[Xymon] Regional Servers to Central

[Jeremy Laidman]

On Tue, May 3, 2016 at 10:19 PM Thomas Eckert <thomas.eckert at it-eckert.de>
wrote:

> On 03 May 2016, at 08:46, Adam Goryachev <
> mailinglists at websitemanagers.com.au> wrote:
>
> On 03/05/16 12:02, Jeremy Laidman wrote:
>
> On Sun, Apr 24, 2016 at 5:44 PM Thomas Eckert <thomas.eckert at it-eckert.de>
> wrote:
>
>> Galen,
>>
>> if egress SSH is allowed you could use an SSH tunnel from the central to
>> the regional Servers opening say port 1985. Then use that for the
>> communication between the Xymon servers.
>>
>> This will weaken the intentions of the security policy of course ...
>>
>> The tunnel can be managed by the ssh-tunnels extension by Padraig Lennon
>> (on Xymonton) or my slightly extended version (on
>> http://www.it-eckert.com/software/patches/ssh-tunnel/). There are also
>> some blog posts on my site on setup and combining with xymonproxy.
>>
> Sorry to be so late to the scene.  I have a similar requirement, but don't
> quite get the proposed solutions.  I have a couple of headless "probe"
> Xymon servers located on less trusted networks, and a pair of Xymon servers
> that primarily probe devices (testing TCP services, ping checks etc) on our
> internal network.  I want to be able to view the results of the probe
> servers on the internal server screens.  I can't have the probe servers
> connect inbound to the internal Xymon servers, except perhaps via ssh
> tunnel.
>
> JC suggested that xymonfetch could be run on the regional (probe) servers
> to send in to the internal servers.  I haven't used xymonfetch before, so
> I'm not intimately familiar with how it's used.  Nevertheless, in reading
> the documentation I can see that xymonfetch is intended to run on a Xymon
> server to connect to a Xymon client that has msgcache running.  This
> doesn't seem to be the model described by JC, where msgcache isn't
> mentioned.  Or am I misunderstanding something?
>
> Perhaps relevant to a solution, the Xymon probe servers periodically
> connect to the Xymon clients over ssh, create a reverse tunnel, and run
> xymonclient.sh with suitable environment variables to push the client data
> through the reverse tunnel.
>
> Perhaps what I need to do is something like this.  I fire up a msgcache on
> each probe server, having everything feed into the msgcache instead of
> xymond, and I periodically run xymonfetch on each probe server to push the
> messages into the real xymond running there.  (I'd probably have xymond
> listen on an alternative port, and have msgcache run on 1984.)  I would
> then have the ability to run a second instance of xymonfetch on the probe
> servers, but being called via ssh from each internal server, complete with
> a reverse tunnel, so that the xymonfetch would inject into the xymond on
> the internal server.  What I can't figure out here is how to allow a
> msgcache queue to be pushed into the probe server's xymond without being
> emptied and hence unavailable for the internal xymon server.
>
>
>
> I would guess you can pass messages to xymonproxy, which will pass
> messages to the local xymon server plus xymonmsgcache, then the remote
> server will connect with ssh and collect the pending messages from the
> xymonmsgcache?
>
> I hope that is what you were asking about.
>
> Yep, that sort of thing.

> I’ve only used `msgcache` on a single client up to now (for isolated
> hosts).
> But I _seem_ to remember that there are limits on the amount of messages
> it can cache (beside other drawbacks, listed here <
> http://www.it-eckert.com/blog/2014/remote-site-monitoring-with-ssh-tunnel/
> >).
>

That's a great point.  Yes, I remember reading about the limitations of
msgcache, now that you mention it.


> As I understand the intended use is
>
> Xymon (central) — Xymon (region1) — multiple clients region1
>                           |
>                           — Xymon (region2) — multiple clients region2
>                           |
>                           — …
>

Yes.


> This would lead to a potential huge number of messages on the regional
> Xymon servers that might well go beyond the capacity of `msgcache`.
>

Agreed.


> Using `xymonproxy` on the regional servers would allow to deliver the
> status-messages to the (local) regional server _and_ the central one.
>

Makes perfect sense.


> This is outlined here <
> http://www.it-eckert.com/blog/2014/combine-ssh-tunnel-with-xymonproxy/>
> (the "remote-datacenter” would be a regional server). To have the data
> available on the regional server as well the `xymond` there has to listen
> on say `127.0.0.1:1986` and xymonproxy report to that location as well
> (xymonproxy-option for sending to multiple servers
> `--server=SERVERIP[:PORT][,SERVER2IP[:PORT]]` — according to
> `xymonproxy(8)` up to 3 servers are possible, configuration it pulled from
> the _last_ in the list!). The order of the xymonproxy “receivers” would
> also allow the configuration of the regions to be either from central or
> from the regional server.
>

I think what's missing from this plan is that the xymonnet probes need to
be diverted to the xymonproxy instance also.  But apart from that, I can't
see why it wouldn't work for me.

The ssh-tunnel is handled by the central server using the `ssh-tunnel`
> extension (either original <
> https://wiki.xymonton.org/doku.php/addons:ssh_tunnel> or extended version
> <http://www.it-eckert.com/software/patches/ssh-tunnel/>) that takes care
> the tunnel is up.
>

I'm not sure why, but I prefer to have transient ssh tunnels for the
duration of the message transfer, rather than persistent tunnels that hang
around forever.  (Either way, ssh encryption and authentication is
provided.)  It looks like xymonproxy has a buffer, and so it may be able to
queue messages destined for the central server for a short period of time,
and the --lqueue size can be adjusted to manage this.  Although I would be
more comfortable if it had the ability to save its queue to disk so as to
survive restarts, and also to allow for a much larger queue.

If xymonproxy offers the same queueing capability as msgcache, without the
shortcomings, then it seems like an easy choice to make.  I just don't know
if it can queue things for long enough if my central server "checks in"
every 5 minutes, instead of having a persistent tunnel.  Come to think of
it, I probably need to know what happens to the queue if a persistent
tunnel goes down for a period of time (eg a firewall crashes and the link
goes down for 30 minutes).  Will the extra messages overflow?  What would
happen if a red-to-green transition was lost?

J
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