[CentOS] GRE based attack?

My new Centso7 server that I am testing out is connected with a public 
address on my dmz.  I am connected to the console port. Overnight the 
following message was logged to the console:

  [44133.679108] conntrack: generic helper won't handle protocol 47. 
Please consider loading the specific helper module.

Protocol 47 is GRE.  So was this 'just' an a ping by an attacker?

No one, but I have any business on this server.  Well other than the 
files I put up for people.

thanks