HopBit — There and Back Again

This guide describes how to set up an SSH tunnel between two computers via an external gateway. A basic understanding of the ssh command (particularly the -R and -p options) is sufficient for following this guide.

The corkscrew utility is used to bypass internal firewalls that block direct SSH access to a VPS over the standard port 22. This can be useful in corporate environments where the IT or security department restricts outbound connections.

Security note: For better protection against unauthorized access, configure SSH key‑based authentication and disable password logins. This is not covered in detail here.

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Network Layout

graph LR
A["Home PC"] <-- port 6666 --> B(VPS) <--> C(Office proxy) <-- port 6644 --> D[Work PC]

n1["Gremlins
o(Tヘoω･ヘ(_|ωノ・(ｼ||д|_￣)▽┌┴┴┌_φ∵∧σ(͡°  ⁠͜ ʖ͜ ⊙)"]
n1@{ shape: hex} -- attack on VPS --> B
style n1 fill:#FF9D80

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Hosts:

• Home PC

  • home_ip
  • home_user

• VPS

  • vps_ip
  • vps_user

• Work PC

  • work_ip
  • work_user

Ports:

• Random port 1 — 6666
• Random port 2 — 6644

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Quick Reference

Tunnel from VPS to Home PC

1. On the Home PC, open a reverse SSH tunnel (keep it running):
   home@ssh -R 6666:localhost:22 vps_user@vps_ip

2. From the VPS, connect to the Home PC:
   vps@ssh -CX home_user@localhost -p 6666

Connect from Work PC to VPS

1. Install corkscrew on the Work PC: https://github.com/bryanpkc/corkscrew

2. Configure SSH on the Work PC, replacing INTERNAL_PROXY_IP and INTERNAL_PROXY_PORT with your office proxy details:

   host name_vps
       HostName vps_ip
       Port 443
       User vps_user
       ProxyCommand /usr/local/bin/corkscrew INTERNAL_PROXY_IP INTERNAL_PROXY_PORT %h %p
   
   host name_home
       HostName localhost
       User home_user
       Port 6666
       ProxyJump name_vps

3. Connect to the VPS:
   work@ssh -CX name_vps

Connect from Work PC to Home PC

1. Ensure the Home PC has the reverse tunnel running:
   home@ssh -R 6666:localhost:22 vps_user@vps_ip

2. From the Work PC:
   work@ssh -CX name_home

Connect from Home PC to Work PC

1. On the Work PC, start a reverse SSH tunnel:
   work@ssh -R 6644:localhost:22 name_vps

2. From the Home PC:
   home@ssh -CX vps_user@vps_ip
   vps@ssh -CX work_user@localhost -p 6644

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Detailed Instructions

1. Connect to VPS

graph LR
A["Home PC"] --> B(VPS)

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From the Home PC:
home@ssh vps_user@vps_ip

Initial setup on VPS (optional)

Verify that networking, package management, and basic tools are working:

vps@apt update
vps@apt install mc htop

If your VPS provider gives you only the root account:

sudo useradd vps_user
sudo usermod -aG sudo vps_user
sudo usermod -aG root vps_user

Disable direct root login in /etc/ssh/sshd_config:

PermitRootLogin no

Restart the SSH service:

sudo service ssh restart

2. Reverse Tunnel from VPS to Home PC

graph LR
A["Home PC"] <-- port 6666 --> B(VPS)

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A VPS cannot initiate an SSH connection to a home machine with a private (NAT) IP address. Instead, the Home PC must establish a reverse tunnel to the VPS.

1. Open Remonte tunnel on Home PC (should be running):
   home@ssh -R 6666:localhost:22 vps_user@vps_ip

2. Connect to Home PC from VPS:
   vps@ssh -CX home_user@localhost -p 6666

3. Keep the tunnel alive automatically:

autossh ins't standart util

home@sudo apt install autossh
home@autossh -f -N -R 6666:localhost:22 vps_user@vps_ip

3. Connect from Work PC to VPS

graph LR
A(VPS) <--> B[corkscrew] <--> C[Work PC]

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If the office network restricts outbound SSH:

1. Install corkscrew:
   https://github.com/bryanpkc/corkscrew

2. Configure SSH on the Work PC:

   host name_vps
       HostName vps_ip
       Port 443
       User vps_user
       ProxyCommand /usr/local/bin/corkscrew INTERNAL_PROXY_IP INTERNAL_PROXY_PORT %h %p

3. Connect:
   work@ssh -CX name_vps

4. Connect from Home PC to Work PC

graph LR
A["home PC"] <-- port 6666 --> B(VPS) <--> C(рабочий proxy) <-- port 6644 --> D[work PC]

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1. On the Work PC:
   work@ssh -R 6644:localhost:22 name_vps

2. On the Home PC:
   home@ssh -CX vps_user@vps_ip
   vps@ssh -CX work_user@localhost -p 6644

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SSH Config Examples

Home PC

host name_vps
    HostName vps_ip
    User vps_user

host name_vps_R
    HostName vps_ip
    User vps_user
    RemoteForward 6666 localhost:22

host *
    ForwardX11 yes
    Compression yes

Work PC

host name_vps
    HostName vps_ip
    Port 443
    User vps_user
    ProxyCommand /usr/local/bin/corkscrew 192.168.172.129 3128 %h %p

host name_vps_R
    HostName vps_ip
    User vps_user
    RemoteForward 6644 localhost:22

host name_home
    HostName localhost
    User home_user
    Port 6666
    ProxyJump name_vps

host *
    ForwardX11 yes
    Compression yes

VPS

host name_home
    HostName localhost
    User home_user
    Port 6666

host name_work
    HostName localhost
    User work_user
    Port 6644

host *
    ForwardX11 yes
    Compression yes

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Using autossh

sudo apt install autossh
export AUTOSSH_DEBUG=1
export AUTOSSH_GATETIME=0
export AUTOSSH_PORT=20037
autossh -f -N vps_user@vps_ip -R 6666:127.0.0.1:22