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Please note that this Citrix KB, https://support.citrix.com/article/CTX110348, states that the public IP address should only be used when there is no VPN connectivity and GSLB has to communicate over the Internet. “When adding a GSLB site, if the site communicates over the internet only then use the "Public IP" field. For example, when there is no site to site VPN connectivity between the GSLB sites.”
Customers GSLB configuration was not working and this was confirmed by a packet capture taken from the Netscaler. Where: 192.168.2.31 = NetScaler IP 192.168.6.22 = Subnet IP 192.168.6.18 = Subnet IP | GSLB site IP 220.127.116.11 = Public IP Remote GSLB Site Configuration Ports = 22, 3008, 3009 Test Lab = 3010 – communicating between two NetScalers.
In the example above we can first see communication from the GSLB site IP to the remote external. Then we see communication attempts from the NSIP to the remote external address. Then we see communication attempts to the SNIP to the remote external VIP.
Netscaler attempts to speak to the external IP of the remote device in this order: -GSLB SITE IP -NSIP -SNIP
The following trace further proves gslb uses ports 3010 and 22. In the example trace below they are not opened ports. Source Address : 172.31.251.120 : NetScaler IP Destination Address : 18.104.22.168 :Remote NetScaler GSLB Public IP Netscaler > System > Network > RPC
Conclusion: GSLB will contact a remote public IP if configured. It will also try in the following order: -GSLB SITE IP -NSIP -SNIP GSLB must have ports 3008,3009 and 3010 open and 22.
Customer had a remote IP configured and this potentially did not need to be present as this was internal GSLB. Customer also did not have the relevant ports opened up between sites.
- ADNS IP: An IP that will listen for ADNS queries. For external, create a public IP for the ADNS IP and open UDP 53 so Internet-based DNS servers can access it. This can be an existing SNIP on the appliance.
- GSLB Site IP / MEP IP: A GSLB Site IP that will be used for NetScaler-to-NetScaler communication, which is called MEP or Metric Exchange Protocol. The IP for ADNS can also be used for MEP / GSLB Site.
- RPC Source IP: RPC traffic is sourced from a SNIP, even if this is different than the GSLB Site IP. It’s less confusing if you use a SNIP as the GSLB Site IP.
- Public IP: For external GSLB, create public IPs that are NAT’d to the GSLB Site IPs. The same public IP used for ADNS can also be used for MEP. MEP should be routed across the Internet so NetScaler can determine if the remote datacenter has Internet connectivity or not.
- MEP Port: Open port TCP 3009 between the two NetScaler GSLB Site IPs. Make sure only the NetScalers can access this port on the other NetScaler. Do not allow any other device on the Internet to access this port. This port is encrypted.
- GSLB Sync Ports: To use GSLB Configuration Sync, open ports TCP 22 and TCP 3008 from the NSIP (management IP) to the remote public IP that is NAT’d to the GSLB Site IP. The GSLB Sync command runs a script in BSD shell and thus NSIP is always the Source IP.
- DNS Queries: The purpose of GSLB is to resolve a DNS name to one of several potential IP addresses. These IP addresses are usually public IPs that are NAT’d to existing Load Balancing, SSL Offload, Content Switching, or NetScaler Gateway VIPs in each datacenter.
- IP Summary: In summary, for external GSLB, you will need a minimum of two public IPs in each datacenter:
- One public IP that is NAT’d to the IP that is used for ADNS and MEP (GSLB Site IP). You only need one IP for ADNS / MEP no matter how many GSLB names are configured. MEP (GSLB Site IP) can be a different IP, if desired.
- One public IP that is NAT’d to a Load Balancing, SSL Offload, Content Switching, or NetScaler Gateway VIP.
- If you GSLB-enable multiple DNS names, each DNS name usually resolves to different IPs. This usually means that you will need additional public IPs NAT’d to additional VIPs.
Bullet points taken from Carl Websters superb site: