sftp and utmp

hvjunk hvjunk at gmail.com
Fri Mar 31 08:12:19 AEDT 2023

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

> 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.
> Thank you,
> François
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