Cisco Cisco Email Security Appliance C160 用户指南
24-3
Cisco AsyncOS 9.1 for Email User Guide
Chapter 24 Configuring Routing and Delivery Features
Routing Email for Local Domains
Defining an SMTP Route
Use the Network > SMTP Routes page (or the
smtproutes
command) to construct routes. When you
create a new route, you first specify the domain or partial domain for which you want to create a
permanent route. You then specify destination hosts. Destination hosts can be entered as fully-qualified
hostnames or as IP addresses. IP addresses can be either Internet Protocol version 4 (IPv4) or version 6
(IPv6).
permanent route. You then specify destination hosts. Destination hosts can be entered as fully-qualified
hostnames or as IP addresses. IP addresses can be either Internet Protocol version 4 (IPv4) or version 6
(IPv6).
For IPv6 addresses, AsyncOS supports the following formats:
•
2620:101:2004:4202::0-2620:101:2004:4202::ff
•
2620:101:2004:4202::
•
2620:101:2004:4202::23
•
2620:101:2004:4202::/64
You can also specify a a special destination host of
/dev/null
to drop the messages that match the entry.
(So, in effect, specifying
/dev/null
for the default route is will ensure that no mail received by the
appliance is ever delivered.)
A receiving domain can have multiple destination hosts, each assigned a priority number, much like an
MX record. The destination host with the lowest number identifies as the primary destination host for
the receiving domain. Other destination hosts listed will be used as backup.
MX record. The destination host with the lowest number identifies as the primary destination host for
the receiving domain. Other destination hosts listed will be used as backup.
Destinations with identical priority will be used in a “round-robin” fashion. The round-robin process is
based on SMTP connections, and is not necessarily message-based. Also, if one or more of the
destination hosts are not responding, messages will be delivered to one of the reachable hosts. If all the
configured destination hosts are not responding, mail is queued for the receiving domain and delivery to
the destination hosts is attempted later. (It does not fail over to using MX records).
based on SMTP connections, and is not necessarily message-based. Also, if one or more of the
destination hosts are not responding, messages will be delivered to one of the reachable hosts. If all the
configured destination hosts are not responding, mail is queued for the receiving domain and delivery to
the destination hosts is attempted later. (It does not fail over to using MX records).
When constructing routes using the
smtproutes
command in the CLI, you can prioritize each destination
host by using
/pri=
, followed by an integer between
0
and
65535
to assign priority (
0
is the highest
priority) after the hostname or IP address. For example,
host1.example.com/pri=0
has a higher priority
than
host2.example.com/pri=10
. Separate multiple entries with commas.
SMTP Routes Limits
You can define up to 40,000 routes. The final default route of
ALL
is counted as a route against this limit.
Therefore, you can define up to 39,999 custom routes and one route that uses the special keyword
ALL
.
SMTP Routes and DNS
Use the special keyword
USEDNS
to tell the appliance to do MX lookups to determine next hops for
specific domains. This is useful when you need to route mail for subdomains to a specific host. For
example, if mail to example.com is to be sent to the company’s Exchange server, you might have
something similar to the following SMTP route:
example, if mail to example.com is to be sent to the company’s Exchange server, you might have
something similar to the following SMTP route:
example.com exchange.example.com
However, for mail to various subdomains (foo.example.com), add an SMTP route that looks like this:
.example.com USEDNS