In message <20031028152912.B12461@yoda.cubidou.net>, cube@cubidou.net writes:
>On Tue, Oct 28, 2003 at 09:14:43AM -0500, Steve Bellovin wrote:
>> I'm having trouble logging in to ftp.netbsd.org by v6.  There is 
>> connectivity, and v4 works, but with v6, ftpd seems to hang after 
>> receiving the PASS command.
>> 
>> # ftp -d -6 -a ftp.netbsd.org
>> Connected to ftp.netbsd.org.
>> 220 ftp.NetBSD.org FTP server (NetBSD-ftpd 20020615) ready.
>> ftp_login: user `<null>' pass `<null>' host `ftp.netbsd.org'
>> ---> USER anonymous
>> 331 Guest login ok, type your name as password.
>> ---> PASS XXXX
>> 230-
>>        <***************************** LONG PAUSE HERE
>
>At that point, ftp.netbsd.org usually replies with a 1500-byte long
>packet. Make sure you negociate a correct MSS in case you have a
>MTU black hole, like in a lot of PPPoE setups.
>

I doubt that there's any PPPoE in the mix -- my machine is a server in 
a colo facility.  Here's a traceroute:

# traceroute6 ftp.netbsd.org        
traceroute6 to ftp.netbsd.org (2001:4f8:4:7:2e0:81ff:fe21:6563) from 2001:418:1::16, 30 hops max, 12 byte packets
 1  2001:418:1::1  0.478 ms  0.506 ms  0.342 ms
 2  2001:418:1:fdff:ffff:ffff:ffff:fffe  23.289 ms  22.749 ms  22.707 ms
 3  p1-5-3-0.r06.plalca01.us.bb.verio.net  23.103 ms  23.136 ms  23.042 ms
 4  3ffe:80a::1  23.162 ms  23.192 ms  23.309 ms
 5  r7.r8.pao1.isc.org  24.289 ms r7-2.r8.pao1.isc.org  23.226 ms  23.224 ms
 6  r8-pao1.r3.sfo2.isc.org  24.006 ms  24.128 ms  24.012 ms
 7  ftp.netbsd.org  23.921 ms  23.981 ms  23.949 ms

But yes, tcpdump shows a very large MSS with v6 -- 33160 -- though I 
thought that Path MTU was mandatory and would cope.

		--Steve Bellovin, http://www.research.att.com/~smb