Re: Feature request: UDP generic protocol testing

[Jan-Henrik Haukeland]

On 22. okt. 2005, at 03.43, Alex Black wrote:

> There are a number of things I'd like to test for using Monit that  
> answer on UDP, for example SIP and IAX (voip protocols).  Could  
> functionality be added so send/expect can be used with UDP sockets  
> as well as TCP stream sockets?

I have studied this and have one question. Are you sure that send/ 
expect does not work over UDP?

The reason I ask is that send/expect should in principle work with  
UDP sockets. I thought it would not, but after reading up on Stevens  
(again) there is nothing in monits code that prevent send/expect to  
work with UDP. I have even written a simple echo-server to test UDP  
with monit and it works perfectly (admittedly it is a simple test).


Technical discussion:

1) We call connect on all sockets, also UDP sockets. This allow us to  
use write/read on UDP as well as TCP sockets. I.e. make i/o  
operations transparent which we use in the socket.c and net.c code.

2) We do not have logic to a) resend lost UDP packets nor b) sequence  
number testing to assembly packets in order when we read. These  
shortcomings makes send/expect over UDP unreliable.

3) Adding reliability to UDP sockets require support for both a) and  
b). This means quite a bit of recoding and redesign in monit's net.c  
and socket.c code. It's possible of course and if anyone would like  
to provide a patch it would be very welcome. I really need to use  
most of my free time now on m/monit.

4) However if you follow these recommendations then testing an UDP  
server may work.

 - Use a send then expect pattern. That is, for each send call expect.

 - Make sure that you do not send or that the server send more text  
than can be
   put into an IP packet. The Ethernet Frame max out at around 1500  
bytes. In case
   of problems, limit the test to only one send/expect pair and send  
max a few
   hundred bytes and hope the server does the same.

 - Since packets may be lost, use the new fail tolerant mode in  
monit version
   4.6 to allow some failure before raising an error. For example,

check host foo with address foo.baz
      if failed port 9877 type udp
           send "Hello World!\r\n"
           expect "Hello World*"
           send "Hello Norway\r\n"
           expect "Hello*"
           for 5 times within 8 cycles # Fail tolerant
      then alert

Does this work?

--
Jan-Henrik Haukeland
Mobil +47 97141255