telnet freeze agony

dear linux.networking people:

i've come to the end of my troubleshooting ability with a
telnet freezing problem:

kernel: 2.4.26 (but it was the same with 21, and is the same with 2.6.6)
os: debian/unstable
telnet: various versions & also with the java telnet app
connection: dhcpcd
ethernet: 10Mbps, half-duplex, lpa 0x0000

i use it primarily for public library access, and one lib is fine,
but the other, the nycity public lib at nyplgate.nypl.org spits
out a number of screens of info, and when the are too many char's,
freeze.

strace -p `pidof telnet` yields:

select(4, [0 3], [], [3], NULL

i've tried many combo's of altering mtu, to no avail. the lower i set
it, the sooner the freeze.

i've tried 8-bit char's in telnet itself, and as i said, several
versions, to no avail.

i've scanned this newsgroup and the web, to no avail.

i did find and try:

echo 0 > /proc/sys/net/ipv4/tcp_ecn

to no avail.

networking is, it appears, extremely detailed, or in other words,
there's actually a ton going on, of which i understand very little.

thank you,

/frank


fmarx@earthlink.net

(E-Mail Removed) wrote:

> i've come to the end of my troubleshooting ability with a
> telnet freezing problem:

> i use it primarily for public library access, and one lib is fine,
> but the other, the nycity public lib at nyplgate.nypl.org spits
> out a number of screens of info, and when the are too many char's,
> freeze.

You should see if it happens from another internet connection.

You might also try typing ^Q (That's Control-Q). If this fixes it, you
have a problem with software flow control.

--
Cameron Kerr
(E-Mail Removed) : http://nzgeeks.org/cameron/
Empowered by Perl!

Thank you for your response:

I just went and tried ^Q, and have tried this before, to no avail.

I should have mentioned:

1. the connection works fine from my cygwin setup on windows;

2. and it *has* worked fine in linux; this problem has recurred
and gone away several times over the past three years.

the closest I can come:

1. reducing mtu causes the freeze to occur earlier;

2. we're in select(...) when the freeze occurs;

3. if mtu may properly be thought of as a *buffer size*,
then it seems to me the essential problem is that there's a
mismatch somewhere along the line - someone thinks it can write
x#bytes, and someone else is reading only x-y#bytes, so finishes
reading and waits for more, whereas the former has already
written more. Does that make sense?

Thank you,

/frank


Cameron Kerr <(E-Mail Removed)> wrote in message news:<(E-Mail Removed)>...
> (E-Mail Removed) wrote:
>
> > i've come to the end of my troubleshooting ability with a
> > telnet freezing problem:
>
> > i use it primarily for public library access, and one lib is fine,
> > but the other, the nycity public lib at nyplgate.nypl.org spits
> > out a number of screens of info, and when the are too many char's,
> > freeze.
>
> You should see if it happens from another internet connection.
>
> You might also try typing ^Q (That's Control-Q). If this fixes it, you
> have a problem with software flow control.

(E-Mail Removed) wrote:

> 1. reducing mtu causes the freeze to occur earlier;

It would seem that your machine may not be dealing correctly with
fragments. Fragments are generated when a packet must be sent through an
interface whose MTU is smaller than the size of the packet. It may be
that DF (don't fragment) may be set.

If you can show us the last screenful of tcpdump output as it freezes,
we may be able to test this theory.

PS. Do you have a firewall in use? What version kernel do you have?

--
Cameron Kerr
(E-Mail Removed) : http://nzgeeks.org/cameron/
Empowered by Perl!

Cameron Kerr <(E-Mail Removed)> wrote in message news:<(E-Mail Removed)>...
> (E-Mail Removed) wrote:
>
> > 1. reducing mtu causes the freeze to occur earlier;
>
> It would seem that your machine may not be dealing correctly with
> fragments. Fragments are generated when a packet must be sent through an
> interface whose MTU is smaller than the size of the packet. It may be
> that DF (don't fragment) may be set.
>
> If you can show us the last screenful of tcpdump output as it freezes,
> we may be able to test this theory.
>
> PS. Do you have a firewall in use? What version kernel do you have?`


Dear networking people:

below please find the results of ``tcpdump -vvv port 23'';
the point at which it hangs is marked;
please note the length fields - they seem to alter at that point;

my linux kernel is: ``2.4.26''; no firewall whatsoever;

i did a little investigating based on your hint about DF;
tried: ``echo 1 >/proc/sys/net/ipv4/ip_no_ptmu_disc'' to no avail.

i can't thank you enough for your help; i've been totally frustrated,
and this has been going on for some time.

thanks again

/frank


tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size
96 bytes

10:26:59.153249 IP (tos 0x10, ttl 64, id 21755, offset 0, flags [DF],
length: 60) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: S [tcp sum ok] 4237429203:4237429203(0) win
5840 <mss 1460,sackOK,timestamp 548136 0,nop,wscale 0>

10:26:59.163580 IP (tos 0x0, ttl 52, id 54829, offset 0, flags [DF],
length: 60) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: S [tcp sum ok]
2743623762:2743623762(0) ack 4237429204 win 31740 <mss
1380,sackOK,timestamp 2559582197 548136,nop,wscale 0>

10:26:59.163627 IP (tos 0x10, ttl 64, id 21756, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 1:1(0) ack 1 win 5840
<nop,nop,timestamp 548137 2559582197>

10:26:59.164471 IP (tos 0x10, ttl 64, id 21757, offset 0, flags [DF],
length: 79) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 1:28(27) ack 1 win 5840
<nop,nop,timestamp 548137 2559582197> [telnet DO SUPPRESS GO AHEAD,
WILL TERMINAL TYPE, WILL NAWS, WILL TSPEED, WILL LFLOW, WILL LINEMODE,
WILL NEW-ENVIRON, DO STATUS, WILL XDISPLOC]

10:26:59.180681 IP (tos 0x0, ttl 52, id 54830, offset 0, flags [DF],
length: 52) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: . [tcp sum ok] 1:1(0) ack 28
win 31740 <nop,nop,timestamp 2559582199 548137>

10:26:59.338246 IP (tos 0x0, ttl 52, id 54832, offset 0, flags [DF],
length: 64) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 1:13(12) ack
28 win 31740 <nop,nop,timestamp 2559582215 548137> [telnet DO TERMINAL
TYPE, DO TSPEED, DO XDISPLOC, DO NEW-ENVIRON]

10:26:59.338261 IP (tos 0x10, ttl 64, id 21758, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 28:28(0) ack 13 win 5840
<nop,nop,timestamp 548154 2559582215>

10:26:59.346893 IP (tos 0x0, ttl 52, id 54833, offset 0, flags [DF],
length: 91) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 13:52(39) ack 28 win 31740
<nop,nop,timestamp 2559582216 548154> [telnet WILL SUPPRESS GO AHEAD,
DO NAWS, DO LFLOW, DONT LINEMODE, WILL STATUS, SB TSPEED SEND SE, SB
XDISPLOC SEND SE]

10:26:59.346903 IP (tos 0x10, ttl 64, id 21759, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 28:28(0) ack 52 win 5840
<nop,nop,timestamp 548155 2559582216>

10:26:59.347348 IP (tos 0x10, ttl 64, id 21760, offset 0, flags [DF],
length: 157) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P 28:133(105) ack 52 win 5840
<nop,nop,timestamp 548155 2559582216> [telnet SB NAWS IS 0x84 0 0x3c
SE, SB TSPEED IS 0x33 0x38 0x34 0x30 0x30 0x2c 0x33 0x38 0x34 0x30
0x30 SE]

10:26:59.356573 IP (tos 0x0, ttl 52, id 54834, offset 0, flags [DF],
length: 52) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: . [tcp sum ok] 52:52(0) ack
133 win 31740 <nop,nop,timestamp 2559582217 548155>

10:26:59.357141 IP (tos 0x0, ttl 52, id 54835, offset 0, flags [DF],
length: 58) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 52:58(6) ack
133 win 31740 <nop,nop,timestamp 2559582217 548155> [telnet SB
TERMINAL TYPE SEND SE]

10:26:59.357175 IP (tos 0x10, ttl 64, id 21761, offset 0, flags [DF],
length: 70) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 133:151(18) ack 58 win 5840
<nop,nop,timestamp 548156 2559582217> [telnet SB TERMINAL TYPE IS 0x78
0x74 0x65 0x72 0x6d 0x2d 0x64 0x65 0x62 0x69 0x61 0x6e SE]

10:26:59.366527 IP (tos 0x0, ttl 52, id 54836, offset 0, flags [DF],
length: 55) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 58:61(3) ack
151 win 31740 <nop,nop,timestamp 2559582218 548156> [telnet DO ECHO]

10:26:59.366552 IP (tos 0x10, ttl 64, id 21762, offset 0, flags [DF],
length: 55) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 151:154(3) ack 61 win 5840
<nop,nop,timestamp 548157 2559582218> [telnet WONT ECHO]

10:26:59.380912 IP (tos 0x0, ttl 52, id 54838, offset 0, flags [DF],
length: 319) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 61:328(267) ack 154 win
31740 <nop,nop,timestamp 2559582219 548157> [telnet WILL ECHO]

10:26:59.380961 IP (tos 0x10, ttl 64, id 21763, offset 0, flags [DF],
length: 55) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 154:157(3) ack 328 win 6432
<nop,nop,timestamp 548158 2559582219> [telnet DO ECHO]

10:26:59.402998 IP (tos 0x0, ttl 52, id 54839, offset 0, flags [DF],
length: 52) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: . [tcp sum ok] 328:328(0) ack
157 win 31740 <nop,nop,timestamp 2559582222 548158>

10:26:59.411401 IP (tos 0x0, ttl 52, id 54840, offset 0, flags [DF],
length: 59) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 328:335(7) ack
157 win 31740 <nop,nop,timestamp 2559582222 548158>

10:26:59.441942 IP (tos 0x10, ttl 64, id 21764, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 157:157(0) ack 335 win 6432
<nop,nop,timestamp 548165 2559582222>

10:26:59.778165 IP (tos 0x10, ttl 64, id 21765, offset 0, flags [DF],
length: 53) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 157:158(1) ack 335 win 6432
<nop,nop,timestamp 548198 2559582222>

10:26:59.787303 IP (tos 0x0, ttl 52, id 54841, offset 0, flags [DF],
length: 53) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 335:336(1) ack
158 win 31740 <nop,nop,timestamp 2559582260 548198>

10:26:59.787325 IP (tos 0x10, ttl 64, id 21766, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 158:158(0) ack 336 win 6432
<nop,nop,timestamp 548199 2559582260>

10:26:59.896762 IP (tos 0x10, ttl 64, id 21767, offset 0, flags [DF],
length: 53) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 158:159(1) ack 336 win 6432
<nop,nop,timestamp 548210 2559582260>

10:26:59.906833 IP (tos 0x0, ttl 52, id 54842, offset 0, flags [DF],
length: 53) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 336:337(1) ack
159 win 31740 <nop,nop,timestamp 2559582272 548210>

10:26:59.906851 IP (tos 0x10, ttl 64, id 21768, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 159:159(0) ack 337 win 6432
<nop,nop,timestamp 548211 2559582272>

10:27:00.055170 IP (tos 0x10, ttl 64, id 21769, offset 0, flags [DF],
length: 53) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 159:160(1) ack 337 win 6432
<nop,nop,timestamp 548226 2559582272>

10:27:00.062790 IP (tos 0x0, ttl 52, id 54843, offset 0, flags [DF],
length: 53) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 337:338(1) ack
160 win 31740 <nop,nop,timestamp 2559582287 548226>

10:27:00.062818 IP (tos 0x10, ttl 64, id 21770, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 160:160(0) ack 338 win 6432
<nop,nop,timestamp 548227 2559582287>

10:27:00.286049 IP (tos 0x10, ttl 64, id 21771, offset 0, flags [DF],
length: 54) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 160:162(2) ack 338 win 6432
<nop,nop,timestamp 548249 2559582287>

10:27:00.294859 IP (tos 0x0, ttl 52, id 54844, offset 0, flags [DF],
length: 54) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 338:340(2) ack
162 win 31740 <nop,nop,timestamp 2559582311 548249>

10:27:00.294877 IP (tos 0x10, ttl 64, id 21772, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 340 win 6432
<nop,nop,timestamp 548250 2559582311>

10:27:00.340112 IP (tos 0x0, ttl 52, id 54845, offset 0, flags [DF],
length: 136) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 340:424(84) ack 162 win
31740 <nop,nop,timestamp 2559582315 548250>

10:27:00.340124 IP (tos 0x10, ttl 64, id 21773, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 424 win 6432
<nop,nop,timestamp 548254 2559582315>

10:27:03.428285 IP (tos 0x0, ttl 52, id 54853, offset 0, flags [DF],
length: 92) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 424:464(40) ack 162 win
31740 <nop,nop,timestamp 2559582624 548254>

10:27:03.428318 IP (tos 0x10, ttl 64, id 21774, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 464 win 6432
<nop,nop,timestamp 548563 2559582624>

10:27:03.439903 IP (tos 0x0, ttl 52, id 54855, offset 0, flags [DF],
length: 74) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 464:486(22)
ack 162 win 31740 <nop,nop,timestamp 2559582625 548563>

10:27:03.439915 IP (tos 0x10, ttl 64, id 21775, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 486 win 6432
<nop,nop,timestamp 548564 2559582625>

10:27:03.485112 IP (tos 0x0, ttl 52, id 54861, offset 0, flags [DF],
length: 106) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 486:540(54) ack 162 win
31740 <nop,nop,timestamp 2559582630 548564>

10:27:03.485123 IP (tos 0x10, ttl 64, id 21776, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 540 win 6432
<nop,nop,timestamp 548569 2559582630>

10:27:03.503938 IP (tos 0x0, ttl 52, id 54863, offset 0, flags [DF],
length: 102) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 540:590(50) ack 162 win
31740 <nop,nop,timestamp 2559582632 548569>

10:27:03.503950 IP (tos 0x10, ttl 64, id 21777, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 590 win 6432
<nop,nop,timestamp 548571 2559582632>

10:27:03.547794 IP (tos 0x0, ttl 52, id 54865, offset 0, flags [DF],
length: 78) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 590:616(26)
ack 162 win 31740 <nop,nop,timestamp 2559582636 548571>

10:27:03.547804 IP (tos 0x10, ttl 64, id 21778, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 616 win 6432
<nop,nop,timestamp 548575 2559582636>

10:27:03.566796 IP (tos 0x0, ttl 52, id 54867, offset 0, flags [DF],
length: 190) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 616:754(138) ack 162 win
31740 <nop,nop,timestamp 2559582638 548575>

10:27:03.566807 IP (tos 0x10, ttl 64, id 21779, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 754 win 7504
<nop,nop,timestamp 548577 2559582638>

10:27:03.668108 IP (tos 0x0, ttl 52, id 54869, offset 0, flags [DF],
length: 58) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 754:760(6) ack
162 win 31740 <nop,nop,timestamp 2559582648 548577>

10:27:03.668126 IP (tos 0x10, ttl 64, id 21780, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 760 win 7504
<nop,nop,timestamp 548587 2559582648>

10:27:03.715155 IP (tos 0x0, ttl 52, id 54871, offset 0, flags [DF],
length: 66) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P [tcp sum ok] 760:774(14)
ack 162 win 31740 <nop,nop,timestamp 2559582653 548587>

10:27:03.715176 IP (tos 0x10, ttl 64, id 21781, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 774 win 7504
<nop,nop,timestamp 548592 2559582653>

10:27:03.722643 IP (tos 0x0, ttl 52, id 54872, offset 0, flags [DF],
length: 1420) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 774:2142(1368) ack 162 win
31740 <nop,nop,timestamp 0 548587>

10:27:03.722657 IP (tos 0x10, ttl 64, id 21782, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 2142 win 9576
<nop,nop,timestamp 548593 0>

10:27:03.736939 IP (tos 0x0, ttl 52, id 54874, offset 0, flags [DF],
length: 393) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 2142:2

<<<<<<<<<<<<<<<<<<<<<<<<<<<<
it hangs right here, until i hit ^] in telnet;

then the remainder
>>>>>>>>>>>>>>>>>>>>>>>>>>>>


483(341) ack 162 win 31740 <nop,nop,timestamp 2559582655 548593>

10:27:03.736987 IP (tos 0x10, ttl 64, id 21783, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 2142 win 9576
<nop,nop,timestamp 548594 0>

10:27:23.396059 IP (tos 0x10, ttl 64, id 21784, offset 0, flags [DF],
length: 53) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 162:163(1) ack 2142 win 9576
<nop,nop,timestamp 550560 0>

10:27:23.424399 IP (tos 0x0, ttl 52, id 54907, offset 0, flags [DF],
length: 52) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: . [tcp sum ok] 2483:2483(0)
ack 163 win 31740 <nop,nop,timestamp 2559584624 550560>

10:27:23.424419 IP (tos 0x10, ttl 64, id 21785, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 163:163(0) ack 2142 win 9576
<nop,nop,timestamp 550563 0>

10:27:23.601938 IP (tos 0x10, ttl 64, id 21786, offset 0, flags [DF],
length: 53) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 162:163(1) ack 2142 win 9576
<nop,nop,timestamp 550581 0>

10:27:23.610336 IP (tos 0x0, ttl 52, id 54908, offset 0, flags [DF],
length: 52) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: . [tcp sum ok] 2483:2483(0)
ack 163 win 31740 <nop,nop,timestamp 2559584642 550581>

10:27:23.610346 IP (tos 0x10, ttl 64, id 21787, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 163:163(0) ack 2142 win 9576
<nop,nop,timestamp 550581 0>

10:27:24.021948 IP (tos 0x10, ttl 64, id 21788, offset 0, flags [DF],
length: 53) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 162:163(1) ack 2142 win 9576
<nop,nop,timestamp 550623 0>

10:27:24.031140 IP (tos 0x0, ttl 52, id 54909, offset 0, flags [DF],
length: 52) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: . [tcp sum ok] 2483:2483(0)
ack 163 win 31740 <nop,nop,timestamp 2559584684 550623>

10:27:24.031159 IP (tos 0x10, ttl 64, id 21789, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 163:163(0) ack 2142 win 9576
<nop,nop,timestamp 550623 0>

10:27:24.041429 IP (tos 0x0, ttl 52, id 54910, offset 0, flags [DF],
length: 393) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: P 2142:2483(341) ack 163 win
31740 <nop,nop,timestamp 2559584685 550623>

10:27:24.041439 IP (tos 0x10, ttl 64, id 21790, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 163:163(0) ack 2142 win 9576
<nop,nop,timestamp 550624 0>

10:27:24.861947 IP (tos 0x10, ttl 64, id 21791, offset 0, flags [DF],
length: 53) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: P [tcp sum ok] 162:163(1) ack 2142 win 9576
<nop,nop,timestamp 550707 0>

10:27:24.871247 IP (tos 0x0, ttl 52, id 54911, offset 0, flags [DF],
length: 52) nyplgate.nypl.org.telnet >
user-0cdfbse.cable.mindspring.com.32954: . [tcp sum ok] 2483:2483(0)
ack 163 win 31740 <nop,nop,timestamp 2559584768 550707>

10:27:24.871264 IP (tos 0x10, ttl 64, id 21792, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.telnet: . [tcp sum ok] 163:163(0) ack 2142 win 9576
<nop,nop,timestamp 550707 0>

10:27:25.909278 IP (tos 0x10, ttl 64, id 21793, offset 0, flags [DF],
length: 52) user-0cdfbse.cable.mindspring.com.32954 >
nyplgate.nypl.org.tel


----------------------------------------------------------------------------
f.g.marx <(E-Mail Removed)>

(E-Mail Removed) wrote:

> i use it primarily for public library access, and one lib is fine,
> but the other, the nycity public lib at nyplgate.nypl.org spits
> out a number of screens of info, and when the are too many char's,
> freeze.

<quote>
TCPMSS target support

This option adds a `TCPMSS' target, which allows you to alter the
MSS value of TCP SYN packets, to control the maximum size for that
connection (usually limiting it to your outgoing interface's MTU
minus 40).

This is used to overcome criminally braindead ISPs or servers which
block ICMP Fragmentation Needed packets. The symptoms of this
problem are that everything works fine from your Linux
firewall/router, but machines behind it can never exchange large
packets:
1) Web browsers connect, then hang with no data received.
2) Small mail works fine, but large emails hang.
3) ssh works fine, but scp hangs after initial handshaking.

Workaround: activate this option and add a rule to your firewall
configuration like:

iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
-j TCPMSS --clamp-mss-to-pmtu

To compile it as a module, choose M here. If unsure, say N.
</quote>

--
Cameron Kerr
(E-Mail Removed) : http://nzgeeks.org/cameron/
Empowered by Perl!

(E-Mail Removed) wrote:

> below please find the results of ``tcpdump -vvv port 23'';
> the point at which it hangs is marked;
> please note the length fields - they seem to alter at that point;

> my linux kernel is: ``2.4.26''; no firewall whatsoever;

> i did a little investigating based on your hint about DF;
> tried: ``echo 1 >/proc/sys/net/ipv4/ip_no_ptmu_disc'' to no avail.

This isn't related to PMTU Discovery. The number in ip_no_ptmu_disc
should be "0" .

....

I don't know what the telnet negotiation messages mean but this is where
things fall apart.

> 10:27:03.722643 IP (tos 0x0, ttl 52, id 54872, offset 0, flags [DF],
> length: 1420) nyplgate.nypl.org.telnet >
> user-0cdfbse.cable.mindspring.com.32954: P 774:2142(1368) ack 162 win
> 31740 <nop,nop,timestamp 0 548587>

nyplgate sends the first large segment, but still smaller than the MSS
it advertised in the SYN negotiation, 1380.

> 10:27:03.722657 IP (tos 0x10, ttl 64, id 21782, offset 0, flags [DF],
> length: 52) user-0cdfbse.cable.mindspring.com.32954 >
> nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 2142 win 9576
> <nop,nop,timestamp 548593 0>

user-0cdfbse ACKs the large segment.

> 10:27:03.736939 IP (tos 0x0, ttl 52, id 54874, offset 0, flags [DF],
> length: 393) nyplgate.nypl.org.telnet >
> user-0cdfbse.cable.mindspring.com.32954: P 2142:2

> <<<<<<<<<<<<<<<<<<<<<<<<<<<<
> it hangs right here, until i hit ^] in telnet;

> then the remainder
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


> 483(341) ack 162 win 31740 <nop,nop,timestamp 2559582655 548593>

nyplgate sends the next segment, containing 341 data octets, but it
appears that telnet at your end fails to get the data, the reason
apparently being associated with the strange behavior you saw above.

> 10:27:03.736987 IP (tos 0x10, ttl 64, id 21783, offset 0, flags [DF],
> length: 52) user-0cdfbse.cable.mindspring.com.32954 >
> nyplgate.nypl.org.telnet: . [tcp sum ok] 162:162(0) ack 2142 win 9576
> <nop,nop,timestamp 548594 0>

> 10:27:23.396059 IP (tos 0x10, ttl 64, id 21784, offset 0, flags [DF],
> length: 53) user-0cdfbse.cable.mindspring.com.32954 >
> nyplgate.nypl.org.telnet: P [tcp sum ok] 162:163(1) ack 2142 win 9576
> <nop,nop,timestamp 550560 0>

These two messages from user-0cdfbse again ACK the full-sized segment
sent by nyplgate. The second ACK also requests that nyplgate send all
the data in it's buffer (P).

> 10:27:23.424399 IP (tos 0x0, ttl 52, id 54907, offset 0, flags [DF],
> length: 52) nyplgate.nypl.org.telnet >
> user-0cdfbse.cable.mindspring.com.32954: . [tcp sum ok] 2483:2483(0)
> ack 163 win 31740 <nop,nop,timestamp 2559584624 550560>

nyplgate responds saying it doesn't have any more data in it's buffer.

This nonsense goes on and on, typical of automation gone wrong. The
transport layer (TCP) has ACKed the large segment but the application,
telnet, didn't get the segment data.

It seems so me that something is preventing telnet from collecting the
segment data on your side, but beware: I'm no expert on telnet or TCP.

I really have no idea just what it is that is wrong, except that it's
not PMTU Discovery. But if whatever the other side sends in the large
segment could be other than ASCII then I'd try using telnet with the -8
option: " telnet -8 <whatever> ". According to the man pages here that
causes telnet to attempt to negotiate an "8-bit data path" by using the
"TELNET BINARY" option.

I will add that a particular telnet implementation may not be expecting
a data segment that large. Telnet usually operates with a text screen
that is 80x25=2000 characters, so perhaps it might be a buffer overflow
(wild speculation).

--
Clifford Kite Email: "echo xvgr_yvahk-(E-Mail Removed)|rot13"
PPP-Q&A links, downloads: http://ckite.no-ip.net/
/* The signal-to-noise ratio is too low in many [news] groups to make
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