wp.jochen.hayek.name/blog-en

Category: UNIX networking

  • setting up VPN with my routers at home

    I have 2 routers at home:

    • one connected to DSL (Deutsche Telekom)
    • one connected to TV cable (Vodafone)

    Therefore I actually have 2 VPN setups, both “L2TP/IPsec”.

    Through time the VPN setups occasionally seem unfunctional. (Password change, …)

    I asked Google AI Mode: How can I check with a networking tool, whether the L2TP/IPsec VPN server is accessible on my router?

    macOS does not come with nmap. I already had macports installed. When I tried to install nmap, macports told me:

    Current platform “darwin 25” does not match expected platform “darwin 24”

    No big deal … – I had to upgrade – it told me how to do this. “_Free_wifi_Berlin” gave me incredible bandwidth, which allowed me to do this upgrade, while sitting in the university restaurant.

    Is there a way to do a VPN connect to a L2TP/IPsec VPN server on the command line, and what command line options are there?

    …

    “L2TP/IPsec” use 3 UDP ports. The router at the TV cable had one of them filtered. I was a little desperate – finally I gave up and turned to an alternative – see below!

    …

    2026-10-01: I gave up using “L2TP/IPsec” to connect to both of them.

    I turned to WireGuard instead – the set-up was easy, on the routers’ end and on my MacBook. (Thanks to Gerald D. for the hint!)

  • macOS AKA OSX domain-specific DNS servers (/etc/resolver)

    If you VPN to work from home (or even VPN to home from work, you may have issues resolving hostnames.

    It’s trivial to solve in macOS AKA OSX.

    E.g.:

    $ cat /etc/resolver/fritz.box
    nameserver 10.0.4.1

    The server 10.0.4.1 (e.g. my fritz.box) will do the nameserving for the domain fritz.box, as soon as macOS can see the file.

    I worried actually quite a while, until I found that solution. It’s very nice.

    I actually have to access each local box with “.fritz.box” appended, but that’s not so bad.

  • how to test ports – Telnet, Expect, Bash, Netcat, Nmap

    For “security reasons” telnet and nmap are not included in the basic set of utilities present on a production server. And why would you install software, that you usually don’t need there? But occasionally these tools are needed and should be available for installation. I certainly like telnet and nmap for the described purposes.