Unable to Access Some Websites

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If some sites will load, but other sites will not, there are a few possible causes.

  1. Check all of the items listed on Connectivity Troubleshooting before proceeding
  2. Ensure the WAN gateway is reachable and set to the proper address
  3. Ensure the subnet mask on the WAN interface of the firewall is correct
  4. Ensure the subnet mask on the client stations and on every interface (and VPN) on pfSense is correct
  5. Ensure the WAN MTU is properly set (See here to determine the MTU), use MSS to lower the MTU if necessary
  6. Use traceroute to determine where the traffic stops. It may be an upstream connectivity issue and not the pfSense firewall or ISP.
  7. Disable hardware checksums and see if the problem disappears
  8. Disable any proxy package such as squid that is in use (if any)
  9. Check Clear invalid DF bits instead of dropping the packets on System > Advanced, Firewall/NAT tab.
  10. Check Disable Firewall Scrub on System > Advanced, Firewall/NAT tab.