Difference between pages "Funtoo ARMv7" and "SFTP Only Access"

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This is currently a scratch page for getting an ARMv7 build working for Funtoo:
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= Context =
 +
 
 +
In some cases, it can be useful to set up an access on your Funtoo box such as a user:
 +
* does not see the whole contents of the machine but, instead, remains "jailed" in a home directory
 +
* is able to transfer files back and forth on the box via SFTP
 +
* does not have access to a shell
 +
 
 +
Such a SFTP only access is easy to setup:
 +
 
 +
# Assign a group (e.g. ''sftponly'') to users that must be restricted to a SFTP-only account
 +
# Change a bit the configuration of OpenSSH so that users belonging to your sftp-only group are given a chrooted access
 +
# Make OpenSSH ignore any other command than running sftp-server on the server side for users belonging to your sftp-only group (this is where the trick lies !)
 +
 
 +
= Quick start =
 +
 
 +
First, a dedicated group must be created. For the sake of the example we use sftponly here, use whatever name fits your preferences:
  
 
<pre>
 
<pre>
# echo 'sys-devel/crossdev **' >> /etc/portage/package.keywords
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# groupadd sftponly
# emerge layman crossdev
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# echo 'PORTDIR_OVERLAY=/usr/local/portage' >> /etc/make.conf
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# install -d /usr/local/portage
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# layman -a gentoo-arm
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# crossdev -t armv7-none-linux-gnueabi
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</pre>
 
</pre>
  
[[Category:ARM]]
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Next in the configuration of OpenSSH (located in '''/etc/sshd/sshd_config''') locate:
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 +
<pre>
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Subsystem      sftp    /usr/lib64/misc/sftp-server
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</pre>
 +
 
 +
and change it for:
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 +
<pre>
 +
Subsystem      sftp    internal-sftp
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</pre>
 +
 
 +
Now the $100 question: ''"how can OpenSSH can be told to restrict a user access to a simple sftp session?"'' Simple! Assuming that ''sftponly'' is the group you use for for your restricted users, just add to the file '''/etc/sshd/sshd_config''' the following statement:
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 +
<pre>
 +
# Restricted users, no TCP connexions bouncing, no X tunneling.
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Match group sftponly
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        ChrootDirectory /home/%u
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        X11Forwarding no
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        AllowTcpForwarding no
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        ForceCommand internal-sftp
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</pre>
 +
 
 +
To understand how it works, you must be aware that, when you open an SSH session, the SSHD process launch a process on the server side which could be:
 +
* a shell => ssh login@host
 +
* a kind of dedicated ftp daemon (sftp-server) => sftp user@host
 +
 
 +
TBC
 +
 
 +
[[Category:HOWTO]]

Revision as of 16:50, 30 August 2011

Context

In some cases, it can be useful to set up an access on your Funtoo box such as a user:

  • does not see the whole contents of the machine but, instead, remains "jailed" in a home directory
  • is able to transfer files back and forth on the box via SFTP
  • does not have access to a shell

Such a SFTP only access is easy to setup:

  1. Assign a group (e.g. sftponly) to users that must be restricted to a SFTP-only account
  2. Change a bit the configuration of OpenSSH so that users belonging to your sftp-only group are given a chrooted access
  3. Make OpenSSH ignore any other command than running sftp-server on the server side for users belonging to your sftp-only group (this is where the trick lies !)

Quick start

First, a dedicated group must be created. For the sake of the example we use sftponly here, use whatever name fits your preferences:

# groupadd sftponly

Next in the configuration of OpenSSH (located in /etc/sshd/sshd_config) locate:

Subsystem      sftp    /usr/lib64/misc/sftp-server

and change it for:

Subsystem      sftp    internal-sftp

Now the $100 question: "how can OpenSSH can be told to restrict a user access to a simple sftp session?" Simple! Assuming that sftponly is the group you use for for your restricted users, just add to the file /etc/sshd/sshd_config the following statement:

# Restricted users, no TCP connexions bouncing, no X tunneling.
Match group sftponly
        ChrootDirectory /home/%u
        X11Forwarding no
        AllowTcpForwarding no
        ForceCommand internal-sftp

To understand how it works, you must be aware that, when you open an SSH session, the SSHD process launch a process on the server side which could be:

  • a shell => ssh login@host
  • a kind of dedicated ftp daemon (sftp-server) => sftp user@host

TBC