TCB13 , 11 months ago (edited 11 months ago)

Then I don’t understand why maintainers all keep using network.target.

Because most services are able to dynamically accommodate networking changes and act accordingly and it’s rare to have cases where we really need a working link with addresses and a proper connection when the service starts. For those special cases, as described, you need to enable systemd-networkd-wait-online.service and include After=network-online.target and/or Wants=network-online.target in your service.

Programs should be designed to detect and react to networking changes and both Apache and Nginx are good examples of software that does that. There are simpler cases like stuff that needs to bind to non-existent IPs at boot (before networking) that can be dealt with ipv4.ip_nonlocal_bind and net.ipv6.ip_nonlocal_bind as described here.

This may also interest you as it includes what “up” means a few other details: systemd.io/NETWORK_ONLINE/

I still find it a solution desperately looking for a problem to solve.

Systemd does a LOT more than “you can control loads of dependencies”, it solves tons of painful issues and provides a cohesive ecosystem of tools to manage stuff like:

• Start services in parallel (while init was one after the other)
• Defer service startup until they are actually needed
• Replace a bunch of poorly integrated tools such as dhcpcd, dhcpv6, chrony, NetworkManager, resolvconf, logrotate among others with a cohesive ecosystem of tools that are way easier to work with;
• Has facilities to deal with IPv6 advanced use cases like IPv6-PD without having to depend on four different packages;
• Centralizes logs and provides journalctl as a unified interface so you can quickly browse them by specific, filters, timestamps and events.
• Replaces crontab with something auditable that actually makes sense;
• Monitors the status of your services and restarts them if they crash according to specific rules. Can also be used to limit execution time and trigger notifications when stuff fails.
• Allows you to isolate your processes with sandboxing (like jails) techniques by restricting what resources (CPU time, RAM, networking, filesystems) they can access by settings everything in a single unit file. Even setup basic firewalls for your services;
• Create full fledged containers both privileged and non-privileged here and here while providing tools such as machinectl to manage them.

Systemd is very useful and most people (including myself) don’t know about half of the things it can do. It isn’t teached because of the current poor state of thing when it comes to software development but if you use it you’ll quickly find how to have systems running very well with less processes than conventionally.