Tag Archives: IPv6 Interface Identifier

Trying to change an IPv6 Link-Local Address on a FortiGate

I got an email where someone asked whether I know how to change the link-local IPv6 addresses on a FortiGate similar to any other network/firewall devices. He could not find anything about this on the Fortinet documentation nor on Google.

Well, I could not find anything either. What’s up? It’s not new to me that you cannot really configure IPv6 on the FortiGate GUI, but even on the CLI I couldn’t find anything about changing this link-local IPv6 address from the default EUI-64 based one to a manually assigned one. Hence I opened a ticket at Fortinet. It turned out that you cannot *change* this address at all, but that you must *add* another LL address which will be used for the router advertisements (RA) after a reboot (!) of the firewall. Stupid design!

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IPv6 Interface ID Structure

While there are many approaches on how to structure your IPv6 prefix into /64 subnets (blogposts, books, talks) there are only a few hints what you can do with the other 64 bits of the addresses, namely the IPv6 interface identifier or IID. To my mind you can put some (but not too much) logic into those IIDs to a) have some structure for your addresses that b) helps you identifying those addresses when seeing them in logs or anywhere else. Hence it is easier for you to remember the IPv6 address behind a name (forward DNS) as well as the host when seeing the address (reverse DNS).

This post just shows the approach I am using in my lab. You might find it useful or you might disagree completely. Anyhow, feel free to comment your experiences or solutions for that. :D I am wondering why there isn’t much discussion about these IIDs at all. Maybe for some good reasons I am not seeing yet?

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