Networking - Basics

🌐 Networking Basics (for Servers)

When running a node or validator, understanding basic networking is very important. This helps you solve problems related to syncing, peer connections, RPC access, and firewalls.

This page covers:

• Public & Private IP

• Ports & Port Forwarding

• Basic tools for networking

• Common network troubleshooting

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🌍 Public IP vs Private IP

• Public IP: The IP address visible to the internet. Your node uses this to communicate with other peers or to expose an RPC/API.

• Private IP: Local IP used inside a private network (like 192.168.x.x). Useful for internal-only services.

VPS servers usually get a Public IP by default. You can see it with:

curl ifconfig.me

🔢 What is a Port?

A port is like a door to your server. Each app/service runs on a different port.

Examples:

Service	Default Port
SSH	22
HTTP	80
HTTPS	443
Cosmos P2P	26656
Cosmos RPC	26657

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🎯 Port Forwarding (NAT)

Relevant mostly for home servers or local VMs, not typical VPS setups.

If you're running a node from your home or local network, your router/firewall might block outside connections.

You’ll need to:

1. Login to your router admin page

2. Go to Port Forwarding / NAT settings

3. Forward the needed port (e.g. 26656) to your local server IP

For example: Forward 26656 to 192.168.1.100

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🚦 Allowing Ports in Ubuntu Firewall (UFW)

By default, Ubuntu might block certain ports. Use ufw to allow them:

# Enable UFW
sudo ufw enable

# Allow SSH (important to avoid locking yourself out!)
sudo ufw allow 22

# Allow Cosmos P2P & RPC ports
sudo ufw allow 26656
sudo ufw allow 26657

# Check status
sudo ufw status

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🧪 Useful Networking Commands

🔍 Check open ports on your server:

sudo lsof -i -P -n | grep LISTEN

📡 Check public IP:

curl ifconfig.me

🔗 Test if a port is open (from outside):

telnet <your-server-ip> 26656

🧱 Check if a port is blocked by firewall:

nc -zv <your-server-ip> 26656

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🔄 Connecting to Peers

Most blockchain nodes need to connect to other peers. Make sure:

• Your p2p port is open

• You're not behind NAT (or have port forwarding)

• You’re advertising your correct external IP in config (e.g. external_address = "<ip>:<port>")

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📊 Monitor Who's Connecting

sudo netstat -tulnp

Or using ss:

sudo ss -tuln

To see SSH login attempts:

journalctl -u ssh

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🚫 Block Dangerous Traffic

Block unused ports and protocols. Example:

sudo ufw deny 8000
sudo ufw deny from 0.0.0.0/0 to any port 23 proto tcp

Don’t expose unnecessary services like FTP, Telnet, or open databases.