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Beginner

Forcing ASA to NAT when using NTP

When giving an ASA an NTP server that is on the outside Internet, is there a way of forcing which NAT rule it uses? It seems to use the default egress interface which is not what we want.

We've tried creating a NAT rule that has a source interface of any and a destination interface of the outside interface, but our specific NAT rule still doesn't get caught.

Has anyone tried to do this before and if so did you get this to work?

It's an ASA5515-X running 9.3x

Thanks!

8 REPLIES 8
Highlighted
Beginner

Hi

In your NTP command, what source interface have you set? e.g.

ntp server 172.1.1.1 key 1 source inside

Have you done a packet tracer from the CLI, with whatever interface you have set in NTP as the SRC_ADDR, and if so can you paste the output?

Example

"packet-tracer input DMZ tcp 10.2.1.1 1024 10.4.2.1 http"

packet-tracer input [src_int] protocol src_addr src_port dest_addr  dest_port [detailed] [xml]

Thanks

James

Highlighted

The NTP command reads as follows:

ntp server x.x.x.x source outside

(no authentication is used)

If we do a packet tracer and we specify the outside interface of the firewall as the source IP address, the NAT rule we want fires correctly. I'm not in a position to undertake a packet tracer on the firewall at present.

Thanks, Anish

Highlighted

I think because the packet is originating on the OUTSIDE interface, it's just doing a route lookup the been forwarded, bypassing NAT - see packet order of operations

ASA Order of Operation IMG 

I could be wrong but try something like this:

object network obj-local
subnet 172.22.1.0 255.255.255.0    (inside interface)

object network obj-remote
subnet x.y.z.1 255.255.255.0     (NTP-destination)

nat (inside,outside) 1 source static obj-local obj-local destination static obj-remote obj-remote

ntp server x.y.z.1 source inside

You'll need a matching ACL obviously. 

Hope it helps

Dont forget to rate :-)

Highlighted

Hi James,

thanks for the coherent response. can I just check a couple of things with you?

with the command:

object network obj-local
subnet 172.22.1.0 255.255.255.0    (inside interface)

Are you proposing that we have the inside interface even though this isn't used as the source IP address?

And the second thing, with the command ntp server x.y.z.1 source inside

should the interface be the interface though which the ntp server is reached? in which case should this read '..... source Outside'

Thanks, Anish

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I was thinking - get the NTP sourced from the inside interface (i.e. ntp server x.y.z.1 source inside) and use that source interface IP in the NAT statement - so you have something to NAT. Obviously doing this you'll need to do an ACL as well.

If you have a default route your routing should still try and pump it out to the Outside.

Like i said, it might not work but at the moment it's the only thing i can think of without seeing the config.

Just wondered - at the moment, as the NTP is been sourced from your Outside interface and using that external IP, why do you need to source NAT it to something else?

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Hi James,

We're going to try and mess about with it. But in answer to your question, we need to NAT it because the outside interface of the firewall isn't the outside routable address.

Cheers, Anish

Highlighted

Right - it makes sense now. Let me know how you get on and if it works can you rate my responses?

Thanks 

James

Highlighted
Cisco Employee

Hi Anish,

Could you share the NAT command you have used on the ASA for the NTP traffic  ?

Regards,

Aditya

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