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

[Tilman Baumann]

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.

Also, the length of the array can be one or two. So that bit is variable
too.

Thanks for any hints
 Tilman Baumann