Re: [SLUG] Telnet and the Internet

From: Derek Glidden (dglidden@illusionary.com)
Date: Tue Feb 05 2002 - 11:21:01 EST


On Mon, 2002-02-04 at 21:47, Paul M Foster wrote:
> On Mon, Feb 04, 2002 at 07:53:16PM -0500, Russ Herrold wrote:
>
> > On Mon, 4 Feb 2002, Paul M Foster wrote:
> >
> > > Anyone have any enlightening info?
> >
> > As a BOFH, this is a safe rule to live by: -- 80% of the time,
> > DNS is not doing what you expect.
> >
> > Another gem: tcpdump is your friend -- and it can monitor
> > ppp0
> >
> > As I recall ppp can set debugging up to kdebug 7, and debug 7
> > -- which show everything but the color of your eyebrows, into
> > /var/log/messages
> >
>
> Good, but that doesn't answer the question of why _telnet_ is making
> pppd dial out rather than simply responding itself. (BTW, after pppd
> dials out, telnet responds with a prompt. Go figure.)

What's likely happening is that the telneted-to box is trying to do a
reverse lookup of who's connecting, the telnet-ing box is ignoring the
fact that the box is local and listed in /etc/hosts and doing a lookup
anyway, the DNS server is ignoring both of them and taking the
opportunity of a DNS lookup to refresh data from the root servers, or
some other box on the network is seeing extra activity and using that as
an excuse to lookup www.playboy.com.

As Russ said, DNS rarely behaves the way you expect. Try to get as much
logging as possible and make liberal use of tcpdump. (I use tcpdump in
conjunction with Ethereal to make packet decoding easy, something like:
'tcpdump -s 6000 -w dns.dump -i ppp0' and then when you're done
capturing, import that dump into Ethereal to do the decode and read the
packets to see what's going on. You should be able to decode the DNS
packets and see what's being looked up.)

-- 
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
#!/usr/bin/perl -w
$_='while(read+STDIN,$_,2048){$a=29;$b=73;$c=142;$t=255;@t=map
{$_%16or$t^=$c^=($m=(11,10,116,100,11,122,20,100)[$_/16%8])&110;
$t^=(72,@z=(64,72,$a^=12*($_%16-2?0:$m&17)),$b^=$_%64?12:0,@z)
[$_%8]}(16..271);if((@a=unx"C*",$_)[20]&48){$h=5;$_=unxb24,join
"",@b=map{xB8,unxb8,chr($_^$a[--$h+84])}@ARGV;s/...$/1$&/;$d=
unxV,xb25,$_;$e=256|(ord$b[4])<<9|ord$b[3];$d=$d>>8^($f=$t&($d
>>12^$d>>4^$d^$d/8))<<17,$e=$e>>8^($t&($g=($q=$e>>14&7^$e)^$q*
8^$q<<6))<<9,$_=$t[$_]^(($h>>=8)+=$f+(~$g&$t))for@a[128..$#a]}
print+x"C*",@a}';s/x/pack+/g;eval 

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