At SharkFest’22 EU, the Annual Wireshark User and Developer Conference, I attended a beginners’ course called “Network Troubleshooting from Scratch”, taught by the great Jasper Bongertz. In the end, we had some high-level discussions concerning various things, one of them was the insight that TCP RSTs are not only sent from a server in case the port is closed, but are also commonly sent (aka spoofed) from firewalls in case a security policy denies the connection. Key question: Can you distinguish between those spoofed vs. real TCP RSTs? Initially, I thought: no, you can’t, cause the firewalls out there do a great job.
It turned out: you can!
Continue reading Who sends TCP RSTs?
Some years ago I wrote a blog post called “Basic syslog-ng Installation“. While I used it myself quite often in my labs or at the customers’ sites, it shows only basic UDP transport which is both unreliable and insecure. So, let’s have a look at a fresh installation of syslog-ng with TLS support for security reasons. However, TCP and UDP as transport are covered as well for the support of legacy systems.
Continue reading syslog-ng with TLS: Installation Guide
The other day I wanted to verify whether a service running on my Linux server was listening on IPv6 as well as IPv4. It turned out that it wasn’t that easy to answer – if at all.
Continue reading Services listening on IPv6 and IPv4 (or maybe not?)
I was missing a generic layer 4 ping in my toolbox. Initially searching for a mere TCP ping, I have found Nping which completely satisfies my needs and gives so much more. ;)
What’s a layer 4 ping, and why? –> A normal ping (= ICMP echo-request) reveals whether the destination IP address, that is: the mere server/VM, is up and running. That’s great for a layer 3 networker since routing to and from the destination is already working. However, it does NOT reveal whether or not a service at layer 4 (TCP or UDP) is up and running as well. That’s what a layer 4 ping is about: sending TCP SYNs to the port in question, waiting for a “SYN ACK” (port is listening) or “RST”/no reply (port is not available). Common use cases: Waiting for a service to start again after an upgrade, or waiting for new firewall policies (to allow or deny) a certain port.
Continue reading Nping aka Layer 4 Ping
I am using the WHOIS client a lot these days since I am migrating some RIPE objects such as ASes, inetnum/inet6num, etc. Meanwhile, I recognized that I have never captured this TCP port 43 protocol, nor looked at it with Wireshark. That’s what this post is all about, incl. a downloadable pcap for your own analysis.
Continue reading Who is WHOIS?
I am currently working on a network & security training, module “OSI Layer 4 – Transport”. Therefore I made a very basic demo of a TCP and UDP connection in order to see the common “SYN, SYN-ACK, ACK” for TCP while none of them for UDP, “Follow TCP/UDP Stream” in Wireshark, and so on. I wanted to show that it’s not that complicated at all. Every common application/service simply uses these data streams to transfer data aka bytes between a client and a server.
That is: Here are the Linux commands for basic lab, a downloadable pcap, and, as always, some Wireshark screenshots:
Continue reading Basic TCP and UDP Demos w/ netcat and telnet
It’s not always this simple DNS thing such as “single query – single answer, both via UDP”. Sometimes you have some more options or bigger messages that look and behave differently on the network. For example: IP fragmentation for larger DNS answers that do not fit into a single UDP datagram (hopefully not after the DNS flag day 2020 anymore), or DNS via TCP, or some newer options within the EDNS space such as “EDNS Client Subnet” (ECS) or DNS cookies.
I won’t explain any details about those options, but I am publishing a pcap with that kind of packets along with some Wireshark screenshots. Feel free to dig into it.
Continue reading DNS Capture: UDP, TCP, IP-Fragmentation, EDNS, ECS, Cookie
One of my readers sent me this question:
We have an internal discussion about whether it is possible to block the 3 way hanshake TCP but allow the JDBC application protocol. In other words we would like to block the test of the port with the command “telent address port” but we would like that the connections via JDBC continue to work. is it possible to do this theoretically? Is it possibile to do it with paloalto firewall?
Let’s have a look:
Continue reading PAN Blocking Details