While debugging an issue last week, I noticed some odd behavior when the Sendmail Smart host configuration directive (DS) was used. Smart hosts allow nodes to delegate mail delivery services to upstream devices, alleviating the need to run a full blown mail relay on each host. Smart hosts are configured with the Sendmail MC SMART_HOST directive, or through the sendmail.cf/submit.cf DS variable:
$ grep DS /etc/mail/submit.cf
DSsmtp-relay.dns.net
While I was performing a packet capture to isolate a problem with mail delivery, I noticed that each outbound message triggered several odd DNS lookups:
$ snoop -d eri0 port 53
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.net. Internet MX ?
dnsserver.dns.net -> s1 DNS R
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.net.internal.dns.net. Internet MX ?
dnsserver.dns.net -> s1 DNS R Error: 3(Name Error)
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.net.corp.dns.net. Internet MX ?
dnsserver.dns.net -> s1 DNS R Error: 3(Name Error)
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.net.dns.net. Internet MX ?
dnsserver.dns.net -> s1 DNS R Error: 3(Name Error)
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.net. Internet Unknown (28) ?
dnsserver.dns.net -> s1 DNS R
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.net. Internet Addr ?
dnsserver.dns.net -> s1 DNS R smtp-relay.dns.net. Internet Addr 1.2.3.4
Upon closer inspection, I noticed that sendmail (actually the resolution libraries) were attempting to resolve the SMTP relay along with each entry in the search path. To fix this issue ( I think it’s an issue, since it’s wasting bandwidth and CPU cycles), I fully qualified the smart host in my submit.cf:
$ grep DS /etc/mail/submit.cf
DSsmtp-relay.dns.net.
Once this was changed, and sendmail was restarted, the resolver only requested entries for the FQDN:
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.com. Internet MX ?
dnsserver.dns.net -> s1 DNS R
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.com. Internet Unknown (28) ?
dnsserver.dns.net -> s1 DNS R
s1 -> dnsserver.dns.net DNS C smtp-relay.dns.com. Internet Addr ?
dnsserver.dns.net -> s1 DNS R smtp-relay.dns.com. Internet Addr 1.2.3.4
I just dusted off my bat book, and hope to find an MC setting to disable MX lookups for the smart host. That should kill four more UDP datagrams. :)