A machine with Wi-Fi, a VPN and a container bridge has several addresses, and a hostname lookup can return the wrong one. Connecting a UDP socket to a destination makes the kernel pick the source address it would use, and getsockname() reads it back without a single packet being sent. Checked on Linux with three interfaces: five destinations, five answers, no IPv4 or ARP frames on the wire.
A runbook for replacing a running daemon without dropping requests, tested on a small TCP server under real systemd. A plain restart refused or reset connections every time; starting the new version beside the old one avoided refusals but still reset a connection in some runs, and a kernel setting made those resets go away; socket activation produced no errors. Also covered: draining in-flight work against TimeoutStopSec, and a release swap that checks the new version and rolls back.
Send a header and a body as two small write() calls, then wait for a reply, and every round trip can stall for about 40 ms on an idle machine with an idle network. Neither Nagle’s algorithm nor delayed ACK is a bug; together they deadlock until a timer fires. This post predicts the stall, reproduces it with a 50-line script, reads it off a packet trace, and compares the four ways out.
epoll’s edge-triggered mode stalls after a partial read; level-triggered mode cannot. The same distinction decides whether a realtime client survives lost, duplicated and reordered events. A reproducible epoll experiment, a five-way simulation on a lossy channel, and a 40-line invalidator that handles the race everyone misses: an event that arrives while the refetch is running.
A machine with Wi-Fi, a VPN and a container bridge has several addresses, and a hostname lookup can return the wrong one. Connecting a UDP socket to a destination makes the kernel pick the source address it would use, and getsockname() reads it back without a single packet being sent. Checked on Linux with three interfaces: five destinations, five answers, no IPv4 or ARP frames on the wire.
A runbook for replacing a running daemon without dropping requests, tested on a small TCP server under real systemd. A plain restart refused or reset connections every time; starting the new version beside the old one avoided refusals but still reset a connection in some runs, and a kernel setting made those resets go away; socket activation produced no errors. Also covered: draining in-flight work against TimeoutStopSec, and a release swap that checks the new version and rolls back.
Send a header and a body as two small write() calls, then wait for a reply, and every round trip can stall for about 40 ms on an idle machine with an idle network. Neither Nagle’s algorithm nor delayed ACK is a bug; together they deadlock until a timer fires. This post predicts the stall, reproduces it with a 50-line script, reads it off a packet trace, and compares the four ways out.
epoll’s edge-triggered mode stalls after a partial read; level-triggered mode cannot. The same distinction decides whether a realtime client survives lost, duplicated and reordered events. A reproducible epoll experiment, a five-way simulation on a lossy channel, and a 40-line invalidator that handles the race everyone misses: an event that arrives while the refetch is running.