Securing and troubleshooting SSTP (Secure Socket Tunneling Protocol) VPN connections requires more than reacting to user complaints. Effective monitoring and log analysis provide visibility into authentication failures, SSL/TLS problems, performance bottlenecks, and attacks that attempt to hide behind TCP/443. This article provides practical, technically detailed guidance for system administrators, developers, and platform owners who operate SSTP endpoints—especially those running on Windows RRAS, Linux SSTP servers, or multi-protocol appliances.

Where SSTP Logs Come From

Understanding the sources of relevant telemetry is the first step. SSTP-related events and artifacts are generated in several places:

  • VPN server application logs — e.g., Windows Routing and Remote Access Service (RRAS), SoftEther, or sstpd on Linux. These logs record tunnel lifecycle events, authentication outcomes, and per-session statistics.
  • System event logs / Event Tracing for Windows (ETW) — useful for deeper diagnostics on Windows; contains SSL/TLS handshake errors, service-level warnings and errors.
  • Authentication & accounting services — Network Policy Server (NPS), RADIUS (FreeRADIUS, radiator), which log authentication requests, responses, and accounting records (start/stop, bytes).
  • Firewall and IDS/IPS logs — capture connection attempts, drops, and blocked packets that affect SSTP over TCP/443.
  • Packet captures — full-packet captures using tcpdump, Wireshark, or Windows netsh traces. Because SSTP rides over TLS, these captures show TCP/TLS level details and allow correlation of retransmissions, handshake phases, and timing.
  • Client-side logs — client OS VPN diagnostics (Windows Event Viewer for SSTP clients, mobile VPN logs) are essential when the server shows no errors.

Key Fields and Indicators to Extract

When you collect logs, extract structured fields that matter for security and troubleshooting:

  • Timestamps — use synchronized clocks (NTP) across servers; logs without accurate timestamps make correlation impossible.
  • User identifiers — username, caller ID, or certificate subject (DN) for certificate-based auth.
  • Client and server IPs — source IP, NAT public IP, and internal mapped IP if applicable.
  • Authentication method — EAP, MSCHAPv2, certificate-based, RADIUS/LDAP source.
  • TLS/SSL details — server certificate CN/SAN, issuer, TLS version, negotiated cipher suite, certificate chain validation results, and TLS alerts (e.g., bad_certificate, decrypt_error).
  • Session metrics — session duration, bytes in/out, packets lost, retransmissions, and throughput.
  • Error codes — vendor-specific codes, Windows VPN error codes, RADIUS result codes, and TCP/TLS error indications.

Enabling and Collecting Logs: Practical Steps

Below are concrete steps to increase visibility for common SSTP installations.

Windows RRAS and Event Tracing

  • Enable verbose RRAS logging via the RRAS console: open Routing and Remote Access → server properties → Logging and Accounting to configure log files and retention.
  • Use ETW and netsh tracing to capture low-level events: run netsh trace start capture=yes tracefile=C:tracessstp_trace.etl before reproducing the issue, then netsh trace stop. The ETL contains TCP and TLS handshake details and can be converted/analyzed with Microsoft tools or exported to pcapng for use in Wireshark.
  • Collect Windows Event Viewer logs (System and Application), and look for warnings/errors from RRAS, TCP/IP, SCHANNEL (TLS) and NPS.

RADIUS / NPS

  • Enable accounting (start/stop) on the RADIUS server so each SSTP session has a corresponding accounting record, including bytes and duration.
  • Log the full RADIUS attributes (User-Name, Calling-Station-Id, Framed-IP-Address, NAS-IP-Address, Acct-Status-Type, etc.) for forensic correlation.

Linux SSTP and Third-Party Servers

  • sstpd implementations usually log to syslog; set a more verbose loglevel in the service config (e.g., /etc/rsyslog.conf or the service’s own config) and forward logs to a central syslog or ELK for analysis.
  • Capture TLS handshake details with tcpdump: tcpdump -i eth0 port 443 -w sstp.pcap and analyze in Wireshark with TLS dissectors enabled. If certificate private keys are available in a test environment, enable TLS decryption in Wireshark for full SSTP payload visibility.

Analyzing Common Failure Modes

Below are common problems, how they manifest in logs/captures, and diagnostic steps.

Authentication Failures

  • Symptoms: repeated RADIUS Access-Reject entries, client-side errors showing authentication error, or RRAS logging “authentication failed.”
  • Checks: confirm username/password or certificate validity, check RADIUS shared secret mismatch, verify that the authentication method configured on server and client match (e.g., EAP vs MSCHAPv2).
  • Logs to examine: RADIUS logs for Access-Request and Access-Accept/Reject, server-side RRAS logs, and client Event Viewer entries.

TLS Handshake / Certificate Problems

  • Symptoms: TLS alert messages in captures (e.g., fatal alerts), client reports “cannot communicate with the remote computer,” or server logs show certificate validation errors.
  • Checks: verify certificate chain and validity dates, confirm the server certificate CN/SAN includes the hostname clients connect to, and ensure the certificate private key is accessible to the VPN service.
  • Packet-level debug: observe ClientHello and ServerHello; check chosen TLS version and cipher. If the ServerHello is missing, the server likely failed to load a cert or aborted the handshake.

Performance Issues and TCP Behavior

  • Symptoms: low throughput, high latency, frequent disconnects.
  • Checks: check TCP retransmissions and windowing in packet captures; look for duplicated ACKs, excessive retransmissions, and high RTTs. On the server, watch for CPU-bound TLS encryption if using many simultaneous SSTP sessions.
  • Mitigations: tune TCP parameters (congestion control, window scaling) at OS level, enable TCP offload features if appropriate, and consider SSL/TLS acceleration or load balancing for high session counts.

NAT and Firewall Interference

  • Symptoms: partial connectivity (can establish but not pass traffic), MTU-related fragmentation, or sessions dropped when crossing certain NAT devices.
  • Checks: inspect MSS/MTU negotiation and ICMP “fragmentation needed” messages in captures. Firewalls performing HTTPS inspection can break SSTP if they intercept or replace certificates.
  • Mitigations: configure path MTU discovery or lower MTU/MSS on endpoints; ensure HTTPS inspection devices are configured to allow passthrough for SSTP or have appropriate CA trust installed on clients if interception is unavoidable (with caution).

Tools and Techniques for Deep Analysis

Here are recommended tools and how to use them effectively:

  • Wireshark / tshark — capture TCP 443 traffic and filter by IP to analyze handshake timing, sequence numbers, retransmissions, and TLS alerts. Use follow TCP stream to inspect exchange sequences. For SSTP-specific payload analysis, decrypt TLS if keys are available (test environments).
  • netsh trace — native Windows trace generation producing ETL files that include kernel-level events and TCP traces. Useful for intermittent issues as it records across the stack.
  • ETW and Perfmon — monitor RRAS counters, CPU, context switches, and socket counts to find resource exhaustion.
  • Log aggregators (Splunk, ELK, Graylog) — centralize RRAS, RADIUS, firewall and syslog events for correlation. Build dashboards for session counts, failed auth rate, cert errors, and unusual client IP spikes.
  • SIEM correlation rules — detect brute-force attempts by correlating many Access-Request entries with repeated failures from multiple usernames or IPs; trigger alerts on spikes in TLS handshake failures which may indicate scanning or MITM devices.

Parsing and Automating Insights

Automate log parsing to extract actionable metrics and generate alerts:

  • Regular expressions or Grok patterns to extract username, NAS-IP-Address, Acct-Status-Type, and bytes from RADIUS/accoounting logs.
  • Create SPL/Elastic queries to compute:
    • Top 10 users by connection count or bandwidth
    • Authentication failure rate and trends (e.g., failed attempts per minute)
    • Distribution of TLS versions and ciphers used — to identify legacy clients
    • Session duration percentiles (p50, p95) and sudden drops in average session duration
  • Set alerts for anomalies: e.g., >X authentication failures in Y minutes for a single account, sudden increase in TLS alert codes, or sustained high retransmission rate on SSTP server IPs.

Forensic and Security Best Practices

When investigating suspected compromise or abuse:

  • Preserve raw packet captures and server log files for the relevant timeframe. Ensure chain-of-custody if required by policy.
  • Correlate VPN session start times with downstream activity logs (web proxies, access logs) to determine what a compromised session accessed.
  • Look for lateral movement patterns: new internal IP allocations, unusual service access patterns, or multi-stage authentication anomalies.
  • Rotate certificates/keys and terminate suspicious sessions proactively. If abuse is confirmed, revoke certificates or reset credentials and perform a forced reauthentication for affected users.

Practical Checklist for Running SSTP in Production

  • Centralize logs (RRAS, RADIUS, firewall, IDS) into a log analysis platform.
  • Keep TLS certificates valid, and use strong TLS versions and ciphers; plan deprecation of weak ciphers.
  • Monitor authentication failure rates and session counts; alert on spikes or patterns that indicate brute-force or credential stuffing.
  • Capture packet traces for intermittent problems and conversely keep long-term accounting for usage and compliance.
  • Document operational runbooks: how to enable traces, relevant commands (netsh trace start/stop), and steps to gather client logs.

Monitoring and analyzing SSTP VPN logs is a multi-layered activity: combine application logs, authentication records, packet traces, and OS-level telemetry. Use centralized aggregation and automated queries to detect problems early, and keep forensic artifacts when investigating security incidents. With the practices outlined here—structured field extraction, packet-level inspection, and robust alerting—you can both improve service reliability and harden your SSTP deployment against abuse.

For more resources and configuration examples tailored to managed SSTP setups, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.