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Therefore I wrote this small article to create some more awareness of the existence of the anti DoS module and hopefully thorugh it help some of my readers to strengthen their server security.
DOSPageCount 30 – Add IP in evasive temporary blacklist if a request for any IP that hits the same page 30 consequential times.
DOSSiteInterval 1 – Interval of 1 second in which if DOSSiteCount of 40 is matched the matched IP will be blacklisted for configured period of time. Using the DOSSystemCommand in relation with iptables, could be configured to filter out any IP addresses which are found to be matching the configured mod-evasive rules.
I decided not to log mod-evasive DoS IP matches as this will just add some extra load on the server, however in debugging some mistakenly blacklisted IPs logging is sure a must. At the company where I administrate some servers, they’re running Nagios to keep track of the servers status and instantly report if problems with connectivity to certain servers occurs. Now one of the servers which had configured UP host checks is up, but because of heavy ICMP denial of service attacks to the servers the ICMP protocol ping is completely disabled. In Nagios this host was constantly showing as DOWN in the usual red color, so nagios reported issue even though all services on the client are running fine.
As this is quite annoying, I checked if Nagios supports host checking without doing the ICMP ping test.
That’s all now Nagious should start checking the down host by doing a query if the webserver on port 80 is up and running instead of pinging it. In my quest for sysctl ?? I found a few more handy sysctl variables apart from the old ones I incorporate on every Linux server I adminstrate. I have been doing a bunch of stuff around disaster recovery (DR) recently, and my storage of choice at both the production site and the recovery site has been VSAN, VMware Virtual SAN. I was fortunate in that I had a number of other vCenter servers around my lab, so I immediately added the 3 ESXi hosts that formed my VSAN cluster to a different vCenter. The command reported a whole bunch of components that were inaccessible, just like we’d observed in the UI.
Now that the cluster was recreated on the new vCenter, I went to theĀ  VSAN General tab, and under Network status, we saw the dreaded Misconfiguration detected message.
So I next examined the multicast setting used by my VSAN Cluster using the esxcli vsan network list command.


We couldn’t see any change initially, so we simply ran some ping commands once again to the VSAN VMkernel ports, and suddenly we could see the correct ESXi hosts in the tcpdump-uw output. Hopefully you will find those command useful when you have to troubleshoot VSAN network issues. If you are interested in getting started with VSAN, the Essential VSAN book is now available via both Amazon and Pearsons. This entry was posted in Cluster, network, Replication, software, Storage, VMDK, VMware, VMworld, VSAN, vSphere, vSphere Replication and tagged deep dive, multicast, RVC, troubleshooting by Cormac. What happens when the other cluster changes its multi cast address another collision occurs? Maybe I’m confused but in your book, I remember it mentioning that if running multiple VSAN clusters, you have to turn on IGMP snooping and setup multiple multicast groups so that the multicast traffic is separated. Cool post, just for my own interest do we plan to allow the end user to have a choice using multicast or unicast in the future ? Would be great having a comparison between both if this was actually implemented in the future. The result in a scenario with a infected botnet running a DoS tool in most of the cases will be a quick exhaustion of system resources available (bandwidth, server memory and processor consumption). I have already done a number of tests already with products like vCenter Server, vCenter Operations Manager and NSX, our network virtualization product. Before doing that I created a new cluster object in the new vCenter’s inventory and enabled VSAN, then added my hosts. We tried the –refresh_state option to this command, just to see if it could give VSAN a nudge to figure out where its components were (in case we had a massive APD (all paths down) event or something similar over the weekend).
I would also recommend watching Christian’s STO3098 session from VMworld for additional troubleshooting tips. The issue is that we deployed multiple clusters on the same VLAN, and forgot to make the multicast change.
The hosts remain part of a VSAN Cluster, so the cluster simply reforms without anything needing to be done at the disk level.
On looking at the error messages, I could see that vCenter had problems accessing it disks.


Once I added the hosts to the VSAN cluster, I was in a position to examine the storage and try to figure out what was wrong with my original vCenter server (and my other VMs). The hosts in the cluster had difficulty in communicating with each other, and in fact the cluster had partitioned. I then ran a tcpdump-uw command to monitor the heartbeats between the hosts on the agent group multicast address in the VSAN cluster.
These are the IP addresses of the VSAN network interface on ESXi hosts that are in a completely different VSAN cluster. The environment was running like this for some time, and the clusters didn’t appear to have a problem.
Once we made the change so that each cluster had its own unique multicast addresses, we were good to go.
Using esxcli vsan cluster list on the ESXi hosts, we saw that in fact none of the hosts were able to communicate, and that there was only a single host in each partition.
So we started off by pinging each VSAN traffic VMkernel port on all the hosts using the ping -I command – all were good and responding. And then we remembered KB article 2075451 which talks about setting up separate multicast addresses for VSAN cluster that might be on the same network.
However we can only assume that having the same multicast addresses across both clusters on the same network led to the issue at hand.
The problem was that my vCenter was running on VSAN (a bit of a chicken and egg type situation), so how do I troubleshoot this situation without my vCenter. The master should send this heartbeat every second and all other hosts in the VSAN cluster should see this.
Once we separated the clusters to have their own unique multicast addresses, we were good to go. I then started to use some of the very useful multicast troubleshooting commands referenced by Christian Dickmann in his 2014 VMworld presentation (STO3098).



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