[2.1.47] Bug in Path MTU or PPP(?)

B. James Phillippe (bryan@Terran.ORG)
Thu, 31 Jul 1997 22:49:37 -0700 (PDT)


I posted this back in 2.1.18~ days or so, and it still appears to
be a problem. Hmm, I must be the only person out there using PPP over a
28.8 modem & masquerading. :)

The problem is that unless I set the MRU on my end and the MTU on
remote end to 1500, my masqueraded machines can't access HTTP data on key
hosts, such as www.nbc.com, events.ticketmaster.com and some others (they
can get to most sites without problem). I can access them from my system
normally, and if I bump the MRU up to 1500, then it will work for them,
too. Here is a sample of a failed tcpdump session under an MRU of 768:

Script started on Thu Jul 31 22:09:39 1997
You have mail.
root has logged on ttypa from :0.0.
root has logged on ttypb from :0.0.
[bryan/root.4@earth]:~% tcpdump -i ppp0 -n -vv
tcpdump: listening on ppp0
22:09:55.682392 > S 865677258:865677258(0) win 8192 <mss 1460> (DF) (ttl 31, id 61767)
22:09:55.926764 > S 1345164000:1345164000(0) ack 865677259 win 8760 <mss 1460> (DF) (ttl 117, id 39792)
22:09:55.927766 > . ack 1345164001 win 8760 (DF) (ttl 31, id 62023)
22:09:55.933339 > P 0:168(168) ack 1 win 8760 (DF) (ttl 31, id 62279)
22:09:59.174307 > P 0:168(168) ack 1 win 8760 (DF) (ttl 31, id 62535)
22:09:59.426764 > . ack 169 win 8592 (DF) (ttl 117, id 21617)

6 packets received by filter
0 packets dropped by kernel
[bryan/root.4@earth]:~% exit

Script done on Thu Jul 31 22:10:15 1997 is the masqueraded host. My machine's external IP address is It appears that the 3-way handshake happens normally, but
the following data is never received. I just wrote my first simple
client/server network program in C yesterday, so I'm not feeling qualified
yet to dig too deep into this. ;) I hope this helps any of you wily
hackers out. If there's any other data that would be useful, please bonk
me on the head.


B. James Phillippe                              Seattle Software Labs, Inc
Network Administrator                           Phone: (206) 521-8346
NIC Handle: BJP4                                Fax: (206) 521-8340
http://w3.terran.org/~bryan                     http://www.sealabs.com