How does OpenVZ know a VE's distribution

Question

With OpenVZ you assign a VE's hostname (among other things) from the host node (HN) using vzctl set CTID --hostname myhost --save. It then sets this in /etc/vz/conf/CTID.conf. During startup of the VE, a distribution-specific file in /etc/vz/dists is consulted containing pointers to files in /etc/vz/dists/scripts. Those are distribution-specific scripts for editing the VE's config files, for instance if the VE uses Debian or Ubuntu, the hostname is written to /etc/hostname. The same happens with the IP address, for example.

My question now is: How does OpenVZ know which file in /etc/vz/dists to use? Does it somehow derive the distribution name from the template's name?

Answer 1

Looking at the source of vzctl, I figured out how it works. It actually does derive the name from the name of the template. When starting up the VE, at some point a method in lib/dist.c is called:

static int get_dist_conf_name(char *dist_name, char *dir, char *file, int len)
{
      char buf[256];
      char *ep;

      if (dist_name != NULL) {
            snprintf(buf, sizeof(buf), "%s", dist_name);
            ep = buf + strlen(buf);
            do {
                  snprintf(file, len, "%s/%s.conf", dir, buf);
                  if (stat_file(file))
                        return 0;
                  while (ep > buf && *ep !=  '-') --ep;
                  *ep = 0;
            } while (ep > buf);
      [...]
}

There it removes dash delimited parts from the end of the name, each time looking for a match. The template I was looking at is arch-2010.05-x86_64-minimal, so here it first tries arch-2010.05-x86_64-minimal.conf, then arch-2010.05-x86_64.conf, then arch-2010.05.conf, until it finally finds a file named arch.conf holding the pointers to the scripts for updating Arch's configuration files.