Latest revision as of 01:54, 1 July 2018

This tutorial is the UDP Sockets continuation of the Sockets (Intro) tutorial. If you haven't read that, please do so first before moving on to this one.

Recall that UDP is a connectionless protocol service. Because of this there are no on sockopen/sockread/sockclose events for the different stages like TCP. The basic idea is you send a message and quit, or you send a message and wait for response.

Sending A Packet[edit]

The /sockudp command allows you to send data to a specific address at a specific port destination. The syntax is:

; Sending some data
/sockudp [-kb] <handle> <ipaddress> <port> [numbytes] [text|%var|&bvar]

By default, /sockudp sends the entire data specified. The -b switch can be used to limit the amount of bytes sent.

If you are expecting some data back, the -k switch can be used to force the UDP socket to remain open. This will allow you to listen to incoming data.

Listening for Incoming Data[edit]

If you are expecting data back (I.E. if you specified the -k switch) you can listen for incoming data via the on udpread event.

on *:udpread:<handle>:{
   ; your code goes here
}

Socket Failure and More Data Sending[edit]

The on sockwrite event can be used to write additional data when the previous data is sent. Additionally, If the sockudp command fails, the on sockwrite event will trigger $sockerr set to a non-zero value.

on *:sockwrite:<handle>:{
   ; your code goes here
}

Examples[edit]

Example 1 - Time Protocol[edit]

This example will use the Time Protocol to display the current time. The Time Protocol is a very simple network protocol that provides site-independent, machine readable date and time. The protocol is defined in RFC 868.

From RFC 868:

When used via UDP the time service works as follows:

 S: Listen on port 37 (45 octal).
 U: Send an empty datagram to port 37.
 S: Receive the empty datagram.
 S: Send a datagram containing the time as a 32 bit binary number.
 U: Receive the time datagram.

 The server listens for a datagram on port 37. When a datagram
 arrives, the server returns a datagram containing the 32-bit time
 value. If the server is unable to determine the time at its site, it
 should discard the arriving datagram and make no reply.

From the instructions above you can see that the first thing we have to do is send an empty datagram to their server. On port 37. &null will hold our NULL byte.

alias getTime {
  ; NULL byte
  bset &null 1 0
  ; Time.nist.gov = 132.163.96.4
  sockudp -k getTime 132.163.96.4 37 &null
}

Let's add a single line of code to print if an error occurred

on *:sockwrite:getTime:{
  if ($sockerr) echo -a /getTime: Error: $sock($sockname).wserr - $sock($sockname).wsmsg
}

Now, all we have to do is sit and wait for the datagram response. Remember that since UDP is connectionless protocol, its header is much smaller, thus much faster (Ideal for a time protocol).

on *:udpRead:getTime: {
   ; read the reply
   sockread -f &time
 
   ; bvar to var
   var %time $bvar(&time,1,$bvar(&time,0))
 
   ; get convert to binary
   var %bin $regsubex(%time,/(\d+)/g,$base(\1,10,2,8))
 
   ; print it and close the socket
   echo -a our 32-bit time value: %bin
   sockclose $sockname
}

Let's make sense of this 32bit time value, shall we?

Once again, from the RFC 868:

The Time

 The time is the number of seconds since 00:00 (midnight) 1 January 1900
 GMT, such that the time 1 is 12:00:01 am on 1 January 1900 GMT; this
 base will serve until the year 2036.

 For example:

 the time 2,208,988,800 corresponds to 00:00 1 Jan 1970 GMT,
 2,398,291,200 corresponds to 00:00 1 Jan 1976 GMT,
 2,524,521,600 corresponds to 00:00 1 Jan 1980 GMT,
 2,629,584,000 corresponds to 00:00 1 May 1983 GMT,
 and -1,297,728,000 corresponds to 00:00 17 Nov 1858 GMT.

Since we know that 2,208,988,800 = 00:00 1 Jan 1970 GMT (Unix epoch). We can just do $calc(%time - 2208988800) to get the current Unix time. Now all we got to do is use $asctime to format it nicely.

on *:udpRead:getTime: {
  ; read the reply
  sockread -f &time
  var %time $bvar(&time,1,$bvar(&time,0))
 
  ; convert to binary, remove spaces
  var %bin $regsubex(%time, /(\d+)\s?/g, $base(\1, 10, 2, 8))
 
  ; get the current unix time in decimal system
  var %time = $base(%bin, 2, 10)
 
  ; print the time and close the socket
  echo -a Currnt Time/Date: $asctime($calc(%time - 2208988800), yyyy-mm-dd hh:nn:ss TT)
  sockclose $sockname
}

Example 2 - QOTD Protocol[edit]

In this example we will use an interesting protocol, the Quote Of The Day, RFC 865. This is a very simple protocol; you send a blank datagram, and the server responds with a quote. The hardest part was actually finding a website that still supports this protocol. (The reason most servers don't have this service enabled is because it is vulnerable to a ping-pong attack, where an attacker spoofs a server's IP (that supports QOTD protocol) and sends a request to a second server that support it, causing both server to flood each other)

Send a request:

alias getQOTD {
  ; NULL byte
  bset &null 1 0
  ; Dns resolved quotes4all.net to 85.25.143.214
  sockudp -k getQOTD 85.25.143.214 17 &null
}

Now wait for the quote:

on *:udpRead:QOTD: {
   var %Quote
   sockread -f %Quote
   echo -ea %Quote
   sockclose $sockname
}