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Default sockets

Varlink has no bus and no resolver: each service listens on a fixed socket path. Generated packages export it as DefaultSocket, and Dial connects to it. The table comes from the generator config, idl/generate.json.

Interface Package Default socket Since
io.systemd systemd — (DialPath only) v257
io.systemd.AskPassword askpassword /run/systemd/io.systemd.AskPassword v258
io.systemd.BootControl bootcontrol /run/systemd/io.systemd.BootControl v257
io.systemd.Credentials credentials /run/systemd/io.systemd.Credentials v257
io.systemd.FactoryReset factoryreset /run/systemd/io.systemd.FactoryReset v258
io.systemd.Hostname hostname /run/systemd/io.systemd.Hostname v257
io.systemd.Import importd /run/systemd/io.systemd.Import v257
io.systemd.Journal journal /run/systemd/journal/io.systemd.journal v257
io.systemd.JournalAccess journalaccess /run/systemd/io.systemd.JournalAccess v260
io.systemd.Login login /run/systemd/io.systemd.Login v258
io.systemd.Machine machine /run/systemd/machine/io.systemd.Machine v257
io.systemd.MachineImage machineimage /run/systemd/machine/io.systemd.MachineImage v257
io.systemd.ManagedOOM managedoom /run/systemd/io.systemd.ManagedOOM v257
io.systemd.Manager manager /run/systemd/io.systemd.Manager v258
io.systemd.Metrics metrics — (DialPath only) v260
io.systemd.MountFileSystem mountfilesystem /run/systemd/io.systemd.MountFileSystem v257
io.systemd.MuteConsole muteconsole /run/systemd/io.systemd.MuteConsole v260
io.systemd.NamespaceResource namespaceresource /run/systemd/io.systemd.NamespaceResource v257
io.systemd.Network network /run/systemd/netif/io.systemd.Network v257
io.systemd.Network.Link networklink /run/systemd/netif/io.systemd.Network v260
io.systemd.oom oom /run/systemd/oom/io.systemd.ManagedOOM v257
io.systemd.oom.Prekill oomprekill — (DialPath only) v260
io.systemd.PCRExtend pcrextend /run/systemd/io.systemd.PCRExtend v257
io.systemd.PCRLock pcrlock /run/systemd/io.systemd.PCRLock v257
io.systemd.Repart repart /run/systemd/io.systemd.Repart v260
io.systemd.Resolve resolve /run/systemd/resolve/io.systemd.Resolve v257
io.systemd.Resolve.Hook resolvehook — (DialPath only) v260
io.systemd.Resolve.Monitor resolvemonitor /run/systemd/resolve/io.systemd.Resolve.Monitor v257
io.systemd.service service — (DialPath only) v257
io.systemd.sysext sysext /run/systemd/io.systemd.sysext v257
io.systemd.Udev udev /run/udev/io.systemd.Udev v258
io.systemd.Unit unit /run/systemd/io.systemd.Manager v258
io.systemd.UserDatabase userdatabase /run/systemd/userdb/io.systemd.Multiplexer v257

Notes

  • Shared sockets. One socket can serve several interfaces. PID 1 serves io.systemd.Manager and io.systemd.Unit on /run/systemd/io.systemd.Manager; systemd-networkd serves io.systemd.Network and io.systemd.Network.Link on /run/systemd/netif/io.systemd.Network. Share a pool between their clients with New; see Concurrency.
  • Every socket also serves org.varlink.service (introspection) and usually io.systemd (generic errors) and io.systemd.service (Ping, SetLogLevel, GetEnvironment, ...).
  • No default socket means the interface is implemented by several daemons or on dynamic paths: io.systemd.Metrics sockets live under /run/systemd/report/; io.systemd.Resolve.Hook and io.systemd.oom.Prekill are implemented by clients that systemd calls. Use DialPath.
  • "Since" is the oldest release among those sdvarlink generates bindings for (v257, v258, v260) whose IDL contains the interface. Some interfaces existed before v257.
  • User services. Per-user instances (e.g. systemd --user) serve sockets under $XDG_RUNTIME_DIR/systemd/. Connect with DialPath.
  • Containers. A container's sockets are reachable from the host under its root directory, e.g. /var/lib/machines/<name>/run/systemd/..., subject to permissions.

Discovering sockets on a host

find /run/systemd /run/udev -type s -name 'io.systemd*' 2>/dev/null

socks, _ := filepath.Glob("/run/systemd/report/*") // every metrics provider
for _, s := range socks {
    fmt.Println(s)
}