From: RainbowHat (nHiATlE_at_blSackholeP.mAit.edMu.invalid)
Date: 07/15/03

Date: Tue, 15 Jul 2003 19:55:45 +0000 (UTC)

< Jason

>if I telnet that mail server from my Linux box, the packets to that
>server is not SYN, it is CWR ECE SYN. And the remote mail server
>respond with nothing back.

--> Reason
RFC3360 Inappropriate TCP Resets Considered Harmful
3. The Specific Example of ECN
In short, iptables, intermediate gateway or target host drop the
legitimate packets.

--> Troubleshoot
Run `tcpdump` and `hping2` <> with SYN|ECE|
CWR TCP flags and traceroute mode.

hping2 -nVTSXYc 32 -p 25 mail.srv.ip.addr

--> Solution
If problem is your own network, re-configure it (Sorry, configuration
is not my business). If not, (e)mail to the owner.

--> Work around (Note: just a work around not a real solution)

echo 0 > /proc/sys/net/ipv4/tcp_ecn

BTW why do you need to send _remote_ mail server (not your ISP?)
_directly_? In most case, spammer is doing (MXware). What's your

"Be liberal in what you accept, and conservative in what you send."
"adaptability to change must be designed into all levels of Internet 
host software" from RFC3360. Hope This Informative, RainbowHat.

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