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cloud engineer

network

Nmap

Here’s how to list all the devices on your network using nmap.

nmap -sn 192.168.0.0/24

If nmap is not installed, install it.

yum install nmap
apt install nmap

Netstat

Netstat is a utility for gathering network statistics. It comes preinstalled on most systems.

If missing, here’s how to install.

# Redhat, CentOS
yum install net-tools
# Debian, Ubuntu, Mint
apt install net-tools
# SLES, SUSE
zypper install net-tools

Here are some common and useful commands.

# show version
netstat -v
# show routing table
netstat -nr
# show network interface statistics
netstat -ai
# show network connections
netstat -ant
# show network services
netstat -pnltu

GCP ILB Issue Targets Not Healthy

I ran into an issue with Google Compute Engine TCP internal load balancer. The targets are unhealthy although all configs were correct installed. At one time the targets were working, but somehow they became unhealthy. In the end, 3 things needed to be checked.

  1. Make sure GCP’s guest environment is installed.
  2. Add the internal load balancer VIP address to the network properties.
  3. Add route via cmd. “route ADD VIP MASK 255.255.255.255”

You will need to change the server’s IP from DHCP to static to add ILB VIP.

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.