Hacker's ramblings

One of my own boxes runs a Fedora Linux. A while back my upgrade failed miserably due to Fedora installer not getting an IP-address from my ISP, Elisa. I had a minor skirmish for an hour or so with the installer, but no avail, Fedora installer beat me on that one. As I love to have that box up and running, I gave up and decided to investigate that later. Now that day came and I'm victorious!

Basics

DHCP is what pretty much everybody has for getting an IPv4 address in 2016. Mobile connections have something different, but everything else including Wi-Fi hotspots, ADSL-routers, Fiber-to-the-Home -connections, etc. etc. issue an IP-address (mostly IPv4, sometimes IPv6) to any well-behaving customer of theirs. Today, the de-facto is that the IP-address is allocated out of a well known broadband address range, or pool. Lists of those are generally available, so that home customers can be differentiated from data centers and companies.

To put DHCP simply, it is a mechanism for allocating an unique address for your Internet connection. The Wikipedia definition for Dynamic Host Configuration Protocol uses more words and isn't as concise as mine, but you'll get the idea.

Details of the problem

In case of mis-use or unpaid internet bill, they'd naturally decline any DHCP-requests for an IP-address. Since everything I tested, including various Windowses, OS Xes and Linuxes worked it wasn't about that. The connection was ok, the DHCP server issued a valid DHCP-lease as it had been doing for couple years, but not for my Fedora installer. Duh?

At this point I went to google for the symptoms and quite soon I landed into RedHat Bugzilla. It contains bug 1154200 which is titled as "not getting a dhcp address assigned". Mr. Krovich reports that his Fedora 21 installer won't get an IP-address from ISP. I pulled up a Fedora 20 installer for the previous version. It worked ok! Yep, they changed something into Fedora 21. The change affects Fedora 22, 23 and the latest 24. It does not affect RedHat nor CentOS (yet).

Fix (aka. trial and error)

In the comments of bug 1154200 they're talking about Option 61 commit which was introduced for Fedora 21 release. A possible fix would be to use DHCP-configuration: send dhcp-client-identifier = hardware;

I downloaded Fedora 24 installer and tested it out. It didn't help any. After a lot of wiresharking the traffic, I isolated this:
In the DHCP Discover -packet, there was an Option 61 present.

More poking for man 5 dhclient-options revealed that it was possible to specify a fixed string for identifier. So, again I edited /etc/dhcp/dhclient.conf (btw. the file didn't exist to begin with, I had to create one) to contain:
send dhcp-client-identifier = "";

That did the trick! Now even Fedora 24 installer got a a valid IP-address and it was possible to install.

Specs

In the early days, all IP-addresses were assigned manually. Everybody was given an IP-address and they punched all the details manually. That was frustrating and error-prone, so somebody invented BOOTP to automate the entire setup. Quite soon, that evolved into DHCP, defined by RFC 1531. When DHCP gained traction and more and more vendors joined the dynamically allocated -game, couple of clarifying iterations later, we're at RFC 2131 for current breed of DHCPing. It is basically the original BOOTP, but with most wrinkles ironed out.

The options are defined at RFC 1533 for DHCP Options and BOOTP Vendor Extensions. Looks like nobody supported Option 61 for a long time. Windowses don't, Apple doesn't, most Linux Distros don't, but RFC 4361 for Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four (DHCPv4) must have done it for Fedora-guys. They chose to implement request 560361 (Dhclient doesn't use client-identifier; may cause issues in certain bridged environments) and make sure everybody uses it, with assumption that all ISPs that won't support Client Identifiers will merrily ignore the option. Nice!

Afterwards

I'll target equal blame to my ISP. The Option 61 is well-defined and it should be possible to ignore it. Looks like they're running Alcatel-Lucent hardware there and for some reason it is configured to spit on Option 61 requests.

Naturally I reported the error to my ISP, but you can assume how well that goes. Any regular customer facing clerk won't know DHCP or what it does, nor any options of it. So all I got back is the classic "we'll investigate" -style response. I'm not keeping my hopes up. I have more hope on my own Bugzilla request 1357469 to have an option to enable or disable usage of Option 61 on Fedora. They might even implement that one day.

Anyway, I'm hoping that this post will help somebody strugging to install their Fedora.