Virtual Private Networks using the Point-to-Point Tunneling Protocol (PPTP) remain in use across legacy systems and some enterprise environments. Although PPTP is widely considered obsolete compared to modern VPN technologies, understanding how PPTP traffic looks on the wire remains a valuable skill for network engineers, security analysts, and developers maintaining compatibility with older equipment. This article provides a hands-on, packet-capture-focused walkthrough for decoding PPTP VPN traffic, with practical tips for capture, analysis, and interpretation of both the control and data planes.

Background: What to expect from PPTP traffic

PPTP separates control and data traffic into two distinct channels:

  • Control channel: A TCP session on port 1723 used for session negotiation and management. This channel carries PPTP control messages (Start-Control-Connection-Request/Reply, Outgoing-Call-Request/Reply, etc.).
  • Data channel: GRE (Generic Routing Encapsulation) packets that carry PPP frames encapsulating user IP traffic. The GRE stream contains PPP negotiation (LCP/Authentication/IPCP) and the encrypted IP payload when encryption is negotiated.

Authentication often uses MS-CHAPv2 (Microsoft Challenge Handshake Authentication Protocol version 2), and encryption—if enabled—is typically Microsoft Point-to-Point Encryption (MPPE). Knowing these building blocks helps you filter and decode captures effectively.

Setting up a safe capture lab

Before capturing live traffic, create an isolated lab to avoid privacy and legal issues. Use two virtual machines (one client, one PPTP server) on an internal network, or mirror traffic to a monitoring host. Basic requirements:

  • A controlled PPTP server (e.g., Windows RRAS, Linux pptpd implementation)
  • A PPTP-capable client (Windows, some mobile devices, or open-source clients)
  • Packet capture tools: tcpdump/tshark for CLI capture, Wireshark for interactive analysis

Capture command example (Linux):

sudo tcpdump -i eth0 -w pptp_capture.pcap 'tcp port 1723 or proto 47'

This captures both the TCP-based control channel and GRE (protocol 47) traffic into a single pcap file for later inspection in Wireshark.

Initial triage with Wireshark

Open the pcap in Wireshark and start with a few focused filters to separate control and data:

  • Control channel: tcp.port == 1723 or pptp (Wireshark has a PPTP dissector)
  • GRE data channel: ip.proto == 47 or simply gre
  • PPP and authentication packets: ppp, ppp_lcp, ppp_pap, ppp_chap, ppp_ipcp, ms-chapv2

Start by following the TCP stream for port 1723 to view the control handshake. Wireshark will decode PPTP control messages, revealing connection establishment, control connection options, call IDs, and peer info.

Decoding the control channel

The TCP-based control channel holds messages that set up the GRE data path. Key fields you should inspect:

  • Call ID: Unique identifiers for the GRE session; you’ll correlate call IDs in TCP control messages with GRE packets’ Call ID fields to pair control and data.
  • PPTP message types: Look for Start-Control-Connection-Request/Reply, Outgoing-Call-Request/Reply, and Call-Disconnect-Notify. These indicate session lifecycle events.
  • Magic cookie and versions: Used for protocol identification; persistent mismatches can indicate incompatible implementations.

Understanding the control exchange is important because it provides the parameters used for the GRE data channel (e.g., call IDs and whether encryption is negotiated).

Decoding the GRE/PPP data plane

GRE encapsulates PPP frames. Use the following steps when analyzing GRE packets:

  • Filter on gre in Wireshark to isolate GRE encapsulated traffic.
  • Match GRE Call ID to the call ID observed on the control channel to ensure you’re analyzing the correct tunnel.
  • Inspect the PPP protocol field inside GRE. Expect PPP LCP, PPP authentication protocols (CHAP/MS-CHAP), IPCP, and finally IP payloads.

When MPPE encryption is negotiated, the PPP payload becomes encrypted, and you will generally not be able to see inner IP headers without keys. However, some negotiation frames (LCP, CHAP) occur before encryption is enabled and can leak metadata.

Analyzing MS-CHAPv2 authentication

MS-CHAPv2 operates inside PPP/CHAP and is usually visible before MPPE starts. In a capture you may observe these stages:

  • CHAP Challenge to client (from server)
  • CHAP Response (client proves knowledge of password-derived secret)
  • Success/Failure codes

Wireshark can dissect MS-CHAPv2 and display challenge and response fields. Those values are sensitive: with a captured MS-CHAPv2 handshake, attackers can attempt offline cracking to recover credentials. Tools such as John the Ripper and Hashcat can be used on offline challenge/response data to attempt password recovery—this is a common reason PPTP/MS-CHAPv2 is considered weak.

Ethical and legal note: Only capture and attempt cracking in environments where you have explicit permission. Unauthorized interception and credential cracking are illegal in many jurisdictions.

Detecting MPPE and understanding encryption limits

MPPE (Microsoft Point-to-Point Encryption) is negotiated after authentication via PPP. In packet captures, you can detect that MPPE is active by examining LCP negotiation and PPP options. When MPPE is present:

  • PPP payloads become encrypted and no longer reveal inner IP/TCP/UDP headers.
  • Control-plane metadata (timestamps, packet sizes, directionality) still exists and may be useful for traffic analysis or QoS troubleshooting.

Even when MPPE is enabled, some negotiation packets and the MS-CHAPv2 exchange remain useful for forensic or diagnostic purposes. However, MPPE uses symmetric keys derived from authentication; without those keys you cannot decrypt the MPPE-encrypted payloads in Wireshark.

Practical packet analysis workflow

Follow a structured approach when analyzing a PPTP capture:

  • 1) Start with a quick overview: use capture summary, protocol hierarchy, and top talkers to find PPTP-related flows.
  • 2) Isolate control traffic: apply tcp.port==1723 and analyze the control handshake, noting Call IDs and options.
  • 3) Isolate GRE traffic: apply ip.proto==47 or gre, and correlate Call IDs to the control channel.
  • 4) Decode PPP layers: inspect LCP, CHAP/MS-CHAPv2, and IPCP payloads visible inside GRE before encryption is enabled.
  • 5) Note timing, packet sizes, and sequence: even encrypted tunnels leak metadata useful for performance or security analysis.

Command-line tips and filters

Common tcpdump/tshark/Wireshark filters and commands used in PPTP analysis:

  • Capture both control and GRE: tcpdump -i eth0 -w pptp.pcap 'tcp port 1723 or proto 47'
  • Wireshark display filters: tcp.port==1723, ip.proto==47, ppp, ms-chapv2
  • Quickly find MS-CHAPv2 packets: ms-chapv2 in display-filter
  • Follow control stream: right-click a TCP control packet and choose “Follow TCP Stream” to visualize control message sequence

Limitations, pitfalls, and security implications

Keep in mind several important caveats:

  • PPTP is insecure: MS-CHAPv2 has known cryptographic weaknesses and MPPE keys are derived from that exchange. Do not rely on PPTP for sensitive or compliance-restricted traffic.
  • Encrypted payloads are opaque: once MPPE is active, payloads are not readable without keys. Only pre-encryption negotiation frames and metadata are available for analysis.
  • Packet fragmentation and ordering: GRE encapsulation and PPP framing can interact with MTU and fragmentation. Correlate TCP control messages and ICMP fragmentation messages when diagnosing problems.
  • False positives: GRE traffic can carry many protocols. Correlate Call IDs and control channel activity to avoid misattributing unrelated GRE flows to PPTP sessions.

When to recommend modernization

For administrators and developers maintaining remote access services, PPTP should be considered a legacy option. Modern alternatives like OpenVPN, WireGuard, and IPsec offer stronger cryptography, better auditing, and active support. If you must support PPTP for compatibility, limit exposure by: restricting endpoints, using it only on internal networks, and logging/monitoring with strong packet-capture practices to detect misuse.

Conclusion

Decoding PPTP traffic requires understanding the split between the TCP-based control plane and the GRE-based data plane, familiarity with PPP and MS-CHAPv2 negotiation, and practical skills with tools such as tcpdump and Wireshark. In many captures, you can extract meaningful diagnostic and authentication metadata even when payload encryption obscures inner IP traffic. However, the inherent weaknesses of MS-CHAPv2 and PPTP mean that long-term solutions should move to modern VPN technologies. For continued reference, testing, and lab builds you can rely on methodical capture and analysis workflows outlined above.

For more in-depth VPN analysis guides and managed dedicated IP VPN solutions, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.