Re-introduce bad_port_check(), which upstream dropped between 2.3.13 and 2.3.14

for it having been "rather antiquated for years", with no justification given

for that claim.

--- xinetd/xinetd/builtins.c

+++ xinetd/xinetd/builtins.c

@@ -52,6 +52,7 @@ static void dgram_daytime(const struct server *) ;

 static void stream_chargen(const struct server *) ;

 static void dgram_chargen(const struct server *) ;

 static void tcpmux_handler(const struct server *) ;

+static int bad_port_check(const union xsockaddr *, const char *);

 /*

  * SG - This is the call sequence to get to a built-in service

@@ -163,6 +164,25 @@ static void stream_echo( const struct server *serp )

       Sclose(descriptor);

 }

+/* For internal UDP services, make sure we don't respond to our ports

+ * on other servers and to low ports of other services (such as DNS).

+ * This can cause looping.

+ */

+static int bad_port_check( const union xsockaddr *sa, const char *func )

+{

+   uint16_t port = 0;

+

+   port = ntohs( xaddrport( sa ) );

+

+   if ( port < 1024 ) {

+      msg(LOG_WARNING, func,

+         "Possible Denial of Service attack from/against %s:%u (likely spoofed)", xaddrname(sa), (unsigned int)port);

+      return (-1);

+   }

+

+   return (0);

+}

+

 static void dgram_echo( const struct server *serp )

 {

    char            buf[ DATAGRAM_SIZE ] ;

@@ -178,6 +198,7 @@ static void dgram_echo( const struct server *serp )

    cc = recvfrom( descriptor, buf, sizeof( buf ), 0, (struct sockaddr *)( &lsin ), &sin_len ) ;

    if ( cc != (ssize_t)-1 ) {

+      if( bad_port_check(&lsin, "dgram_echo") != 0 ) return;

       (void) sendto( descriptor, buf, (size_t)cc, 0, SA( &lsin ), sizeof( lsin ) ) ;

    }

 }

@@ -303,6 +324,8 @@ static void dgram_daytime( const struct server *serp )

    if ( val == (ssize_t)-1 )

       return ;

+   if( bad_port_check(&lsin, "dgram_daytime") != 0 ) return;

+

    daytime_protocol( time_buf, &buflen ) ;

    (void) sendto( descriptor, time_buf, buflen, 0, SA(&lsin), sizeof( lsin ) ) ;

@@ -369,6 +392,8 @@ static void dgram_time( const struct server *serp )

    if ( val == (ssize_t)-1 )

       return ;

+   if( bad_port_check(&lsin, "dgram_time") != 0 ) return;

+

    time_protocol( time_buf ) ;

    (void) sendto( fd, (char *) time_buf, 4, 0, SA( &lsin ), sin_len ) ;

 }

@@ -476,6 +501,8 @@ static void dgram_chargen( const struct server *serp )

    if ( val == (ssize_t)-1 )

       return ;

+   if( bad_port_check(&lsin, "dgram_chargen") != 0 ) return;

+

 #if BUFFER_SIZE < LINE_LENGTH+2

    bad_variable = 1 ;      /* this will cause a compilation error */

 #endif