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Can someone enlighten me on how the below is possible?


I have a remote site, L2 connection to HQ. Clients are configured like this...

  • Client (rc01) IP address 10.1.1.1/24
  • Gateway 10.1.1.254/24 (The catch: I have no access or control of this device)

HQ Edge firewall details...

  • internal 192.168.1.1/24
  • external 100.1.0.1/30

HQ example server...let's call it hq01

  • 192.168.1.2/24
  • Gateway 192.168.1.1/24

When I run a tracert from rc01 -> hq01 the first response is from hq01. But if I do the reverse (hq01 -> rc01) the first response is from the internal interface of the hq firewall as expected.

How is rc01 finding hq01 without first going through a gateway?

  • 2
    because your gateway is a firewall, and it isn't messing with TTL. (firewalls are known to do this crap to "hide") – Ricky Beam Jul 7 '15 at 3:59
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Firewalls can be setup to not decrement the ttl(time to live) as a weak security feature, to make them invisible. This is the default on the ASA.

Explanation for your scenario,

The FW(10.1.1.254) has a interface in the server range 192.168.1.0/24. Then traceroute with ttl=1 will hit hq01 and get a response as FW delivers packet to directly connected subnet without decrementing the ttl.

In the reverse direction your server hq01 uses a real router, and sees it as the first hop, the FW is again hidden (no ttl decrement) and you see the pc next.

Traceroute Overview

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