For full instructions, see Cisco.
To send Logging Information to a Syslog Server
logging host interface_name ip_address [tcp[/port] | udp[/port]] [format emblem]logging trap severity_levellogging facility number A server that runs a syslog application is required in order to send syslog messages to an external host. ASA sends syslog on UDP port 514 by default, but protocol and port can be chosen. If TCP is chosen as the logging protocol, this causes the ASA to send syslogs via a TCP connection to the syslog server. If the server is inaccessible, or the TCP connection to the server cannot be established, the ASA will, by default, block ALL new connections. This behavior can be disabled if you enable logging permit-hostdown. See the configuration guide for more information about the logging permit-hostdown command.