ISP port blocking practice

William Herrin bill at
Thu Sep 2 22:45:47 UTC 2010

On Thu, Sep 2, 2010 at 5:59 PM, Zhiyun Qian <zhiyunq at> wrote:
> One of the high-level findings is that we developed probing techniques
> to verify that indeed most ISPs are only blocking 1) "outgoing traffic
> of destination port 25" instead of 2) "incoming traffic with source
> port 25", which means that these ISPs are vulnerable to the
> assymetric routing attack.

If I understand your idea correctly:

1. GoodNet filters TCP destination port 25 packets from his customer
PwndBox, preventing PwndBox from spamming.

2. BadGuy on BadNet sends a forged TCP SYN packet to SpamVictim
allegedly from PwndBox on GoodNet.

3. PwndBox receives the response packets from SpamVictim and tunnels
them to BadGuy allowing BadGuy to complete the spam.

4. GoodNet didn't stop it because PwndBox never sent any packets to TCP port 25.

5. Since the IP address used was GoodNet's, GoodNet's reputation is damaged.

6. GoodNet could prevent this attack vector by also blocking packets
with TCP source port 25 sent -to- PwndBox.

Is that correct?

I observe that if PwndBox is behind a stateful firewall such as a COTS
NAT box, that also prevents this attack.

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

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