Identtcp, identtcplimited, identtcpnumeric, identtcpsymbolic, identnone, identnonelimited, identnonenumeric, identnonesymbolic, forwardchecknone, forwardchecktag, forwardcheckdelete Channel Options

Reverse DNS and IDENT lookups on incoming SMTP connections
The  channel option tells the MTA to perform a connection and lookup using the IDENT protocol (RFC 1413). The information obtained from the IDENT protocol (usually the identity of the user making the SMTP connection) is then inserted into the Received: header for the message, with the hostname corresponding to the incoming IP number, as reported from a DNS reverse lookup, and the IP number itself.

The  channel option tells the MTA to perform a connection and lookup using the IDENT protocol (RFC 1413). The information obtained from the IDENT protocol (usually the identity of the user making the SMTP connection) is then inserted into the Received: header for the message, with the hostname corresponding to the incoming IP number, as reported from a DNS reverse lookup; the IP number itself is not included in the Received: header.

The  channel option tells the MTA to perform a connection and lookup using the IDENT protocol (RFC 1413). The information obtained from the IDENT protocol (usually the identity of the user making the SMTP connection) is then inserted into the Received: headers of the message, with the actual incoming IP number --- no DNS reverse lookup on the IP number is performed.

Note that the remote system must be running an IDENT server in order for the IDENT lookup caused by the,  , or   options to be useful. In addition, be aware that IDENT query attempts may incur a serious performance hit. Increasingly routers simply "black hole" attempted connections to ports that they don&#x27;t recognize; if this happens on an IDENT query, then the MTA does not hear back until the connection times out (a TCP/IP package controlled timeout, typically on the order of a minute or two). A lesser performance factor is that when comparing  or   vs., note that the DNS reverse lookup called for with   or   incurs some additional overhead to obtain the more "user-friendly" hostname.

The  channel option disables this IDENT lookup, but does do IP to hostname translation, and both IP number and hostname will be included in the Received: header for the message. The  channel option disables this IDENT lookup, but does do IP to hostname translation; only the hostname will be included in the Received: header for the message. The  channel option disables this IDENT lookup and inhibits the usual DNS reverse lookup translation of IP number to hostname, and may therefore result in a performance improvement at the cost of less user-friendly information in the Received: headers. is the default.

The  and   channel options have the same effect as   and , respectively, as far as IDENT lookups, reverse DNS lookups, and information displayed in Received: header lines. Where they differ is that with  or   the IP literal address is always used as the sole basis for any channel switching due to use of the   channel option, regardless of whether the DNS reverse lookup succeeds in determining a host name. Note that since channel switching is always performed preferentially based on IP address rather than host name, the effect of  or   is merely to disable ever trying host name switching in case all IP address rewriting failed.

The,  , and   channel options can modify the effects of doing reverse lookups, controlling whether the MTA does a forward lookup of an IP name found via a DNS reverse lookup, and if such forward lookups are requested, further control what the MTA does in case the forward lookup of the IP name does not match the original IP number of the connection. The  channel option is the default, and means that no forward lookup is done. The  channel option tells the MTA to do a forward lookup after each reverse lookup and to tag the IP name with an asterisk, , if the number found via the forward lookup does not match that of the original connection. The  channel option tells the MTA to do a forward lookup after each reverse lookup and to ignore (delete) the reverse lookup returned name if the forward lookup of that name does not match the original connection IP address, and stick with the original IP address instead. (Note that having the forward lookup not match the original IP address is normal at many sites, where a more "generic" IP name is used for several different IP addresses.)

These options are only useful on SMTP channels that run over TCP/IP.

See also:
 * TCPIP channels
 * TCPIP connections and DNS lookups channel options
 * Channel options