02-05-2014
My bet is that the defaults mean that the client connection as a whole times out before the DNS failure is noticed and redirected to the secondary DNS. An incoming request is just an IP address, so your server will try to resolve that to a name for you.
Have a closer look at the manual for resolv.conf
The defaults will mean a 20 second timeout, however this might be extended if you have to look up the name of the DNS server than does respond, and that goes to the first one again.
For local sessions, you can also set the environment variables RES_RETRANS & RES_RETRY to something more like 1000 & 1 respectively.
I hope that this helps. It might not be the cause, but it's something we have hit in the past.
Robin
Liverpool/Blackburn
UK
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LEARN ABOUT DEBIAN
smtpdcheck
smtpdcheck(1) Mail Avenger 0.8.3 smtpdcheck(1)
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
Mail Avenger 0.8.3 2012-04-05 smtpdcheck(1)