Many system administrators intentionally remove telnet from production systems due to security concerns. The telnet protocol transmits data in plaintext, making it vulnerable to eavesdropping. Modern security practices recommend using encrypted alternatives like SSH.

CentOS provides several native tools that can serve as telnet alternatives:

# Using netcat (nc)
nc -zv target_host 22

# Using bash built-ins
timeout 1 bash -c "cat < /dev/null > /dev/tcp/target_host/port"

# Using curl for HTTP/HTTPS services
curl -I http://target_host:port
curl -I https://target_host:port --insecure

If you have sudo privileges, you can install more robust tools:

# Install netcat (often preferred over telnet)
sudo yum install nc

# Install nmap for advanced port scanning
sudo yum install nmap
nmap -p 22,80,443 target_host

# Install socat for complex connections
sudo yum install socat

For environments where you can't install packages, Python provides excellent alternatives:

# Simple port checker
python -c "import socket; s = socket.socket(socket.AF_INET, socket.SOCK_STREAM); print(s.connect_ex(('target_host', 22)))"

# Persistent connection test
import socket
with socket.create_connection(('target_host', 22), timeout=2) as sock:
    print(sock.recv(1024))

Remember that failed connections might indicate firewall rules rather than actual service availability. Check these before troubleshooting further:

# Check local firewall rules
sudo iptables -L -n

# Check connection tracking
sudo conntrack -L

Many system administrators intentionally remove telnet from production systems due to security concerns. While this is a wise security practice, it leaves us needing alternative ways to test basic TCP connectivity between servers. Here are several reliable methods you can use on CentOS when telnet isn't available.

Netcat is often called the "Swiss Army knife" of networking tools. Most CentOS systems have it installed by default:

# Test connection to machine B on port 80
nc -zv machineB.example.com 80

# For multiple ports
for port in {80,443,22}; do nc -zv machineB.example.com $port; done

The -z flag sets zero-I/O mode (just scan), while -v enables verbose output.

Modern bash includes built-in network capabilities through /dev/tcp and /dev/udp:

# Test TCP connection
timeout 1 bash -c "</dev/tcp/machineB.example.com/80" && echo "Port open" || echo "Port closed"

# More detailed version
if timeout 1 bash -c "cat </dev/null >/dev/tcp/machineB.example.com/3306"; then
    echo "MySQL port is accessible"
else
    echo "Connection failed"
fi

If you have SSH access but want to verify other ports:

# Test if you can reach port 5432 through SSH
ssh -T -o ConnectTimeout=5 -o BatchMode=yes user@machineB.example.com -p 22 "echo 2>/dev/null >/dev/tcp/localhost/5432 && echo PostgreSQL port open || echo Port closed"

For web services, cURL provides excellent debugging capabilities:

# Basic HTTP test
curl -I http://machineB.example.com:8080

# More detailed HTTP test with timeout
curl --connect-timeout 3 -v http://machineB.example.com:8080/api/health

Most CentOS systems have Python installed. Here's a quick connectivity test:

python -c "import socket; s = socket.socket(); s.settimeout(3); print('Port open' if s.connect_ex(('machineB.example.com', 3306)) == 0 else 'Port closed')"

If nmap is available, it's excellent for comprehensive port testing:

# Basic port scan
nmap -Pn -p 80,443,22 machineB.example.com

# Fast scan with service detection
nmap -T4 -F -sV machineB.example.com

For teams that regularly need to test connectivity, consider these permanent solutions:

• Install a minimal netcat package: yum install nmap-ncat
• Set up a dedicated monitoring container with all needed tools
• Implement proper service health checks using HTTP endpoints