Posts Tagged ‘asap’

How to filter dhcp traffic between two networks running separate DHCP servers to prevent IP assignment issues and MAC duplicate addresses

Tuesday, February 8th, 2022

Tracking the Problem of MAC duplicates on Linux routers

If you have two networks that see each other and they're not separated in VLANs but see each other sharing a common netmask lets say or, it might happend that there are 2 dhcp servers for example (isc-dhcp-server running on and dhcpd running on can broadcast their services to both LANs (netmask and Local Net LAN The result out of this is that some devices might pick up their IP address via DHCP from the wrong dhcp server.

Normally if you have a fully controlled little or middle class home or office network (10 – 15 electronic devices nodes) connecting to the LAN in a mixed moth some are connected via one of the Networks via connected Wifi to others are LANned and using static IP adddresses and traffic is routed among two ISPs and each network can see the other network, there is always a possibility of things to go wrong. This is what happened to me so this is how this post was born.

The best practice from my experience so far is to define each and every computer / phone / laptop host joining the network and hence later easily monitor what is going on the network with something like iptraf-ng / nethogs  / iperf – described in prior  how to check internet spepeed from console and in check server internet connectivity speed with speedtest-cliiftop / nload or for more complex stuff wireshark or even a simple tcpdump. No matter the tools network monitoring is only part on solving network issues. A very must have thing in a controlled network infrastructure is defining every machine part of it to easily monitor later with the monitoring tools. Defining each and every host on the Hybrid computer networks makes administering the network much easier task and  tracking irregularities on time is much more likely. 

Since I have such a hybrid network here hosting a couple of XEN virtual machines with Linux, Windows 7 and Windows 10, together with Mac OS X laptops as well as MacBook Air notebooks, I have followed this route and tried to define each and every host based on its MAC address to pick it up from the correct DHCP1 server (that is distributing IPs for Internet Provider 1 (ISP 1), that is mostly few computers attached UTP LAN cables via LiteWave LS105G Gigabit Switch as well from DHCP2 – used only to assigns IPs to servers and a a single Wi-Fi Access point configured to route incoming clients via Linux NAT gateway server.

To filter out the unwanted IPs from the DHCPD not to propagate I've so far used a little trick to  Deny DHCP MAC Address for unwanted clients and not send IP offer for them.

To give you more understanding,  I have to clear it up I don't want to have automatic IP assignments from DHCP2 / LAN2 to DHCP1 / LAN1 because (i don't want machines on DHCP1 to end up with IP like or DHCP2 (to have, as such a wrong IP delegation could potentially lead to MAC duplicates IP conflicts. MAC Duplicate IP wrong assignments for those older or who have been part of administrating large ISP network infrastructures  makes the network communication unstable for no apparent reason and nodes partially unreachable at times or full time …

However it seems in the 21-st century which is the century of strangeness / computer madness in the 2022, technology advanced so much that it has massively started to break up some good old well known sysadmin standards well documented in the RFCs I know of my youth, such as that every electronic equipment manufactured Vendor should have a Vendor Assigned Hardware MAC Address binded to it that will never change (after all that was the idea of MAC addresses wasn't it !). 
Many mobile devices nowadays however, in the developers attempts to make more sophisticated software and Increase Anonimity on the Net and Security, use a technique called  MAC Address randomization (mostly used by hackers / script kiddies of the early days of computers) for their Wi-Fi Net Adapter OS / driver controlled interfaces for the sake of increased security (the so called Private WiFi Addresses). If a sysadmin 10-15 years ago has seen that he might probably resign his profession and turn to farming or agriculture plant growing, but in the age of digitalization and "cloud computing", this break up of common developed network standards starts to become the 'new normal' standard.

I did not suspected there might be a MAC address oddities, since I spare very little time on administering the the network. This was so till recently when I accidently checked the arp table with:

Hypervisor:~# arp -an     5c:89:b5:f2:e8:d8      (Unknown)    00:15:3e:d3:8f:76       (Unknown)


and consequently did a network MAC Address ARP Scan with arp-scan (if you never used this little nifty hacker tool I warmly recommend it !!!)
If you don't have it installed it is available in debian based linuces from default repos to install

Hypervisor:~# apt-get install –yes arp-scan

It is also available on CentOS / Fedora / Redhat and other RPM distros via:

Hypervisor:~# yum install -y arp-scan



Hypervisor:~# arp-scan –interface=eth1    00:16:3e:0f:48:05       Xensource, Inc.    00:16:3e:04:11:1c       Xensource, Inc.    00:15:3e:bb:45:45       Xensource, Inc.    00:15:3e:59:96:8e       Xensource, Inc.    00:15:3e:d3:8f:77       Xensource, Inc.    8c:89:b5:f2:e8:d8       Micro-Star INT'L CO., LTD     5c:89:b5:f2:e8:d8      (Unknown)    00:15:3e:d3:8f:76       (Unknown)

192.168.x.91     02:a0:xx:xx:d6:64        (Unknown)
192.168.x.91     02:a0:xx:xx:d6:64        (Unknown)  (DUP: 2)

N.B. !. I found it helpful to check all available interfaces on my Linux NAT router host.

As you see the scan revealed, a whole bunch of MAC address mess duplicated MAC hanging around, destroying my network topology every now and then 
So far so good, the MAC duplicates and strangely hanging around MAC addresses issue, was solved relatively easily with enabling below set of systctl kernel variables.

1. Fixing Linux ARP common well known Problems through disabling arp_announce / arp_ignore / send_redirects kernel variables disablement


Linux answers ARP requests on wrong and unassociated interfaces per default. This leads to the following two problems:

ARP requests for the loopback alias address are answered on the HW interfaces (even if NOARP on lo0:1 is set). Since loopback aliases are required for DSR (Direct Server Return) setups this problem is very common (but easy to fix fortunately).

If the machine is connected twice to the same switch (e.g. with eth0 and eth1) eth2 may answer ARP requests for the address on eth1 and vice versa in a race condition manner (confusing almost everything).

This can be prevented by specific arp kernel settings. Take a look here for additional information about the nature of the problem (and other solutions): ARP flux.

To fix that generally (and reboot safe) we  include the following lines into


Hypervisor:~# cp -rpf /etc/sysctl.conf /etc/sysctl.conf_bak_07-feb-2022
Hypervisor:~# cat >> /etc/sysctl.conf

# LVS tuning


Press CTRL + D simultaneusly to Write out up-pasted vars.

To read more on Load Balancer using direct routing and on LVS and the arp problem here

2. Digging further the IP conflict / dulicate MAC Problems

Even after this arp tunings (because I do have my Hypervisor 2 LAN interfaces connected to 1 switch) did not resolved the issues and still my Wireless Connected devices via network (ISP2) were randomly assigned the wrong range IPs 192.168.0.XXX/24 as well as the wrong gateway (ISP1).
After thinking thoroughfully for hours and checking the network status with various tools and thanks to the fact that my wife has a MacBook Air that was always complaining that the IP it tried to assign from the DHCP was already taken, i"ve realized, something is wrong with DHCP assignment.
Since she owns a IPhone 10 with iOS and this two devices are from the same vendor e.g. Apple Inc. And Apple's products have been having strange DHCP assignment issues from my experience for quite some time, I've thought initially problems are caused by software on Apple's devices.
I turned to be partially right after expecting the logs of DHCP server on the Linux host (ISP1) finding that the phone of my wife takes IP in 192.168.0.XXX, insetad of IP from (which has is a combined Nokia Router with 2.4Ghz and 5Ghz Wi-Fi and LAN router provided by ISP2 in that case Vivacom). That was really puzzling since for me it was completely logical thta the iDevices must check for DHCP address directly on the Network of the router to whom, they're connecting. Guess my suprise when I realized that instead of that the iDevices does listen to the network on a wide network range scan for any DHCPs reachable baesd on the advertised (i assume via broadcast) address traffic and try to connect and take the IP to the IP of the DHCP which responds faster !!!! Of course the Vivacom Chineese produced Nokia router responded DHCP requests and advertised much slower, than my Linux NAT gateway on ISP1 and because of that the Iphone and iOS and even freshest versions of Android devices do take the IP from the DHCP that responds faster, even if that router is not on a C class network (that's invasive isn't it??). What was even more puzzling was the automatic MAC Randomization of Wifi devices trying to connect to my ISP1 configured DHCPD and this of course trespassed any static MAC addresses filtering, I already had established there.

Anyways there was also a good think out of tthat intermixed exercise 🙂 While playing around with the Gigabit network router of vivacom I found a cozy feature SCHEDULEDING TURNING OFF and ON the WIFI ACCESS POINT  – a very useful feature to adopt, to stop wasting extra energy and lower a bit of radiation is to set a swtich off WIFI AP from 12:30 – 06:30 which are the common sleeping hours or something like that.

3. What is MAC Randomization and where and how it is configured across different main operating systems as of year 2022?

Depending on the operating system of your device, MAC randomization will be available either by default on most modern mobile OSes or with possibility to have it switched on:

  • Android Q: Enabled by default 
  • Android P: Available as a developer option, disabled by default
  • iOS 14: Available as a user option, disabled by default
  • Windows 10: Available as an option in two ways – random for all networks or random for a specific network

Lately I don't have much time to play around with mobile devices, and I do not my own a luxury mobile phone so, the fact this ne Androids have this MAC randomization was unknown to me just until I ended a small mess, based on my poor configured networks due to my tight time constrains nowadays.

Finding out about the new security feature of MAC Randomization, on all Android based phones (my mother's Nokia smartphone and my dad's phone, disabled the feature ASAP:

4. Disable MAC Wi-Fi Ethernet device Randomization on Android

MAC Randomization creates a random MAC address when joining a Wi-Fi network for the first time or after “forgetting” and rejoining a Wi-Fi network. It Generates a new random MAC address after 24 hours of last connection.

Disabling MAC Randomization on your devices. It is done on a per SSID basis so you can turn off the randomization, but allow it to function for hotspots outside of your home.

  1. Open the Settings app
  2. Select Network and Internet
  3. Select WiFi
  4. Connect to your home wireless network
  5. Tap the gear icon next to the current WiFi connection
  6. Select Advanced
  7. Select Privacy
  8. Select "Use device MAC"

5. Disabling MAC Randomization on MAC iOS, iPhone, iPad, iPod

To Disable MAC Randomization on iOS Devices:

Open the Settings on your iPhone, iPad, or iPod, then tap Wi-Fi or WLAN


  1. Tap the information button next to your network
  2. Turn off Private Address
  3. Re-join the network

Of course next I've collected their phone Wi-Fi adapters and made sure the included dhcp MAC deny rules in /etc/dhcp/dhcpd.conf are at place.

The effect of the MAC Randomization for my Network was terrible constant and strange issues with my routings and networks, which I always thought are caused by the openxen hypervisor Virtualization VM bugs etc.

That continued for some months now, and the weird thing was the issues always started when I tried to update my Operating system to the latest packetset, do a reboot to load up the new piece of software / libraries etc. and plus it happened very occasionally and their was no obvious reason for it.


6. How to completely filter dhcp traffic between two network router hosts
IP / to stop 2 or more configured DHCP servers
on separate networks see each other

To prevent IP mess at DHCP2 server side (which btw is ISC DHCP server, taking care for IP assignment only for the Servers on the network running on Debian 11 Linux), further on I had to filter out any DHCP UDP traffic with iptables completely.
To prevent incorrect route assignments assuming that you have 2 networks and 2 routers that are configurred to do Network Address Translation (NAT)-ing Router 1:, Router 2:

You have to filter out UDP Protocol data on Port 67 and 68 from the respective source and destination addresses.

In firewall rules configuration files on your Linux you need to have some rules as:

# filter outgoing dhcp traffic from to
-A INPUT -p udp -m udp –dport 67:68 -s -d -j DROP
-A OUTPUT -p udp -m udp –dport 67:68 -s -d -j DROP
-A FORWARD -p udp -m udp –dport 67:68 -s -d -j DROP

-A INPUT -p udp -m udp –dport 67:68 -s -d -j DROP
-A OUTPUT -p udp -m udp –dport 67:68 -s -d -j DROP
-A FORWARD -p udp -m udp –dport 67:68 -s -d -j DROP

-A INPUT -p udp -m udp –sport 67:68 -s -d -j DROP
-A OUTPUT -p udp -m udp –sport 67:68 -s -d -j DROP
-A FORWARD -p udp -m udp –sport 67:68 -s -d -j DROP

You can download also with above rules from here

Applying this rules, any traffic of DHCP between 2 routers is prohibited and devices from Net: will no longer wrongly get assinged IP addresses from Network range: as it happened to me.

7. Filter out DHCP traffic based on MAC completely on Linux with arptables

If even after disabling MAC randomization on all devices on the network, and you know physically all the connecting devices on the Network, if you still see some weird MAC addresses, originating from a wrongly configured ISP traffic router host or whatever, then it is time to just filter them out with arptables.

## drop traffic prevent mac duplicates due to vivacom and bergon placed in same network –
dchp1-server:~# arptables -A INPUT –source-mac 70:e2:83:12:44:11 -j DROP

To list arptables configured on Linux host

dchp1-server:~# arptables –list -n

If you want to be paranoid sysadmin you can implement a MAC address protection with arptables by only allowing a single set of MAC Addr / IPs and dropping the rest.

dchp1-server:~# arptables -A INPUT –source-mac 70:e2:84:13:45:11 -j ACCEPT
dchp1-server:~# arptables -A INPUT  –source-mac 70:e2:84:13:45:12 -j ACCEPT

dchp1-server:~# arptables -L –line-numbers
Chain INPUT (policy ACCEPT)
1 -j DROP –src-mac 70:e2:84:13:45:11
2 -j DROP –src-mac 70:e2:84:13:45:12

Once MACs you like are accepted you can set the INPUT chain policy to DROP as so:

dchp1-server:~# arptables -P INPUT DROP

If you later need to temporary, clean up the rules inside arptables on any filtered hosts flush all rules inside INPUT chain, like that

dchp1-server:~#  arptables -t INPUT -F

Psychological instability and the gap between desire and reality

Wednesday, August 7th, 2013

sad reality - bulgarians see no future, protests in bulgaria are the least people can do

I'm in Sofia just for a few days and I'm stunned how expensive is life here if compared to people's revenues. I think it is almost impossible to live normal live in Sofia with money lower than 500 eur. I'm here for a  4 day with my Belarusian girlfriend and I'm trying not to spend too much and already spend about 200 EUR. This makes 50 euro per two person a day and this is just for going outside public transport Metro, Buses and taxis. This makes about 50 euro per day or 25 euro a day required for person to live normal (not at all luxury life) … It is no strange that people are protesting for already about 50 days here in Sofia and that Bulgarians are heavily drinking and smoking and don't want to work. Our Bulgarian nation is depressed and there is a good reason for that. We are forced to live almost like animals. There are no money for renovation the buildings, no money to eat properly, young people's morale is totally messed up for the reason culture here is almost dead or even the one that's left is heavily  connected with godlessness, sexuality and consumerism. It is no strange our structures politics and infrastructure here is so messed up. It all starts with people's messed heads and all this gets materialized in our daily lives … Money are the main aim of living of young people and life here is made in a way that you have to only strive after money and for money you should learn to do anytihng … I just don't understand how we ended up here, its quite a pain especially as I lived in end up communistic Soviet time sand I remember people was not so much after money as there was money for normal almost middle class life in Socialistic Bulgaria … There are plenty of knowledgeable people in my dear homeland. Plenty of brilliant and good people but most emigrate for the reason here conditions of living has become extremely abnormal. Truth is European union totally smashed us. With all this shop chains in Western model like Lidl and Billa  made just to collect even our last money and the lack of industry, if something doesn't change ASAP we will simply be cleaned up of the face of the world or at least noone will stay in this country. There are plenty of emigrants already, the problem is even in western europe and most other countries there is big problem with unemployment so even if we Bulgarians try to go out we can't find job. I know this post is a big rant but I feel sad for my beautiful homeland. We have one of the most beautiful natures in Europe, we have Sea Beaches and Mountains, we have culture of 15 centuries and own well developed language. But it seems we're loosing our identity day by day. Almost everywhere you go you hear some music in English – even though most people in Bulgaria do not speak decent English and are far from understanding music in English I more and more think there is almost straight logic information in English. Most song lyrics are just fractured and incomplete or messed up, mostly speaking about evil than good. It is tragedy how young people spend there time here in Bulgaria for nothing hanging in pubs and bars spending money for alcohol and cigarettes trying to waste themselves for the only reason they see no future …

I blog again :)

Tuesday, April 28th, 2009

I haven’t blogged for quite some time. First I glorify the Almighty God our Blessed Holy Trinity for his abundant mercy towards me!! Glory be to the Father, The Son and The Holy Spirit! Now and forever and ever! Amen. I start with this loud words and I have so many things to say. But yes yesterday a joyful news came to me it seems my Exemption letter for Internship has been Approved! I prayed to God that he make them approve this exemption letter because I want to graduate asap and go back to Bulgaria. Here in the Netherlands I feel really terrible, the spiritual state of the country is simply softly said terrible, even though they seem to be an advanced country from the tangible aspect of the things from the intangible/spiritually poor. Not to say that I feel like the devil is controlling most of their lives already. The complete mix of negroes, chinese, indonesians and all other type of races makes the country mixed. Here in the air it feels like a spirits of gluttony are crawling around all the time, also quite often I feel like madness crossing around the air. Sometimes I have that strange thoughts in my mind that something is really wrong with that country. Maybe I had a nice point about that.This SHR project is getting schizophrenic, anyways glory be to God for his abundant mercy towards me and sustaining me always. Yesterday was a terrible day I felt so confused such a profound spiritual sorrow was rulling me that I can hardly bear it, I had a couple of terrible days this days. Since some time I am suspecting there is something wrong here, everytime I have classes with most of the teachers here I feel terrible afterwards and I usually need a couple of days to recover to some sane state. In their presence I experience profound spiritual sorrow and suffering, I’ve been in a similar spiritual states before and I know that this simply can be described in the biblical word hell. Since some time I suspect something is wrong with this guys (I mean the tachers), a couple of days Mr. Joop Vinke the guy who seems to be like a dean to us mentioned during some of the theater sports answered my question where have been yesterday “to the rotary club” and then he added like every other day before, it was not clear is he mean it or not. My suspects became even stronger, because I know that one of my employers used to be attending rotary club as well I know some really terrible things happened in his life and I think he quit that club, anyways. Last week on Friday I met one of my other teachers (Mr. Da Ponte) and I spoke with him, the conversation flowed as he mentioned something about the Lord making the sun circle around the earth, I was interested by his statement so asked him if he believes in God and if he is a roman catholic. He said he is not roman catholic and then what followed was a sort of preach about what he believes and his God as I continusly asked him questions. From his description I left with the impression that he is probably believing in the same God of the masonry (I’ve red about that just a couple of days before). So many things matched, the teacher even mentioned that a lot of teachers in that school are also believing in God and I was left with the impression that he meant the same God as he believes, so I make the connection that they are probably rotarians, masons or taking participation in some sort of organization like that which has to deal pretty much with the occult. A couple of days before I spoke with a brother in Christ (Stelian) and I explained him what is happening and about this BHC (business ethics classes), I explained him how much they want you to accept what they say and if you don’t you are not worthy, I also explained to Stelio the whole story and how this guys are able to make you feel really bad. Since this guys tried to teach us their methods and I tried a couple of times their methods and saw the effect how by doing “something” “unconsciously” you can alter the other into a state of broken spiritness and terrible suffering while at the same time you feel overflowed with joy, a sort of stealing his living power or Angels so to say. I don’t want to enter into details about that since its to me surely related with demonic manifestation. I’ve also remembered that one of the guest lecturers that Mr. Vinke has brought here mentioned that he is rotarian, the coincidences started becoming too much seriously. I also spoke with a student who has graduated and I asked him if he feels that bad thing inside of him, I was stunned when he confirmed. Also in that SHR project it really is schizophrenic I feel that spiritually something really wrong is happening there I started thinking and I could recognize many of the things done in classes of Mr. Vinke has to deal with the paranormal even though not openly showed, even his theater sports has a lot of unconscious spiritism involved, not to mention his Werewolf games including vampires, whitches etc. a lot of the theater sport games include games which include things with dying, you play dead etc., etc. I also have noticed that teachers often are pointing me and saying that I’m not changing, many of the students here are changing seriously for bad. I know by my saviour Jesus Christ who said “by their fruits you will know them”, seeing their fruit suffering, confusion, hate, lies etc. I started being more and more convinced that this guys are dealing with the devil. So if my theory is right and I think it probably is, most of the teachers are members of the rotary club. Maybe they even see it as harmless way to improve their business contacts but I know this is not the case, and this guys are giving oaths, having their strange believes spiritual leaders and do worship the devil even though not openly. I shared all that with a couple of my colleagues and many of them probably just thought that I’m out of my mind. But the holy spirit in me testified all that the things I am thinking are true. I’ve shared what is happening with a brother in Christ (a priest) in the orthodox Church Bulgaria and he said he is gonna mention my name on the altar before God on the Divine Liturgies. I guess this matters because today even though I am not completely okay I feel much relieved and better and I feel God! Glory be to the Immortal and Holy of Israel now and unto ages of ages. I try to learn the gospel a bit early in the morning and a bit late at night before I go to bed I also try to pray a bit each morning and evening and trust the Lord to keep me and protect me from the schemes of the evil one! Yesterday I was at Ina’s place and tried to explain her that this project we do now has something to deal with evil spiritism. I even have suspected that this guys from the rotary club ask their members to share information about certain people that the rotary members work with and then try to bring some curses and spells if they see somebody as a problem to their practices. I suspect that this guys somehow use their members as a channels to spread their evil spirituality. I’ve also taken the advice of Stelio to start caring an Icon in me whenever I go to school. Quite often here and especially in the dormitory I feel something is happening inside of me, my heart starts beating unsteady I also feel spirits flying around and trying sort of trying to conquer me this more or less has to deal with their broken spirituality here. Often I feel completely exhausted like somebody stealing my living power and willingness. Also I have noticed that here in their discoteques, they don’t allow you to enter with a hat? My assumption is that there are somethings placed in the discoteques which has to deal with inducing thoughts in you. On saturday I and Sali entered into a discoteque, I was not so willing so I’ve removed my hat I’ve borrowed temporary from Sali. After being in the discoteque on the day after and on Monday I had terrible headache and felt weak and pretty much like almost dying, Also I felt something on my forehead happening, just like I felt on a numerous times the Holy Spirit annointance and the Lord’s spiritual sign on my forehead, I’ve red in revelation that the Antichrist is going to put something on ppl’s forehead and and their right hands I’m more and more starting to think that this in some phase is already working, I’ve felt aches on my right hand on a numerous occasions, here often Its like I fight for my spiritual survivance. People around I see as they’re dead and just living to consume “limed tombs” as they are called in the gospel. I know that all I’ve written here might seem like too much of a conspiracy, however I’m pretty much sure that many of the things I suppose are true to a certain high degree or even completely.END—–