Uly.me

cloud engineer

GCP SDK Firewall Update

Here’s how to update an existing GCP firewall.

Ingress

gcloud compute firewall-rules update "firewall-rule-name" \
--description="firewall description" \
--priority="1000"
--target-service-accounts="service-account@gserviceaccount.com" \
--source-ranges="10.0.0.0/8"
--rules tcp:80,tcp:443,udp:1000-1100

Egress

gcloud compute firewall-rules update "firewall-rule-name" \
--description="firewall description" \
--priority="1000"
--target-service-accounts="service-account@gserviceaccount.com" \
--destination-ranges="10.0.0.0/8"
--rules tcp:80,tcp:443,udp:1000-1100

GCP Setup NLB

Here’s how to setup a Network Load Balancer in GCP.

Setup your instances.

# Instance 1
gcloud compute instances create www1 \
  --image-family debian-9 \
  --image-project debian-cloud \
  --zone us-central1-b \
  --tags network-lb-tag \
  --metadata startup-script="#! /bin/bash
    sudo apt-get update
    sudo apt-get install apache2 -y
    sudo service apache2 restart
    echo '<!doctype html><html><body><h1>www1</h1></body></html>' | tee /var/www/html/index.html
    EOF"
# Instance 2
gcloud compute instances create www2 \
  --image-family debian-9 \
  --image-project debian-cloud \
  --zone us-central1-b \
  --tags network-lb-tag \
  --metadata startup-script="#! /bin/bash
    sudo apt-get update
    sudo apt-get install apache2 -y
    sudo service apache2 restart
    echo '<!doctype html><html><body><h1>www2</h1></body></html>' | tee /var/www/html/index.html
    EOF"
# Instance 3
gcloud compute instances create www3 \
  --image-family debian-9 \
  --image-project debian-cloud \
  --zone us-central1-b \
  --tags network-lb-tag \
  --metadata startup-script="#! /bin/bash
    sudo apt-get update
    sudo apt-get install apache2 -y
    sudo service apache2 restart
    echo '<!doctype html><html><body><h1>www3</h1></body></html>' | tee /var/www/html/index.html
    EOF"

Create a firewall to allow external traffic to reach port 80.

gcloud compute firewall-rules create www-firewall-network-lb \
    --target-tags network-lb-tag --allow tcp:80

Configure your network load balancer.

# Create an external IP address.
gcloud compute addresses create network-lb-ip-1 \
    --region us-central1
# Add a legacy HTTP health check.
gcloud compute http-health-checks create basic-check
# Add a target pool.
gcloud compute target-pools add-instances www-pool \
    --instances www1,www2,www3 \
    --instances-zone us-central1-b
# Add a forwarding rule.
gcloud compute forwarding-rules create www-rule \
    --region us-central1 \
    --ports 80 \
    --address network-lb-ip-1 \
    --target-pool www-pool
# Lookup external IP address.
gcloud compute forwarding-rules describe www-rule \
    --region us-central1

Finally, use the curl command to send traffic to the NLB external IP address.

while true; do curl -m1 [IP_ADDRESS]; done

Tcpdump

Tcpdump is a command line utility that allows you to capture and analyze network traffic going through your system. It is often used to help troubleshoot network issues. 

# Find out if tcpdump is installed
$ which tcpdump
/usr/sbin/tcpdump
 
# Install tcpdump
sudo yum install -y tcpdump
 
# Find which interface is available to you
tcpdump -D
 
# Capture on eth0. Use Ctrl-C to end capture.
tcpdump -i eth0
 
# Capture after 10 packets
tcpdump -i eth0 -c10
 
# Filter by port
tcpdump -i any -c10 -nn port 80
 
# Filter by ip address
tcpdump -i any -c10 -nn host 192.168.1.23
 
# Filter by source or destination ip
tcpdump -i any -c10 -nn src 192.168.1.23
tcpdump -i any -c10 -nn dst 192.168.1.23
 
# Filter by destination ip and port
tcpdump -i any -c5 -nn src 192.168.1.23 and port 80
 
# Save output to a file (binary format)
tcpdump -i any -c10 -nn -w http.pcap port 80
 
# Save output to a file (text format)
tcpdump -nn -r http.pcap

Here’s a good intro article about tcpdump.