lokinet/llarp/router/systemd_resolved.cpp

#include "systemd_resolved.hpp"
#include <llarp/util/logging/logger.hpp>

#include <stdexcept>

extern "C" {
#include <systemd/sd-bus.h>
#include <net/if.h>
}

using namespace std::literals;

namespace llarp {

#ifndef WITH_SYSTEMD

  bool systemd_resolved_set_dns(std::string, llarp::SockAddr, bool) {
    LogDebug("lokinet is not build with systemd support, cannot set systemd resolved DNS");
    return false;
  }

#else

  namespace {
    template <typename... T>
    void resolved_call(sd_bus* bus, const char* method, const char* arg_format, T... args) {
      sd_bus_error error = SD_BUS_ERROR_NULL;
      sd_bus_message *msg = nullptr;
      int r = sd_bus_call_method(bus,
          "org.freedesktop.resolve1",
          "/org/freedesktop/resolve1",
          "org.freedesktop.resolve1.Manager",
          method,
          &error,
          &msg,
          arg_format,
          args...);

      if (r < 0)
        throw std::runtime_error{"sdbus resolved "s + method + " failed: " + strerror(-r)};

      sd_bus_message_unref(msg);
      sd_bus_error_free(&error);
    }

    struct sd_bus_deleter { void operator()(sd_bus* ptr) const { sd_bus_unref(ptr); } };
  }

  bool systemd_resolved_set_dns(std::string ifname, llarp::SockAddr dns, bool global) {
    unsigned int if_ndx = if_nametoindex(ifname.c_str());
    if (if_ndx == 0) {
      LogWarn("No such interface '", ifname, "'");
      return false;
    }

    // Connect to the system bus
    sd_bus *bus = nullptr;
    int r = sd_bus_open_system(&bus);
    if (r < 0) {
      LogWarn("Failed to connect to system bus to set DNS: ", strerror(-r));
      return false;
    }
    std::unique_ptr<sd_bus, sd_bus_deleter> bus_ptr{bus};

    try {
      // This passing address by bytes and using two separate calls for ipv4/ipv6 is gross, but the
      // alternative is to build up a bunch of crap with va_args, which is slightly more gross.
      if (dns.isIPv6()) {
        auto ipv6 = dns.getIPv6();
        static_assert(sizeof(ipv6) == 16);
        auto* a = reinterpret_cast<const uint8_t*>(&ipv6);
        resolved_call(bus, "SetLinkDNSEx", "ia(iayqs)",
            (int32_t) if_ndx,
            (int) 1, // number of "iayqs"s we are passing
            (int32_t) AF_INET6, // network address type
            (int) 16, // network addr byte size
            a[0], a[1], a[2], a[3], a[4], a[5], a[6], a[7], a[8], a[9], a[10], a[11], a[12], a[13], a[14], a[15], // yuck
            (uint16_t) dns.getPort(),
            nullptr // dns server name (for TLS SNI which we don't care about)
        );
      }
      else
      {
        auto ipv4 = dns.getIPv4();
        static_assert(sizeof(ipv4) == 4);
        auto* a = reinterpret_cast<const uint8_t*>(&ipv4);
        resolved_call(bus, "SetLinkDNSEx", "ia(iayqs)",
            (int32_t) if_ndx,
            (int) 1, // number of "iayqs"s we are passing
            (int32_t) AF_INET, // network address type
            (int) 4, // network addr byte size
            a[0], a[1], a[2], a[3], // yuck
            (uint16_t) dns.getPort(),
            nullptr // dns server name (for TLS SNI which we don't care about)
        );
      }

      if (global)
        // Setting "." as a routing domain gives this DNS server higher priority in resolution
        // compared to dns servers that are set without a domain (e.g. the default for a
        // DHCP-configured DNS server)
        resolved_call(bus, "SetLinkDomains", "ia(sb)",
            (int32_t) if_ndx,
            (int) 1, // array size
            "." // global DNS root
            );
      else
        // Only resolve .loki and .snode through lokinet (so you keep using your local DNS server for
        // everything else, which is nicer than forcing everything though lokinet's upstream DNS).
        resolved_call(bus, "SetLinkDomains", "ia(sb)",
            (int32_t) if_ndx,
            (int) 2, // array size
            "loki", // domain
            (int) 1, // routing domain = true
            "snode", // domain
            (int) 1 // routing domain = true
            );

      return true;

    } catch (const std::exception& e) {
      LogWarn("Failed to set DNS via systemd-resolved: ", e.what());
    }
    return false;
  }

#endif // WITH_SYSTEMD

} // namespace llarp