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