DBus message sender

[nicomuns]

Hello

My use case is the following, I have one systemd service acting as a DBus server, exposing a DBus method that will be called by several clients (also systemd services). I need to know which client called the method, so I used getObject().getCurrentlyProcessedMessage().getSender().
This gives me the unique identifier of the sender (:1.217 for example) but I don't know how to link it to my systemd services, I have no idea who :1.217 is. Is there any solution to this?

I'm using sdbus-cpp v1.17

DBus method:

void MyClass::myMethod(const std::string& arg0, const std::string& arg1, int32_t arg2) {
    const char* sender = getObject().getCurrentlyProcessedMessage().getSender();
    std::cout << "Sender of this method call: " << sender << std::endl;

    // ...
}

DBus server log example:
[21-04-2026 17:23:28.529] [myService] [info] [myFile.cpp:121] Sender of this method call: :1.217

Monitoring:

sudo busctl monitor Service
‣ Type=method_call  Endian=l  Flags=4  Version=1 Cookie=2  Timestamp="Tue 2026-04-21 15:23:28.529481 UTC"
  Sender=:1.217  Destination=Service  Path=ObjectPath  Interface=Interface  Member=Method
  UniqueName=:1.217
  MESSAGE "ssi" {
          STRING "";
          STRING "";
          INT32 0;
  };

Thank you

[sangelovic]

When you get unique connection name of the sender (":1.217") as a result of getSender(), it most likely means that the sender has not registered a well-known connection name for his D-Bus connection (this is done with IConnection::requestName()). Otherwise I think you would get that more readable, well-known name as the result of getSender() (but I am not 100% sure, haven't tried it).

It is completely OK and very typical that not all senders do register a well-known name. D-Bus services do, but pure clients or short-lived clients don't have to. If you have control over the code of senders, you may try to call IConnection::requestName() for their connections.

Otherwise you can use Message::getCreds*() family of functions. Not sure it's part of sdbus-c++ v1 API though. sdbus-c++ v2 has it for sure. For example, calling getObject().getCurrentlyProcessedMessage().getCredsPid() shall get you the pid of the sender process. Then you can use that pid to get information via /proc/<pid> like executable path, cmdline, etc.

Maybe there are more options, but this is what I am currently aware of.

[nicomuns]

Thanks for the quick answer.

I missed requestName() but it looks promising, I will test it out thanks.
Will also check out the Message::getCreds*() family of functions, as I'm using the v2 of the API. Using the pid could be a workaround if resquestName() does not work.

[nicomuns]

Hello again,

So I tested this with a small DBus client but unfortunately requestName() doesn't seem to do what was expected. I'm still getting sender name in the form :x.yz

const sdbus::ServiceName serviceName{"com.foo.bar"};
const sdbus::ObjectPath objectPath{"/com/foo/bar"};

std::unique_ptr<sdbus::IConnection> dbusConnection = sdbus::createSystemBusConnection();

auto proxy = sdbus::createProxy(*dbusConnection, std::move(serviceName), std::move(objectPath));

const sdbus::ServiceName wellKnownName{"com.foo.dbusclient"};
dbusConnection->requestName(wellKnownName);

...

Will try the getCreds*() family next

[nicomuns]

Yes I called dbusConnection->enterEventLoopAsync(). I didn't put the whole code but basically the rest is starting the event loop and calling a dbus method that is exposed by the server which is running on the side.

[sangelovic]

Yes, I tried now myself and looks like D-Bus broker by design always puts the unique connection name into that field of the message. So another approach I can think of is to query D-Bus broker interface for the list of unique and well-known names and find the well-known name by the unique connection name from the D-Bus message. Or maybe using sd-bus sd_bus_get_name_creds() function with SD_BUS_CREDS_WELL_KNOWN_NAMES could help. sdbus-c++ doesn't have a wrapper for this lower level sd-bus function. Perhaps I could try to implement one quickly... (or you can too if you want)

[nicomuns]

Thanks for this other suggestion, I might try this before getCreds*() as this seems more elegant. So you mean org.freedesktop.DBus, some equivalent to busctl call org.freedesktop.DBus /org/freedesktop/DBus org.freedesktop.DBus ListNames? It looks like ListNames only gives me all connected names on the bus.

[sangelovic]

Yes, this is what I meant.

[nicomuns]

Ok, so I need to find a way to link whatever ListNames returns and the unique connection name