If you've got a lot of security groups and you need Consul to connect to each - here's the brute force way to do it.

consul-sg-sync.sh

#!/bin/bash

# Make a list of all security groups - one on each line.
# Save as `security-groups.txt`

# Yes this would be easier in VPC or by launching with multiple Security Groups
# but you might not have that option and you might have to do this.

SGROUPS=$(cat security-groups.txt)

for SG in $SGROUPS;
do
  echo "===== Working on '$SG' ====="
  for ILOOP in $SGROUPS;
  do
    echo "$SG with $ILOOP"
    aws ec2 authorize-security-group-ingress --group-id $SG --protocol tcp --port 8300-8301 --source-group $ILOOP
    aws ec2 authorize-security-group-ingress --group-id $SG --protocol udp --port 8300-8301 --source-group $ILOOP
  done
done