Altering read timeouts

It is possible to time out when reading the standard input or when reading from a remote SMTP server. These timeouts are set using the Timeout (r) option in the configuration file. The option names are all of the form Timeout.suboption. The recognized suboptions, their default values, and the minimum values allowed by RFC 1123 section 5.3.2 are:

connect

The time to wait for an SMTP connection to open (the connect(SSC) system call) [0, unspecified]. If zero, uses the kernel default. In no case can this option extend the timeout longer than the kernel provides, but it can shorten it. This is to get around kernels that provide an absurdly long connection timeout (90 minutes in one case).

iconnect

The same as connect, except it applies only to the initial attempt to connect to a host for a given message [0, unspecified]. The concept is that this should be very short (a few seconds); hosts that are well connected and responsive will thus be serviced immediately. Hosts that are slow will not hold up other deliveries in the initial delivery attempt.

initial

The wait for the initial 220 greeting message [5m, 5m].

helo

The wait for a reply from a HELO or EHLO command [5m, unspecified]. This may require a host name lookup, so five minutes is probably a reasonable minimum.

mail+

The wait for a reply from a MAIL command [10m, 5m].

rcpt+

The wait for a reply from an RCPT command [1h, 5m]. This should be long because it could be pointing at a list that takes a long time to expand.

datainit+

The wait for a reply from a DATA command [5m, 2m].

datablock+

The wait for reading a data block (that is, the body of the message). [1h, 3m]. This should be long because it also applies to programs piping input to sendmail which have no guarantee of promptness.

datafinal+

The wait for a reply from the dot terminating a message. [1h, 10m]. If this is shorter than the time actually needed for the receiver to deliver the message, duplicates will be generated. This is discussed in RFC 1047.

rset

The wait for a reply from a RSET command [5m, unspecified].

quit

The wait for a reply from a QUIT command [2m, unspecified].

misc

The wait for a reply from miscellaneous (but short) commands such as NOOP (no-operation) and VERB (go into verbose mode). [2m, unspecified].

command+

In server SMTP, the time to wait for another command. [1h, 5m].

ident

The timeout waiting for a reply to an IDENT query [30s, unspecified].

fileopen

The timeout for opening .forward and :include: files [60s, none].

control

The timeout for a complete control socket transaction to complete [2m, none].

hoststatus

How long status information about a host (such as host down) will be cached before it is considered stale [30m, unspecified].

resolver.retrans

The resolver's retransmission time interval (in seconds) [varies]. Sets both Timeout.resolver.retrans.first and Timeout.resolver.retrans.normal.

resolver.retrans.first

The resolver's retransmission time interval (in seconds) for the first attempt to deliver a message [varies].

resolver.retrans.normal

The resolver's retransmission time interval (in seconds) for all resolver lookups except the first delivery attempt [varies].

resolver.retry

The number of times to retransmit a resolver query. Sets both Timeout.resolver.retry.first and Timeout.resolver.retry.normal [varies].

resolver.retry.first

The number of times to retransmit a resolver query for the first attempt to deliver a message [varies].

resolver.retry.normal

The number of times to retransmit a resolver query for all resolver lookups except the first delivery attempt [varies].

For compatibility with old configuration files, if no ``suboption'' is specified, all the timeouts marked with + are set to the indicated value.

Many of the RFC 1123 minimum values may well be too short. sendmail was designed to the RFC 822 protocols, which did not specify read timeouts; hence, versions of sendmail prior to 8.1 does not guarantee to reply to messages promptly. In particular, a RCPT command specifying a mailing list will expand and verify the entire list; a large list on a slow system may take more than five minutes. (This verification includes looking up every address with the name server; this involves network delays and can be considerable in some cases.) A one hour timeout is recommended. Because this failure is rare, a long timeout is not onerous and may ultimately help reduce network load.

For example, the following line sets the server SMTP command timeout to 25 minutes and the input data block timeout to three hours:

   O Timeout.command=25m
   O Timeout.datablock=3h

   Orcommand=25m,datablock=3h