Point-to-point tunneling protocol (PPTP) remains in use because of its simplicity and wide client support, but it can produce stubborn DNS and routing problems that frustrate administrators and users alike. This guide walks through practical, technical troubleshooting steps for diagnosing and fixing PPTP VPN DNS and routing issues on clients and servers. It targets webmasters, enterprise admins, and developers who need dependable connectivity and predictable name resolution across PPTP tunnels.

Start with a clear problem statement and reproduction steps

Before changing configuration, gather a concise description: which client OS, VPN client, server software/version (pptpd, Windows RRAS, pfSense, etc.), symptoms (unable to resolve names, partial routing, only internal IPs reachable), and whether the issue is reproducible from multiple clients. Ask the user to provide:

  • Exact error messages or the output of failed pings and nslookups.
  • Whether the client can reach remote IPs by address (e.g., 8.8.8.8) but not by hostname.
  • Full timing and sequence: connect → success → traffic behavior.

Basic client-side checks

Start on the client machine. Many DNS problems are client-side policy or cache issues.

Network state and routing table

Check assigned addresses and routes immediately after connecting.

  • Windows: run ipconfig /all and route print.
  • Linux/macOS: use ip addr or ifconfig, and ip route or netstat -rn.

Look for the PPTP/PPP adapter (often named PPP adapter or ppp0) and confirm whether a default route is set via the tunnel. If the tunnel provides an IP but there is no route for 0.0.0.0/0 via the PPP device, name resolution to remote DNS may fail depending on server pushes and client policy.

DNS server assignments and search domains

Use nslookup or dig to test both forward and reverse resolution and to verify which DNS server is being queried:

  • Windows: nslookup example.com shows the default server. Check ipconfig /all for DNS servers on the VPN adapter.
  • Linux: resolvectl status or inspect /etc/resolv.conf (note: systemd-resolved may make this indirect).

If the DNS server is the public ISP or the client’s previous DNS rather than the VPN’s internal DNS, either the server did not push DNS settings or the client ignored them due to OS policy.

DNS cache and Winsock issues

Clear local caches as these can mask changes:

  • Windows: ipconfig /flushdns, and if necessary netsh winsock reset.
  • Linux: restart systemd-resolved or nscd, or clear DNS cacher (dnsmasq) caches.
  • macOS: sudo killall -HUP mDNSResponder (or correct command for the OS version).

Server-side PPTP/pppd configuration to push DNS and routes

Common server implementations include pptpd on Linux, Microsoft RRAS, and embedded platforms. On Linux, pppd options control what the client receives.

Pushing DNS with pppd

Add DNS pushes in the pppd configuration or the PPTP server options. Typical entries:

  • ms-dns 10.0.0.10 — instructs PPP clients to use 10.0.0.10 as a DNS server.
  • For Windows clients, you’ll see the server pushed as the DNS server for the PPP adapter.

Ensure your /etc/ppp/options or /etc/ppp/chap-secrets setup includes these pushes or that your server sends them in per-user scripts. If you are using a distro-specific script (like /etc/ppp/ip-up), verify it runs and updates resolvconf/systemd-resolved.

Default route behavior

PPTP clients may receive a default route via the PPP interface. pppd often adds a default route by default; however, some clients prefer split tunneling and will ignore or deprioritize the pushed default route. Options to control routing behavior:

  • defaultroute — pppd adds a default route through the PPP link.
  • nodefaultroute — prevents adding a default route (useful for split tunnels).
  • On Windows RRAS, check “Use default gateway on remote network” in IPv4 properties.

If the server intentionally leaves routing to the client (split-tunnel), you must ensure DNS server IPs are still reachable via the tunnel; otherwise, the client will continue to query its local ISP DNS.

Common causes and fixes

1. PPTP connects, can ping IPs, but DNS fails

Symptoms: ping 10.0.0.1 (internal DNS) works; nslookup times out or queries an ISP DNS.

Fixes:

  • Confirm the server pushes DNS via ms-dns and the client applied it. If not applied, manually set DNS on the PPP adapter or add a route to the internal DNS through the tunnel.
  • On Linux clients using resolvconf or systemd-resolved, ensure the ip-up script updates resolvconf with the pushed DNS. Example ip-up script line: echo "nameserver $DNS1" > /run/resolvconf/resolv.conf or use resolvconf hook scripts.

2. Split tunneling leads to mixed DNS queries

If the VPN is split-tunnel and internal resources require internal DNS, the client may still query public DNS for names that should resolve internally.

Fixes:

  • Push internal DNS servers and ensure the client uses them for the domains in question (set DNS search suffixes if necessary).
  • Use DNS conditional forwarding on internal DNS servers or use a DNS proxy that forwards internal queries appropriately.

3. GRE blocked by firewall — data doesn’t flow

PPTP control channel uses TCP 1723, but the actual tunneled data uses GRE (protocol 47). If a firewall allows TCP 1723 but blocks GRE, the session establishes but traffic stops or routing appears broken.

Fixes:

  • Allow GRE (protocol 47) and TCP/1723 through intermediate firewalls and NAT devices.
  • On Linux NAT gateways, ensure iptables rules allow forwarding of GRE and that connection tracking is enabled.

4. MTU/MSS issues causing fragmented DNS/UDP failure

Large DNS responses or TCP connections may be blocked if MTU is too large over the PPP link, causing silent failures.

Fixes:

  • Lower MTU on the PPP interface (e.g., mtu 1400 in pppd options) or use MSS clamping on the gateway (iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu).

Advanced diagnostics — packet capture and logging

If the configuration checks don’t reveal the culprit, capture traffic on both client and server sides.

  • Use tcpdump or Wireshark to capture PPP/GRE traffic: tcpdump -i ppp0 -w client-ppp.pcap or on the server capture on the physical interface and ppp interfaces.
  • Inspect whether DNS queries leave the PPP interface and whether DNS responses come back. If requests are sent to the wrong server, you’ll see that on the capture.
  • Enable verbose logging for pppd (debug option) and check /var/log/messages or syslog for pppd negotiation details, MS-DNS push, and route additions.

Environment-specific tips

Windows clients

Windows may ignore DNS pushed by the server unless the PPP adapter becomes the preferred network in the interface metric. Fixes:

  • Open Network Connections → PPP adapter Properties → IPv4 → Advanced and check “Use default gateway on remote network” if you want all traffic routed through the VPN.
  • Adjust interface metric: uncheck automatic metric and set a lower metric on the PPP interface.
  • Use netsh interface ipv4 add dnsservers <ifname> <dnsip> index=1 to set DNS explicitly.

Linux servers and clients

Watch out for resolvconf and systemd-resolved interactions. If pppd doesn’t call resolvconf hooks, the system will not update DNS. Place scripts in /etc/ppp/ip-up.d and ensure they are executable. Example script should parse $DNS1 and add to resolvconf.

Embedded or hardware VPN gateways

Many appliances have their own DNS and routing settings. Ensure DNS push options are enabled and that firewall policies include GRE. Check vendor docs for how DNS push is implemented (some only push to Windows clients).

Checklist: quick remediation workflow

  • Confirm connectivity to remote IPs over VPN (ping remote DNS IP).
  • Check PPP adapter IP and routes (ipconfig/ifconfig + route print/ip route).
  • Verify DNS servers assigned to PPP adapter (ipconfig /all or resolvectl).
  • Flush DNS caches (ipconfig /flushdns, systemd-resolved restart).
  • Inspect pppd/pptpd server config for ms-dns and routing options; enable defaultroute if appropriate.
  • Check firewall/NAT for GRE (protocol 47) and port 1723; ensure ip_forward is enabled and forwarding rules permit PPP traffic.
  • Capture packets (tcpdump/Wireshark) to confirm queries and responses traverse the tunnel.
  • Adjust MTU/MSS if fragmentation or large transfers fail.

Diagnosing PPTP DNS and routing issues requires methodical checks across client and server configurations, firewall policies, and OS-specific DNS handling. Following the steps above will resolve the majority of problems: ensure the server pushes correct DNS and routes, verify clients accept and apply them, and check for GRE/firewall or MTU-related failures when traffic stalls despite a successful control connection.

For more practical configuration examples and advanced troubleshooting scripts, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.