SSH/Socks5 Proxy

Creating a proxy with SSH to a server one has full control over in order to get content that is restricted to a certain coutry or region.

Prerequisites

• A server runnning SSH with a global address.
• Full control on the server to create users a.k.a root access.
• SSH client that understands the -D option e.g. OpenSSH or PuTTY.
• A web browser capable of using a Socks5 proxy. e.g. Firefox
• A proxy switcher like FoxyProxy [optional].

Howto

Server Setup

The first thing do do is create a restricted or jailed users. Although the jailed users is more secure a restricted user is used as it is more universally applicable.

User Creation

The assumption is that this is Linux server on other Unix-like hosts the commands need to be adjusted. The user's home directory will be located under /var/restricted which is probably not present and has to be created first. The user's name is for this example going to be socks5.

 mkdir -p /var/restricted
 useradd -d /var/restricted/socks5 -u 25002 -s /bin/rbash socks5

To make this users truly restricted a few things need to be put in place and changed. First password authentication is not recommended here hence the .ssh directory. Further a .hushlogin for faster logins without the motd. And a few tools running a while loop while the connection is open to prevent timeouts. Using rbash prevents this users from using any binaries not in the path and the user can't change directories.

chmod 500 /var/restricted/socks5
mkdir /var/restricted/socks5/.ssh
chown socks5 /var/restricted/socks5/.ssh
chmod 100 /var/restricted/socks5/.ssh
touch /var/restricted/socks5/.huslogin 
chown root:root /var/restricted/socks5/.hushlogin
chmod 444 /var/restricted/socks5/.hushlogin

Additionally the login profile and the bin directory with the necessary binaries are created.

cat <<PROFILE > /var/restricted/socks5/.bash_profile
PATH="\${HOME}/bin"
export PATH
PROFILE

chown root:root /var/restricted/socks5/.bash_profile
chmod 444 /var/restricted/socks5/.bash_profile
mkdir /var/restricted/socks5/bin
chown root:root /var/restricted/socks5/bin
chmod 555 /var/restricted/socks5/bin
cd /var/restricted/socks5/bin
ln -s /bin/date
ln -s /bin/true
ln -s /bin/sleep

Copy the public ssh key to the /var/restricted/socks5/.ssh/authorized_keys file. See man ssh-keygen for more information how to achieve this. Finally adjust the permission of the authorized_keys file.

cat id_rsa.pub >> /var/restricted/socks5/.ssh/authorized_keys
chown socks5:root /var/restricted/socks5/.ssh/authorized_keys
chmod 400 /var/restricted/socks5/.ssh/authorized_keys

SSHD Setup

This only applies if the AllowUsers configuration is already in used. Due to frequent ssh brute force attack it is one the recommended measures to take curb unwanted logins. The other one is only allow ssh-key authentication. After the change restart the sshd process (sample below is from a debian squeeze system, your mileage may vary).

sed -i.bak.$( date +%F ) \
  -e 's/^\(AllowUsers.*\)/\1 socks5/' \
  /etc/ssh/sshd_config

/etc/init.d/ssh restart