[LARTC] Weird problem, not 100% routing based.

[LARTC] Weird problem, not 100% routing based.

[Bill Williamson]

I current have a dsl modem.  I'm running a nexland brand router for it.
I'm going to eventually move to using my linux box as my main router, but
that's for a later day, and scouring for info/wisdom is actually why I
joined this, but this isn't about that yet :)

The router has a cool "loopback" function, where I can hit my external IP
from internal, and it passes back through the hits.

simple network topology to deal with for the problem:
inet---dslmodem---router---hub---win box
                                              |---linux box
if it isn't aligned, just win and linux in a hub to the router.

Situations that work:
windows box IMAP to linux imap server, through loopback on router.
windows box ssh to linux ssh server, through loopback on router.
port 80 fw'd to linux box, windows box hits web server through loopback
port 80 fw'd to windows box, windows box doing loopback to itself

(by loopback i mean through the router, NOT 127.0.0.1)

Situation(s) that doesn't work:
linux box->loopback->linux box, ANY ports
port 80 fw'd to linux box, no work with lynx or wget
port 22 fw'd, no work with ssh from linux to linux
etc...

What's going on here?  It looks like the linux box is not liking talking to
itself while going through an external router, or something crazy?

What should I look for to diagnose this?

I've just about eliminated the problem being at the router since it works in
every combo but linux box->itself.

Thanks!

--Bill Williamson


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Re: [LARTC] Weird problem, not 100% routing based.

[bert hubert]

On Thu, Dec 06, 2001 at 05:18:35PM -0600, Bill Williamson wrote:

> Situation(s) that doesn't work:
> linux box->loopback->linux box, ANY ports
> port 80 fw'd to linux box, no work with lynx or wget
> port 22 fw'd, no work with ssh from linux to linux
> etc...

Never *ever* complain 'doesn't work', we are not clairvoyant! What does it
do? Timeout? Connection denied? No route to host? Host unreachable?

> What's going on here?  It looks like the linux box is not liking talking to
> itself while going through an external router, or something crazy?

Run 'tcpdump -e -s 1500 -n -i eth0' while you try to connect to yourself and
supply us with the IP addresses. Replace eth0 with the right interface.

Regards,

bert hubert


-- 
http://www.PowerDNS.com          Versatile DNS Software & Services
Trilab                                 The Technology People
Netherlabs BV / Rent-a-Nerd.nl           - Nerd Available -
'SYN! .. SYN|ACK! .. ACK!' - the mating call of the internet

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Re: [LARTC] Weird problem, not 100% routing based.

[bert hubert]

On Thu, Dec 06, 2001 at 05:54:36PM -0600, Bill Williamson wrote:

Bill, please do not silently move discussions away from the mailinglist! I
am not a free consulting firm!

> > Run 'tcpdump -e -s 1500 -n -i eth0' while you try to connect to yourself
> and
> > supply us with the IP addresses. Replace eth0 with the right interface.
> 

Sanitized output of the tcpdump you provided:

5.4 a8:d7 1b:94  192.168.0.3.53395 > x.y.z.w.80: S 912730624:912730624(0) win 5840 <mss 1460,sackOK,timestamp 43698736 0,nop,wscale 0> (DF)
5.4 1b:94 a8:d7  x.y.z.w.53395 > 192.168.0.3.80: S 912730624:912730624(0) win 5840 <mss 1432,sackOK,timestamp 43698736 0,nop,wscale 0> (DF)

This part is good, your .0.3 host tries to connect to the external address, 
your router immediately sends a reply back, properly NATted.

5.4 a8:d7 1b:94  192.168.0.3.53395 > x.y.z.w.80: . ack 1 win 5840 <nop,nop,timestamp 43698736 43698736> (DF)

Linux doesn't go for it. It basically says 'I know this session already'!
This trace is all very very broken, and I think parts of it are missing.

I suspect that your router gets confused by timestamp and SACK options, but
I'm not sure.

Regards,

bert

-- 
http://www.PowerDNS.com          Versatile DNS Software & Services
Trilab                                 The Technology People
Netherlabs BV / Rent-a-Nerd.nl           - Nerd Available -
'SYN! .. SYN|ACK! .. ACK!' - the mating call of the internet

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