Dynamic Domain Name Server Updates

This chapter discusses DNS update methods and Server Address assignment, and provides configuration details of those features.

This chapter contains the following sections:

•IP Reachability

•Configuring IP Reachability

•DNS Server Address Assignment

–Support DNS Remapping on Home Agent

–DNS Redirection with Monitoring

•Examples

IP Reachability

TIA/EIA/IS-835-D describes dynamic DNS update method by the home AAA server and the Home Agent. DNS update by AAA is applicable to both Simple IP and Mobile IP service, while DNS update by the Home Agent is only applicable to Mobile IP service. The following describes the IP Reachability feature on Home Agent.

When the HA receives an initial Registration Request it sends a RADIUS Access-Request to the Home RADIUS server. If the RADIUS server is configured to request Home Agent-based DNS updates, the Home RADIUS server will include the DNS-Update-Required attribute in the RADIUS Access-Accept message returned to the HA. If the initial Mobile IP registration is successful, the HA sends a DNS Update message to the DNS server to add an A Resource Record for the MS. The HA sends a DNS Update message to the primary and secondary DNS server, if present.

When the HA receives a Mobile IP RRQ with lifetime timer set to zero, or the Mobile IP lifetime expires, or administrative operations invalidate the mobility binding for the MS, the Home Agent will send a DNS Update message to DNS server to delete the associated Resource Record. The following commands will enable the IP Reachability feature on Home Agent for the specified realm.

Note DNS updates are not sent for each Re-registration.

Note This feature is supported for Proxy Mobile IP flows as well.

The following call flow describes the IP Reachability on Home Agent - mobile registration scenario:

1. Home Agent receives a registration request from the PDSN/FA.

2. Home Agent sends an access request to RADIUS Server. The HA includes DNS Server Update Capability VSA.

3. The RADIUS server sends access accept with DNS Update Required VSA.

4. The HA sends Registration response to the PDSN/FA. If the HA is configured for redundancy, the active Home Agent will sync the binding creation to the standby Home Agent.

5. The HA creates a binding, and sends DNS Update request message to DNS Server

6. The DNS Server creates a DNS entry for the NAI, and sends DNS Update response message to the HA.

The following call flow describes the IP Reachability on Home Agent - Mobile deregistration scenario:

1. Home Agent receives a registration request with lifetime zero from PDSN/FA.

2. Home Agent sends an access request to RADIUS Server, if SA is not stored locally (optional).

3. RADIUS Server sends access accept (optional).

4. Home Agent deletes the binding. Home Agent sends Registration response to PDSN/FA. If Home Agent is configured for redundancy, the active Home Agent will sync the binding deletion to standby Home Agent.

5. Home Agent sends DNS Update request message to DNS Server, to delete the DNS entry.

6. DNS Server deletes the DNS entry for the NAI. DNS Server sends DNS Update response message to Home Agent.

Configuring IP Reachability

To enable this feature for the specified realm, issue the following commands:

 
Command
Purpose

Step 1 

Router(config)# ip name-server x.x.x.x

Specifies the address of one or more name servers to use for name and address resolution.

Step 2 

Router(config)# ip mobile realm @ispxyz1.com dns dynamic-update method word

Enables the DNS Update procedure for the specified realm. word is the dynamic DNS update method name.

Step 3 

Router(config)# ip mobile realm realm dns server primary dns server address secondary dns server address

Enables you to locally configure the DNS Server address.

To verify that this feature is enabled for a binding, use the following command:

 
Command
Purpose

Step 1 

Router# show ip mobile binding

Displays the mobility binding table.

The following example illustrates the realm configuration for IP reachability: 

ip ddns update method sit-ha2-ddns2

 DDNS both

ip mobile realm @ispxyz2.com dns dynamic-update method sit-ha2-ddns2

DNS Server Address Assignment

IS835D defines a method to push the home DNS server address to a mobile as an NVSE in a mobileip registration response. This procedure allows the Mobile Station to learn the primary and secondary DNS server address of its home domain.

The RADIUS server will include DNS Server VSA in an access response to the HA during mobile authentication. The HA forms a DNS server NVSE from the DNS Server VSA and adds it to mobileip registration response. If the DNS Server VSA is not received at the time of authentication, and DNS server address is configured locally on the Home Agent will form a DNS server NVSE from the local configuration and add it to mobileip registration response.

The DNS Server VSA and DNS Server NVSE carry primary and secondary DNS IP addresses.

DNS Server VSA will be synced to the standby if the HA is deployed in redundant mode.

To enable this feature for the specified realm, issue the following commands:

ip mobile realm realm dns server assign

ip name-server x.x.x.x

To locally configure the DNS Server address, issue the following command:

ip mobile realm realm dns server primary dns server address secondary dns server address

To verify that this feature is enabled for a binding, use the show ip mobile binding command.

Note If the DNS server address is configured both locally and downloaded from AAA, then preference will be given to the local configuration on the HA.

Support DNS Remapping on Home Agent

In Cisco Mobile Wireless Home Agent Release 5.0, the Home Agent supports Stateful NAT capability with scaling to the number of subscribers supported by the Home Agent. This involves matching to a specific protocol and port so that DNS requests from a user can be recognized. Once recognized, the destination IP address is modified so that the DNS request is sent to the IP address defined by the operator. Similarly, the response has a source IP address of the DNS server that responded to the request. This is then mapped back to the original address used by the subscriber.

MN is initially configured with a DNS server IP address of the visited network during session setup. Later, MN tries to resolve hostname by sending DNS message to this IP address which cannot reach the destination via the home network (i.e. reverse tunneled to the HA). In order to address this issue, in HA 5.0, "DNS remapping" feature is added.

DNS Redirection with Monitoring

One problem with DNS remapping is when the primary DNS server fails, the DNS query is not redirected on the secondary DNS server configured on the HA. Additionally, the HA does not use a NAT configuration for remapping the destination address of the DNS query to the configured DNS address on the HA.

The DNS Redirection feature, on the top of the existing DNS Remapping functionality, enables the Home Agent to support Stateful NAT capability with scaling to the number of subscribers supported by the Home Agent.

As part of this feature support, the HA now takes care of remapping the destination address as well as DNS servers monitoring for their availability. The HA rewrites the destination IP address of the DNS messages from the MN to a configured IP address of the primary or secondary DNS server, depending on which one is available. If both primary and secondary DNS are available, the primary will play the role of active DNS. If the primary DNS server is unavailable, the HA starts remapping the destination IP address to the secondary DNS server configured on the HA.

This solution solves the potential problem of when a primary DNS server fails; the DNS query needs to be redirected on the secondary DNS server configured on the HA.

The HA uses the functionality of IP SLA to detect the availability of the primary and secondary DNS server from the Home Agent. Since the IP SLA only informs the CP about the connectivity of the monitored node, the CP informs all of the TPs (through IPC) about the connectivity which the CP has received from IP SLA.

If the HA finds the primary DNS server is available, the primary DNS server is used as an active DNS server and used for remapping the DNS queries coming from the FA on the tunnel. If primary DNS server is down, the secondary DNS server is used as an active DNS server for remapping DNS queries. In case when both primary and secondary DNS servers are reachable from the Home Agent, the primary server is used for DNS remapping. Additionally, if the secondary DNS server is the active DNS server, and the primary DNS server comes up or connectivity resumes with the Home Agent, the primary DNS server takes over the role of active DNS server again.

Here are some important considerations about this feature:

•When switchover occurs, all pending DNS queries that are awaiting responses at the HA from the DNS server are lost on the new, active HA. Mobile nodes need to resend DNS query in this scenario.

•If the destination address of the DNS query matches with the addresses of the DNS servers configured on the HA, DNS redirection does not come into picture, and the HA treats this packet as a normal data packet.

•There is no need to use a NAT configuration for DNS redirection.

To enable realm-based DNS Redirection perform the following tasks;

 
Command
Purpose

Step 1 

Router(config)# ip mobile realm word dns server primary DNS ip secondary DNS ip

Configures the primary and secondary DNS server for a realm.

Step 2 

Router(config)# ip mobile realm word dns server redirect {all}

Enables the DNS redirection feature for this realm.

Behavior of Above Two Commands:

•If ip mobile realm word dns server redirect {all}is configured before ip mobile realm word dns server primary DNS ip secondary DNS ip, the HA will display the following error message. 

Error Message    Error: Primary and Secondary DNS not configured for realm

•Since DNS redirection feature is realm based therefore only "@" or "@domain" will be valid realm. E.g xyz@domain, xyz or xyz@ will not be a valid realm option. In case of an error, the HA will display the following error message: 

Error Message    DNS Redirection is allowed for realm only (e.g. @word)

•If no command to unconfigure the primary DNS server and secondary DNS server is run for a particular realm, this will automatically disable DNS redirection for that realm.

•When unconfiguring the DNS redirection feature using the no version of the ip mobile realm word dns server redirect command, it will not remove the existing binding for that realm from the HA. Only the DNS redirection feature will be disabled

To enable DNS servers monitoring for their availability, configure the following IP SLA CLIs. This set of IP SLA configuration commands are required for all the DNS server nodes which need to be monitored by the HA. These IP SLA commands are existing commands that are available in all 7600 series routers.

 
Command
Purpose

Step 1 

Router(config)# ip sla ipsla-number

icmp-echo ip-addr

frequency freq

Assigns a IPSLA number, and configures and IP address that needs to be monitored.

Step 2 

Router(config)# ip sla reaction-configuration ipsla-number react timeout threshold-type immediate action-type trapAndTrigger

Configures the IP sla to notify if the above configured DNS server is not available.

Step 3 

router(config)#ip sla reaction-configuration ipsla-number react connectionLoss threshold-type immediate action-type trapAndTrigger

Configures ip sla to notify if the above configured DNS server is available.

Step 4 

router(config)#ip sla enable reaction-alerts

Configures the ip sla to generate notification for availability and unavailability of DNS servers configured above.

Step 5 

router(config)#ip sla sch ipsla-number start-time now life forever

Configures the ip sla to start monitoring configure DNS server configured above.

Where:

–ipsla-number—IP SLA number that has been assigned for checking the DNS server.

–ip-addr—The IP address of the DNS server.

–freq—The frequency of the probe in seconds (default 60).

DNS Query Matching PDNS or SDNS

This section explains the redirection behavior when the DNS query matches either the configured PDNS or SDNS.

Requests matching PDNS:

If the DNS request matches the PDNS and if it is alive, then that request is skipped. But if PDNS is down, then the request is redirected to SDNS, if it is active. Otherwise the request is ignored (treated as a normal data packet).

Requests matching SDNS:

The behavior pertaining to requests matching SDNS is controlled through the configuration CLI. The following is the CLI used to configure DNS redirect:

ip mobile realm @realm dns server redirect {all}

When redirect alone is configured, the requests that are sent to SDNS are not redirected, if it is up. They are sent to SDNS server only. Other DNS requests are redirected to PDNS.

When redirect all is configured, all the DNS requests (including the requests that are matching the configured SDNS IP) are redirected to PDNS.

Monitor DNS servers Through IP SLA

Whenever IP SLA detects a connection loss or a connection up event with any of the configured primary and secondary DNS servers, it invokes the registry API on the CP. When the CP gets the notification, it notifies all of the TPs through IPC about this event. When the TPs get this notification from the CP, it sets the active DNS between the primary DNS and secondary DNS.

DNS Redirection supports redundancy. After a switchover, when HA becomes active, it starts monitoring the configured DNS servers for their availability. When any DNS query comes it is remapped to the configured DNS server on the HA.

The only limitation is when a switchover occurs, all pending DNS queries that are awaiting DNS responses at the HA will be lost on the new, active HA. The mobile nodes need to resend a DNS query in this scenario.

Examples

The following example illustrates how to configure a User profile for DNS: 

[ //localhost/Radius/Profiles/mwts-mip-r20sit-haslb1-prof/Attributes ]

    CDMA-DNS-Server-IP-Address = 01:06:0A:4D:9B:0A:02:06:0A:4D:9B:09:03:03:01:04:03:01

    CDMA-DNS-Update-Required = "HA does need to send DNS Update"

    CDMA-HA-IP-Addr = 20.20.225.1

    CDMA-MN-HA-Shared-Key = ciscociscociscoc

    CDMA-MN-HA-SPI = 00:00:10:01

    CDMA-Reverse-Tunnel-Spec = "Reverse tunneling is required"

    class = "Entering the World of Mobile IP-3"

    Service-Type = Framed

Here is a sample configuration of the DNS server address assignment realm: 

ip mobile realm @ispxyz2.com dns server 10.77.155.10 2.2.2.2

ip mobile realm @ispxyz2.com dns server assign

The following example illustrates how to configure the same in AR user profile: 

set  CDMA-DNS-Server-IP-Address 01:06:0A:4D:9B:0A:02:06:0A:4D:9B:09:03:03:01:04:03:01

The ones marked in bold text are primary and secondary DNS server address.

Here is a sample configuration of both IP Reachability and DNS Server Address Assignment: 

ha2#show run

Building configuration...


Current configuration : 10649 bytes

!

! Last configuration change at 22:45:21 UTC Fri Nov 11 2005

!

version 12.3

service timestamps debug datetime msec

service timestamps log datetime msec

no service password-encryption

service internal

service udp-small-servers

!

hostname tb1-6513-ha2

!

boot-start-marker

boot-end-marker

!

!

aaa new-model

!

!

aaa group server radius MOT

 server 150.2.0.1 auth-port 1645 acct-port 1646

!         

aaa authentication ppp default local group MOT

aaa authorization config-commands

aaa authorization ipmobile default group MOT 

aaa authorization network default group MOT 

aaa authorization configuration default group MOT 

aaa accounting session-duration ntp-adjusted

aaa accounting update newinfo periodic 3

aaa accounting network ha start-stop group MOT

aaa accounting system default start-stop group MOT

!

aaa server radius dynamic-author

 client 150.2.0.1

 server-key cisco

!

aaa session-id common

!

resource policy

!

ip subnet-zero

no ip gratuitous-arps

!

!

ip cef    

ip dfp agent ipmobile

 port 400

 interval 15

 inservice

!

ip ftp source-interface GigabitEthernet0/0.10

ip ftp username root

ip ftp password pdsnmwg

no ip domain lookup

ip name-server 10.77.155.10

ip name-server 1.1.1.1

ip name-server 6.6.6.6

no ip dhcp use vrf connected

no ip dhcp conflict logging

ip dhcp ping packets 0

!

ip dhcp pool Subnet-Pool1

   utilization mark high 75

   utilization mark low 25

   origin dhcp subnet size initial /30 autogrow /30

!

!

ip vrf forwarding

!

ip vrf ispxyz

!

ip vrf ispxyz-vrf1

 rd 100:1

!

ip vrf ispxyz-vrf2

 rd 100:2

!

!

ip ddns update method sit-ha2-ddns1

 DDNS both

!

ip ddns update method sit-ha2-ddns2

 DDNS both

!

vpdn enable

vpdn ip udp ignore checksum

!

vpdn-group testsip1-l2tp

! Default L2TP VPDN group

! Default PPTP VPDN group

 accept-dialin

  protocol any

  virtual-template 1

 l2tp tunnel hello 0

!

username user-ha2 password 0 cisco

!

!

!

interface Tunnel10

 no ip address

 ip access-group 150 in

!

interface Loopback0

 ip address 20.20.225.1 255.255.255.0

!

interface Loopback1

 description address of the LNS server

 ip address 20.20.206.20 255.255.255.0

!

interface Loopback2

 ip address 170.12.0.102 255.255.0.0

!

interface GigabitEthernet0/0

 no ip address

 no ip route-cache cef

 no ip route-cache

 no keepalive

 no cdp enable

!

interface GigabitEthernet0/0.10

 description TFTP vlan

 encapsulation dot1Q 10

 ip address 10.77.155.5 255.255.255.192

 no ip route-cache

 no snmp trap link-status

 no cdp enable

!

interface GigabitEthernet0/0.172

 description HAAA interface

 encapsulation dot1Q 172

 ip address 170.2.0.20 255.255.0.0

 no ip route-cache

 no snmp trap link-status

 no cdp enable

 standby delay minimum 15 reload 15

 standby version 2

 standby 2 ip 170.2.0.102

 standby 2 follow sit-ha2

!

interface GigabitEthernet0/0.202

 description PI interface 

 encapsulation dot1Q 202

 ip address 20.20.202.20 255.255.255.0

 no ip route-cache

 no snmp trap link-status

 no cdp enable

 standby delay minimum 15 reload 15

 standby version 2

 standby 2 ip 20.20.202.102

 standby 2 ip 20.20.204.2 secondary

 standby 2 ip 20.20.204.3 secondary

 standby 2 ip 20.20.204.4 secondary

 standby 2 ip 20.20.204.5 secondary

 standby 2 ip 20.20.204.6 secondary

 standby 2 timers msec 750 msec 2250

 standby 2 priority 130

 standby 2 preempt delay minimum 180

 standby 2 name sit-ha2

!

interface GigabitEthernet0/0.205

 description REF interface 

 encapsulation dot1Q 205

 ip address 20.20.205.20 255.255.255.0

 no ip route-cache

 no snmp trap link-status

 no cdp enable

 standby delay minimum 15 reload 15

 standby version 2

 standby 2 ip 20.20.205.102

 standby 2 follow sit-ha2

!

interface Virtual-Template1 

 description To be used by VPDN for PPP tunnel

 ip unnumbered Loopback1

 peer default ip address pool LNS-pool

 no keepalive

 ppp accm 0

 ppp authentication chap pap optional

 ppp accounting none

!         

router mobile

!

ip local pool LNS-pool 7.0.0.1 7.0.0.255

ip local pool ispxyz-vrf1-pool 50.0.0.1 50.0.0.255

ip local pool mobilenodes 40.0.0.1 40.0.100.255

ip default-gateway 10.77.155.1

ip classless

ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0.202

ip route 10.77.139.29 255.255.255.255 10.77.155.1

ip route 150.2.0.0 255.255.0.0 170.2.0.1

no ip http server

!

!

ip mobile debug include username

ip mobile home-agent template Tunnel10 address 20.20.202.102

ip mobile home-agent revocation timeout 5 retransmit 4

ip mobile home-agent dynamic-address 20.20.202.102

ip mobile home-agent accounting ha broadcast lifetime 3600 replay 8 suppress-unreachable 
unknown-ha deny

ip mobile home-agent redundancy sit-ha2 virtual-network address 20.20.202.102 
periodic-sync

ip mobile radius disconnect

ip mobile virtual-network 50.0.0.0 255.0.0.0

ip mobile virtual-network 40.0.0.0 255.0.0.0

ip mobile host nai mwts-pmp-r20sit-base-user1@ispxyz1.com virtual-network 40.0.0.0 
255.0.0.0 aaa load-sa lifetime 600

ip mobile host nai @ispxyz2.com address pool local mobilenodes virtual-network 40.0.0.0 
255.0.0.0 aaa lifetime 180

ip mobile realm mwts-pmp-r20sit-base-user1@ispxyz1.com dns server 10.77.155.10 1.1.1.1

ip mobile realm mwts-pmp-r20sit-base-user1@ispxyz1.com dns server assign

ip mobile realm mwts-pmp-r20sit-base-user1@ispxyz1.com dns dynamic-update method 
sit-ha2-ddns1

ip mobile realm @ispxyz2.com vrf ispxyz-vrf2 ha-addr 20.20.204.6

ip mobile realm @ispxyz2.com dns server 10.77.155.10 2.2.2.2

ip mobile realm @ispxyz2.com dns server assign

ip mobile realm @ispxyz2.com dns dynamic-update method sit-ha2-ddns2

ip mobile secure foreign-agent 20.20.201.10 20.20.201.100 spi 100 key ascii cisco replay 
timestamp within 7 algorithm md5 mode prefix-suffix

ip mobile secure foreign-agent 20.20.210.10 20.20.210.100 spi 100 key ascii cisco replay 
timestamp within 5 algorithm md5 mode prefix-suffix

ip mobile secure home-agent 20.20.202.10 20.20.202.95 spi 100 key ascii cisco replay 
timestamp within 7 algorithm md5 mode prefix-suffix

!         

ip radius source-interface Loopback2 

no logging trap

logging source-interface GigabitEthernet0/0.201

access-list 150 permit ip host 40.0.0.1 host 20.20.205.220 log

access-list 150 permit ip host 20.20.205.220 host 40.0.0.1 log

access-list 150 deny   ip any any log

snmp-server community public RO

snmp-server community private RW

snmp-server trap-source Loopback0

snmp-server host 150.2.0.100 version 2c private 

snmp-server host 150.2.0.100 public 

no cdp run

!

!

radius-server attribute 44 include-in-access-req

radius-server attribute 8 include-in-access-req

radius-server attribute 32 include-in-access-req 

radius-server attribute 55 access-request include

radius-server host 150.2.0.1 auth-port 1645 acct-port 1646 key 7 121A0C041104

radius-server host 150.2.0.100 auth-port 1645 acct-port 1646 key cisco

radius-server retransmit 4

radius-server timeout 2

radius-server deadtime 5

radius-server key cisco

radius-server vsa send accounting

radius-server vsa send authentication

radius-server vsa send accounting 3gpp2

radius-server vsa send authentication 3gpp2

!

control-plane

!

alias exec shc sh cdma pdsn 

alias exec ua undebug all

alias exec ui undebug ip packet

!

line con 0

 exec-timeout 0 0

line vty 0 4

 exec-timeout 0 0

line vty 5 15

 exec-timeout 0 0

!         

!

end

ha2#