Introduction

Secure Socket Tunneling Protocol (SSTP) is a VPN transport layer protocol that uses TLS over TCP port 443 to encapsulate PPP traffic. It combines strong encryption with the ubiquity of HTTPS to penetrate firewalls and NAT devices reliably. When paired with a proxy server, SSTP can provide additional routing, caching, and access control benefits. This guide provides a practical, step‑by‑step approach to configuring an SSTP VPN server and integrating it with a proxy server for enhanced flexibility and control, aimed at site administrators, enterprise IT staff, and developers.

Why SSTP and When to Use a Proxy

SSTP is especially useful when UDP‑based VPNs (like OpenVPN UDP or IPsec) are blocked or unreliable. Because SSTP runs over TCP/443 (TLS), it often bypasses strict network filters. A proxy server—HTTP, HTTPS, or SOCKS—can complement SSTP by:

  • Offloading web traffic caching and reducing bandwidth usage.
  • Implementing granular access or content filtering rules at the application layer.
  • Providing per‑application routing (split‑tunneling) or chaining for privacy.
  • Facilitating logging and auditing for compliance and troubleshooting.

Prerequisites and Environment

Before starting, ensure you have:

  • A server (Linux recommended—Ubuntu/Debian or CentOS) with a public IP and domain name (A record).
  • Root or sudo access to the server.
  • Valid TLS certificate (Let’s Encrypt or commercial CA) for the server domain to prevent client certificate warnings.
  • Firewall rules allowing TCP 443 (for SSTP) and proxy ports (e.g., 3128 for Squid, 1080 for SOCKS) as needed.
  • Knowledge of your client platforms (Windows has built‑in SSTP client; Linux and macOS may require additional software).

Overview of the Setup

This guide will cover:

  • Installing and configuring an SSTP server (using sstpd or strongSwan with SSTP module).
  • Setting up a TLS certificate for SSTP.
  • Configuring authentication (password vs certificate‑based).
  • Routing and firewall configuration to forward and NAT traffic.
  • Installing a proxy server (Squid for HTTP/HTTPS or Dante for SOCKS) and integrating it with SSTP clients.
  • Example client configuration and testing steps.

Step 1 — Obtain and Install a TLS Certificate

SSTP relies on TLS for security. Use Certbot to obtain a Let’s Encrypt certificate. Example for Ubuntu:

Install Certbot:

  • sudo apt update
  • sudo apt install certbot

Obtain certificate:

  • sudo certbot certonly –standalone -d vpn.example.com

The certificate files are usually located at /etc/letsencrypt/live/vpn.example.com/. Configure your SSTP server to point to the fullchain.pem and privkey.pem files.

Step 2 — Install and Configure SSTP Server

There are several SSTP server implementations. On Linux, sstpd is a lightweight implementation, while strongSwan can be extended to support SSTP. This section uses sstpd for clarity.

Install sstpd:

  • sudo apt install sstpd

Configure sstpd: Edit /etc/sstpd/sstpd.conf (path may vary) to include TLS certificate paths and authentication backend. Minimal example:

  • listener “sstpd” {
  • tls_certificate = “/etc/letsencrypt/live/vpn.example.com/fullchain.pem”
  • tls_private_key = “/etc/letsencrypt/live/vpn.example.com/privkey.pem”
  • listen_addr = “0.0.0.0:443”
  • pppd_options = “/etc/ppp/options.sstp”
  • }

PPP (pppd) configuration: Create /etc/ppp/options.sstp with options for ip allocation, DNS, and authentication:

  • noauth
  • local
  • ms-dns 8.8.8.8
  • ms-wins 10.0.0.5
  • lock

Authentication: use a simple username/password via /etc/ppp/chap-secrets or integrate with RADIUS/LDAP for enterprise setups. Example chap-secrets entry:

  • vpnuser sstpd mysecret *

Start and enable sstpd

sudo systemctl enable –now sstpd

Check status and logs:

  • sudo journalctl -u sstpd -f

Step 3 — Networking and IP Forwarding

Enable IP forwarding and configure NAT so VPN clients can reach the internet through the server.

  • Enable forwarding temporarily: sudo sysctl -w net.ipv4.ip_forward=1
  • Persist setting: edit /etc/sysctl.conf and set net.ipv4.ip_forward=1.
  • Configure iptables (IPv4):
  • sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
  • sudo iptables -A FORWARD -i ppp+ -o eth0 -j ACCEPT
  • sudo iptables -A FORWARD -i eth0 -o ppp+ -j ACCEPT

Replace eth0 with your external interface. For systems using nftables or firewalld, configure equivalent rules. Persist iptables rules via iptables-persistent or firewall configuration tools.

Step 4 — Install and Configure Proxy Server

Choose a proxy based on needs: Squid for HTTP/HTTPS caching and filtering, Dante for SOCKS5. This example covers Squid (HTTPS interception and forward proxy).

Install Squid:

  • sudo apt install squid

Basic Squid configuration adjustments (in /etc/squid/squid.conf):

  • http_port 3128
  • acl localnet src 10.0.0.0/8 # adjust to VPN subnet
  • http_access allow localnet
  • cache_mem 256 MB
  • maximum_object_size_in_memory 512 KB
  • access_log /var/log/squid/access.log

To allow VPN clients to use the proxy, ensure the ACL matches the PPP address pool assigned by SSTP.

Optional: Transparent Proxying

If you prefer transparent proxying (intercept traffic without client proxy settings), set up iptables to redirect port 80/443 to Squid’s intercept port and configure Squid with http_port 3129 intercept. Note: intercepting HTTPS requires SSL bumping, which has legal and technical implications and requires a trusted CA on clients.

Step 5 — Integrating SSTP with Proxy Usage

There are multiple integration strategies depending on policy:

  • Client‑side proxy configuration: Easiest approach—configure browsers or system proxy settings to point to the proxy (IP:3128 or hostname). SSTP handles routing; proxy handles HTTP/HTTPS.
  • Forced proxy via firewall: Redirect web traffic from VPN client subnet to proxy transparently (note HTTPS implications as above).
  • Proxy chaining: Squid can forward to upstream proxies for egress control or anonymization; configure cache_peer entries accordingly.
  • SOCKS + SSTP: For application‑level tunneling (e.g., SSH, tools that only support SOCKS), run a SOCKS server (Dante) and have clients point selected apps to SOCKS on the VPN gateway.

Step 6 — Authentication and Access Control

For production environments, integrate authentication with enterprise systems:

  • Use RADIUS or LDAP for SSTP authentication so credentials are centrally managed.
  • Configure Squid to use LDAP/RADIUS for web access control and logging.
  • Use client certificates for SSTP if you need two‑factor or certificate‑based authentication—configure pppd and sstpd to validate client certs.

Example of enabling RADIUS with pppd: use pppd plugin radius.so or underlying SSTP auth module documentation to attach to RADIUS server.

Step 7 — Client Configuration and Testing

Windows (built‑in):

  • Network > VPN > Add a VPN connection.
  • VPN provider: Windows (built‑in).
  • Connection name: Your label.
  • Server name or address: vpn.example.com.
  • VPN type: Secure Socket Tunneling Protocol (SSTP).
  • Type of sign‑in info: Username and password (or certificate).

Linux (NetworkManager): install network-manager-sstp plugin and configure similarly.

After connection, verify:

  • Public IP changed to server egress IP (if routing all traffic).
  • DNS resolution either via VPN DNS or split DNS as configured.
  • HTTP(S) requests reach Squid and are logged in /var/log/squid/access.log if proxy used.
  • Check sstpd logs for handshake and pppd logs for IP assignment.

Troubleshooting Tips

  • If the client fails to connect, inspect TLS certificate chain and ensure the domain matches the certificate Common Name (CN) or SAN.
  • Use tcpdump or tshark on port 443 to ensure traffic hits the server: sudo tcpdump -i eth0 port 443 -n.
  • Check for MTU issues—PPP over SSTP over TCP can encounter MTU/MSS problems. Reduce MTU on the PPP interface (e.g., 1400) or enable MSS clamping in firewall.
  • For intermittent disconnects, examine TCP retransmissions and firewall timeouts; consider keepalive settings in pppd.
  • If proxy access is blocked, ensure ACLs include client IP range and Squid is listening on the correct interface.

Security Considerations

Keep the following in mind to maintain a secure deployment:

  • Protect server certificates and private keys; use file permissions and limited access.
  • Prefer certificate‑based authentication for clients where feasible.
  • Limit open ports to only what’s necessary and sinkhole or rate‑limit repeated authentication failures.
  • Monitor logs for suspicious activity and integrate with SIEM tools for larger deployments.
  • Keep sstpd, pppd, Squid/Dante and underlying OS updated to mitigate vulnerabilities.

Scaling and High Availability

For enterprise needs, consider:

  • Load balancing SSTP across multiple servers using a reverse proxy or TCP load balancer that supports session persistence (because SSTP uses TCP and PPP stateful sessions).
  • Using a clustered proxy architecture or multiple Squid nodes behind a load balancer with cache hierarchy.
  • Centralized authentication and logging (RADIUS, LDAP, syslog/ELK) to manage users and audit trails.
  • Automated certificate renewal (Certbot) and coordinated reloads across SSTP nodes.

Conclusion

Combining SSE‑grade TLS transport via SSTP with a robust proxy server offers a flexible, firewall‑friendly VPN solution suitable for scenarios where UDP or IPsec is blocked. By following the steps above—TLS provisioning, SSTP server installation, PPP and routing setup, proxy configuration, and careful authentication integration—you can build a reliable and secure remote access platform. Ensure that you harden, monitor, and scale the solution according to your organization’s policy and demand.

Published by Dedicated‑IP‑VPN: https://dedicated-ip-vpn.com/