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Types of Cyberattacks and other terms from the world of cyber security


It’s convenient to name drop different types of cyber attacks at a party. I often struggle to name more than a few. I will try to maintain a running list of them.

But I find you cannot speak about cybersecurity unless you also have a basic understanding of information technology so I am including some of those terms as well.

As I write this I am painfully aware that you could simply ask ChatGPT to generate a list of all relevant terms in cybersecurity along with their definitions – at least I think you could – and come up with a much better and more complete list. But I refuse to go that route. These are terms I have personally come across so they have special significance for me personally. In other words, this list has been organically grown. For instance I plowed through a report by a major vendor specializing in reviewing other vendor’s offerings and it’s just incredible just how dense with jargon and acronyms each paragragh is: a motherlode of state-of-the-art tech jargon.


I guess an attack which has a bait such as a plum job offer combined with some kind of extortion? The usage was not 100% clear.

Collision attack

I.e., against the MD5 hash algorithm as done in the Blast RADIUS exploit.

Credential Stuffing Attack

I.e., password re-use. Takes advantage of users re-using passwords for different applications. Nearly three of four consumers re-use password this way. Source: F5. Date: 3/2024

Password spraying

A type of attack in which the threat actor tries the same password with multiple accounts, until one combination works. 

Supply Chain attack
Social Engineering

I suppose that would be an activitst who uses hacking to further their agenda.

Living off the land
Data Breach
Click farms

This is one of my favorite terms. Imagine crooks implanted malware into an ATM and were able to convince it to dispense all its available cash to them on the spot! something like this actually happened. Scary.

Overlay Attack

Example: When you open a banking app on your phone, malware loads an HTML phishing page that’s designed to look just like that particular app and the malware’s page is overlaid on top.

Anti-bot, bot defense
Selenium (Se) or headless browser
PII, Personally Identifiable Information
api service
Reverse proxy
endpoint, e.g., login, checkout
Layer 7
Carpet bombing DDOS attack

Many sources hitting many targets within the same subnet. See:,entire%20CIDR%20or%20multiple%20ASNs.

SYN flood
Browser Fingerprint
AICPA Trust Services
(JavaScript) Injection
Command Injection
GET|POST Request
Virtual Server
Clear text
Threat Intelligence
Use case
Carding attack
Source code
CEO Fraud

(Voice Phishing) A form of cyber-attack where scammers use phone calls to trick individuals into revealing sensitive information or performing certain actions.

Business email compromise (BEC)
Threat Intelligence
Social engineering
SIM box
Command and control (C2)
Typo squatting
Voice squatting

A technique similar to typo squatting, where Alexa and Google Home devices can be tricked into opening attacker-owned apps instead of legitimate ones.

Control flow obfuscation
Indicators of Compromise
AMSI (Windows Antimalware Scan Interface)
Polymorphic behavior
Protocol handler
Security Service Edge (SSE)
Secure Access Service Edge (SASE)
Zero Trust

Zero Trust is a security model that assumes that all users, devices, and applications are inherently untrustworthy and must be verified before being granted access to any resources or data.

Zero Trust Network Access (ZTNA)
Zero Trust Edge (ZTE)
Secure Web Gateway (SWG)
Cloud Access Security Broker (CASB)
Remote Browser Isolation (RBI)
Content Disarm and Reconstruction (CDR)
Firewall as a service
Egress address
Data residency
Data Loss Prevention (DLP)
Magic Quadrant
Managed Service Provider (MSP)
0-day or Zero day
User Experience (UX)
Remote Access Trojan (RAT)

2024. A remote access trojan.

IoC (Indicators of Compromise)
Object Linking and Embedding
(Powershell) dropper
TTP (Tactics, Techniques and Procedures)
Shoulder surfing
Pig butchering

This is particularly disturbing to me because there is a human element, a foreign component, crypto currency, probably a type of slave trade, etc. See the Bloomberg Businessweek story about this.

Forensic analysis
Attack vector
Attack surface
Economic espionage
Gap analysis
AAL (Authentication Assurance Level)
IAL (Identity Assurance Level)
CSPM (Cloud Security Posture Management)
Trust level
Network perimeter
DMZ (Demilitarized zone)
Defense in depth
Lateral movement
Access policy
Micro segmentation
Least privilege
Elevated privileges
Insider threat
Cache poisoning

I know it as DNS cache poisoning. If an attacker manages to fill the DNS resolver’s cache with records that have been altered or “poisoned.”

Verify explicitly
Network-based attack
Adaptive response
Identity Provider (IDP)
Consuming entity
Behavior analysis
Lifecycle management
Flat network
Inherent trust
Cloud native
Data encryption
EDR (Endpoint Detection and Response)
BSI (Bundesamt für Sicherheit in der Informationstechnik)

German Federal Office for Information Security (Bundesamt für Sicherheit in der Informationstechnik)

ICS (Industrial Control System)
Reverse shell

A text-based interfaces that allow for remote server control.

Crypto Miner
A RCE (Remote Code Execution)
Threat Actor
APT (Advanced Persistent Threat)
Remote Access VPN (RAVPN)
XDR (Extended Detection and Response)
SIEM (Security Information and Event Management)
User Entity Behavior Analytics (UEBA)
Path traversal vulnerability

An attacker can leverage path traversal sequences like “../” within a request to a vulnerable endpoint which ultimately allows access to sensitive files like /etc/shadow.

Post-exploit persistence technique
MFA bomb

Bombard a user with notifications until they finally accept one.

Use-after-free (UAF)

use-after-free vulnerability occurs when programmers do not manage dynamic memory allocation and deallocation properly in their programs.

Cold boot attack

A cold boot attack focuses on RAM and the fact that it is readable for a short while after a power cycle.

Famous named attacks

Agent Tesla
Morris Worm

Explanations of exploits

Famous attackers

APT29 (Cozy Bear)

A Russia-nexus threat actor often in the news

Volt Typhoon

2024. A China-nexus threat actor

IT terminology

Active Directory
BGP (Border Gateway Protocol)
CISA (Cybersecurity and Infrastructure Security Agency)

CVEs, or Common Vulnerabilities and Exposures, are a maintained list of vulnerabilities and exploits in computer systems. These exploits can affect anything, from phones to PCs to servers or software.  Once a vulnerability is made public, it’s given a name in the format CVE–. There are also scoring systems for CVEs, like the CVSS (Common Vulnerability Scoring System), which assigns a score based on a series of categories, such as how easy the vulnerability is to exploit, whether any prior access or authentication is required, as well as the impact the exploit could have.

Data at rest
Data in motion
Data Remanence

The residual representation of data that remains even after attempting to erase or initialize RAM.

DDI (DNS, DHCP and IP address management)
DLP (Data Loss Prevention)
DoH (DNS over HTTPS)
Enhanced Factory Reset (EFR)
Exact Data Matching (EDM)
GSLB (global Server Load Balancing)
Modbus protocol
NSA (National Security Agency)
OCR (Optical Character Recognition)
OT (Operational Technology)
PaaS (Platform as a Service)
PLC (programmable logic controller)
Portable Executable (PE)
Private Cloud
Proof of Concept (POC)

An open-source unified compute framework used by the likes of OpenAI, Uber, and Amazon which simplifies the scaling of AI and Python workloads, including everything from reinforcement learning and deep learning to tuning and model serving.

Retrieval-Augmented Generation (RAG)
SaaS (Software as a Service)
VPN – Virtual Private Network
Admin Network Technologies

Ping sweep for network security engineers


I swear my bash programming skills are getting worse and worse. What I really need is a bash scripting tips blog entry to remind myself of my favorite bash scripting tips. I have this for python and I refer toit and add to it all the time. I don’t care if anyone else never uses it, it’s worth having all my used tips in one place as I find I constantly forget the basics due to infrequent usage.

Oh. So to the point. What this blog post is nominally about is to provide a useable medium-quality ping swep that a network security engineer would find useful.

  • access to host on the subnet in question
  • this accessible host has a bash shell CLI, e.g., a Checkpoint firewall
  • ping and arp programs available
What it does

This script is designed to sweep through a /24 subnet, politely pausing one second per attempt. It send s a single PING to each IP. This is the things that makes it appealing to network security engineers. it does not require a reply, which is a common situation for network security appliances. It immediately checks the arp table afterwards to see if there is an arp entry (before that has a chance to age out). If so, it reports the IP as up.

The code

I call the program


  ping -c 1 -W 1 $1 > /dev/null
# arp -an output looks like: ? ( at 01:c0:ed:78:b3:dc [ether] on eth0
# or if not present, like ? ( at <incomplete> on eth0
  arp -an|grep -iv incomplete|grep -qi $1\)
  [ $? -eq 0 ] && echo Node with IP: $i is up.

if [[ ! -n $1 ]];
  echo "No subnet passed. Pass three octects like 10.29.129"
for i in ${subnet}.{1..254}
is_alive_ping $i
sleep 1

Apologies for the lousy programming. But it gets the job done.

./ 10.29.129
Node with IP: is up.
Node with IP: is up.
Node with IP: is up.
Node with IP: is up.
Node with IP: is up.
Node with IP: is up.
Node with IP: is up.

As a network security engineer you may be asked if it’s safe to use a paricular IP on one of your subnets where you have your equipment plus equipment frmo other groups. I provide a ping sweep script which reports which IPs are taken, not relying on an ICMP REPLY, but just on the ARP table entry which gets created if a device is on the network.

References and related

None so far!

Admin Apache Linux

Cloudflare: an added layer of protection for your personal web site


I was looking at what Cloudflare could do for my web site. A colleague pointed out that they have a free usage tier which supplies a web application firewall and some anti-bot measures. I checked it out and immedaitely signed up!

The details

What Cloudflare is supplying at no cost (for personal web sites like mine) is amazing. It’s not just a world-class dns service. That would already be amazing. Run against and you will see several different IPs mentioned around the world- just like the big guns! I also get for free some level of mitigation against dns-based attackes.

Web site protections

I don’t fully understand their products so I don’t know what level of protections I am getting in the free tier, but there are at least some! They say they’ve blocked 10 requests in the last few days

Web usage stats

I have to admin using raw linux tools against my apache access file hasn’t bee n the most illuminating until now. Now that I use Cloudflare I get a nice visual presentation showing where (which country) my visitors came from, where the bots come from, how much data was transmitted.

Certificate for HTTPS

Cloudflare automatically takes care of the web site certificate. I had to do nothing at all. So now I can forget my call out to LetsEncrypt. I wonder if GoDaddy is still charging $69 annually for their certificates.


Yeah my web site just feels faster now since the switch. It just does. And Cloudflare stats say that about 30% of the content has been served from their cache – all with zero setup effort on my part! I also believe they use certain tcp acceleration techniques to speed things up.


And Cloudflare caches some of my objects to boost performance. Considering that I pay for data transfer at Amazon AWS, it’s a fair question to ask if this caching could even be saving me money? I investigated this and found that I get billed maybe $ .02 per GByte, and in a busy month I might use .8 GB or so, so $ .02 per month. So I might occasionally save a penny or so – nothing substantial though!


Even with this free tier you get some geoDNS functionality for free, namely, visitors from around the world will see an IP address which is geographically close to where they are, bossting their performance when using your site. Stop to think about that. That’s a whole lot of infrastructure sophistication that they’re just giving you for free!

Why are they giving this much away?

I think they have the noble aim of improving the security posture of the Internet writ large. Much as letsencrypt greatly accelerated the adoptipon of web page encyrption (https) by making certificates free, Cloudflare hopes to accelerate the adoption of basic security measures for every web site, thereby lifting the security posture of the Internet as a whole. Count me as a booster!

What’s their business model. How will they ever make money?

Well, you’re only supposed to use the free tier for a personal web site, for one. My web sites don’t really have any usage and do not display ads so I think I qualify.

More importantly, the free security protections and acceleration are a kind of teaser and the path to upgrading to profesisonal tier is very visibly marked. So they’re not 100% altruistic.

Why I dislike GoDaddy

Let’s contrast this with offerings from GoDaddy. GoDaddy squeezes cents out of you at every turn. They make it somewhat mysterious what you are actually paying for so they’re counting on fear of screwing up (FOSU, to coin a term). After all, except for the small hit to your wallet, getting that upgraded tier – whois cloaking, anyone? – might be what you need. Who knows. Won’t hurt, right? But I get really tired of it. Amazon AWS is perhaps middle tier in this regards. They do have a free tier virtual server which I used initially. But it really doesn’t work except as a toy. My very modest web site overwhlemed it on too many occasions. So, basically useless. Everything else: you pay for it. But somehow they’re not shaking the pennies out of you at every turn unlike GoDaddy. And AWS even shows you how to optimize your spend.

How I converted my live site to Cloudflare

After signing up for Cloudflare I began to enter my dns domains, e.g.,,, plsu a few others. They explained how at GoDaddy I had to update the nameserver records for these domains, which I did. Then Cloudflare has to verify these updates. Then my web sites basically stopped working. So I had to switch the encryption mode to full. This is done in Web sites > > SSL/TLS > Overview. This mode encrypts the back-end data to my web server, but it accepts a self-signed certificate, no matter if it’s expired or not and no matter who issued it. That is all good because you still get the encrypted channel to your content server.

Then it began to work!

Restoring original visitor IPs to my apache web server logs

Very important to know from a technical standpoint that Cloudflare acts as a reverse proxy to your “content server.” Knowing this, you will also know that your content server’s apache logs get kind of boring because they will only show the Cloudflare IPs. But Cloudflare has a way to fix that so you can see the original IPs, not the Cloudlfare IPs in your apache logs.

Locking down your virtual server

If Internet users can still access the web server of your virtual server directly (bypassing Cloudflare), your security posture is only somewhat improved. To go further you need to use a local firewall. I debated whether to use AWS Network Security Groups or iptables on my centos virtual server. I went with iptables.

I lossely followed this developer article. Did I mention that Cloudflare has an extensive developer community?

Actually I had to install iptables first because I hadn’t been using it. So my little iptables script I created goes like this.

# from
# For IPv4 addresses
curl -s|while read ip; do
 echo adding $ip to iptables restrictions
 iptables -I INPUT -p tcp -m multiport --dports http,https -s $ip -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s $ip -j ACCEPT
# maybe needed it just once??
#iptables -A INPUT -p tcp -m multiport --dports http,https -j DROP
# list all rules
iptables -S

I believe I just need to run it the one time, not, e.g., after every boot. We’ll soon see. The output looks like this:

-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -s -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -p tcp -m multiport --dports 80,443 -j DROP

Note that this still leaves ssh open, but that’s ok since it is locked down via Network Security Group rules. No urgent need to change those.

Then I made sure that direct access to my content server freezes, which it does, and that access through the official DNS channels which use Cloudflare still works, which it did. So… all good. The setup was not hard at all. But since I have several hosted web sites for the iptables to make any sense I had to be sure to migrate all my hosted sites over to Cloudflare.

Not GoDaddy

I was dreading migrating my other zones (dns domains) over to Cloudflare. Still being in the GoDaddy mindframe I figured, sure, Cloudflare will permit me one zone for free, but then charge me for a second one.

So I plunged ahead. No charge!

And a third one: Also no charge!

And a fourth, and a fifth and a sixth.

I thought perhaps five will be the threshold. But it wasn’t. I only have six “zones” as Cloudflare now calls them. But they are all in my account and all free. Big relief. This is like the anti-GoDaddy.

DNS changes

Making DNS changes is quite fast. The changes are propagated within a minute or two.

api access

Everything you can do in the GUI you can do through the api. I had previously created and shared some model python api scripts.


As if all the above weren’t already enough, I see Cloudflare also gives my web site accessibility via ipv6:

$ dig +short aaaa


I guess it’s accessible through ipv6 but I haven’t quite proven that yet.

Mail forwarding

I originally forgot that I had set up mail forwarding on GoDaddy. It was one of the few free things you could get. I think they switched native Outlook or something so my mail forwarding wasn’t working. On a lark I checked if Cloudflare has complementary mail forwarding for my domains. And they do! So that’s cool – another free service I will use.

Sending mail FROM this Cloudflare domain using your Gmail account

This is more tricky than simple mail forwarding. But I think I’ve got it working now. You use Gmail’s own server ( as your relay. You also need to set up an app password for Gmail. Even though you need to specify a device such as Windows, it seems once enabled, you can send from this new account from any of your devices. I’ve found that you also need to update your TXT record (see link below) with an expanded SPF information:

v=spf1 ~all

In words it means the Google and Cloudflare sending servers are authorized to sends emails with this domain in the sender field, mail from elsewhere will be marked.

Even after all that I wasn’t seeing my sent message at work where Microsoft 365 is in use. It landed in the Junk folder! Why? The sending email “appears similar to someone who previously sent you email, but may not be that person.” Since I am a former mail admin I am sympathetic to what they’re trying to do – help hapless users avoid phishing; because it’s true – the characters in my test email did bear similarities to my regular email. My regular email is first_name.last_name @, while mail from this domain was first_name @ last_name + s .com Mail sent to a fellow Gmail user suffered no such fate however. Different providers, different approaches. So I can accept that. Once it’s set up you get a drop-down menu of sending addresses every time you compose a new message! The detailed instructions are at the Cloudflare community site.

Cost savings using Cloudflare

Suppose like me you only use GoDaddy as your registrar and get all your other services in some other way. Well, Cloudflare began to pitch me on transferring my domains to them. I thought, Aha, this is the moment they will make money off me. So I read their pitch. Their offer is to bill me for the charges they incur from ICANN or wherever, i.e., pass-through charges without any additional middleman overhead. It’s like, what? So let’s say at GoDaddy I pay $22 per year per domain. Well with Cloudflare I’d be paying something like $10 per year. For one domain I wouldn’t bother, but since I have more than five, I will be bothering and gladly leaving GoDaddy in the dust. I have just transferred the first two domains. GoDaddy seems to drag out the process as long as possible. I found I could expedite it by approving the transfer in the GoDaddy portal ( The trick there is that that one URL looks very different depending on whether or not a domain transfer is pending. If GoDaddy perceives a domain transfer has been initiated by an other registrar, it will show that page with a Transfer In and Transfer Out tabs. Just select Transfer Out and approve your domain for transfer. Then the transfer happens within five minutes. Otherwise that page is shown with no possibility to do a transfer out. So I guess you have to be patiennt, refresh it, or I don’t know what to get it to draw correctly. Once approved in the GoDaddy transfer out portal, Cloudflare had them within 5 minutes. It’s not super-easy to do a transfer, but also not impossble.

In typical GoDaddy style, executing a domain transfer to another registrar seems essentially impossible if you use their latest Domain portfolio app. Fortunately I eventually noticed the option to switch from “beta” to the old Domain manager, which still has the option and looks a bit more like their documentation. I’ve generated auth codes and unlocked, etc. And I even see the correct domain status (ok as opposed to client transfer prohibited) when I do a whois, but now Cloudflare, which is usually so quick to execute, seems to be lagging in recognizing that the domains have been unlocked and suggests to check back in some hours. Weird. The solution here was to provide my credit card info. Even 12 hours later I was having this trouble where it said none of my domains were eligible for transfer. As soon as I provided my payment information, it recognized two of my domains as eligible for transfer. In other cases Cloudflare recognized that domains were unlocked in a matter of 15 minutes or so. It may help to first unlock the domain in GoDaddy, then to view it in Cloudflare. Not sure.

A plug for GoDaddy

As my favorite sport seems to be bashing GoDaddy I wanted to balance that out and say a few kind words about them. Someone in my houisehold just started a job with a startup who uses GoDaddy. It provides desktop Outlook Email, MS Teams, Sharepoint, helps with consulting, etc. And on day one this person was up and running. So if you use their services, they definitely offer value. My issue is that I tried to restrict my usage to just one service – domain registrar – and they pushed me to use it more extensively, which I resisted. But for a small business which needs those thnigs, it’s fine.

How many domains are you sharing your IP with?

The thnig with Cloudflare is that they assign you to a couple of their IP addresses, often beginning with either 172.67 or 104…. . Now did you ever wonder with how many other web sites you’re sharing those IPs? If not, you should! I found a tool that provides the answer: So for this free tier they seem to keep the number around 500 unique domains per IP! Yes that’s a lot, but I’d only be concerned if there was evidence of service degradation, which so far I have not seen. What’s nice about the dnsyltics site is that it lists a few of the domains – far from all of them, but at least it’s 20 or 30 – associated with a given IP. That can be helpful during truobleshooting.


What Cloudflare provides for protective and performance services represents a huge forward advance in the state of the art. They do not niggle you for extra charges (entice is more the word here) for Fear of Screwing Up.

All in all, I am amazed, and I am something of an insider – a professional user of such services. So I heartily endorse using Cloudflare for all personal web servers. I have not been sponsored or even in contact with Cloudflare, by the way!

References and related

Cloudlfare tip: Restoring original visitor IPs to your apache web server.

Locking your virtual server down to just Cloudflare IPs:

Using the Cloudflare python api: working examples

Sending Gmail with your Cloudlflare domain as sending address

Cloudflare’s analysis of the exploit HTTP/2 Rapid Reset is extremely detailed. See and .

I remember being so excited to discover free certificates from LetsEncrypt.

A good explanation of SPF records

Turn an IP addres into a list of associated domain names:

Admin Linux Network Technologies Web Site Technologies

The IT Detective Agency: This site can’t be reached


It’s been awhile since I’ve had the opportunity to relatean IT mystery. After awhile they are repates of what’s already happened in the past, or it’s too complex to relate, or I was only peripherally involved. But today I came across a good one. It falls into the never been seen before category.

The details

A web server behind my web application firewall became unreachable. In the browser they get a message This site can’t be reached. The app owners came to me looking for input. I checked the WAF and it was fine. The virtual server was looking healthy. So I took a packet trace, something to this effect:

$ tcpdump -nni 0.0 host

14:00:45.180349 IP > Flags [S], seq 1106553901, win 23360, options [mss 1460,sackOK,TS val 3715803515 ecr 0], length 0 out slot1/tmm3 lis=/Common/ port=0.53 trunk=
14:00:45.181081 IP > ICMP host unreachable - admin prohibited filter, length 64 in slot1/tmm2 lis= port=0.47 trunk=
14:00:45.181239 IP > Flags [R.], seq 1106553902, ack 0, win 0, length 0 out slot1/tmm3 lis=/Common/ port=0.53 trunk=

I’ve never seen that before, ICMP host unreachable – admin prohibited filter. But I know ICMP can be used to relay out-of-band routing information on occasion, though I do not see it often. I suspect it is a BAD THING and forces the connection to be shut down. Question is, where was it coming from?

The communication is via a firewall so I check the firewall. I see a little more traffic so I narrow the filter down:

$ tcpdump -nni 0.0 host host 443

And then I only see the initial SYN packet followed by the RST – from the same source IP! So since I didn’t see the bad ICMP packet on the firewall, but I do see it on the WAF, I preliminarily conclude the problem exists on the WAF.

Rookie mistake! Did you fall for it? So very, very often, in the heat of debugging, we invent some unit test which we’ve never done before, and we have to be satisified with the uncertainty in the testing method and hope to find a control test somehow, somewhere to validate our new unit test.

Although I very commonly do compound filters, in this case it makes no sense, as I realized a few minutes later. My port 443 filter would of course exclude logging the bad ICMP packets because ICMP does not use tcp port 443! So I took that out and re-run it. Yup. bad ICMP packet still present on the firewall, even on the interface of the firewall directly connected to the server.

So at this point I have proven to my satisfaction that this packet, which is ruining the communication, really comes frmo the server.

What the server guys say

Server support is outsourced. The vendor replies

As far as the patching activities go , there is nothing changed to the server except distro upgrading from 15.2 to 15.3. no other configs were changed. This is a regular procedure executed on almost all 15.2 servers in your environment. No other complains received so far…

So, the usual It’s not us, look somewhere else. So the app owner asks me for further guidance. I find it’s helpful to create a test that will convince the other party of the error with their service. And what is one test I would have liked to have seen but didn’t cnoduct? A packet trace on the server itself. So I write

I would suggest they (or you) do a packet trace on the server itself to prove to themselves that this server is not behaving ini an acceptable way, network-wise, if they see that same ICMP packet which I see.

The resolution

This kind of thing can often come to a stand-off, or many days can be wasted as an issue gets escalated to sufficiently competent technicians. In this case it wasn’t so bad. A few hours later the app owners write and mention that the home-grown local firewall seemed suspect to them. They dsabled it and this traffic began to work.

They are reaching out to the vendor to understand what may have happened.

Case: closed!


An IT mystery was resolved today – something we’ve never seen but were able to diagnose and overcome. We learned it’s sometimes a good thing to throw a wider net when seeing unexpected reset packets because maybe just maybe there is an ICMP host unreachable packet somewhere in the mix.

Most firewalls would just drop packets and you wait for a timeout. But this was a homegrown firewall running on SLES 15. So it abides by its own ways of working, I guess. So because of the RST, your connection closes quickly, not timing out as with a normal network firewall.

As always, one has to maintain an open mind as to the true source of an issue. What was working yesterday does not today. No one admits to changing anything. Finding clever ad hoc unit tests is the way forward, and don’t forget to validate the ad hoc test. We use curl a lot for these kinds of tests. A browser is a complex beast and too much of a black box.

Admin Network Technologies TCP/IP

Verizon Airspeed Hotspot uses ipv6 and interferes with VPN client Global Protect


The headline says it all. I got my shiny brand new Verizon hotspot from Walmart. I managed to activate it and add it to my Verizon account (not super easy, but after a few stumbles it did work.) I tried it out my home PC – works fine. I tried it out on my work PC. No good. My Global Protect connection was unstable. It connects for about a minute, then disconnects, then connects, etc. Basically unusable.

The details

I have heard of possible problem with the GP client (version 5.2.11) and IPv6. So I looked to see if this hotspot could be handing out IPv6 info. Yes. It is. But is that really making a difference? I concocted a simple test. I disabled IPv6 on my Wi-Fi adapter, then re-tested the GP client. The connection was smooth as glass! No disconnects!

Disable ipv6 on your Wi-Fi adapter

Bring up a powershell as administrator. Then:

get-netadapterbinding -componentid ms_tcpip6

will show you the current state of ipv6 on your adapters.

disable-netadapterbinding -Name “Wi-Fi” -ComponentID ms_tcpip6

will disable ipv6 on your Wi-Fi. And

enable-netadapterbinding -Name “Wi-Fi” -ComponentID ms_tcpip6

will re-enable it.

ipconfig /all output

For the record, here are some interesting bits from running ipconfig /all:

Wireless LAN adapter Wi-Fi:

Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Intel(R) Dual Band Wireless-AC 8265
Physical Address. . . . . . . . . : 0C-BD-94-98-11-5B
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Temporary IPv6 Address. . . . . . : 2600:1001:b004:2b78:8ab:145c:d014:2edd(Deprecated)
IPv6 Address. . . . . . . . . . . : 2600:1001:b004:2b78:2cc0:71b0:7f1e:a973(Deprecated)
Link-local IPv6 Address . . . . . : fe80::2cc0:71b0:7f1e:a973%30(Preferred)
IPv4 Address. . . . . . . . . . . :

Subnet Mask . . . . . . . . . . . :
Lease Obtained. . . . . . . . . . : Thursday, April 21, 2022 4:54:04 PM
Lease Expires . . . . . . . . . . : Friday, April 22, 2022 4:54:04 AM
Default Gateway . . . . . . . . . :
DHCP Server . . . . . . . . . . . :
DHCPv6 IAID . . . . . . . . . . . : 302832932
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-28-89-F6-8E-B0-5C-DA-E6-09-0A
DNS Servers . . . . . . . . . . . : fe80::50ae:caff:fea8:1dbc%30
NetBIOS over Tcpip. . . . . . . . : Enabled

But, having done all that, I can only occasionally connect to GP. It seems to work slightly better at night. ipv6 does not seem to be the sole hiccup. No idea what the recipe for reliable success is. If I ever learn it I will publish it. Meanwhile, my phone’s hotspot, also VErizon, also handing out ipv6 info, usually permits me to connect to GP. It’s hard to see the difference.


The Verizon Airspeed Hotspot sends out a mix of IPv6 and IPv4 info to dhcp clients. Palo Alto Networks’ Global Protect client does not play well with that setup and wil not have a stable connection.

I do not think there is a way to disable IPv6 on the hotspot. However, for those with admin access it can be disabled on a Windows PC. And then GP will work just fine. Or not.

Oh, and by the way, otherwise the Airspeed works well and is an adequate solution where you need a good reliable hotspot. Well, in fact, don’t expect reliability like you have from a wired connection. After a couple hours, all users just got dropped for no apparent reason whatsoever.

Admin Linux Raspberry Pi

Scripts checker


Imagine an infrastructure team empowered to create its own scripts to do such things as regularly update external dynamic lists (EDLs) or interact with APIs in an automated fashion. At some point they will want to have a meta script in place to check the output of the all the automation scripts. This is something I developed to meet that need.

I am getting tired of perl, and I still don’t know python, so I decided to enhance my bash scripting for this script. I learned some valuable things along the way.

I call the script Here it is.

# DrJ 2021/12/17, updated 2023/7/26
# it is desired to run this using the logrotate mechanism
# logrotate invokes with /bin/sh so we have to do this trick...
if [ ! "$BASH_VERSION" ] ; then
  exec /bin/bash "$0" "$@"
DIR=$(cd $(dirname $0);pwd)
DAY=2 # Day of week to analyze full week of logs. Monday is 1, Tuesday 2, etc
recipients="[email protected]"
checklog2() {
  [[ "$DEBUG" -eq "1" ]] && echo ID, $ID, LPATH, $LPATH, START, $START, ERROR, $ERROR, END, $END
# the Ec switches mean (E) extnded regular expressions, (c) count of matching lines
  zgrep -Ec "$START" ${LPATH}|cut -d: -f2|while read sline; do starts=$((starts + sline));echo $starts>starts; done
  zgrep -Ec "$END" ${LPATH}|cut -d: -f2|while read sline; do ends=$((ends + sline));echo $ends>ends; done
# Outlook likes to remove our newline characters - double up on them with this sed trick!
  zgrep -Ec "$ERROR" ${LPATH}|cut -d: -f2|sed 'a\\'|while read sline; do errors=$((errors + sline));echo $errors>errors; done
  exampleerrors=$(zgrep -E "$ERROR" ${LPATH}|head -10)
  starts=$(cat starts)
  ends=$(cat ends)
  errors=$(cat errors)
  Total starts: $starts
  Total finishes: $ends
  Total errors: $errors
  Most recent errors: "

  unset NEW
# get cumulative totals
  starttot=$((starttot + starts))
  endtot=$((endtot + ends))
  errortot=$((errortot + errors))
  [[ "$DEBUG" -eq "1" ]] && echo starttot, $starttot, endtot, $endtot, errortot, $errortot
  [[ "$DEBUG" -eq "1" ]] || rm starts ends errors
} # end of checklog2 function

checklog() {
# clear out stats and some variables
#this IFS and following line is trick to preserve those darn backslash charactes in the input file
for line in $(<$INI); do
  [[ "$line" =~ ^# ]] || {
  pval=$(echo "$line"|sed s'/: */:/')
  lhs=$(echo $pval|cut -d: -f1)
  rhs=$(echo "$pval"|cut -d: -f2-)
  lhs=$(echo $lhs|tr [:upper:] [:lower:])
  [[ "$DEBUG" -eq "1" ]] && echo line is "$line", pval is $pval, lhs is $lhs, rhs is "$rhs"
  if [ "$lhs" = "identifier" ]; then
    [[ "$DEBUG" -eq "1" ]] && echo matched lhs = identifer section
    [[ -n "$NEW" ]] && checklog2
  [[ "$lhs" = "path" ]] && LPATH="$rhs" && NEW=false
  [[ "$lhs" = "error" ]] && ERROR="$rhs"
  [[ "$lhs" = "start" ]] && START="$rhs"
  [[ "$lhs" = "end" ]] && END="$rhs"
# call one last time at the end
} # end of checklog function

anomalydetection() {
# a few tests - you can always come up with more...
  diff=$((starttot - endtot))
  [[ $diff -gt $maxdiff ]] || [[ $starttot -lt $minstarts ]] || [[ $errortot -gt $maxerrors ]] && {
    [[ "$DEBUG" -eq "1" ]] && echo ANOMALIES, $ANOMALIES, starttot, $starttot, endtot, $endtot, errortot, $errortot
} # end function anomalydetection

sendsummary() {
  subject="Weekly summary of sesamstrasse automation scripts - please review"
  [[ -n "$ANOMALIES" ]] && subject="${subject} - ANOMALIES DETECTED PLEASE REVIEW CAREFULLY!!"

  intro="This summarizes the results from the past week of running automation scripts on sesamstrasse.
Please check that values seem reasonable. If things are out of range, check with Heiko or look at
sesamstrasse yourself.


  [[ "$DEBUG" -eq "1" ]] && echo subject, $subject, intro, "$intro", info, "$info"
  [[ "$DEBUG" -eq "1" ]] && args="-v"
  echo "${intro}${info}"|mail "$args" -s "$subject" "$recipients"
} # end function sendsummary

# always check the latest log

# only check all logs if it is certain day of the week. Monday = 1, etc
day=$(date +%u)
[[ "$DEBUG" -eq "1" ]] && echo day, $day
[[ $day -eq $DAY ]] || [[ -n "$ANOMALIES" ]] && {
  [[ "$DEBUG" -eq "1" ]] && echo calling checklog with wildcard set

[[ "$DEBUG" -eq "1" ]] && echo message so far is "$info"


# The suggestion: To have a configuration file with log identifiers
#(e.g. “anydesk-edl”) and per identifier: log file path (“/var/log/anydesk-edl.log”),
# error pattern (“.+\[Error\].+”), start pattern (“.+\[Notice\] Starting$”) end pattern (“.+\[Notice\] Done$”).
#Then just count number of executions (based on start/end) and number of errors.

# the start/end/error values are interpreted as extended regular expressions - see regex(7) man page
identifier: anydesk-edl
path: /var/log/anydesk-edl.log
error: .+\[Error\].+
start: .+\[Notice\] Starting$
end: .+\[Notice\] Done$

identifier: firewall-requester-to-edl
path: /var/log/firewall-requester-to-edl.log
error: .+\[Error\].+
start: .+\[Notice\] Starting$
end: .+\[Notice\] Done$

identifier: sase-ips-to-bigip
path: /var/log/sase-ips-to-bigip.log
error: .+\[Error\].+
start: .+\[Notice\] Starting$
end: .+\[Notice\] Done$

What this script does

So when the guy writes an automation script, he is so meticulous that he follows the same convention and hooks it into the syslogger to create uniquely named log files for it. He writes out a [Notice] Starting when his script starts, and a [Notice] Done when it ends. And errors are reported with an [Error] details. Some of the scripts are called hourly. So we agreed to have a script that checks all the other scripts once a week and send a summary email of the results. I look to see that the count of starts and ends is roughly the same, and I report back the ten most recent errors from a given script. I also look for other basic things. That’s the purpose of the function anomalydetection in my script. It’s just basic tests. I didn’t want to go wild.

But what if there was a problem with one of the scripts, wouldn’t we want to know sooner than possibly six days later? So I decided to have my script run every day, but only send email on the off days if an anomaly was detected. This made the logic a tad more complex, but nothing bash and I couldn’t handle. It fits the need of an overworked operational staff.

Techniques I learned and re-learned from developing this script

cron scheduling – more to it than you thought

I used to naively think that it suffices to look into the crontab files of all users to discover all the scheduled processes. What I missed is thinking about how log rotate works. How does it work? Turns out there is another section of cron for jobs run daily, weekly and monthly. logrotate is called from cron.daily.

logrotate – potential to do more

The person who wrote the automation scripts is a much better scripter than I am. I didn’t want to disappoint so I put in the extra effort to discover the best way to call my script. I reasoned that logrotate would offer the opportunity to run side scripts, and I was absolutely right about that! You can run a script just before the logs rotate, or just after. I chose the just before timing – prerotate. In actual fact logrotate calls the prerotate script with all the log files to be rotate as arguments, which you notice we don’t take advantage of, because at the time we were unsure how we were going to interface. But I figure let’s just leave it now. man logrotate to learn more.

By the way although I developed on a generic Debian system, it should work on a Raspberry Pi as well since it is Debian based.

BASH – the potential to do more, at a price

You’ll note that I use some bash-specific extensions in my script. I figure bash is near universal, so why not? The downside is that when logrotate invokes an external script, it calls is using old-fashioned shell. And my script does not work. Except I learned this useful trick:

if [ ! "$BASH_VERSION" ]; then
  exec /bin/bash "$0" "$@"

Note this is legit syntax in SHELL and a legit conditional operator expression. So it means if you – and by you I mean the script talking about itself – are invoked via SHELL, then invoke yourself via BASH and exit the parent afterwards. And this actually does work (To do: have to check which occurs first, the syntax checking or the command invocation).


Speaking of that conditional, if you want to know all the major comparison tests, do a man test. I have around to use the double bracket expressions [[ more and more, though they are BASH specific I believe. The double bracket can be followed by a && and then an open curly brace { which can introduce a block of code delimited of course by a close curly brace }. So for me this is an attractive alternative to SHELL’s if conditional then code block fi syntax, and probably just slightly more compact. Replace && with || to execute the code block when the condition does not evaluate to be true.

zgrep is grep for compressed files, but we knew that right? But it’s agnostic – it works like grep on both compressed and uncompressed files. That’s important because with rotated logs you usually have a combination of both.

Now the expert suggested a certain regular expression for the search string. It wasn’t working in my first pass. I reasoned that zgrep may have a special mode to act more like egrep which supports extended regular expressions (EREs). EREs aren’t really the same as perl-compatible regular expressions (PCREs) but for this kind of simple stuff we want, they’re close enough. And sure enough zgrep has the -E option to force it to interpret the expression as an ERE. Great.


So in the log.ini file the regular expression has a \[…\] syntax. The backslash is actually required because otherwise the […] syntax is interpreted as a character class, where all the characters between the brackets get tried to match a single character in the string to be matched. That’s a very different match!

My big thing was – will I have to further escape those lines read in from log.ini, perhaps to replace a \[ with a \\[? Stuff like that happens. I found as long as I used those double quotes around the variables (see below) I did not need to further escape them. Similarly, I found that the EREs in log.ini did not need to be placed between quotes though the guy initially proposed that. It looks cleaner without them.

Variable scope

I wasted a lot of time on a problem which I thought may be due to some weird variable scoping. I’ve memorized this syntax cat file|while read line; do etc, etc so I use it a lot in my tiny scripts. It’s amazing I got away with it as much as I have because it has one huge flaw. if you start using variables within the loop you can’t really suck them out, unless you write them to a file. So while at first I thought it was a problem of variable scoping – why do my loop variables have no values when the code comes out of the loop? – it really isn’t that issue. It’s that the pipe, |, created a forked process which has its own variables. So to avoid that I switched to this weird syntax for line in $(<$INI); do etc. So it does the line-by-line file reading as before but without the pipe and hence without the “variable scope” problem.

But in another place in the script – where I add up numbers – I felt I could not avoid the pipe. So there I do write the value to a file.

The conclusion is that with the caveat that if you know what you’re doing, all variables have global scope, and that’s just as it should be. Hey, I’m from the old Fortran 66/77 school where we were writing Monte Carlos with thousands of lines of code and dozens of variables in a COMMON block (global scope), and dozens of contributors. It worked just fine thank you very much. Because we knew what we were doing.

Adding numbers in bash

Speaking of adding, I can never remember how to add numbers (integers). In bash you can do starts=$((starts + sline)) , where starts and sline are integers. At least this worked in Debian linux Stretch. I did not really get the same to work so well in SLES Linux – at least not inside a loop where I most needed it.

When you look up how to add numbers in bash there are about a zillion different ways to do it. I’m trying to stick to the built-in way.

Sending mail in Debian linux

You probably need to configure a smarthost if you haven’t used your server to send emails up until now. You have to reconfigure of the exim4 package:

dpkg-reconfigure exim4-config

This also can be done on a RPi if you ever find you need for it to send out emails.


If a variables includes linebreaks and you want to see that, put it between double-quotes, e.g., echo “$myVariableWithLineBreaks”. If you don’t do that it seems to remove the linebreaks. Use of the double quotes also seems to help avoid mangling variables that contain meta characters found in regular expressions such as .+ or \[.

Result of executing the commands

I grew up using the backtick metacharacter, `, to indicate that the enclosed command should be executed. E.g., old way:

DIR=`dirname $0`

But when you think about it, that metacharacter is small, and often you are unlucky and it sits right alongside a double quote or a single quote, making for a visual trainwreck. So this year I’ve come to love the use of $(command to be executed) syntax instead. It offers much improved readability. But then the question became, could I nest a command within a command, e.g., for my DIR assignment? I tried it. Now this kind of runs counter to my philosophy of being able to examine every single step as it executes because now I’m executing two steps at once, but since it’s pretty straightforward, I went for it. And it does work. Hence the DIR variable is assigned with the compound command:

DIR=$(cd $(dirname $0);pwd)

So now I wonder if you can go more than two levels deep? Each level is an incrementally bad idea – just begging for undetectable mistakes, so I didn’t experiment with that!

By the way the reason I needed to do that is that the script jumps around to another directory to create temporary files, and I wanted it to be able to reference the full path to its original directory, so a simpler DIR=$(dirname $0) wasn’t going to cut it if it’s called with a relative path such as ./


I make mistakes left and right. But I know what results I expect. So I generously insert statements as variables get assigned to double check them, prefacing them with a conditional [[ $DEBUG -eq 1 ]] && print out these values. As I develop DEBUG is set to 1. When it’s finally working, I usually set it to 0, though in some script I never quite reach that point. It looks like a lot of typing, but it’s really just cut and paste and not over-thinking it for the variable dump, so it’s very quick to type.

Another thing I do when I’m stuck is to watch as the script executes in great detail by appending -xv to the first line, e.g., #!/bin/bash -xv. But the output is always confusing. Sometimes it helps though.

Compensating for Outlook’s newline handling

Outlook is too clever for its own good and “helpfully” removes what it considers extra linefeeds. Thanks Microsoft. Really helpful. So if you add extra linefeeds you can kind of get around that, but then you go from 0 linefeeds in the displayed output to two. Again, thanks Microsoft.

Anyway, I disocvered sed ‘a/\\/’ is a way to add an extra linefeed to my error lines, where the problem was especially acute and noticeable.

Techniques I’d like to use in the future

You can assign a function to a variable and then call that variable. I know that will have lots of uses but I’m not used to the construct. So maybe for my next program.


This fairly simple yet still powerful script has forced me to become a better BASH shell scripter. In this post I review some of the basics that make for successful scripting using the BASH shell. I feel the time invested will pay off as there are many opportunities to write such utility scripts. I actually prefer bash to perl or python for these tasks as it is conceptually simpler, less ambitious, less pretentious, yes, far less capable, but adequate for my tasks. A few rules of the road and you’re off and running! bash lends itself to very quick testing cycles. Different versions of bash introduced additional features, and that gets trying. I hope I have found and utilized some of the basic stuff that will be available on just about any bash implementation you are likely to run across.

References and related

The nitty gritty details about BASH shell can be gleaned by doing a man bash. It seems daunting at first but it’s really not too bad once you learn how to skim through it.

This post shows how to properly use the syslog package within python to create these log files that I parse.

Admin Firewall Linux

Linux: how to estimate bandwidth usage to a particular subnet


Let’s say someone asks you to estimate the total bandwith used by a particular subnet, or a particular service such as https on port 443. I provide a crude way to do that using tcpdump on a not-too-busy server.

The code

I call it By the way, I ran it on a Checkpoint Gaia appliance so it works there as well.

# DrJ 11/21
sum() {
cat $file|while read line; do
 length=$(echo $line|awk '{print $17}'|sed 's/)//')
 sum=$(expr $sum + $length)
 echo $sum
while /bin/true; do
tcpdump -c1000 -v -nni eth1 net 216.71/16 > $file
#10:29:49.471455 IP (tos 0x0, ttl 126, id 32399, offset 0, flags [none], proto: UDP (17), length: 105) > UDP, length 77
total=$(sum|tail -1)
t0=$(head -1 $file|awk '{print $1}')
t1=$(tail -1 $file|awk '{print $1}')
h0=$(echo $t0|cut -d: -f1|sed 's/^0//')
h1=$(echo $t1|cut -d: -f1|sed 's/^0//')
m0=$(echo $t0|cut -d: -f2|sed 's/^0//')
m1=$(echo $t1|cut -d: -f2|sed 's/^0//')
s0=$(echo $t0|cut -d: -f3|sed 's/^0//')
s1=$(echo $t1|cut -d: -f3|sed 's/^0//')
s0=$(echo $s0|cut -d\. -f1|sed 's/^0//')
s1=$(echo $s1|cut -d\. -f1|sed 's/^0//')
[ -z "$h0" ] && h0=0
[ -z "$h1" ] && h1=0
[ -z "$m0" ] && m0=0
[ -z "$m1" ] && m1=0
[ -z "$s0" ] && s0=0
[ -z "$s1" ] && s1=0
#echo total bytes: $total
#echo elapsed time: $elapsed
echo $kbps kbps at $(date)
sleep $sleep

The idea

Running tcpdump with the -v switch gives us packet length. We find that length and sum it up. Here we used a filter epxression of 216.71/16 to capture only the traffic from that subnet.

The number of packets to capture has to be tuned to how busy it gets. Now it’s set to only capture 1000 packets. And you see my crude timings are truncated at the second. So 1000 packets in one second or about 1.5 MBytes/sec = 12 Mbps is the maximum sensitivy of this approach. I doub it will really work for interfaces with more thn 100 Mbps, even after you scaled up the count (and don’t forget to change the denominator in the kbps line!

Here’s a sample output:

1000 packets captured
2002 packets received by filter
0 packets dropped by kernel
5 kbps at Wed Nov 3 12:09:45 EDT 2021

I think it’s important to note the number of packets dropped by the kernel. So if it gets too busy as I underatdn it, it will at least try to tell yuo that it couldn’t capture all the data and at that point you can no longer trust this method. Perhaps with enhanced statistical methods it could be salvaged.

I don’t run it continuously to also give the kernel a breather. It probably doesn’t make much difference, but every two minutes seems plenty frequent to me…


We have demonstrated a crude but better-than-nothing script to calculate bandwidth for a given tcpdump filter expression. It won’t win any awards, but it contains some worthwhile ideas. And it seems to work at low bandwidth levels.

Admin DNS Firewall Network Technologies TCP/IP

The IT detective agency: named times out tcp queries


I’ve been reliable running ISC’s BIND server for eons. Recently I had a problem getting my slave servers updated after a change to the primary master. What was going on there?

The details

This was truly a team effort. I saw that the zone file had differing serial numbers on the master versus the slave servers. My attempts to update via an rndc refresh zone was having no effect.

So I tried a zone transfer by hand: dig axfr @

That timed out!

Yet, regular dns qeuries went through fine: dig ns @

I thought about it and remembered zone transfers use TCP whereas standard queries use UDP. So I tried a TCP-based simple query: dig +tcp ns @ It timed out!

So of course one suspects the firewall, which is reasonable enough. And when I looked at the firewal I found some funny drops, though i cuoldn’t line them up exactly with my failed tests. But I’m not a firewall expert; I just muddle through.

The next day someone from the DNS group asked how local queries behaved? Hmm. never tried that. So I tried it: dig +tcp ns @localhost. That timed out as well! That was a brilliant suggestion as we now could eliminate the firewall and all that complexity from the equation. Because I had tried to do packet traces on two different machines at the same time and line up the results. It wasn’t easy.

The whole issue was very concerning to us because we feared our secondaries would be unable to pudate their slave zones and ultimately time them out. The result would be devastating.

We have support, fortunately. A company that hearkens frmo the good old days, with real subject matter experts. But they’re extremely busy. We did not get a suggestion for a couple weeks. But eventually we did. They had seen this once before.

named time to respond to TCP-based queries

The above graph is from a Zabbix monitor showing how long it takes that dns server to respond to that simple query. 6 s is a time-out. I actually set dig to timeout at 2 s, but in wall-clock time it actually takes 6 s.

The fix

We removed this line from the options block of named.conf:

keep-response-order {any; };

The info fmo the experts is that most likely that was configured as a workaround to CVE-2019-6477 but that issue was fixed since 9.15.6.


We encountered the named daemon in a situation where it was unable to respond to TCP-based DNS queries and hence unable to do zone transfers. So although most queries use UDP, this was a serious issue for us and prevented zones from being updated on all authoritative nameservers.

As is the case with so many modern IT problems, the effect was not black or white. Failures were intermittent, and then permanent. A restart fixed ths issue (forgot to mention so far!). But we involved an expert to find the root cause and it was the presence of a single configuration line in our named.conf. After removing that all was good.


Git commands cheat sheet


This is the list of git commands I compiled.

Create new local GIT repository

git init [project name]

Copy a repository

git clone username@host:/path/to/repository

Add a file to the staging area – must be done or won’t be saved in next commit

git add temp.txt or git add -A (add everything at once)

My working style is to change one to three files, and only add those.

Suppose you run git add, then change the file again, and then run git commit: what version will you push? The original! Run git add, etc all over again to pick up the latest changes. And note that your git adds can be run over the course of time – not all at once!

Create a snapshot of the changes and save to git directory??

Note that any committed changes won’t make their way to the remote repo

git commit –m “Message to go with the commit here”

Nota bene: git does not allow to add empty folders! If you try, you’ll simply see: nothing to commit, working tree clean

Put some “junk file” like .gitkeep in your empty folder for git add/git commit to work.

Set user-specific values

git config –global youremail

Displays the list of changed files together with the files that are yet to be staged or committed

git status

Undo that git add you just did

git status to see what’s going on. Then

git restore –staged filename

and then another git status to see that that worked.

Send local commits to the master branch of the remote repository

  (Replace <master> with the branch where you want to push your changes when you’re not intending to push to the master branch)

git push origin <master>

For real basic setups like mine, where I work on branch master, it suffices to simply do git push

Merge all the changes present in the remote repository to the local working directory

git pull

Create branches and helps you to navigate between them

git checkout -b <branch-name>

Switch from one branch to another

git checkout <branch-name>

View all remote repositories

git remote -v

Connect the local repository to a remote server

git remote add origin <host-or-remoteURL>

Delete connection to a specified remote repository

git remote rm <name-of-the-repository>

List, create, or delete branches

git branch

Delete a branch

git -d <branch-name>

Merge a branch into the active one

git merge <branch-name>

List file conflicts

git diff –base <file-name>

View conflicts between branches before a merge

git diff <source-branch> <target-branch>

List all conflicts

git diff

Mark certain commits, i.e., v1.0

git tag <commitID>

View repository’s commit history, etc

git log, e.g., git log –oneline

Reset index?? and working directory to last git commit

git reset –hard HEAD

Remove files from the index?? and working directory

git rm filename.txt

Revert (undo) changes from a commit as per hash shown by git log –oneline

git revert <hash>

Temporarily save the changes not ready to be committed??

git stash

View info about any git object

git show

Fetch all objects from the remote repository that don’t currently reside in the local working directory

git fetch origin

View a tree object??

git ls-tree HEAD

Search everywhere

git grep <string>

Clean unneeded files and optimize local repository

git gc

Create zip or tar file of a repository

Git archive –format=tar master

Delete objects without incoming pointers??

git prune

Identify corrupted objects

git fsck

Merge conflicts

Today I got this error during my usual git pull:

error: Pulling is not possible because you have unmerged files.
hint: Fix them up in the work tree, and then use 'git add/rm '
hint: as appropriate to mark resolution and make a commit.
fatal: Exiting because of an unresolved conflict.

I did not wish to waste too much time. I tried a few things (git status, etc), none of which worked. As it is a small repository without much at stake and I know which files I changed, I simply deleted the clone and re-cloned the repo, put back the newer versions of the changed files, did a usual git add and git commit and git push. All was good. Not too much time wasted in becoming a gitmaster, a fate I wish to avoid.

Ignore a file

Put the unqualified name of the file in .gitignore at same level as .git. It will not be added to the project. Use this for keeping passwords secret.

Test a command

Most commands have a –dry-run option which prevents it from taking any action. Useful for debugging.

References and related

Admin Apache CentOS Python Raspberry Pi Web Site Technologies

Traffic shaping on linux – an exploration


I have always been somewhat agog at the idea of limiting bandwidth on my linux servers. Users complain about slow web sites and you want to try it for yourself, slowing your connection down to meet the parameters of their slower connection. More recently I happened on librespeed, an alternative to, where you can run both server and client. But in order to avoid transferring too much data and monopolizing the whole line, I wanted to actually put in some bandwidth throttling. I began an exploration of available methods to achieve this and found some satisfactory approaches that are readily available on Redhat-type linuxes.

bandwidth throttling, bandwidth rate limiting, bandwidth classes – these are all synonyms for what is most commonly called traffic shaping.

What doesn’t work so well

I think it’s important to start with the walls that I hit.


I stumbled on cgroups first. The man page starts in a promising way

cgroup - control group based traffic control filter

Then after you research it you see that support was enabled for cgroups in linux kernels already long ago. And there is version 1 and 2. And only version 1 supports bandwidth limits. But if you’re just a mid-level linux person such as myself, it is confusing and unclear how to take advantage of cgroup. My current conclusion is that it is more a subsystem designed for use by systemctl. In fact if you’ve ever looked at a status, for instance of crond, you see a mention of a cgroup:

sudo systemctl status crond
? crond.service - Command Scheduler
Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)
Active: active (running) since Mon 2021-08-09 15:44:24 EDT; 5 days ago
Main PID: 1193 (crond)
Tasks: 1 (limit: 11278)
Memory: 2.1M
CGroup: /system.slice/crond.service
mq1193 /usr/sbin/crond -n

I don’t claim to know what it all means, but there it is. Some nice abilities to schedule and allocate finite resources, at a very high level.

So I get the impression that no one really uses cgroups to do traffic shaping.

apache web server to the rescue – not

Since I was mostly interested in my librespeed server and controlling its bandwidth during testing, I wondered if the apache web server has this capability built-in. Essentially, it does! There is the module mod_ratelimit. So, quest over, and let the implementation begin! Except not so fast. In fact I did enable that module. And I set it up on my librespeed server. It kind of works, but mostly, not really, and nothing like its documented design.

    SetOutputFilter RATE_LIMIT
    SetEnv rate-limit 400 
    SetEnv rate-initial-burst 512

That’s their example section. I have no interest in such low limits and tried various values from 4000 to 12000. I only got two different actual rates from librespeed out of all those various configurations. I could either get 83 Mbps or around 162 Mbps. And that’s it. Merely having any statement whatsoever starts limiting to one of these strange values. With the statement commented out I was getting around 300 Mbps. So I got rate-limiting, but not what I was seeking and with almost no control.

So the apache config approach was a bust for me.


There are some linux programs that are perhaps promoted too heavily? Within a minute of posting my first draft of this someone comes along and suggests trickle. Well, on CentOS yum search trickle gives no results. My other OS was SLES v 15 and I similarly got no results. So I’m not enamored with trickle.

tc – now that looks promising

Then I discovered tc – traffic control. That sounds like just the thing. I had to search around a bit on one of my OSes to find the appropriate package, but I found it. On CentOS/Redhat/Fedora the package is iproute-tc. On SLES v15 it was iproute2. On FreeBSD I haven’t figured it out yet.

But it looks unwieldy to use, frankly. Not, as they say, user-friendly.

tcconfig + tc – perfect together

Then I stumbled onto tcconfig, a python wrapper for tc that provides convenient utilities and examples. It’s available, assuming you’ve already installed python, through pip or pip3, depending on how you’ve installed python. Something like

$ sudo pip3 install tcconfig

I love the available settings for tcset – just the kinds of things I would have dreamed up on my own. I wanted to limit download speeds, and only on the web server running on port 443, and noly from a specific subnet. You can do all that! My tcset command went something like this:

$ cd /usr/local/bin; sudo ./tcset eth0 --direction outgoing --src-port 443 --rate 150Mbps --network

$ sudo ./tcshow eth0

"eth0": {
"outgoing": {
"src-port=443, dst-network=, protocol=ip": {
"filter_id": "800::800",
"rate": "150Mbps"
"incoming": {}

More importantly – does it work? Yes, it works beautifully. I run a librespeed cli with three concurrent streams against my AWS server thusly configured and I get around 149 Mbps. Every time.

Note that things are opposite of what you first think of. When I want to restrict download speeds from a server but am imposing traffic shaping on the server (as opposed to on the client machine), from its perspective that is upload traffic! And port 443 is the source port, not the destination port!

Raspberry Pi example

I’m going to try regular librespeed tests on my home RPi which is cabled to my router to do the Internet monitoring. So I’m trying

$ sudo tcset eth0 --direction incoming --rate 100Mbps
$ sudo tcset eth0 --direction outgoing --rate 9Mbps --add

This reflects the reality of the asymmetric rate you typically get from a home Internet connection. tcshow looks a bit peculiar however:

"eth0": {
"outgoing": {
"protocol=ip": {
"filter_id": "800::800",
"delay": "274.9s",
"delay-distro": "274.9s",
"rate": "9Mbps"
"incoming": {
"protocol=ip": {
"filter_id": "800::800",
"delay": "274.9s",
"delay-distro": "274.9s",
"rate": "100Mbps"
Results on the RPi

Despite the strange delay-distro appearing in the tcshow output, the results are perfect. Here are my librespeed results, running against my own private AWS server:

Time is Sat 21 Aug 16:17:23 EDT 2021
Ping: 20 ms Jitter: 1 ms
Download rate: 100.01 Mbps
Upload rate: 9.48 Mbps


Problems creep in on RPi

I swear I had it all working. This blog post is the proof. Now I’ve rebooted my RPi and that tcset command above gives the result Illegal instruction. Still trying to figure that one out!

March, 2022 update. My RPi had other issues. I’ve re-imaged the micro SD card and all is good once again. I set traffic shaping policies as shown in this post.

Conclusion about tcconfig

It’s clear tcset is just giving you a nice interface to tc, but sometimes that’s all you need to not sweat the details and start getting productive.

Possible issue – missing kernel module

On one of my servers (the CentOS 8 one), I had to do a

$ sudo yum install kernel-modules-extra

$ sudo modprobe sch_netem

before I could get tcconfig to really work.

To do list

Make the tc settings permanent.

Verify tc + tcconfig work on a Raspberry Pi. (tc is definitely available for RPi.)


We have found a pretty nice and effective way to do traffic shaping on linux systems. The best tool is tc and the best wrapper for it is tcconfig.

References and related

Librespeed is a great alternative for hard-code linux types who love command line and being in full control of both ends of a speed test. I describe it here.

tcconfig’s project page on PyPi.

Power cycling one’s cable modem automatically via an attached RPi. I refer to this blog post specifically because I intend to expand that RPi to also do periodic, automated speedtesting of my home braodband connection, with traffic shaping in place if all goes well (as it seems to thus far).

Bandwidth management and “queueing discipline” in all its gory detail is explained in this post, including example raw tc commands. I haven’t digested it yet but it may represent a way for me to get my RPi working again without a re-image: