sftp and utmp

[François Ouellet]

Le Friday, 31 March 2023, 17:47:14 EDT John-Mark Gurney a écrit :
> hvjunk wrote this message on Thu, Mar 30, 2023 at 23:12 +0200:
> > I've been battling similar issues, and the only methods I've found (with sftp) was to use
> > software like pureftd or crushftp (using crushftp lately as production) that does handle these
> > issues "out of the box"
> > Other than that, I'd expect you'll need to write your own PAM modules to track the accounting part to 
> > enforce the limits yourself, as you'll need to account for the sftp different from the terminal sessions
> 
> You could use an sftp-server wrapper script that creates a lock file/dir
> or another way to detect if a connection is already present, and then
> force the use of that script via the sshd_config Subsystem directive.

I'm using the internal-sftp server, because it's a chrooted setup.  
Not sure this can easily be done with this setup

Thanks,

François


> 
> > > On 30 Mar 2023, at 22:43, François Ouellet <franco at sol.mpact.tv> wrote:
> > > 
> > > Hi,
> > > 
> > > We need to limit concurrent sftp logins to one per user (because of bad
> > > client behaviour).  Is there any way to achieve this I have overlooked?
> > > 
> > > It seems it could be possible with pam_limits, if sftp sessions were
> > > recorded in utmp (a guess from what I found googling around).  If I
> > > configure /etc/security/limits.conf with
> > > 
> > >  testuser hard maxlogins 1
> > > 
> > > and connect with ssh, and try a second connection with sftp, the sftp
> > > fails because there is already one session open.  But if I connect with
> > > sftp and try a second sftp connection, it is allowed.
> > > 
> > > Is there some way to have sftp connections recorded in utmp?  I haven't
> > > found any reference to this.  There are some posts from 10+ years ago
> > > where others were trying the same thing but there's no reply about how
> > > to do it.  Would it be possible to add this option?
> > > 
> > > We're using ChrootDirectory and ForceCommand internal-sftp, if it makes
> > > a difference (I've tried without and had the same results).
> > > 
> > > Tried this on Debian bookworm's openssh-server (9.2).  The changelog
> > > from 9.3 does not mention anything related to this.