HIDS 5703
Rule ID
5703
Status
Possible breakin attempt (high number of reverse lookup errors).
Description
This rule is detects when an application, such as sshd, has reported a high number of reverse lookup errors. A reverse lookup error occurs when your application attempts to determine what the fully qualified DNS name is for an IP address, and then looks up the fully qualified name to see if it matches the IP address. If they do not match, this may indicate that someone is spoofing the fully qualified domain name to try to trick your system into allowing them to log in.
For example, when this occurs with SSH you may see an error message such as this:
servername sshd[12345]: reverse mapping checking getaddrinfo for www.example.com failed - POSSIBLE BREAK-IN ATTEMPT!
In this example, a system has connected to your ssh server. That connection has an IP address. For the purposes of example, lets say that IP address is 1.2.3.4. The sshd service lookups that IP address, conducting what is called a "reverse lookup" to determine that the fully qualified domain name is for 1.2.3.4. The DNS server for 1.2.3.4 returns the name "www.example.com". Because anyone can return any name they want from a DNS server, this method is not an accurate way of determining if the answer is correct. You now have to reverse the process to see if "www.example.com" will resolve to 1.2.3.4. The sshd service then conducts a DNS query to ask the authoritative DNS server for www.example.com what the IP address is for www.example.com. If that DNS server returns an address that is different from 1.2.3.4, then the reverse mapping has failed. 1.2.3.4 is not the IP address for www.example.com, so someone may be trying to spoof the DNS address. This could also occur if someone made a mistake with their DNS names. Contact the DNS operators for both the domain name and IP address if you believe they have made a mistake.
False Positives
There are no known false positive for this rule. This rule simply reports when your application reports that this has occurred. If your application is in error, please contact your application vendor for assistance. If the DNS servers are in error, please contact the DNS operators. And if the DNS software is incorrectly reporting this information to your application, please contact your DNS vendor.
Tuning Recommendations
None.
Similar Rules
Knowledge Base Articles
None.
Outside References
