Virtual Services???

[Stanley Hopcroft]

Dear Sir,

If you mean a 'virtual host' in the sense of 'vip' or 'virtual server' 
used by Load Balancer vendors such as Foundry Networks (with the 
ServerIron product) and you want to check that there is a sufficient 
number of 'Real Servers' available then I think the approaches are

1 Set up Nag to receive SNMP traps from the load balancer and process 
those relating to 'Real Servers' (probably port unreachable traps)

This is a bit hard to do since the process that handles these traps has 
to record previous 'real server' down traps.

2 Use check_snmp in a creative way to check the status of 'Real Servers' 
by polling the load balancer for the 'Real Server' availability

This assumes that check_snmp will take the OIDs corresponding to the Ok 
state of _all_ the real servers.

3 a custom check that checks all the real servers and reacts to the
number that are Ok (your custom check program would accept a variable
number of arguments specifying the real server names/addresses eg
check_virt_server -s real_1 -s real_2 -s real_3 ...)

Here is a diagram of the situation your words suggest to me.




------- LB           --- Real Server Real IP 1
        Virtual IP
                     --- Real Server Real IP 2 ...

ie the LB accepts requests for the service on address VIP and farms them 
out (according to some method such as 'round robin', 'least loaded' etc) 
to the duplicate Real servers (which may be directly connected to the LB 
[because the LB is also a L2 switch] or not (if the LB does source NAT).

I beg your pardon if I have missed the point.

Yours sincerely.


-- 
------------------------------------------------------------------------
Stanley Hopcroft
------------------------------------------------------------------------

'...No man is an island, entire of itself; every man is a piece of the
continent, a part of the main. If a clod be washed away by the sea,
Europe is the less, as well as if a promontory were, as well as if a
manor of thy friend's or of thine own were. Any man's death diminishes
me, because I am involved in mankind; and therefore never send to know
for whom the bell tolls; it tolls for thee...'

from Meditation 17, J Donne.


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