Prefer AF_INET results on IPv4-only systems per @thomasvincent suggestion.

On IPv4-only systems, getaddrinfo() can still return AF_INET6 mapped
addresses (e.g. ::ffff:127.0.0.1). The code would format these as
udp6:[addr] peername that Net-SNMP cannot connect to.

When IPv6 is not explicitly requested (no bracket notation), prefer
AF_INET results. If no suitable address is found, fall back to the
original hostname and let Net-SNMP resolve it itself, matching the
behavior of snmprealwalk and other Net-SNMP CLI tools.

Author: devnexen

ext/snmp/snmp.c

@@ -886,6 +886,10 @@ static bool netsnmp_session_init(php_snmp_session **session_p, int version, zend
 		return false;
 	}
 
+	/* Save hostname before clearing peername, since host_ptr may point into the same buffer */
+	char saved_hostname[MAX_NAME_LEN];
+	strlcpy(saved_hostname, host_ptr, MAX_NAME_LEN);
+
 	/* we have everything we need in psal, flush peername and fill it properly */
 	*(session->peername) = '\0';
 	res = psal;
@@ -896,6 +900,13 @@ static bool netsnmp_session_init(php_snmp_session **session_p, int version, zend
 			res++;
 			continue;
 		}
+		/* When IPv6 is not explicitly requested, prefer IPv4 results to avoid
+		   issues on systems where getaddrinfo() returns AF_INET6 mapped addresses
+		   but IPv6 networking is not actually available */
+		if (!force_ipv6 && (*res)->sa_family == AF_INET6) {
+			res++;
+			continue;
+		}
 		if ((*res)->sa_family == AF_INET6) {
 			if (inet_ntop((*res)->sa_family, &(((struct sockaddr_in6*)(*res))->sin6_addr), name, sizeof(name))) {
 				snprintf(pptr, MAX_NAME_LEN, "udp6:[%s]", name);
@@ -910,13 +921,11 @@ static bool netsnmp_session_init(php_snmp_session **session_p, int version, zend
 		break;
 	}
 
-	if (strlen(session->peername) == 0) {
-		php_error_docref(NULL, E_WARNING, "Unknown failure while resolving '%s'", ZSTR_VAL(hostname));
-		return false;
+	/* If no suitable address was found, fall back to the original hostname
+	   and let Net-SNMP resolve it (matches behavior of snmprealwalk et al.) */
+	if (session->peername[0] == '\0') {
+		strlcpy(session->peername, saved_hostname, MAX_NAME_LEN);
 	}
-	/* XXX FIXME
-		There should be check for non-empty session->peername!
-	*/
 
 	/* put back non-standard SNMP port */
 	if (remote_port != SNMP_PORT) {