systemd has a feature called socket activation where initially systemd
manages and listens on ports/uds and only invokes a service when traffic
appears. to then hand over the involved sockets, systemd will pass the
relevant FDs into the invoked process and defines them in the environment.
use coreos/go-systemd to grab the active listeners passed by systemd and
use them to serve the api routes. only one listener may be specified right
now as we also only support one -listen argument for the binary.
this allows admins to craft a systemd socket and service file for aptly
where systemd manages the socket, its permission and its live time, and
lazy start aptly when needed.
`unix://$PATH` as listen argument will bind aptly to a unix domain socket
rather than TCP.
This allows binding the API to a UDS rather than a port.
Since aptly has no concept of authentication or any amount of high level
API hardening one needs to bottle it up in some other manner. Binding
to a localhost port is often a step in the right direction, ultimately is
still a scary insecure setup as any user on that host getting compromised
would mean that the entire archive is compromised as well.
UDS on the other hand are basically files and have their access managed
by regular file permission. As such, binding to a socket is in fact
the least insecure way to listen as you'd have to explicitly open up the
socket permissions to an access qualified group. In the most conservative
scenario that means no one but the aptly user can talk to the API, in a
more practical setup apache might get access as well and proxy the UDS
with authentication or limited to GET operations.
Using UDS allows reducing the attack surface of the API server while
preserving all the flexibility.
