NRPE vs. check_by_ssh

[Andreas Ericsson]

Kevin Keane wrote:
> Christopher McAtackney wrote:
>> 2009/3/25 Kevin Keane <subscription at kkeane.com>:
>>   
>>> I think you are comparing apples and oranges here, because in most
>>> situations that I can think of, the decision is dictated by the network
>>> topology. If you are exclusively on a trusted private network,
>>> check_by_ssh really doesn't offer any benefits. Conversely, if your
>>> topology involves the Internet or some other untrusted network (WiFi),
>>> then you wouldn't want NRPE in the first place.
>>>
>>> The only exception to the above that I can think of is when it comes to
>>> deciding between using check_by_ssh over an untrusted network, vs. NRPE
>>> through some other kind of tunnel or VPN. But in that case, you'd incur
>>> encryption overhead either way, and the comparison is very different
>>> from the question you asked.
>>>
>>> All that said: I don't have any first-hand experience, but I suspect
>>> that the impact of establishing 2200 ssh connections in a five-minute
>>> span (assuming that you are using a five-minute check interval) is
>>> pretty substantial. The main impact actually lies in establishing and
>>> tearing down the connections, key negotiations etc.; the encryption
>>> during the data phase probably has only limited impact because most
>>> checks only transmit a few bytes back and forth.
>>>
>>> SSH does much better with longer-duration connections when the keys are
>>> already exchanged. This is even more true if you have a router-based
>>> VPN, because in that case the overhead is offloaded to a different machine.
>>>
>>> So if you have the option of sending the checks as NRPE through one or a
>>> few long-term VPNs: you are probably going to be better off. Of course,
>>> in the big picture, your mileage may vary.
>>>     
>> Firstly, thanks for the detailed explanation of the issues involved in
>> this choice Kevin, it's been very helpful.
>>
>> I'm curious though, could you elaborate on why NRPE is unsuitable if
>> communication with my remote hosts is going to go via the Internet? Is
>> it not sufficient that NRPE uses SSL? This may be more of a network
>> security question than a Nagios one, but I've no real experience in
>> either area unfortunately, so I appreciate any info you can give here.
>>   
> No, you are right. I wasn't aware that NRPE could use SSL. In that case, 
> NRPE would be pretty much the same in terms of performance as SSL.
> 
> That said, I am generally concerned from a security standpoint about any 
> kind of active checks going over the Internet. This is because if you 
> are monitoring, in your example, 200 hosts, you have to poke holes into 
> 200 firewalls (or into one firewall, and then set up SSL or SSH keys on 
> 200 hosts). That's 200 potential security holes all over the place with 
> little or no control, and on machines that may not necessarily be 
> hardened for access from the outside world. Worse - active checks, by 
> nature, cause a program to be launched and executed on the monitored 
> client, and usually with very high permissions. You said that you check 
> 2000 services, so that's 2000 plugins (give or take a few). What if a 
> hacker found a way to compromise one of your 2000 plugins? You'd have a 
> privilege escalation issue along with remote-launch capability. On 200 
> clients.
> 

Very high permissions are normally not needed. I prefer using NRPE because
of two reasons:
1. It provides a rather simple way of specifying exactly which commands
   can be run, and with which arguments (don't enable argument parsing
   in nrpe if the receiving end isn't duly protected by firewalls etc)
2. If someone breaks into the Nagios server, he or she does not get the
   public keys required for running commands on the remote servers.

> Because of these concerns, I am using passive checks almost exclusively 
> over the Internet (except for publicly available services such as HTTP 
> or SMTP, of course); I wrote an agent that resides on the client as a 
> wrapper around the excellent NSClient++ and performs the actual checks. 
> It then forwards the checks to the Nagios server via NSCA over HTTPS. A 
> second benefit is that this agent collects about 40 or so check results, 
> and then sends all of them at once through a single SSL connection. That 
> reduces the overhead of establishing a secure connection by a factor of 
> 40. BTW, the agent is available as Open Source. Go to 
> http://www.tntmonitoring.com .
> 

Sounds like a rather neat solution, although I suppose it has to be
configured in both ends before it's actually useful (although all other
agents require some configuration anyways, so perhaps it's not such a big
deal). I'm not too fond of relinquishing the re-check logic of Nagios
though, but I guess you can't get everything.

-- 
Andreas Ericsson                   andreas.ericsson at op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

Considering the successes of the wars on alcohol, poverty, drugs and
terror, I think we should give some serious thought to declaring war
on peace.

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