I'm using NetworkManager 1.6.2-3 on Debian stretch x86_64.

We have implemented a DHCP client in Ada (ADHCP, see here https://www.codelabs.ch/adhcp/) which is able to replace dhclient as NM's DHCP client of choice. The client is spawned via a wrapper binary as explained in "Using adhcp_client on a Linux Desktop" on the project website.

The client calls an external notify binary which sends lease information via D-Bus over the private D-Bus socket unix:path=/var/run/NetworkManager/private-dhcp.

Since the interface change from signals to method calls, we encounter some problems.

The problem can be reproduces via dbus-monitor as well:

$ sudo dbus-monitor --address unix:path=/var/run/NetworkManager/private-dhcp

The expected outcome is:

Failed to register connection to bus at unix:path=/var/run/NetworkManager/private-dhcp: No such interface 'org.freedesktop.DBus' on object at path /org/freedesktop/DBus

There is no monitor facility on this socket, so far so good.

But also another behavior can be observed:

$ sudo dbus-monitor --address unix:path=/var/run/NetworkManager/private-dhcp
Failed to register connection to bus at unix:path=/var/run/NetworkManager/private-dhcp: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.

The server does not reply and syslog shows:

Feb  2 10:37:34 hostname NetworkManager[PID]: Error adding entry to keyring: 
Error creating directory '/root/.dbus-keyrings': Read-only file system

This looks like glib tries to perform some sort of DBUS_COOKIE_SHA1 authentication with the client where EXTERNAL should be used.