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Question 84

The LTM device is configured to provide load balancing to a set of web servers that implement access control lists (ACL) based on the source IP address of the client. The ACL is at the network level and the web server is configured to send a TCP reset back to the client if it is NOT permitted to connect.

The virtual server is configured with the default OneConnect profile.

The ACL is defined on the web server as:

Permit: 192.168.136.0/24 -

Deny: 192.168.116.0/24 -

The packet capture is taken of two individual client flows to a virtual server with IP address 192.168.136.100.

Client A - Src IP 192.168.136.1 - Virtual Server 192.168.136.100:

Clientside:

09:35:11.073623 IP 192.168.136.1.55684 > 192.168.136.100.80: S 869998901:869998901(0) win 8192

09:35:11.073931 IP 192.168.136.100.80 > 192.168.136.1.55684: S 2273668949:2273668949(0) ack 869998902 win 4380

09:35:11.074928 IP 192.168.136.1.55684 > 192.168.136.100.80: . ack 1 win 16425

09:35:11.080936 IP 192.168.136.1.55684 > 192.168.136.100.80: P 1:299(298) ack 1 win 16425

09:35:11.081029 IP 192.168.136.100.80 > 192.168.136.1.55684: . ack 299 win 4678

Serverside:

09:35:11.081022 IP 192.168.136.1.55684 > 192.168.116.128.80: S 685865802:685865802(0) win 4380

09:35:11.081928 IP 192.168.116.128.80 > 192.168.136.1.55684: S 4193259095:4193259095(0) ack 685865803 win 5840

1460,nop,nop,sackOK,nop,wscale 6>

09:35:11.081943 IP 192.168.136.1.55684 > 192.168.116.128.80: . ack 1 win 4380

09:35:11.081955 IP 192.168.136.1.55684 > 192.168.116.128.80: P 1:299(298) ack 1 win 4380

09:35:11.083765 IP 192.168.116.128.80 > 192.168.136.1.55684: . ack 299 win 108

Client B - Src IP 192.168.116.1 - Virtual Server 192.168.136.100:

Clientside:

09:36:11.244040 IP 192.168.116.1.55769 > 192.168.136.100.80: S 3320618938:3320618938(0) win 8192

09:36:11.244152 IP 192.168.136.100.80 > 192.168.116.1.55769: S 3878120666:3878120666(0) ack 3320618939 win 4380

0,sackOK,eol>

09:36:11.244839 IP 192.168.116.1.55769 > 192.168.136.100.80: . ack 1 win 16425

09:36:11.245830 IP 192.168.116.1.55769 > 192.168.136.100.80: P 1:299(298) ack 1 win 16425

09:36:11.245922 IP 192.168.136.100.80 > 192.168.116.1.55769: . ack 299 win 4678

Serverside:

09:36:11.245940 IP 192.168.136.1.55684 > 192.168.116.128.80: P 599:897(298) ack 4525 win 8904

09:36:11.247847 IP 192.168.116.128.80 > 192.168.136.1.55684: P 4525:5001(476) ack 897 win 142

Why was the second client flow permitted by the web server?

    Correct Answer: B

    The second client flow was permitted by the web server because SNAT automap was enabled on the virtual server. This configuration allows the source IP address of the client to be translated to an address within the 192.168.136.0/24 range, which is allowed by the ACL on the web server. This is why the server permits the connection from the client.

Discussion
WassuOption: B

Why not B

SimonR2

It’s not B because SNAT auto map is not enabled. You can see the client IP is preserved on the server side, it’s not using a self IP

SimonR2

Sorry, it looks like it does after all! The IP on the server side is snatted which is why it’s allowed through.

GVKDOption: B

B - Correct SNAT changes Client IP to 192.168.136.0/24 range.

MelnikovOption: C

Maybe because same TCP source port on server side are used...?