Merge #19024

19024: pkg/paho-mqtt: add support for DNS r=aabadie a=benpicco

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### Contribution description

This is pretty straightforward, we only need to hook up `dns_query()`.

Since paho supports both IPv4 and IPv6 we use `AF_UNSPEC` in the query.
This can however give us a A record if we only have IPv6 enabled, so translate the request based on what IP address module is enabled in `dns_query()`.
### Testing procedure

connecting by name works

```
> con test.mosquitto.org 1883
mqtt_example: Connecting to MQTT Broker from test.mosquitto.org 1883
mqtt_example: Trying to connect to test.mosquitto.org , port: 1883
user: clientId: password:
mqtt_example: Connection successfully
```

connecting by IP still works

```
> con 2001:41d0:1:925e::1 1883
mqtt_example: Connecting to MQTT Broker from 2001:41d0:1:925e::1 1883
mqtt_example: Trying to connect to 2001:41d0:1:925e::1 , port: 1883
user: clientId: password:
mqtt_example: Connection successfully
```

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### Issues/PRs references

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Co-authored-by: Benjamin Valentin <benjamin.valentin@bht-berlin.de>

examples/paho-mqtt/Makefile

@@ -17,6 +17,12 @@ QUIET ?= 1
 WIFI_SSID ?= "Your_WiFi_name"
 WIFI_PASS ?= "Your_secure_password"
 
+# Optionally include remoteDNS support. This includes resolution of names at an
+# upstream DNS server and the handling of RDNSS options in Router Advertisements
+# to auto-configure that upstream DNS server.
+# USEMODULE += sock_dns              # include DNS client
+# USEMODULE += gnrc_ipv6_nib_dns     # include RDNSS option handling
+
 ifneq (,$(DEFAULT_MQTT_CLIENT_ID))
   CFLAGS += -DDEFAULT_MQTT_CLIENT_ID=\"$(DEFAULT_MQTT_CLIENT_ID)\"
 endif

examples/paho-mqtt/main.c

@@ -166,7 +166,7 @@ static int _cmd_con(int argc, char **argv)
 
     printf("mqtt_example: Connecting to MQTT Broker from %s %d\n",
             remote_ip, port);
-    printf("mqtt_example: Trying to connect to %s , port: %d\n",
+    printf("mqtt_example: Trying to connect to %s, port: %d\n",
             remote_ip, port);
     ret = NetworkConnect(&network, remote_ip, port);
     if (ret < 0) {

pkg/paho-mqtt/contrib/riot_iface.c

@@ -22,6 +22,7 @@
 #ifdef MODULE_IPV4_ADDR
 #include "net/ipv4/addr.h"
 #endif
+#include "net/dns.h"
 #include "net/sock/tcp.h"
 #include "paho_mqtt.h"
 #include "MQTTClient.h"
@@ -120,18 +121,20 @@ int NetworkConnect(Network *n, char *addr_ip, int port)
 {
     int ret =-1;
     sock_tcp_ep_t remote = SOCK_IPV4_EP_ANY;
-    char _local_ip[IP_MAX_LEN_ADDRESS];
 
-    strncpy(_local_ip, addr_ip, sizeof(_local_ip));
-    if (IS_USED(MODULE_IPV4_ADDR) &&
-        ipv4_addr_from_str((ipv4_addr_t *)&remote.addr, _local_ip)) {
+    ret = dns_query(addr_ip, &remote.addr, AF_UNSPEC);
+    if (ret > 0) {
+        remote.port = port;
+        remote.family = ret == 4 ? AF_INET : AF_INET6;
+    }
+
+    if (IS_USED(MODULE_IPV4_ADDR) && (remote.port == 0) &&
+        ipv4_addr_from_str((ipv4_addr_t *)&remote.addr, addr_ip)) {
             remote.port = port;
     }
 
-    /* ipvN_addr_from_str modifies input buffer */
-    strncpy(_local_ip, addr_ip, sizeof(_local_ip));
-    if (IS_USED(MODULE_IPV6_ADDR) && (remote.port == 0)  &&
-        ipv6_addr_from_str((ipv6_addr_t *)&remote.addr, _local_ip)) {
+    if (IS_USED(MODULE_IPV6_ADDR) && (remote.port == 0) &&
+        ipv6_addr_from_str((ipv6_addr_t *)&remote.addr, addr_ip)) {
             remote.port = port;
             remote.family = AF_INET6;
     }

sys/include/net/dns.h

@@ -74,6 +74,15 @@ static inline int dns_query(const char *domain_name, void *addr_out, int family)
 {
     int res = -ENOTSUP;
 
+    if (family == AF_UNSPEC) {
+        if (!IS_USED(MODULE_IPV4_ADDR)) {
+            family = AF_INET6;
+        }
+        else if (!IS_USED(MODULE_IPV6_ADDR)) {
+            family = AF_INET;
+        }
+    }
+
     if (res <= 0 && IS_USED(MODULE_GCOAP_DNS)) {
         res = gcoap_dns_query(domain_name, addr_out, family);
     }