Static Hostname Mappings
Services are the construct used to define the properties and destination for traffic traversing the SSR. An example service for a printer can be seen below.
config
authority
service printer
name printer
tenant trusted-users
address 192.168.0.123
service-policy printer-policy
exit
exit
exit
It is common to define a service for every destination of traffic within a network so as to shape traffic and affect policy in a way that assures quality of experience. In deployments of dozens or even hundreds of locations, this may result in duplicate or redundant configuration if a similar service exists at every site. Given the printer example, it is likely that the service configuration would be similar at every location, except for perhaps the address. The overall configuration can be simplified by using a hostname as the service address instead of an IP address.
config
authority
service printer
name printer
tenant trusted-users
address local.printer.service
service-policy printer-policy
exit
exit
exit
This single service template of sorts can now be used for every location, however it requires a DNS server at the location to correctly resolve local.printer.service to the location-specific address. If it not possible to leverage a DNS server to resolve hostnames, the SSR can be used to define a static mapping to achieve the same result.
A mapping of hostname to a list of addresses can be added to the router configuration which in turn adds entries to /etc/hosts. SSR will use these addresses as resolutions for the hostnames as if they were directly added to /etc/hosts or returned by dynamic DNS lookup. The hostname/address mappings are used for all hostnames within the SSR configuration, not only on service addresses.
Configuration
config
authority
router burl-corp
name burl-corp
static-hostname-mapping
static-entry local.printer.service
hostname local.printer.service
ip-address 192.168.0.123
exit
exit
exit
exit
exit
The service configuration from above, in combination with this router-specific hostname mapping will allow the printer service to be properly defined at each respective location.
Troubleshooting
show dns resolutions displays whether hostnames provisioned within the SSR configuration have been resolved, either through configuration, leveraging static-hostname-mapping or manual through the PCLI.
This output from show dns resolutions is performed with the service configured with an address of local.printer.service but does not yet have a static-hostname-mapping configured.
admin@test1.Fabric128# show dns resolutions
Mon 2020-06-29 19:21:19 UTC
=========== ======= ======================= ========== =============== ============
Router Node Hostname Resolved Last Resolved Expiration
=========== ======= ======================= ========== =============== ============
Fabric128 test1 local.printer.service N - -
Completed in 0.10 seconds
As can be seen from the output, the hostname has not been resolved. After the mapping has been configured:
admin@test1.Fabric128# configure authority router Fabric128 static-hostname-mapping static-entry local.printer.service
admin@test1.Fabric128 (static-entry[hostname=local.printer.service])# ip-address 192.168.0.123
*admin@test1.Fabric128 (static-entry[hostname=local.printer.service])# commit
*admin@test1.Fabric128 (static-entry[hostname=local.printer.service])# top
The output of show dns resolutions will show that the hostname now resolves to a valid address.
admin@test1.Fabric128# show dns resolutions
Mon 2020-06-29 19:22:20 UTC
=========== ======= ======================= ========== =============== ============
Router Node Hostname Resolved Last Resolved Expiration
=========== ======= ======================= ========== =============== ============
Fabric128 test1 local.printer.service Y - -
Completed in 0.11 seconds
set dns resolutions can be used to override or set a value learned from a DNS resolver.
admin@test1.Fabric128# show dns resolutions
Mon 2020-06-29 19:00:05 UTC
=========== ======= ========== ========== =============== ============
Router Node Hostname Resolved Last Resolved Expiration
=========== ======= ========== ========== =============== ============
Fabric128 test1 foo Y - -
Fabric128 test1 bar N - -
Completed in 0.11 seconds
admin@test1.Fabric128# set dns resolution bar 2.2.2.2
Successfully set hostname resolution on node test1
admin@test1.Fabric128# show dns resolutions
Mon 2020-06-29 19:00:12 UTC
=========== ======= ========== ========== =============== ============
Router Node Hostname Resolved Last Resolved Expiration
=========== ======= ========== ========== =============== ============
Fabric128 test1 bar Y - -
Fabric128 test1 foo Y - -
If using set dns resolution on a hostname resolved by a DNS server, the SSR will no longer attempt to resolve those addresses as static entries do not have a TTL.