, after a “journalctl -f” on the virtualization host we find: : 

Mar 17 13:21:53 mytux kernel: RULE 16 -- DENY IN=virbr6 OUT=virbr6 PHYSIN=vk63 PHYSOUT=vethb2 MAC=52:54:00:f2:a4:8d:52:54:00:b1:5d:1f:08:00 SRC=192.168.50.13 DST=192.168.50.14 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=16140 DF PROTO=ICMP TYPE=8 CODE=0 ID=1756 SEQ=1   
Mar 17 13:21:54 mytux kernel: RULE 16 -- DENY IN=virbr6 OUT=virbr6 PHYSIN=vk63 PHYSOUT=vethb2 MAC=52:54:00:f2:a4:8d:52:54:00:b1:5d:1f:08:00 SRC=192.168.50.13 DST=192.168.50.14 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=16252 DF PROTO=ICMP TYPE=8 CODE=0 ID=1756 SEQ=2

Our test example shows that rules for border ports help to isolate bridges against misguided packets.

Rule 16 deserves a closer look as it contains a logical negation of 2 separately defined groups of hosts. We see that FWbuilder compiles the negation internally correctly: The related subchain definition contains all required hosts.

As described in the previous articles we stop the attack by the command “killall arpspoof” on kali2. Remember that due to time limits for ARP and port caching information on the guests and the bridge, respectively, it may take some time until normal operation is possible again. See the first article of this series for more information on this topic.

Example 2: kali2 of virbr4 attacks the communication between kali3 and the virtualization host

In this scenario a regular (!) packet would propagate from virbr6 through virbr4 and then to the host. Therefore, neither border port rules for virbr6 nor for virbr4 can block the traffic. We must, instead, rely on the analysis of redirected packets following an OUT direction to port vk42 – this is rule 5.

Therefore, this example is just a repetition of what we learned in the first article of this series
Linux bridges – can iptables be used against MiM attacks based on ARP spoofing ? – I

Actually, after another spoofing attack by kali2 

root@kali2: ~# arpspoof -i eth3 -t 192.168.50.1 192.168.50.13 & 2> /dev/null  
root@kali2: ~# arpspoof -i eth3 -t 192.168.50.13 192.168.50.1 & 2> /dev/null

and sending pings from kali3 to the host we get: 

Mar 17 18:44:51 mytux kernel: RULE 32 -- ACCEPT IN=virbr6 OUT=virbr6 PHYSIN=vk63 PHYSOUT=vethb2 MAC=54:00:f2:a4:8d:52:54:00:b1:5d:1f:08:00 SRC=192.168.50.13 DST=192.168.50.1 LEN=84 TOS=0x00 PREC=0x00 =64 ID=48428 DF PROTO=ICMP TYPE=8 CODE=0 ID=2872 SEQ=2   
Mar 17 18:44:51 mytux kernel: RULE 5 -- DENY IN=virbr4 OUT=virbr4 PHYSIN=vethb1 PHYSOUT=vk42 MAC=52:50:f2:a4:8d:52:54:00:b1:5d:1f:08:00 SRC=192.168.50.13 DST=192.168.50.1 LEN=84 TOS=0x00 PREC=0x00 TTL ID=48428 DF PROTO=ICMP TYPE=8 CODE=0 ID=2872 SEQ=2

We see that the transition from bridge virbr6 to virbr4 works as planned – however the packets redirected to the MiM kali2 are stopped at vk42. Good!

Example 3: kali3 of virbr6 attacks the communication between kali4 of virbr4 and the virtualization host

We look at pings issued from the host to kali4 after an attack of kali3. In this case the border port rules again must not block. Instead, we rely on local port rules at port vk63, .i.e. rule 2. Indeed: 

Mar 17 19:00:39 mytux kernel: RULE 43 -- ACCEPT IN= OUT=vmh2 SRC=192.168.50.1 DST=192.168.50.14 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=53576 DF PROTO=ICMP TYPE=8 CODE=0 ID=2981 SEQ=1 
Mar 17 19:00:39 mytux kernel: RULE 40 -- ACCEPT IN=virbr4 OUT=virbr4 PHYSIN=vmh1 PHYSOUT=vethb1 MAC=52:54:00:b1:5d:1f:7a:ff:fc:bd:68:b6:08:00 SRC=192.168.50.1 DST=192.168.50.14 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=53576 DF PROTO=ICMP TYPE=8 CODE=0 ID=2981 SEQ=1       
Mar 17 19:00:39 mytux kernel: RULE 2 -- DENY IN=virbr6 OUT=virbr6 PHYSIN=vethb2 PHYSOUT=vk63 MAC=52:54:00:b1:5d:1f:7a:ff:fc:bd:68:b6:08:00 SRC=192.168.50.1 DST=192.168.50.14 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=53576 DF PROTO=ICMP TYPE=8 CODE=0 ID=2981 SEQ=1

Example 4: kali3 of virbr6 attacks the communication between kali2 and the virtualization host

In this case border rules should stop redirected packets. For our test case this would in particular be rule 18.

And – after the initialization of the attack by kali3 and the trial to send pings from kali2 to the host, we get: 

Mar 18 16:47:30 mytux kernel: RULE 18 -- DENY IN=virbr4 OUT=virbr4 PHYSIN=vk42 PHYSOUT=vethb1 MAC=52:54:00:b1:5d:1f:52:54:00:f2:a4:8d:08:00 SRC=192.168.50.12 DST=192.168.50.1 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=4405 DF PROTO=ICMP TYPE=8 CODE=0 ID=2420 SEQ=1

And vice versa : 

Mar 18 16:48:42 mytux kernel: RULE 43 -- ACCEPT IN= OUT=vmh2 SRC=192.168.50.1 DST=192.168.50.12 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=63251 DF PROTO=ICMP TYPE=8 CODE=0 ID=21778 SEQ=1  
Mar 18 16:48:42 mytux kernel: RULE 18 -- DENY IN=virbr4 OUT=virbr4 PHYSIN=vmh1 PHYSOUT=vethb1 MAC=52:54:00:b1:5d:1f:f2:be:a1:5a:cd:6e:08:00 SRC=192.168.50.1 DST=192.168.50.12 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=63251 DF PROTO=ICMP TYPE=8 CODE=0 ID=21778 SEQ=1

As expected!

Example 5: The host attacks communication between guests attached to an inner bridge

One may think such an example is just academic. Actually, in my opinion it is not. Although the administrator of a virtualization host has in principle a variety of means available to follow any communication across a bridge, ARP spoofing should NOT be such a measure under normal operation conditions. In addition, there may be legal aspects in a professional hosting situation.

But more important: From the perspective of the involved bridges, in our setup the host is attached to bridge virbr4 as an external guest over a border port. Rules for the virtualization host are, therefore, only an example for similar rules applied to other external hosts which may have the allowance to communicate with bridge guests – via forwarding and a respective route defined on the virtualization host.

We expect rule 20 to stop packages redirected by the MiM: 

Mar 18 17:38:38 rux kernel: RULE 32 -- ACCEPT IN=virbr6 OUT=virbr6 PHYSIN=vk63 PHYSOUT=vethb2 MAC=f2:be:a1:5a:cd:6e:52:54:00:b1:5d:1f:08:00 SRC=192.168.50.13 DST=192.168.50.12 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=36173 DF PROTO=ICMP TYPE=8 CODE=0 ID=2218 SEQ=1   
Mar 18 17:38:38 rux kernel: RULE 20 -- DENY IN=virbr4 OUT=virbr4 PHYSIN=vethb1 PHYSOUT=vmh1 MAC=f2:be:a1:5a:cd:6e:52:54:00:b1:5d:1f:08:00 SRC=192.168.50.13 DST=192.168.50.12 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=36173 DF PROTO=ICMP TYPE=8 CODE=0 ID=2218 SEQ=1

And vice versa 

Mar 18 17:39:39 rux kernel: RULE 20 -- DENY IN=virbr4 OUT=virbr4 PHYSIN=vk42 PHYSOUT=vmh1 MAC=f2:be:a1:5a:cd:6e:52:54:00:f2:a4:8d:08:00 SRC=192.168.50.12 DST=192.168.50.13 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=10910 DF PROTO=ICMP TYPE=8 CODE=0 ID=2730 SEQ=1    
Mar 18 17:39:40 rux kernel: RULE 20 -- DENY IN=virbr4 OUT=virbr4 PHYSIN=vk42 PHYSOUT=vmh1 MAC=f2:be:a1:5a:cd:6e:52:54:00:f2:a4:8d:08:00 SRC=192.168.50.12 DST=192.168.50.13 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=11113 DF PROTO=ICMP TYPE=8 CODE=0 ID=2730 SEQ=2

Summary

So, all in all, for our few examples we could verify that our recipe for setting up iptables rules in case of several linked Linux bridges with guests on one [KVM] virtualization host guided us correctly. After associating unique IP addresses with bridge ports we can define rules that block the transport of packets redirected to a MiM system – even when multiple bridges are present on the virtualization host. Additional and special rules for the bridges’ border ports help to prevent irregular traffic between defined groups of guests and/or external hosts.

Note that we only demonstrated this for specific allowance rules for the ICMP protocol. Yet, it is easy to understand that the very same principles should work for any protocol on level 4.