What is UDP?
User Datagram Protocol
UDP (User Datagram Protocol) is a simple, connectionless transport protocol that sends messages between applications with no handshake or delivery guarantee.
UDP, defined in RFC 768, adds very little to IP: an 8-byte header with a source port, a destination port, a length and a checksum, carried as IP protocol 17. There is no connection set-up, no ordering, no acknowledgement and no congestion control. A datagram either arrives whole or is lost, and the application must decide what to do about loss. That makes UDP a good fit for short request-and-reply exchanges and for traffic where late data is worthless. Common examples are DNS (port 53), DHCP (67 and 68), NTP (123), SNMP (161), syslog (514), VXLAN (4789), WireGuard and QUIC, which builds its own reliability on top of UDP. Linux traceroute sends UDP probes to high ports by default.
The usual comparison is with TCP, which provides reliable, ordered streams at the cost of a handshake and per-connection state. Routing protocols split both ways: BGP uses TCP port 179, BFD uses UDP, and OSPF uses neither, running directly over IP as protocol 89. Because UDP has no connection, firewalls and NAT devices track UDP "flows" by addresses and ports with an idle timeout, and a long-lived but quiet UDP session can be dropped from the state table, which is why tunnels such as WireGuard offer keepalives. UDP's lack of a handshake also makes spoofed sources easy, which is the basis of reflection and amplification attacks using open services like DNS and NTP. When testing UDP, remember that no reply does not mean the packet arrived. The Site-to-site VPN with WireGuard on VyOS lesson shows a UDP-based tunnel in use.
Lessons that use UDP
- Site-to-site VPN with WireGuard on VyOSIntermediate · 35 min
Join two sites' private LANs across the internet with WireGuard on VyOS: keys and peers, the AllowedIPs mistake and what it teaches, what the internet actually sees, and the tunnel's smaller MTU.
- Operating VyOS: monitoring, backups and automationIntermediate · 50 min
Everything after it works: remote syslog, SNMP counters, NetFlow, config archives, commit-confirm and Ansible, all on one VyOS router and an ops server, including three places where this VyOS build differs from older guides.
- EVPN and VXLAN: layer 2 over a routed fabricAdvanced · 45 min
Stretch a layer-2 segment across a routed leaf-spine fabric: VXLAN for the data plane, BGP EVPN for the control plane. Built with FRR: the underlay, VNIs, type-2 and type-3 routes, route targets, and the VXLAN packets on the wire.
- Linux Network ConnectivityBeginner · 80 min
Find out why a Linux machine can't reach something, layer by layer: interfaces and addresses, routes and ip route get, ARP and the neighbour table, ping and traceroute, DNS, and listening sockets — with real output from a three-host lab.
- OSPF FundamentalsBeginner · 70 min
The link-state IGP that runs inside most networks: neighbour adjacencies, LSA flooding, SPF, costs, DR/BDR, areas and LSA types, a hands-on FRR lab, and how OSPF underpins BGP.