Re: Extensions to Tar

[Chris Wilson]

Hi Paul,

> Yes, it is difficult to get right, isn't it?

No kidding, this is my first time using zlib and first look at the tar 
source code. I must say I'm very impressed with the cleanness of tar's 
code, this hack only took me 24 hours =)

> For an extensive change like that we'd need legal papers.  Have you done
> this before with the FSF?  If not I can send you the forms.

I haven't done this before; please send me the forms.

> It would have to be buildable without zlib.  Not every platform has
> zlib.  'configure' should have a --with-zlib=PATH option (see how
> OpenSSH does it, for example).

OK, I will try and add this, although I'm not that familiar with autoconf 
either =)

> You need to be able to fall back to plan B (compressing it twice);
> otherwise the code won't be robust enough in practice.

OK, I'll add this.

> > I have implemented support for these data types using custom values of the 
> > typeflag header field: 'Y' for zlib-filtered data and 'E' for 
> > GPG-encrypted data. I hope this is the "right" way to do it.

> Look at the 'pax' command for a description of the format.

I've had a look at this document, and it says that A-Z typeflags are 
reserved for custom implementations. I noticed that all the GNUTYPE_* 
typeflags were also in the A-Z range with a similar comment, and that Y 
and E were unused, so I added mine to that list. Is that okay?

> With ssh you can give them only the ability to run a certain command,
> presumably rmt or a wrapper for rmt.  Isn't that enough to give you
> want you want?

Maybe, I'll look into it. The main problem is that it doesn't work for 
multi-volume file archives (which is a pain if you're backing up an entire 
machine to a system which has a 2Gb file size limit). I can try working 
around this with --info-script, but I think it'll be kludgy (though not as 
bad as my previous solution, which was `cat | split' =)

> Not me.  :-)  You should be able to do that by telling tar to read
> the archive from stdin and write it to stdout, updating as you go.

How do I tell tar to do that?

> > Finally, for differential/incremental backups, I was thinking about using 
> > md5sums to detect changed files,
> 
> Why?  You don't trust the time stamps?

That's why I ask. I know rsync uses checksums (md5 I think) to compare 
files, and I know programs like tar can restore an archive and fudge the 
timestamps so it wouldn't be backed up by an incremental restore. Or am I 
mistaken?

> here.  And if you're paranoid enough to want a cryptographic checksum,
> you should use a better one than MD5; SHA1 say.

Is MD5 really that insecure?

> Anyway, thanks for your thoughts.  I hope I haven't scared you away
> from contributing....

Not at all, thanks for replying so quickly and completely =)

Cheers, Chris.
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