The (good) deal with freebsd-update(8)

Earlier today, I stumbled across a blog post by Radu Cristian Fotescu entitled The (bad) deal with freebsd-update(8), which (as the title suggests) casts FreeBSD Update in a rather unfavourable light. Since the author is misinformed about several details, I'm taking this opportunity to set the record straight.

First, the author points out that there is an older version of FreeBSD Update available in the ports tree, which he states "can only fetch updates for FreeBSD 6.1". In fact, the version in the ports tree works for releases dating back to FreeBSD 4.7 (although it obviously doesn't provide binary updates to fix bugs which were uncovered after a release ceased to be supported by the FreeBSD Security Team). The only releases which the version of FreeBSD Update in the ports tree does not support are FreeBSD 6.2 and up -- versions of FreeBSD which contain a new (and vastly improved) version of FreeBSD Update in the base system. Once FreeBSD Update is in all supported FreeBSD releases (i.e., in June) I'll remove the old FreeBSD Update code from the ports tree.

Next, the author questions the logic of having "64-byte keys" (actually, 64 hexadecimal digit keys) as file names, and suggests that this makes FreeBSD Update overly complex. Nothing could be further from the truth: In fact, as I described in my BSDCan'07 talk, the "Reference by [SHA256] hash" method makes both FreeBSD Update and Portsnap far simpler than they would otherwise be.

The author then moves on to speaking of "a patch applied to a given release and patch level", thereby demonstrating a fundamental misunderstanding of how FreeBSD Update works. In the author's mind (apparently), to update a system from FreeBSD 6.2-RELEASE-p9 to FreeBSD 6.2-RELEASE-p10, FreeBSD Update downloads a (single) patch and applies it. Not so; rather, FreeBSD Update fetches a file which tells it what FreeBSD 6.2-RELEASE-p10 looks like. FreeBSD Update then makes the system look like that: It can leave files alone if they are already up to date (or if the user has asked it to leave those files alone); or it can download or generate the new versions of files. Put another way, in most patching systems, the server will answer the question "how do I get there from here?" -- with FreeBSD Update, the server merely answers the question "where should I be going?" and leaves it up to the FreeBSD Update client to figure out how to get there.

Related to this error is another mistake which immediately follows: The author asserts that the "full new binaries" are not available. In fact, for every file which appears in a (recent) FreeBSD release, or in a FreeBSD release plus patches, is available via the FreeBSD Update server. (I was concerned that I might be technically violating the GPL on some files by this fact, until I remembered that the FreeBSD source code is also distributed via FreeBSD Update.) FreeBSD Update uses patches in exactly the same way as Portsnap: As I described in my BSDCan'07 talk (linked above), FreeBSD Update and Portsnap rely on "opportunistic patching" -- they start out by attempting to fetch patches and apply them, but if anything goes wrong (the patch isn't available, the file generated by patching has the wrong SHA256 hash, et cetera), they gracefully fall back to fetching the complete file.

Next, the author points out that the list of binary patches used for updating to FreeBSD 6.3 is publicly visible. Oops -- this is fixed now. I don't have any desire to keep this list of file names secret, but there are two very good practical reasons for turning off the directory indexing: First, Apache processes chew up lots of RAM when generating large directory listings; and second, I was having problems with robots ignoring my "don't crawl here" directives in robots.txt and loading down my server with large numbers of pointless requests.

Moving on, the author points to the approach of RedHat, Debian, and Mandriva, of distributing entirely new package tarballs, as a model to be emulated. I don't know how fast the author's internet connection is, but I know one of the most frequent comments I hear about FreeBSD Update is how incredibly fast it is. This is what binary patches do for you -- provide a fifty-fold reduction in the bandwidth needed to download security updates. The tool I wrote for this purpose -- bsdiff -- is now used by Apple, FireFox, Sophos, and probably Amazon's Kindle (in this last case, I haven't heard from any developers, but they have bsdiff code on the device, so presumably they're using it) in addition to FreeBSD, and in the summer of 2006 I calculated that it had saved users upwards of 100 person-years of waiting for updates to download. Returning to downloading complete tarballs every time a small change is made might be simple, but it wouldn't be very popular with many people who have to wait for said tarballs to download!

Finally, the author complains that he can't find the FreeBSD Update server code. As a comment to the blog entry points out, the server code is in the FreeBSD projects repository.

Posted at 2008-02-28 08:50 | Permanent link | Comments
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