[systemd-devel] sd-bus example code for SetLinkDNS()

[Lennart Poettering]

On Wed, 19.07.17 11:20, Tilman Baumann (tilman at baumann.name) wrote:

> Hi folks,
> 
> I'm trying to teach a vpn software (openfortivpn) how to properly set up
> DNS in a systemd-resolve environment.
> 
> I'm trying to set up a equivalent to this in C.
> busctl call org.freedesktop.resolve1 /org/freedesktop/resolve1
> org.freedesktop.resolve1.Manager SetLinkDNS 'ia(iay)' 16 2 2 4 10 10 10
> 10 2 4 10 10 10 11
> [https://gist.github.com/tbaumann/d484efb2e27613654a52dbe11cfe53b8]
> 
> I came up with this quick proof of concept code based on the example
> code in the sd-bus docu.
> Of course it segfaults. No surprise, I have done nothing to hint at the
> length of the inner byte array. (ay)
> 
> I was unable to find any example code that would give me a hint on how
> to pass such more complex data structures into sd_bus_call_method()
> 
> int SetLinkDNSv4(sd_bus *bus, int if_index, struct in_addr ns1, struct
> in_addr ns2) {
>   sd_bus_error error = SD_BUS_ERROR_NULL;
>   sd_bus_message *m = NULL;
>   int r;
>   struct dns_address {
>     int sin_family;
>     struct in_addr ip_addr;
>   };
>   struct dns_address addresses[2];
> 
> 
>   addresses[0].sin_family = AF_INET;
>   addresses[0].ip_addr = ns1;
>   addresses[1].sin_family = AF_INET;
>   addresses[1].ip_addr = ns2;
> 
>   r = sd_bus_call_method(bus,
>                          "org.freedesktop.resolve1",           /*
> service to contact */
>                          "/org/freedesktop/resolve1",          /* object
> path */
>                          "org.freedesktop.resolve1.Manager",   /*
> interface name */
>                          "SetLinkDNS",                         /* method
> name */
>                          &error,                               /* object
> to return error in */
>                          &m,                                   /* return
> message on success */
>                          "ia(iay)",                            /* input
> signature */
>                          if_index,
>                          2,                                    /* Array
> size */
>                          addresses);
> }
> 
> [Full code:
> https://gist.github.com/tbaumann/0f466c984c858767c966458d53483697]
> 
> My guess is that I can have it easier if I somehow use
> sd_bus_message_append() to assemble the message. But I don't see a clear
> path either.

You have to do something like this:

sd_bus_message_new_method(..., &m);
sd_bus_message_append(m, "i", ifindex);
sd_bus_message_open_container(m, 'a', '(iay)');
for (i = 0; i < n_addresses; i++) {
        sd_bus_message_open_container(m, '(', "iay"));
        sd_bus_message_append(m, "i", addresses[i].sin_family);
        sd_bus_message_append_array(m, 'y', &addresses[i].ip_addr, sizeof(addresses[i].ip_addr));
        sd_bus_message_close_container(m);
}
sd_bus_message_close_container(m);
sd_bus_message_send(..., m);
sd_bus_message_unref(m);

(not tested, just written down from the top of my head, and of course,
don't forget to add proper error checking)

Lennart

-- 
Lennart Poettering, Red Hat