There is a problem with the SSH configuration, resulting in slow logon and transmission data

Phenomenon: There is a user feedback in our Aliyun bought 2 servers, through the intranet transmission data and SSH Landing are very slow, suspected that our intranet communication problems, to our investigation.

Analysis process and solution: After the discovery is the user's server SSH configuration file settings caused. The Usedns in the/etc/sshd/sshd_config configuration file is set to Yes and modified to No.

Explain the next usedns, when the client attempts to log on to the SSH server, the server side is based on the client's IP address DNS PTR reverse query the client's host name, and then according to the query out of the client host name DNS forward a record query, verify that its original IP address is consistent, This is a way to prevent client spoofing, but generally we are dynamic IP does not have PTR records, open this option is a waste of time, it is better to close it.