[augeas-devel] Partial answer to augeas not being able to parse / deal with sshd_config Match blocks...

[Mol, Xavier (SCC)]

Hi Spike.
 
> The point is -- in general, augeas barfs whenever there's a Match block.  With the lens file that ships with augtool.  (This is documented in the lens file.)
 
What version of Augeas are you using? My version, 1.8.1 has a different documentation in the lens file:
 
“””
About: CAVEATS
 
  In sshd_config, Match blocks must be located at the end of the file.
  This means that any new "global" parameters (i.e. outside of a Match
  block) must be written before the first Match block. By default,
  Augeas will write new parameters at the end of the file.
 
  I.e. if you have a Match section and no ChrootDirectory parameter,
  this command:
 
     > set /files/etc/ssh/sshd_config/ChrootDirectory "foo"
 
  will be stored in a new node after the Match section and Augeas will
  refuse to save sshd_config file.
 
  To create a new parameter as the right place, you must first create
  a new Augeas node before the Match section:
 
     > ins ChrootDirectory before /files/etc/ssh/sshd_config/Match
 
  Then, you can set the parameter
 
     > set /files/etc/ssh/sshd_config/ChrootDirectory "foo"
“””
 
No mention of a general problem that the lens could not handle, except for ensuring that all Match blocks are positioned at the end of the file. And, as Dan said, there are tools to handle such situations easier. In our case, we also have Puppet to manage (open-)ssh, though we employ basic erb-templates, which don’t have the same pitfalls as Augeas.
 
Ciao,
Xavier.
 
 
Von: Spike White <spikewhitetx at gmail.com> 
Gesendet: Samstag, 24. April 2021 21:30
An: Mol, Xavier (SCC) <xavier.mol at kit.edu>
Cc: augeas-devel at redhat.com
Betreff: Re: [augeas-devel] Partial answer to augeas not being able to parse / deal with sshd_config Match blocks...
 
Xavier,
 
The point is -- in general, augeas barfs whenever there's a Match block.  With the lens file that ships with augtool.  (This is documented in the lens file.)
 
That's a problem for a general-purpose parsing of a sshd_config file, when you don't know a priori whether the sshd_config file has a Match block or not.
 
Yes, there are some exotic edge cases in which augeas can tolerate Match blocks.  I've found 1-2 myself.  But in the main,  the lens file doesn't tolerate Match blocks.
 
I was proposing a miniscule restriction to the sshd_config syntax that should allow the lens file to be embellished to handle Match blocks.
 
Spike
 
On Fri, Apr 23, 2021 at 2:06 AM Mol, Xavier (SCC) <xavier.mol at kit.edu <mailto:xavier.mol at kit.edu> > wrote:
Hi Spike,
 
I don’t know your use case, but wouldn’t it be easier to tell Augeas to add UsePrivilegeSeparation before the first Match block?
 
augtool> ins UsePrivilegeSeparation before /files/etc/ssh/sshd_config/Match[1]
augtool> set /files/etc/ssh/sshd_config/UsePrivilegeSeparation yes
 
Of course, this doesn’t work out anymore when you are not sure UsePrivilegeSeparation wasn’t set before already. sshd_config will be correct anyway, but this approach would add another ‘UsePrivilegeSeparation = yes’ line every time.
 
Ciao,
Xavier.
 
Von: augeas-devel-bounces at redhat.com <mailto:augeas-devel-bounces at redhat.com>  <augeas-devel-bounces at redhat.com <mailto:augeas-devel-bounces at redhat.com> > Im Auftrag von Spike White
Gesendet: Donnerstag, 22. April 2021 20:00
An: augeas-devel at redhat.com <mailto:augeas-devel at redhat.com> 
Betreff: [augeas-devel] Partial answer to augeas not being able to parse / deal with sshd_config Match blocks...
 
Augeas people,
 
augeas has a problem dealing with Match blocks in /etc/ssh/sshd_config.  It's even documented in the notes in the lens file for that file.
 
I know that a general purpose parsing of a Match block in augeas is a problem.  I've tried.  The problem is that augeas by default appends new lines (where they're missing in original file) to the end of the file.
 
Thus if you have an existing Match block at the end of the sshd_config file such as:
 
Match User servicepptprdvra
   MaxSessions 999
   ClientAliveInterval 360
   ClientAliveCountMax 3
 
and you want augeaus to add UsePrivilegeSeparation yes as so:
 
  Match User servicepptprdvra
   MaxSessions 999
   ClientAliveInterval 360
   ClientAliveCountMax 3
  UsePrivilegeSeparation yes 
  
then this UsePrivilegeSeparation will not apply to all users & hosts (as desired), but only to the user specified in the Match block condition.
 
In a nutshell, I think that's the problem with augeas dealing with a Match block.  
 
Is this a partial solution?:
 
If you restrict augeas so that every match block it writes and every Match block it parses ends with a line:
 
Match All
 
to close an existing Match block,   That is if you have this:
 
  Match User servicepptprdvra
   MaxSessions 999
   ClientAliveInterval 360
   ClientAliveCountMax 3
  Match All
 
  and you want augeaus to add UsePrivilegeSeparation yes as so: 
 
  Match User servicepptprdvra
   MaxSessions 999
   ClientAliveInterval 360
   ClientAliveCountMax 3
  Match All
  UsePrivilegeSeparation yes 
 
Then this UsePrivilegeSeparation will be applied to all users &   hosts, as desired.
 
This is a small restriction on the sshd_config syntax when using augeas, but not egregiously so.
 
Spike White
 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/augeas-devel/attachments/20210426/d16fb47b/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 6198 bytes
Desc: not available
URL: <http://listman.redhat.com/archives/augeas-devel/attachments/20210426/d16fb47b/attachment.p7s>