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We have a Zenoss server in our main office monitoring (among many other things) an Apache server in a remote network, with a Tinc link between the two networks. The monitoring simply involves making an HTTP request to a URL once every 5 minutes and confirming that a response page comes back. Most of the requests to this particular web server succeed (and similar requests to other web servers

...sing one's compliment arithmetic, the above code generates new values that are off by one from the correct checksum for the packet in cases where the calculations wrap around zero in one direction but not the other.... According to Eqn 3 given in RFC 1624 (e.g. at https://tools.ietf.org/html/rfc1624#section-3 ) and the UpdateTTL() example function given RFC 1141, it looks like the calculation should instead be something like uint32_t csum = packet->data[50] << 8 | packet->data[51]; [...] csum ^= 0xffff; csum += oldmss ^ 0xffff; csum += newmss; csum = (...