sendto error: 64

64 EHOSTDOWN Host is down. A socket operation failed because the destination host was down.

In this case, the firewall is unable to reach the a target host directly connected at layer 2 (No ARP response), or it received a similar error response from an upstream source. Generally this only happens due to remote problems, indicating that the target is actually down or the L1/L2 link to the target is down.