# Essential Protocols: Quick Guide for SREs & SysAdmins

In infrastructure, you don't guess—you verify. Whether you're debugging a Kubernetes CNI or a cloud-native database, knowing the underlying protocol is the key to faster troubleshooting.

Here is a cheat sheet of the 25 most important protocols and the commands to test them directly from your terminal.

| **Protocol** | **Role in Infrastructure** | **Command to Test** |
| --- | --- | --- |
| **HTTPS** | Secure web traffic & API entry points. | `curl -I` [`https://google.com`](https://google.com) |
| **SSH** | Secure remote server management. | `ssh -v user@host` |
| **DNS** | The backbone of service discovery. | `dig +short` [`google.com`](http://google.com) |
| **TCP** | Connection-oriented, reliable delivery. | `nc -zv host 443` |
| **UDP** | Fast delivery for VoIP, VPN, & DNS. | `nc -zuv host 1194` |
| **ICMP** | Network reachability & latency testing. | `ping -c 4 8.8.8.8` |
| **BGP** | Internet-scale routing (AS path). | `mtr --aslookup 8.8.8.8` |
| **NTP** | Log & certificate synchronization. | `chronyc sources` |
| **TLS/SSL** | Encryption layer for all secure traffic. | `openssl s_client -connect host:443` |
| **DHCP** | Dynamic IP allocation & management. | `ip addr show` |
| **ARP** | IP to MAC resolution (Layer 2). | `ip neighbor` |
| **NFS** | Network file sharing (Linux/Unix). | `showmount -e host` |
| **SMB** | Network file sharing (Windows/Azure Files). | `smbclient -L //host` |
| **SFTP** | Secure file transfer over SSH. | `sftp user@host` |
| **SMTP** | Outbound mail & alert notifications. | `swaks --to` [`user@domain.com`](mailto:user@domain.com) |
| **SNMP** | Monitoring hardware & network devices. | `snmpwalk -v2c -c public host` |
| **LDAP** | Centralized user authentication. | `ldapsearch -x -h host` |
| **RDP** | Remote desktop access (Windows/VDI). | `telnet host 3389` |
| **iSCSI** | Storage over IP (SAN connectivity). | `iscsiadm -m discovery` |
| **gRPC** | High-performance microservices. | `grpcurl host:port list` |
| **Redis** | Fast caching & session management. | `redis-cli -h host ping` |
| **SQL** | Database connectivity (MySQL/PostgreSQL). | `mysql -h host -P 3306` |
| **MQTT** | IoT & event-driven messaging. | `mosquitto_pub -h host -t test` |
| **VXLAN** | Overlay networks (CNIs like Flannel/Calico). | `ip -d link show type vxlan` |
| **HTTP/2** | Modern web performance (Multiplexing). | `curl -I --http2` [`https://google.com`](https://google.com) |