Use BitTorrent To Verify, Clean Up Files

jweatherley writes "I found a new (for me at least) use for BitTorrent. I had been trying to download beta 4 of the iPhone SDK for the last few days. First I downloaded the 1.5GB file from Apple's site. The download completed, but the disk image would not verify. I tried to install it anyway, but it fell over on the gcc4.2 package. Many things are cheap in India, but bandwidth is not one of them. I can't just download files > 1GB without worrying about reaching my monthly cap, and there are Doctor Who episodes to be watched. Fortunately we have uncapped hours in the night, so I downloaded it again. md5sum confirmed that the disk image differed from the previous one, but it still wouldn't verify, and fell over on gcc4.2 once more. Damn." That's not the end of the story, though — read on for a quick description of how BitTorrent saved the day in jweatherley's case.

jweatherley continues: "I wasn't having much success with Apple, so I headed off to the resurgent Demonoid. Sure enough they had a torrent of the SDK. I was going to set it up to download during the uncapped night hours, but then I had an idea. BitTorrent would be able to identify the bad chunks in the disk image I had downloaded from Apple, so I replaced the placeholder file that Azureus had created with a corrupt SDK disk image, and then reimported the torrent file. Sure enough it checked the file and declared it 99.7% complete. A few minutes later I had a valid disk image and installed the SDK. Verification and repair of corrupt files is a new use of BitTorrent for me; I thought I would share a useful way of repairing large, corrupt, but widely available, files."

Nice

Awesome idea. I've done this in the past with stuff. If a corrupt version was on one tracker, I'd save the files, get a new torrent and import the old files. Saves a lot of bandwidth wasting.

Re:Nice

Okay, I had some AVI's and a bunch of them had issues. All I did was copy them out to a different directory, then find a GOOD torrent (with the same rips) then make sure the filenames match exactly. Chucked them in the directory and voila. It checks them all and uses what data it can that you already have and replaces the rest.

Done this with RAR archived stuff as well. (Multipart rars on torrents are retarded, but that's another issue entirely.)

Re:Nice

Okay, my friend had some AVI's and a bunch of them had issues.

There, fixed it for you. You're welcome.

The first rule

The first rule of Usenet: don't talk about Usenet.

Re:The first rule

Second rule: dont mod up the person talking about the first rule.

Re:Nice

I do not know what GP meant precisely, but I had similar experience.

Some game (very old RPG) was available on Overlord and on BitTorrent. Not sold anymore. Problem was that BitTorrent had only single seed which minuscule upload speed - in several day I have downloaded only few megs. I tried then Overlord and in few days I got the game almost complete - but another snag had hit me: whether by mistake or intentionally, file was poisoned and three parts couldn't be downloaded. I was ready to throw everything away - antique games interest me little (but friend was recommending it as milestone RPG I had to play). Then suddenly I was enlightened: I fed the incomplete ISO of game to BitTorrent. BT client happily announced something like 98% of file complete and in less than one night downloaded rest of the file.

Re:Nice

I assume you then continued seeding? :)

Re:Nice

It should be quite simple. Let's say torrentA leaves you with a corrupt/incomplete filesetA (one or more files, it doesn't really matter). Let's supose torrentB contains the files in filesetA, perhaps with different names in its own filesetB.

Ok, you load torrentB in your favorite Bittorrent client, and start it up. It will automatically create 0-sized files with the names in filesetB (at least, all clients I know do that). Stop the transfer of torrentB, and substitute the 0-sized files in filesetB with the corresponding files in filesetA (may require some renaming). As you restart torrentB, your Bittorrent client will recheck the whole filesetB, keeping the valid parts in order to avoid downloading them. Voilá! You have migrated files from one torrent to another.

Note: You should make sure that the files you are substituting in are the same files you want to download through torrentB or, at least, keep a copy around until you see that the restart check accepts most of their contents.

Re:Nice

Any modern file system will fragment if you expand an existing file. It simply has no way to guess how big the file will get when it is created unless your application chooses the proper allocation.

To give you an extreme example, imagine a 100 GB volume which has no files. You create a 1 MB file, and your filesystem places it near the top. Now you create a second file, and your filesystem places it... well, it could place it anywhere except that first 1 MB, so let's say it places it right next to the first file. Uh oh, it turn out that you need to write 1 GB of data to that first file and extend it. Now you have two fragments.

Ok, let's assume our file system is magical and knows that you like to extend files to huge sizes. So it places the second file at the end of the disk, instead. Oops, you fooled you file system: this time, you wanted to extend the second file by 1 GB. There is no room to append to the end of the file, so a second extent is created somewhere else and linked to the second file. You have two fragments again.

This is why performance tuning requires that you anticipate data requirements and allocate space accordingly; for example, by setting the initial size of database files to one that should reasonably accommodate the data requirements for the foreseeable future (and not automatically shrinking the database down when records are deleted).

Good for seeding stuck torrents, too

If I happen to see a stuck torrent (many leechers, no seeds), sometimes I can find a good version of the file I already have - so I start the torrent, stop it, replace the single good file (sometimes you need more if the file is smaller than the part size), and upload a few Kb to finish the torrent. Then sit back and watch as everyone fills up.

Scheduling

Many things are cheap in India, but bandwidth is not one of them. I can't just download files > 1GB without worrying about reaching my monthly cap, and there are Doctor Who episodes to be watched. Fortunately we have uncapped hours in the night

I don't know about other bittorrent clients, but uTorrent lets you set download speed caps by hour(like 0 during the day and unlimited at night).

Any MD5s on Apple's page?

Are their even MD5 hashes on Apple's download pages for such large files? Jusging by how the article was written and the lack of hashes on the QuickTime and iTunes download sites, it doesn't seem like they even bother.

Hardware Failure is your bigger concern

One should be more concerned as to why your files are becoming corrupted.

I'd say its a safe bet that the files from apple.com are in perfect condition.

Which means it either became corrupted in transit to, or on arrival to your machine.

Which leads the question, is your memory defective
run memtest86 to check your memory.
http://www.memtest86.com/ [memtest86.com]

Check if your Harddrives have SMART and are reporting anything. A disk checker would also be a good idea.

The other idea that springs to mind is if your behind some proxy with the above problems, although i doubt anyone would want to proxy a 1.5gig file.

Fact is, if files are being corrupted on your disk, its just a matter of time before something more important is hit by corruption.

Re:Hardware Failure is your bigger concern

could also be one's routers.

There was a problem w/ dlink routers back in the day that hit alot of p2p users. If you placed your machine in the dmz, the router basically did a search and replace on all packets replacing the bitstring representing the global address w/ the bitstring representing the local address. On large files, this didn't just hit in the ip header, but in the data as well corrupting it. If you didn't use dmz functionality, just port mapping, it worked fine, so if you were using bittorrent, you'd get repeated hash fails on some parts that would never fix, because bitorrent has no capability to work around that (as opposed to eMule's extensions)

Re:Hardware Failure is your bigger concern

Maybe, maybe not.

IIRC TCP/IP has a guaranteed maximum error rate of at least 10^-5 bits. Well, the thing is, 1.5 Gigabytes is over 10^10 bits in length. So even at such an error rate, it is not guaranteed that your file will arrive without bit errors.

Been using bittorrent and rsync for this for years

I've used bittorrent for this purpose many times in years gone by.

Especially with our slow links, or worse yet, on dialup (if I go enough years back) in Australia.

Before bittorrent I would use rsync. That required me to download the large file to a server in the US on a fast connection, then rsync my copy to the server's copy to fix what is corrupt in my copy.

It works beautifully. :)

Good for game files too

We have been doing this for ages [ausgamers.com] for certain high-demand games file that we mirror. While offering torrents for some of our download mirrors is only mildly useful (as we're in Australia we're trying to keep bandwidth on-shore to cut down international traffic, and BT doesn't really help this), it is extremely helpful for the VAST amount of users that appear to either have massively crazy Internet problems or are simply unable to drive a HTTP based downloader and resume downloads.

When a large number of users are having problems downloading or resuming a particular file, I simply create a torrent for them and give them some vague instructions about how to resume it and then generally I never hear from them again. They're happy because they don't have to download a 4gb game client again from scratch, they don't have to worry about resuming/corrupt downloads, and because its a torrent it probably feels like they're getting something for free that they shouldn't be.

Or synchronize with yourself...

For even more fun, if you have two differently-corrupted copies of a file and a torrent to go with it, then you can have BitTorrent stitch them together into a valid file without involving any third parties.

I used Azureus's internal tracker ability and two computers on a local network with the torrent modified to track on one of the machines, and one corrupted copy of the file on each.

Obviously only works if they don't have corruption in common, but it also doesn't require the original torrent file tracker to work anymore.

Re:What broken software were you using?

I asked the same question. Wikipedia answered it [wikipedia.org].

Re:What broken software were you using?

Its networking - shit happens. Some of his bits got thrown out of a router somewhere as heat, or maybe a packet timed out and didn't quite make it.

Re:What broken software were you using?

Flipped bits happen, but they are detected by multiple checksums which make it astronomically unlikely for corrupt data to remain undetected.

I actually saw this happen once ... the astronomically unlikely [1]. TCP accepted the corrupt packet. I'm sure it will never happen again. Fortunately, rsync caught it in the next run.

One problem I ran into once with a certain Intel NIC was that a certain data pattern was always being corrupted. TCP always caught it and dropped the packet. There was no progress beyond that point because of the hardware defect always corrupted that data pattern. Turns out there was a run of zeros followed by a certain data byte (I tried a different data byte and with different run lengths and those never got corrupted). What the NIC did was drop 4 bytes, and put 4 bytes of garbage at the end. I suspect it was a clocking syncronization error. I got around the problem by adding the -z option to rsync (which I normally would not have done with an ISO of mostly compressed files). Another way would have been to do the rsync through ssh, either as a session agent (like rsync itself can do) or as a forwarded port (how I do it now for a lot of things).

[1] ... approximately 1 in 2^31-1 chance that the TCP checksum will happen to match when the data is wrong (variance depending on what causes the error in the first place) ... which approaches astronomically unlikely. Take 1 Terabyte of random bits. Calculate the CRC-32 checksum for each 256 byte block. Sort all these checksums. You will find 2 (or more) data blocks with the same checksum (or a repeating pattern in your RNG). Why? Because CRC-32 has 2^32-1 possible states, and you have 2^32 random checksums.

But whatever the cause, it's almost certain that software is to blame.

Agreed. Since it is at least software's responsibility to detect and fix it, if the problem happens, the famous finger of fault points at the software.

I'd bet $100 that if he did the same download over HTTPS, thus preventing software meddling of the packet contents, it would come out perfect.

Your $100 is safe.

Re:What broken software were you using?

Those who have never developed P2P software might never understand why they all need to use strong checksums to detect data corruption, and why bad blocks actually do appear in the wild; frequently.

You'd be shocked - SHOCKED - at how much data gets corrupted routinely - by errant antivirus software, flaky network equipment, plain ol' line noise that the checksums don't detect (which will happen much more often than you expect, see also birthday paradox), or misbehaving routers who think that any occurence of 0xC0A80102 obviously must be an internal IP address and needs to be changed to your external one. Even if that's in the middle of a ZIP file. Oops.

Encryption actually aids this somewhat, as the same byte patterns don't get repeated, so if there's an errant IDS changing things for example, it tends not to fire the second time.

I've done this before for file repairs. Works a treat, but you sort of wish that torrent used a Merkle hash tree such as the modified THEX standard Tiger Tree Hash. SHA-1's so last century.

Re:What broken software were you using?

The TCP checksum offloading on nForce 4 motherboards (I have one) were notorious for corrupting TCP packets and allowing them to be received by the application. That's the most likely kind of failure that would be able to reproduce this problem.

Re:!new

I don't think this tactic is very common, though, as most people seem to have no fucking clue how BitTorrent works. I've seen torrents with gigantic multipart RARs, with an SFV of those. Let's see... so, my torrent software is already checksumming everything, and RAR has a builtin checksum too, or at least, acts like it does (it says "ok" or not) -- and on top of that, there's an SFV checksum (crappy CRC32), too. Never mind that RAR saves you at most a few megabytes (video is already compressed), which, based on the size of these files, you'll spend more time unpacking the RAR than you would downloading the extra couple megs. Or that, once you unpack and throw away the RAR, you can't seed that torrent from the working video. Or that multipart anything is retarded on BitTorrent, as the torrent is splitting it into 512k-4meg chunks anyway.

People who aren't aware of the full situation often make this complaint. These multipart rar files are "scene releases".

First of all, scene releases are _never_ compressed; it's always done with the -0 argument, this makes is basically equivalent to the unix split program. If a file is to be compressed, it is done with a zip archive, and the zip archive is placed inside the rar archive. This is because rar archives can be created/extracted easily with FOSS software, but cannot easily be de/compressed. This was more of an issue before Alexader Roshal released source code (note:not FOSS) to decompress rar archives.

Second, people often have parts of, or complete, scene releases and they are unwilling to unrar them (often because it's an intermediary, like a shell account somewhere where law isn't a problem).

Third, people follow "the scene" and try and download the exact releases that are chosen by the social customs of the scene (I am not going to detail those here), thus, "breaking up" (ie, altering) the original scene release is seen as rude.

Fourth, the archive splitting is in precise sizes so that fitting the archives onto physical media works better; typically the archive size is some rough factor of 698, ~4698 and ~8500.

Fifth, archives are split due to poor data integrity on some transfer protocols (though this is largely historical nowadays); redownloading a corrupted 14.3mb archive is easier than redownloading a 350mb file.

Sixth, traffic of the size is measured in terabytes, with some releases being tens, or sometimes hundreds of gigabytes in size. Thus, there become efficiency arguments for archive splitting; effective use of connections, limited efficiency of software(sftp scales remarkably poorly, though that is beginning to change - not that sftp is used everywhere), use of multiple coordinated machines and so on. This is an incomplete list of reasons; it is almost as though every time a new challenge is presented to the scene, splitting in some way helps to solve it.

AC because I'm not stupid enough to expose my knowledge of this either to law enforcement, or to the scene (who might just hand me over for telling you this - it has been done). Suffice to say that this is more complex than you understand, and that even this level of incomplete explanation is rare.

Re:!new

First of all, scene releases are _never_ compressed; it's always done with the -0 argument, [...] This was more of an issue before Alexader Roshal released source code (note:not FOSS) to decompress rar archives.

So, historical, and pointless. And anyway, just an excuse if there's any point in using RAR anyway. Let's see..

Second, people often have parts of, or complete, scene releases and they are unwilling to unrar them (often because it's an intermediary, like a shell account somewhere where law isn't a problem).

So they should use BitTorrent. Run a seed on your [strike]compromised windows host[/strike] "shell account".

Third, [....] social customs of the scene (I am not going to detail those here), thus, "breaking up" (ie, altering) the original scene release is seen as rude.

Oh, I think we're at the core of the problem. Pale teenagers in their mothers basements getting hurt feelings. I appreciate that someone will rip the Lost episodes in HD pretty much as they are being broadcast, and I actually look for some "group names" in the torrents I get - because they provide one file, not a RAR. In other words, provide what people want, and they will respect you for that. Make their life hard, and they will not care about your 1998 social customs. Like anything else in life.

Fourth, [...]fitting the archives onto physical media works better

Yawn. 1998 called, they want their infrastructure back. Harddrives are cheaper than dirt. Five years ago "the scene" at my college exchanged 250 gb harddrives.

Fifth, archives are split due to poor data integrity on some transfer protocols

SO USE BITTORRENT! It easier and faster and better and more fun, but of course less 'leet than using [strike]compromised windows hosts[/strike] "shell accounts"

Sixth, [...] Thus, there become efficiency arguments for archive splitting;[...]it is almost as though every time a new challenge is presented to the scene, splitting in some way helps to solve it.

No, BitTorrent does ALL this for you. ALL of it.

AC because I'm not stupid enough to expose my knowledge of this either to law enforcement, or to the scene (who might just hand me over for telling you this - it has been done).

Badass gangster!

Suffice to say that this is more complex than you understand, and that even this level of incomplete explanation is rare.

What? Moving files around on the internet is "more complex" than we understand? It probably the simplest fucking thing there is. Let me put it very simple for you: 1) Multi-file RARs made sense back when people got their stuff from FTPs and newsgroups. 2) It's the past. It's pure nostalgia. Get over it. If you're not using your "scene" FTP servers as Torrent seeds instead, you're wasting your resources.