What ports should I leave open for MySQL - Ubuntu server
You need to block all ports excepts the one you need 3306, and 22 if you need your servers to get accessed by MySQL and SSH only, but you need to keep in mind if there are some connections that should be opened in order to update your server and install new packages and that can be only determined by you. here is the list of known ports maybe you are use some of them.


  20 – FTP Data (For transferring FTP data)
  
  21 – FTP Control (For starting FTP connection)
  
  22 – SSH (For secure remote administration which uses SSL to encrypt
  the transmission)
  
  23 – Telnet (For insecure remote administration)
  
  25 – SMTP (Mail Transfer Agent for e-mail server such as SEND mail)
  
  53 – DNS (Special service which uses both TCP and UDP)
  
  67 – Bootp
  
  68 – DHCP
  
  69 – TFTP (Trivial file transfer protocol uses udp protocol for
  connection less transmission of data)
  
  80 – HTTP/WWW(Apache)
  
  88 – Kerberos
  
  110 – POP3 (Mail delivery Agent)
  
  123 – NTP (Network time protocol used for time syncing uses UDP
  protocol)
  
  137 – NetBIOS (nmbd)
  
  139 – SMB-Samba (smbd)
  
  143 – IMAP
  
  161 – SNMP (For network monitoring)
  
  389 – LDAP (For centralized administration)
  
  443 – HTTPS (HTTP+SSL for secure web access)
  
  514 – Syslogd (udp port)
  
  636 – ldaps (both ctp and udp)
  
  873 – rsync
  
  989 – FTPS-data
  
  990 – FTPS
  
  993 – IMAPS
  
  1194 – openVPN
  
  1812 – RADIUS
  
  995 – POP3s
  
  2049 – NFS (nfsd, rpc.nfsd, rpc, portmap)
  
  2401 – CVS server
  
  3306 – MySql
  
  3690 – SVN
  
  6000-6063-X11