Fixing networking after a Tunnelblick crash

Dickon Reed wrote “If you are running tunnelblick to access a VPN using OpenVPN on a High Sierra Mac, and for instance your whole system crashes, once you’ve logged back in you may lose all network access, even when you have the VPN down. In my case the fix was to open Terminal and run: networksetup -setdnsservers "iPhone…”

Automagical port allocation for tests

Ceri Storey wrote “It’s quite common to want to test a net­work ser­vice from the out­side, as if it was being ac­cessed from a cli­ent. Quite of­ten, people will pick a “well-­known” port to use, eg: port 8080 or 8888 for a HTTP ser­vice. But that means that if you leave a stray service process lying around, you’ll need to hunt it…”

Sturmfront auf Doppler-Radar-Schirm, public domain, von

Adventures in TCP latency measurement

Ceri Storey wrote “Re­cently, Google have pub­lished an art­icle on BRR, an al­gorithm that ex­pli­citly meas­ures the round-trip latency and band­width ca­pa­city of the link between two ma­chines (be it in a data­center, or a mo­bile phone) to avoid sending more traffic than is use­ful, causing queues to build up in the net­work that need­lessly in­crease latency. So…”

By User:Salimfadhley (Own work) [GFDL ( or CC-BY-SA-3.0 (], via Wikimedia Commons

4-way TCP handshake and firewalls

Jarek Siembida wrote “This is one of those pieces that you keep in your head for ages but never get around to write up. Tcpdumping I was doing of late brought it back so here it is. We all know the 3-way handshake in TCP: SYN + SYN/ACK + ACK and voila! But this is not the end…”