Interpreting DHCP related message from rsyslog
Solution 1:
An XMT Solicit is basically a DHCPv6 (IPv6 DHCP) request. If you're not using IPv6 at all disable it because your instance is trying to request an IPv6-IP.
The problem seems to be that your instance is running an older version of dhclient which is logging with log-level "normal". Normally dhclient should only log from the log-level "warning" and above.
You could also disable logging messages for dclient and ec2net by adding this code
:programname,isequal,"dhclient" ~
:programname,isequal,"ec2net" ~
to a new file (e.g. "dhclient") under /etc/rsyslog.d/.
See more here: https://en.wikipedia.org/wiki/DHCPv6#Example