[Pkg-exim4-users] Problems concerning IPv6 Port 25's opening to the Internet

Marc Haber mh+pkg-exim4-users at zugschlus.de
Tue Oct 30 12:05:35 UTC 2012

On Tue, Oct 30, 2012 at 01:48:25AM +0900, Satoru Otsubo wrote:
> > On Sun, Oct 28, 2012 at 03:54:38AM +0900, Satoru Otsubo wrote:
> > >  Why is Port 25 shown as "filtered" ?
> > 
> > Many ISPs filter tcp/25 to protect the internet from their users' open
> > relays. Use tcpdump to check whether your connections from outside
> > come through.
> (1) With the command "tcpdump -vv dst host eeee:....:ffff and port 25",
>       (eeee:....:ffff is my PC's ipv6 address)
> I was able to see ipv6 packets coming from bendel.debian.org.
>   (bendel.debian.org is debian-user at lists.debian.org, which I subscribe.)

What exactly does the tcpdump line show?

Please do not obfuscate the IP address. I would have been able to
tcptraceroute to your host if you hadn't.

> Therefore, it seems that my PC's ipv6 port 25 is open.

Non sequitur.

> (2) Next, I investigated bendel.debian.org's IPv6 port 25 with the probing PC:
>   (My PC and the probing PC are connected to the same ISP. )
> # host bendel.debian.org
>     bendel.debian.org has address
>     bendel.debian.org has IPv6 address 2001:41b8:202:deb:216:36ff:fe40:4002
> # nmap -6 -p 25 2001:41b8:202:deb:216:36ff:fe40:4002
> This resulted in "filtered", although debian-user at lists.debian.org
> accepts mailings from its users.

You're breaking a nut with a sledgehammer. Please try telnetting to
bendel's Port 25, that will give a more universal error message. And
please have a tcpdump running while you do this (_without_ filtering
away ICMP as incoming "host unreachable" packets may give hints as


Marc Haber         | "I don't trust Computers. They | Mailadresse im Header
Mannheim, Germany  |  lose things."    Winona Ryder | Fon: *49 621 31958061
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