Keywords: identnone, identnonelimited, identtnonnumeric, identnonesymbolic, identtcp, identtcpnumeric, identtcpsymbolic, identtcplimited
The identtcp, identtcpsymbolic, and identtcpnumeric keywords tell the MTA to perform a connection and lookup using the IDENT protocol. The information obtained from the IDENT protocol (usually the identity of the user making the SMTP connection) is inserted into the Received: header of the message as follows:
identtcp inserts the host name corresponding to the incoming IP number, as reported from a DNS reverse lookup and the IP number itself.
identtcpsymbolic inserts the host name corresponding to the incoming IP number, as reported from a DNS reverse lookup; the IP number itself is not included in the Received: header.
identtcpnumeric inserts the actual incoming IP number—no DNS reverse lookup on the IP number is performed.
The remote system must be running an IDENT server for the IDENT lookup caused by identtcp, identtcpsymbolic, or identtcpnumeric to be useful.
Be aware that IDENT query attempts may incur a performance hit. Increasingly routers will “black hole” attempted connections to ports that they don’t recognize. If this happens on an IDENT query, then the MTA does not hear back until the connection times out (a TCP/IP stack controlled time-out, typically on the order of a minute or two).
Another performance factor occurs when comparing identtcp, indenttcplimited, or identtcpsymbolic to identtcpnumeric. The DNS reverse lookup called for with identtcp, identtcplimited, or identtcpsymbolic incurs some additional overhead to obtain the more user-friendly host name.
The identnonenumeric keyword disables this IDENT lookup and inhibits the usual DNS reverse lookup translation of IP number to host name, and might result in a performance improvement at the cost of less user-friendly information in the Received: header. This is the default.
The identtcplimited and identnonelimited keywords have the same effect as identtcp and identnone, respectively, as far as IDENT lookups, reverse DNS lookups, and information displayed in Received: header. Where they differ is that with identtcplimited or identnonelimited the IP literal address is always used as the basis for any channel switching due to use of the switchchannel keyword, regardless of whether the DNS reverse lookup succeeds in determining a host name.