NAME

       smtpdcheck - check SMTP servers

SYNOPSIS

       smtpdcheck [--stop {ip-addr|name}] [--timeout seconds]
                  [prio:]server [[prio:]server]

DESCRIPTION

       smtpdcheck checks to see if SMTP servers are available.  The intent is for use on
       secondary mail servers, which have no reason to accept mail when the primary server is
       available.

       The argument consists of a list of server names, each of which may optionally be prefixed
       by a numeric MX priority and a colon.  (This is exactly the format for MX records returned
       by the avenger dns command.)  smtpdcheck will attempt to connect to each server in
       succession.

       If one of the servers specified on the command line is available, smtpdcheck will print
       its name to standard output and exit with status 1.  If smtpdcheck cannot connect to any
       of the servers, it will exit with status 0.  If a system error occurs, smtpdcheck will
       exit with status 2.

   OPTIONS
       --stop {ip-addr|name}
           Tells smtpdcheck to stop before checking a server with IP address ip-addr or hostname
           name.  If such a host is encountered in the list of servers and prio is specified,
           then smtpdcheck will consider it acceptable for other servers with the same priority
           to be available, even if those servers were first in the list.  In other words, given
           the following arguments:

                  smtpdcheck --stop s2.domain.com \
                             10:s1.domain.com 10:s2.domain.com \
                             20:s3.domain.com

           This command will always succeed, regardless of whether "s1.domain.com" is up, because
           "s2.domain.com" has the same priority.  On the other hand, the following command will
           fail and output "s1.domain.com" if "s1.domain.com" is up:

                  smtpdcheck --stop s2.domain.com \
                             10:s1.domain.com 20:s2.domain.com \
                             30:s3.domain.com

           If a gethostbyname lookup for the argument name fails, smtpdcheck will exit
           immediately with status 2.

       --timeout {seconds]
           By default, smtpdcheck spends 10 seconds probing each server.  This includes the time
           to do a DNS lookup, to establish a TCP connection to port 25 of the server, and to
           read the "220" SMTP code from the server's SMTP greeting message.  To use a different
           value, specify it with the --timeout option.  The value 0 disables the timeout
           completely, which is dangerous since smtpdcheck might then end up waiting forever to
           read the "220" string.

EXAMPLES

       To refuse to relay mail at a secondary MX server when the primary server is not down, you
       might place the following in /etc/avenger/secondary (assuming MxLocalRcpt is 1):

               dns RECIP_MXES mx "$RECIPIENT_HOST"
               setvars
               server=`smtpdcheck --stop $MYIP $RECIP_MXES`
               test -n "$server" && defer "Please use server $server"

SEE ALSO

       avenger(1) dbutil(1) asmtpd.conf(5),

       The Mail Avenger home page: <http://www.mailavenger.org/>.

BUGS

       smtpdcheck could achieve much lower latency by probing all the servers simultaneously.  It
       should also include some kind of caching, to avoid repeatedly trying to contact an
       unavailable server.  Finally, hosts with multiple IP addresses could be handled more
       cleanly, though what smtpdcheck does should probably work in most cases.

AUTHOR

       David Mazieres