The log shows "IPSec packet to or from illegal host".

03/26/2020 7 12854

DESCRIPTION:
Troubleshooting and resolutions for the "IPSec packet from illegal host” message.

RESOLUTION:

An “IPSec packet from illegal host” message can be due to a number of causes, including but not limited to any one of the following:

1. Box-to-box VPN tunnel has NOT been established. In this instance, the message may indicate an incorrectly configured VPN box-to-box SA. Usually, manual key SPIs do not match or are reversed.
   
2. Box-to-box VPN tunnel HAS been established.In this case, this message can be due to having one SA configured to allow for NetBIOS and another SA configured not to allow for NetBIOS. Both should be configured the same for NetBIOS. If the SAs are allowed for NetBIOS pass-through, make sure that the ending range under DESTINATION NETWORK (if configured in manual key) is x.x.x.255. This will allow for broadcast traffic through the tunnel.
   
3. You have a client computer using our VPN client software trying to connect to previously existing SAs on the SonicWall.Verify that you do not have any VPN client software in the field active.
   
4. A router between the source and destination is configured to ignore the DF bit and fragments the frame anyway. This results in fragmentation issues. To enable fragmented packet handling on the SonicWall following these steps:
   
• Select Enable Fragmented Packet Handling and uncheck the option Ignore DF bit.
  
• Click Apply.

Additionally, try upgrading the firmware to the latest version. You may also want to delete both SAs and recreate them.

Fragmentation may still occur on the network under certain circumstances.For example, a host will set a bit flag in the IP header of all TCP frames it transmits which informs routers that fragmentation is not allowed. This is known as the “Don’t Fragment” or DF bit. When a router receives a frame that is too large to be transmitted onto the next network, it will check to see if the DF bit is set. If it is not, then the frame is fragmented and forwarded on to the destination. If the DF bit is set, then the router should discard the frame and return an ICMP message to the sender indicating that fragmentation was required but the DF bit was set. This process will fail if a router between the source and destination needs to fragment the frame and either fails to return the ICMP message to the sender, or the message gets blocked due to packet filtering. This is known as a “black hole router”. In this case, the frame will be discarded silently and the sender will retransmit the frame several times until the TCP session terminates.