This is probably caused by a firewall or DNS timeout. By default ProFTPD will try to do both DNS and ident lookups against the incoming connection. If these are blocked or excessively delayed a slower than normal login will result. To turn off DNS and ident use:
UseReverseDNS off
IdentLookups off
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也就是在proftpd的配置文件里,加入
UseReverseDNS off
IdentLookups off
这两行,重启proftpd就OK