Category: DHCP

Chinar Trivedi asked an interesting question about DHCP relaying in VXLAN/EVPN world on Twitter and my first thought was “that shouldn’t be hard” but when I read the first answer that turned into “wait a minute, how exactly does DHCP relaying works?”

I’m positive there’s a tutorial out there somewhere, but I decided to go back to the sources of wisdom: the RFCs. It turned out to be a long walk down the IETF history lane.

I was happily munching popcorn while watching the latest season of Lack of DHCPv6 on Android soap opera on v6ops mailing list when one of the lead actors trying to justify the current state of affairs with a technical argument quoted an RFC to prove his rightful indignation with DHCPv6 and the decision not to implement it in Android:

[…not having multiple IPv6 addresses per interface…] is also harmful for a variety of reasons, and for general purpose devices, it’s not recommended by the IETF. That’s exactly what RFC 7934 is about - explaining why it’s harmful.

In case you missed it, there’s a new season of Lack of DHCPv6 on Android soap opera on v6ops mailing list. Before going into the juicy details, I wanted to look at the big picture: why would anyone care about lack of DHCPv6 on Android?

The requirements for DHCPv6-based address allocation come primarily from enterprise environments facing legal/compliance/other layer 8-10 reasons to implement policy (are you allowed to use the network), control (we want to decide who uses the network) and attribution (if something bad happens, we want to know who did it).

Last week we explored the basics of unnumbered IPv4 Ethernet interfaces, and how you could use them to save IPv4 address space in routed access networks. I also mentioned that you could simplify the head-end router configuration if you’re using DHCP instead of per-host static routes.

Obviously you’d need a smart DHCP server/relay implementation to make this work. Simplistic local DHCP server would allocate an IP address to a client requesting one, send a response and move on. Likewise, a DHCP relay would forward a DHCP request to a remote DHCP server (adding enough information to allow the DHCP server to select the desired DHCP pool) and forward its response to the client.

Elisa Jasinska covered several IPAMs in her overview of open-source network automation tools, and we had Jeremy Stretch talking about NetBox in the Building Network Automation Solutions online course, but if you’re looking for a really robust easy-to-implement solution, check out this document from 1998 (deployment experience, including a large-scale one).

If you’ve been a networking engineer (or a sysadmin) for a few years, you must be pretty familiar with DHCP and might think you know everything there is to know about this venerable protocol. So did I… until I read the article by Chris Marget in which he answers two interesting questions:

• How does the DHCP server (or relay) send DHCP offer to the client that doesn’t have an IP address (and doesn’t respond to ARP)?
• How does the DHCP client receive the DHCP responses if it doesn’t have an IP address?

Martin Bernier has decided to open another can of IPv6 worms: how do you address multiple subnets in a very typical setup where you use a firewall (example: ASA) to connect a SMB network to the outside world?

The murky details of IPv6 implementations never crop up till you start deploying it (or, as Randy Bush recently wrote: “it is cheering to see that the ipv6 ivory tower still stands despite years of attack by reality”).

Here’s another one: in theory the prefixes delegated through DHCPv6 should be static and permanently assigned to the customers for long periods of time.

Jernej Horvat sent me the following question:

I know DHCPv6-based prefix delegation should be as stable as possible, so I plan to include the delegated prefix in my RADIUS database. However, for legacy reasons each username can have up to four concurrent PPPoE sessions. How will that work with DHCPv6 IA_PD?

Short answer: worst case, DHCPv6 prefix delegation will be royally broken.

Years ago the IT of the organization I worked for assigned a /28 to my home office. It seemed enough; after all, who would ever have more than ~10 IP hosts at home (or more than four computers at a site).

When the number of Linux hosts and iGadgets started to grow, I occasionally ran out of IPv4 addresses, but managed to kludge my way around the problem by reducing DHCP lease time. However, when the start of school holidays coincided with the first snow storm of the season (so all the kids used their gadgets simultaneously) it was time to act.