Secure Socket Tunneling Protocol (SSTP) is a solid choice for enterprises and developers who need VPN connectivity that traverses restrictive networks reliably. Built on top of SSL/TLS over TCP port 443, SSTP blends strong cryptography with broad network compatibility. However, SSL-over-TCP comes with its own operational nuances: connection drops, TCP meltdown across nested connections, and certificate or TLS negotiation failures can interrupt services. This article dives into practical, technical guidance on configuring Always-On SSTP, robust auto-reconnect behavior, and reliability tuning so that your site-to-site or remote-access VPNs stay available for users and services.

Why focus on Always-On and auto-reconnect for SSTP?

For businesses and developers running critical applications across VPNs, transient interruptions can degrade user experience and break automated workflows. SSTP is especially attractive because it is commonly allowed through corporate and public firewalls, but its reliance on TLS/TCP introduces failure modes that need explicit handling. Implementing an Always-On approach with robust auto-reconnect and reliability settings ensures persistent tunnels, predictable failover, and minimized operational intervention.

Key reliability goals

  • Minimize downtime by automatically reestablishing sessions after network blips.
  • Prevent data leaks during reconnection windows (DNS leaks, split-tunnel exposure).
  • Maintain routing integrity so critical routes persist after reconnection.
  • Preserve security posture with strict certificate/TLS checks and secure rekeying.

Core technical mechanisms to enable Always-On SSTP

Always-On behaviour requires coordinating client-side connection management, PPP/TLS settings, and server-side resilience. Below are the core mechanisms and concrete configurations to tune.

TLS settings: enforce modern versions and strong ciphers

SSTP is TLS-based. On both server and client, enforce TLS 1.2 or later and prefer robust cipher suites (e.g., AES-GCM, ECDHE key exchange). Disable legacy ciphers and SSLv3/1.0/1.1. Certificate validation must be strict — enable OCSP/CRL checks where possible. Certificate pinning on managed clients can prevent MITM attempts during reconnection.

PPP/LCP keepalive and failure detection

PPP runs atop SSTP and offers Link Control Protocol (LCP) mechanisms to detect link failure quickly. Configure the PPP implementation (pppd on Linux, Ras/SSTP stack on Windows) to use LCP echo intervals and failure counts:

  • Set a relatively short lcp-echo-interval (for pppd) — e.g., 5–10 seconds.
  • Set lcp-echo-failure to a conservative count (3–5) so a transient packet loss doesn’t trigger disconnects prematurely.

These values trade sensitivity for false positives; tune them according to your link quality.

TCP/TLS keepalive and socket timeouts

Because SSTP uses TCP, operating system TCP keepalive and application-level TLS timeouts matter. On both ends:

  • Enable TCP keepalive with appropriate intervals (e.g., initial probe at 60–120s for mobile networks or 15–30s for stable LANs depending on needs).
  • Adjust TCP retransmission timeouts (RTO) conservatively to avoid long stalls. Avoid overly long RTO values on mobile clients.

Preventing DNS and routing leaks during reconnection

Always-On SSTP must avoid exposing local network traffic when the tunnel drops. Techniques include:

  • Use VPN-managed DNS servers pushed by the server. Ensure the client applies DNS settings atomically when the tunnel is up and restores them only after verifying the tunnel has been reestablished.
  • Implement a strict default route via the tunnel (force all traffic over VPN) on managed devices, or use firewall rules that block non-VPN traffic until SSTP is up.
  • On Windows, enforce “Use default gateway on remote network” in the PPP settings for full tunneling.

Auto-reconnect strategies: client-side and server-side

Auto-reconnect is best implemented as a layered strategy: built-in client mechanisms augmented with process supervision and scripting to handle edge cases.

Windows client: native tools and scripting

Windows SSTP clients can leverage built-in features and lightweight automation:

  • For managed Windows devices, use Group Policy to configure VPN profiles with “Always On” behavior (Enterprise environments can use Windows 10/11 Always On VPN features with SSTP profiles).
  • For standalone systems, use a combination of rasdial scripts and Task Scheduler triggers. A simple approach is a scheduled task that runs at network state changes (Event ID triggers) or periodically to verify the VPN state and call rasdial to reconnect if needed.
  • Monitor RasClient logs (Event Viewer) to detect repetitive failures and escalate or rotate credentials/certs as needed.

Example Windows reconnection logic (conceptual):

  • On network connect event: test tunnel by pinging a protected internal IP.
  • If test fails: run rasdial ProfileName /phonebook:path to start connection.
  • On repeated failures: collect logs, rotate cert checks, notify operators.

Linux client: systemd, pppd and sstp-client supervision

On Linux, use the sstp-client with pppd, and manage the process with systemd for robust auto-restart:

  • Create a systemd service that wraps sstpc/pppd and set Restart=on-failure and an appropriate RestartSec delay to avoid reconnection loops.
  • Use pppd options like persist, maxfail, lcp-echo-interval, and lcp-echo-failure to control link persistence.
  • Incorporate a network reachability check (e.g., ping to an internal IP or HTTP probe) plus DNS verification before declaring the tunnel up.

Automated reconnection backoff strategies

Implement exponential backoff with jitter for reconnection attempts to avoid thundering herd problems when many clients reconnect after an outage. Example pattern:

  • Attempt 1: immediate
  • Attempts 2–4: wait 2, 4, 8 seconds
  • Subsequent attempts: scale to minutes with random jitter

Backoff prevents overwhelming authentication servers, certificate validation endpoints, or middleboxes.

Server-side reliability: scaling, session persistence, and failover

Server configuration plays a major role in connection stability for many concurrent clients.

Load balancers and TLS session persistence

If you front SSTP servers with a TCP/TLS load balancer, ensure session persistence (TCP/SSL sticky sessions) to avoid expensive TLS renegotiation or dropping active PPP links. Consider:

  • Using a TCP passthrough mode for SSTP to preserve client-server TCP state.
  • Enabling session ticket or session-ID reuse to lower TLS handshake cost.

High availability and stateful failover

Stateless load balancing will break SSTP sessions on failover. Use stateful HA mechanisms that replicate session state or rely on fast Layer 2 failover within a cluster so existing TCP connections remain intact. If stateful failover is not possible, ensure your clients are prepared for reconnection with quick session re-authentication (short rekey times, efficient auth methods).

Authentication and rekey policies

Short-lived session keys and periodic rekeying are good for security but must be balanced with connection reliability. Set reasonable rekey intervals so that rekey events do not become frequent sources of disruption. Also ensure servers support OCSP/CRL and that clients have a fast path for validating server certificates during reconnection.

Operational practices: monitoring, logging, and testing

The best configuration still needs operational oversight. Implement the following:

Active monitoring

  • Monitor connection counts, per-client uptime, TLS handshake latencies, and authentication failure rates.
  • Probe both from inside and outside the VPN to ensure bi-directional reachability.

Logging and root-cause traces

  • Enable verbose logs selectively on pppd/SSTP for troubleshooting. Capture TLS alerts, LCP failure reasons, and TCP retransmission statistics.
  • Correlate client logs with server logs to identify patterns (e.g., particular NAT devices or mobile carriers causing issues).

Test reconnection scenarios

  • Simulate network flaps, TLS handshake failures, and certificate expiry to validate auto-reconnect behavior.
  • Measure time-to-reconnect and packet loss during failover to tune keepalive and backoff parameters.

Advanced tuning recommendations

For high-demand environments and developers building custom clients, consider these advanced measures:

MTU and MSS clamping

Because of encapsulation and TLS overhead, reduce MTU or clamp MSS to avoid fragmentation. Typical values:

  • Set MTU on the PPP interface to 1400–1420 for typical networks; tune down for mobile/DS-Lite environments.
  • Enforce MSS clamping on routers or the client so that TCP flows do not fragment inside the tunnel.

Session resumption and fast reconnect

Where possible, leverage TLS session resumption and PPP-level session identifiers to reduce handshake costs on reconnection. Some enterprise SSTP stacks and managed clients support faster reconnect via preserved session cookies or tickets.

Certificate lifecycle automation

Automate certificate issuance and renewal for clients and servers (e.g., internal PKI with SCEP/ACME adaptation). Expired certs are a common cause of “sudden disconnects” that look like network issues but are policy-driven.

Summary and rollout checklist

Bringing all pieces together, here is a compact checklist for deploying Always-On SSTP with robust auto-reconnect and reliability:

  • Enforce TLS 1.2+ and strong ciphers on both ends; enable OCSP/CRL checks.
  • Tune PPP LCP echo intervals/failure counts for rapid detection without false positives.
  • Adjust TCP keepalive and RTOs appropriately for your network characteristics.
  • Implement DNS/Destination blocking to prevent leaks between disconnect and reconnect.
  • Use process supervision (systemd on Linux, Task Scheduler/GPO on Windows) to auto-restart clients with exponential backoff.
  • Design server-side HA to preserve TCP/TLS sessions or prepare clients for fast re-authentication.
  • Monitor actively, log thoroughly, and run regular reconnection tests.

Implementing Always-On SSTP is a systems exercise as much as a configuration task: it requires client policies, network engineering, server resilience, and operational monitoring. With the settings and strategies listed above, administrators and developers can significantly improve session persistence and user experience while maintaining strong security.

For more practical guides and configuration examples tailored to enterprise deployments, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.