[Borgbackup] bypassing "Cache newer than repo" error

[Marian Beermann]

Hi Sitaram

On 08.10.2016 14:17, Sitaram Chamarty wrote:
>> This sounds like you created one repository and copied it to multiple
>> drives/locations?
>
> I was going to say "no way" but it appears that is what I did.  Now I
> also understand why it's happening only to one specific repo (a rather
> large one I got lazy about creating the first time, and simply did a
> copy).  I'd clean forgotten!
>
>> You can change the repository ID in the "config" file of the repository
>> (it's hex, keep it the same length), which separates the repositories.
>
> I assume it has no other semantics so I can just randomly change some
> hex digits into others?

Yes.

Caveat: if you use key-file mode you'll have to make the same change in
the key files you use. The default location is
~/.config/borg/keys/<names>. Every key file starts with "BORG_KEY
<repository ID>", that's where you need to make the change.

In repokey or unencrypted mode this doesn't matter.

>> Note: for encrypted repositories it's a very unsafe thing to have
>> multiple independently updated copies of a repository; if they diverge
>> (minutely different contents) and an attacker gains access to more than
>> one copy, the privacy of the repository contents may be compromised.
>
> I do have multiple independently updated copies of a repo, but -- other
> than this one where #2 was created by a file-system level copy of #1,
> all the others are "borg init"-ed independently on each disk.
>
> The passphrase I use is the same, but I assume the internal key was
> randomly generated each time and would not be the same, so the attack
> you speak of should not happen for those repos even if someone got hold
> of them.
>
> Is that understanding correct?

Repositories independently "borg init"-ed are really independent, no
problems there.

But in the case, where you "borg init" and then copy (cp -r / rsync /
etc.) the repo the keys will also be the same. This possibly leads to
the situation where different data is encrypted with the same key, which
is highly problematic.

Cheers, Marian

> Thanks again and best regards
> sitaram
>
>>
>> Cheers, Marian
>>
>> FAQs:
>>
http://borgbackup.readthedocs.io/en/stable/faq.html#can-i-copy-or-synchronize-my-repo-to-another-location
>>
>> On 08.10.2016 13:06, Sitaram Chamarty wrote:
>>> Hi
>>>
>>> Some of the directories are backed up to two (in one case three)
>>> different external (USB) hard disks.  When I finish with the first USB
>>> drive, unmount it, and mount the next one and try the backup, borg tells
>>> me the cache is newer.
>>>
>>> I could not find anything about how to bypass this in the docs.  I have
>>> now created a complicated system of separately maintaining the cache
>>> directories for each external disk (labelled in some way that correlates
>>> with the physical disk in question) and manually shuffle them around.
>>>
>>> Any pointers would be appreciated.
>>>
>>> regards
>>> sitaram
>>>
>>> PS: Yes removing ~/.cache/borg works and is generally harmless.  But
>>> that makes ALL the files show up as "A ...", whereas I like to eyeball
>>> the list to see if something got updated which I did not expec to be
>>> updated based on what I have been working on since the last backup.
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>>>
>>
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>