Overview

This article provides information on how to resolve Kerio VPN tunnel issues when establishing a connection.

Diagnosis

The following error shows in the VPN Debug logs:

  [09/Oct/2019 10:29:31] {vpntunnel} Tunnel[0001]('VPN tunel z Ostravy') - UDP
  packets not coming from the other side, VPN tunnel probably broken, disconnecting

  [09/Oct/2019 10:29:31] {vpntunnel} Tunnel[0001]('VPN tunel z Ostravy') - SSL_free
  ID:2

  [09/Oct/2019 10:29:31] {vpncore} Refcount for IP address 10.235.254.1 decremented
  to 1

  [09/Oct/2019 10:29:31] {vpncore} oldapi: KVpnConnectionDelete(2) -> SUCCESS

  [09/Oct/2019 10:29:31] {vpntunnel} Tunnel[0001]('VPN tunel z Ostravy') - connection
  deleted in driver

  [09/Oct/2019 10:29:31] {vpnippool} IP 10.235.254.3 freed

  [09/Oct/2019 10:29:31] {vpntunnel} Tunnel[0001]('VPN tunel z Ostravy') - tunnel
  deregistered

  [09/Oct/2019 10:29:31] {vpncore} oldapi: KVpnGetVpnAdapterIndex() -> SUCCESS

  [09/Oct/2019 10:29:31] {vpntunnel} Tunnel[0001]('VPN tunel z Ostravy') - flushing
  ARP cache for adapter 0xC, IP 10.235.254.3

  [09/Oct/2019 10:29:31] {vpntunnel} TUNNEL_STATUS_CHANGE 'VPN tunel z Ostravy'
  - 0:0

  [09/Oct/2019 10:29:31] {vpntunnel} Tunnel[0001]('VPN tunel z Ostravy') - tunnel
  closed

  [09/Oct/2019 10:29:36] {vpncore} UdpSocket: socket 0x15A closed

Root Cause

One of the remote parts has a problem with the fragmentation of IP-packets.

Workaround

1. Log in to Kerio Control Administration.

2. Manually set the MTU value (for WAN interface) for both ends to 1500:

   

Additional Information

VPN tunnel is broken when opening RDP-Client

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