Fix: Add wrapers on const values to use in hosts_ctl()

To prevent compilation warnings in snmp_agent.c and snmptrapd.c about discard 'const' qualifier:

snmp_agent.c:1082:29: warning: passing argument 2 of 'hosts_ctl' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1082 |         if (hosts_ctl(name, STRING_UNKNOWN, sbuf, STRING_UNKNOWN)) {
      |                             ^~~~~~~~~~~~~~
In file included from snmp_agent.c:94:
/usr/include/tcpd.h:131:42: note: expected 'char *' but argument is of type 'const char *'
  131 | extern int hosts_ctl(char *daemon, char *client_name, char *client_addr,
      |                                    ~~~~~~^~~~~~~~~~~
snmp_agent.c:1082:51: warning: passing argument 4 of 'hosts_ctl' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1082 |         if (hosts_ctl(name, STRING_UNKNOWN, sbuf, STRING_UNKNOWN)) {
      |                                                   ^~~~~~~~~~~~~~
/usr/include/tcpd.h:132:23: note: expected 'char *' but argument is of type 'const char *'
  132 |                 char *client_user);
      |                 ~~~~~~^~~~~~~~~~~

Author: KuzinAndrey

agent/snmp_agent.c

@@ -1037,6 +1037,11 @@ netsnmp_agent_check_packet(netsnmp_session * session,
     char *tcpudpaddr = NULL, *name;
     short not_log_connection;
 
+    /* 'char *' wrapers on 'const char *' STRING_UNKNOWN value for hosts_ctl */
+    char name_unknown[sizeof(STRING_UNKNOWN)] = STRING_UNKNOWN;
+    char addr_unknown[sizeof(STRING_UNKNOWN)] = STRING_UNKNOWN;
+    char user_unknown[sizeof(STRING_UNKNOWN)] = STRING_UNKNOWN;
+
     name = netsnmp_ds_get_string(NETSNMP_DS_LIBRARY_ID,
                                  NETSNMP_DS_LIB_APPTYPE);
 
@@ -1079,7 +1084,7 @@ netsnmp_agent_check_packet(netsnmp_session * session,
         if (xp)
             *xp = '\0';
 
-        if (hosts_ctl(name, STRING_UNKNOWN, sbuf, STRING_UNKNOWN)) {
+        if (hosts_ctl(name, name_unknown, sbuf, user_unknown)) {
             if (!not_log_connection) {
                 snmp_log(allow_severity, "Connection from %s\n", addr_string);
             }
@@ -1096,7 +1101,7 @@ netsnmp_agent_check_packet(netsnmp_session * session,
          */
         if (0 == strncmp(addr_string, "callback", 8))
             ;
-        else if (hosts_ctl(name, STRING_UNKNOWN, STRING_UNKNOWN, STRING_UNKNOWN)){
+        else if (hosts_ctl(name, name_unknown, addr_unknown, user_unknown)){
             if (!not_log_connection) {
                 snmp_log(allow_severity, "Connection from <UNKNOWN> (%s)\n", addr_string);
             };

apps/snmptrapd.c

@@ -284,6 +284,11 @@ pre_parse(netsnmp_session * session, netsnmp_transport *transport,
 {
 #ifdef NETSNMP_USE_LIBWRAP
     char *addr_string = NULL;
+    /* 'char *' wrapers on 'const char *' STRING_UNKNOWN value for hosts_ctl */
+    char name[sizeof("snmptrapd")] = "snmptrapd";
+    char name_unknown[sizeof(STRING_UNKNOWN)] = STRING_UNKNOWN;
+    char addr_unknown[sizeof(STRING_UNKNOWN)] = STRING_UNKNOWN;
+    char user_unknown[sizeof(STRING_UNKNOWN)] = STRING_UNKNOWN;
 
     if (transport != NULL && transport->f_fmtaddr != NULL) {
         /*
@@ -308,17 +313,15 @@ pre_parse(netsnmp_session * session, netsnmp_transport *transport,
         if (xp)
             *xp = '\0';
 
-        if (hosts_ctl("snmptrapd", STRING_UNKNOWN, 
-		      sbuf, STRING_UNKNOWN) == 0) {
+        if (hosts_ctl(name, name_unknown, sbuf, user_unknown) == 0) {
             DEBUGMSGTL(("snmptrapd:libwrap", "%s rejected", addr_string));
             SNMP_FREE(addr_string);
             return 0;
         }
       }
       SNMP_FREE(addr_string);
     } else {
-        if (hosts_ctl("snmptrapd", STRING_UNKNOWN,
-                      STRING_UNKNOWN, STRING_UNKNOWN) == 0) {
+        if (hosts_ctl(name, name_unknown, addr_unknown, user_unknown) == 0) {
             DEBUGMSGTL(("snmptrapd:libwrap", "[unknown] rejected"));
             return 0;
         }