In DHCP servers you can configure IP address reservations, meaning that you statically configure the IP addresses that you want the specific DHCP clients to get. This is sometimes desirable when you know your devices and want to ensure that their DHCP-assigned IP addresses won’t change, because of, you know, reasons. (Printers may or may […]
What has this SHA-384 to do with AES-256-GCM? I thought GCM contained integrity checks already, so why is SHA-384 there anyway? With AES-CBC (= non-GCM) in IKEv2 phase 1, you have to use an authentication (hashing) algorithm that ensures the integrity of the data exchange. Nowadays the common ones are SHA-256, SHA-384 and SHA-512. So, […]
Following my earlier post about introduction to DHCPv6, let’s see how DHCPv6 relaying works in practice. In VLAN 60 there is the DHCPv6 client but no DHCPv6 server. The subnet-attached router is configured as DHCPv6 relay, using the DHCPv6 server address from VLAN 41. The components in this setup are: I’ll capture the traffic on […]
Dynamic Host Configuration Protocol for IPv6 (DHCPv6) is just that: a protocol that can be used for host configuration, defined in RFC 8415. In this post I will introduce some basics of the protocol. DHCPv6 protocol A DHCPv6 message for client-server conversation consists of message type (1 byte), transaction ID (3 bytes) and variable number […]
In my earlier posts I have shown the Microsoft Windows DHCP server failover configuration behavior where the servers independently decide who will answer to which DHCP clients, using a hashing algorithm based on client hardware address field (usually containing MAC address) in the DHCP packets. In this post I will show how the failover system […]
One of the built-in security features in Zabbix is TLS (Transport Layer Security) support for external connections. This means that when your distributed Zabbix proxies or Zabbix agents connect to the Zabbix server (or vice versa), TLS can be used to encrypt all the connections. When the connections are encrypted, third parties cannot read the […]
In a network I was faced with exceptionally large number of IP address DHCP leases for short-time usage. Even though the lease duration was set as relatively short, the DHCP server still occasionally complained about the DHCP scope being full, being unable to assign an IP address to a client. Let’s demonstrate the situation with […]
A user has a Zabbix agent that collects the used disk space information on a host. The item interval is one minute: However, the user complains that Zabbix fails to collect to data appropriately as the graph has empty areas with occasional dots: In Zabbix implementations with very high NVPS (new values per second) this […]
Interesting question: Why does Windows DHCP server first assign me a lease for only one hour, and after that extend it to one day (or whatever was actually configured in the DHCP scope)? First, let’s see that it really happens as described. Replicating the setup from my previous DHCP posts, I have here two VLANs: […]
This is part three of the DHCP relay mini-series: In this setup we still have two DHCP servers in load balancing configuration, but we also have two routers in the DHCP client subnet acting as DHCP relays. The routers are running HSRP (Hot Standby Routing Protocol) for first-hop routing redundancy, but it’s really not relevant […]