[stunnel-users] Connection problems and TCP frame checksum errors

Mon Oct 9 18:40:00 CEST 2006

I have the following kind of test environment: SSL clients
call a public ip address from which the calls are forwarded
to a linux server with Stunnel. The linux server is in a
private network. Stunnel decrypts the data and sends it
forward.

I have been testing this with a browser, monitoring the
traffic on the server to see that Stunnel forwards the
calls. Some strange things happen there that I can't
explain. First of all: sometimes the calls go through
the server as expected, sometimes the server doesn't
respond in any way to the client. If I have two terminal
windows open, one with tcpdump and another with
tail -f stunnel.log - nothing comes into the log in spite
of the incoming connections attempts.

Then on other occasions when the calls come to the server,
it forwards them beautifully to the address and port set in
the configuration file.

Does anyone have any clue, what this could be due to? I
haven't been able to explain why it sometimes works and
sometimes doesn't.

Another thing that bothers me is, that sometimes there
are TCP frames with incorrect checksum. I've monitored
with Ethereal and tcpdump. Both show incorrect frames,
and they are always from the stunnel-end of the connection.
What could be the cause of those broken frames?

Tommi Nieminen

--------------------------------------------------------
My stunnel.conf looks like this:
(any constructive criticism would be welcome :-))

CAfile = /home/tommi/cert/7/demoCA/cacert.pem
cert = /home/tommi/cert/7/newcert.pem
key = /home/tommi/cert/7/newkey.pem

socket = l:TCP_NODELAY=1
socket = r:TCP_NODELAY=1

output = /var/log/stunnel/stunnel.log
pid = /var/run/stunnel/stunnel.pid
debug = 7
client = no

[https]
accept  = 443
connect = 192.168.10.17:5010
TIMEOUTclose = 0

--------------------------------------------------------
A succesful connection from stunnel.log:

2006.10.09 18:01:43 LOG7[11889:3083744960]: https accepted FD=7 from
131.177.254.92:2689
2006.10.09 18:01:43 LOG7[11889:3083742128]: https started
2006.10.09 18:01:43 LOG7[11889:3083742128]: FD 7 in non-blocking mode
2006.10.09 18:01:43 LOG7[11889:3083742128]: TCP_NODELAY option set on local
socket
2006.10.09 18:01:43 LOG5[11889:3083742128]: https connected from
131.177.254.92:2689
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept):
before/accept initialization
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3 read
client hello A
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3 write
server hello A
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3 write
certificate A
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3 write
server done A
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3
flush data
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3 read
client key exchange A
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3 read
finished A
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3 write
change cipher spec A
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3 write
finished A
2006.10.09 18:01:43 LOG7[11889:3083742128]: SSL state (accept): SSLv3
flush data
2006.10.09 18:01:43 LOG7[11889:3083742128]:    2 items in the session cache
2006.10.09 18:01:43 LOG7[11889:3083742128]:    0 client connects
(SSL_connect())
2006.10.09 18:01:43 LOG7[11889:3083742128]:    0 client connects that
finished
2006.10.09 18:01:43 LOG7[11889:3083742128]:    0 client renegotiations
requested
2006.10.09 18:01:43 LOG7[11889:3083742128]:    8 server connects
(SSL_accept())
2006.10.09 18:01:43 LOG7[11889:3083742128]:    7 server connects that
finished
2006.10.09 18:01:43 LOG7[11889:3083742128]:    0 server renegotiations
requested
2006.10.09 18:01:43 LOG7[11889:3083742128]:    4 session cache hits
2006.10.09 18:01:43 LOG7[11889:3083742128]:    1 session cache misses
2006.10.09 18:01:43 LOG7[11889:3083742128]:    1 session cache timeouts
2006.10.09 18:01:43 LOG6[11889:3083742128]: SSL accepted: new session
negotiated
2006.10.09 18:01:43 LOG6[11889:3083742128]: Negotiated ciphers: AES256-SHA
            SSLv3 Kx=RSA      Au=RSA  Enc=AES(256)  Mac=SHA1
2006.10.09 18:01:43 LOG7[11889:3083742128]: FD 8 in non-blocking mode
2006.10.09 18:01:43 LOG7[11889:3083742128]: https connecting
192.168.10.17:5010
2006.10.09 18:01:43 LOG7[11889:3083742128]: connect_wait: waiting 10 seconds
2006.10.09 18:01:43 LOG7[11889:3083742128]: connect_wait: connected
2006.10.09 18:01:43 LOG7[11889:3083742128]: Remote FD=8 initialized
2006.10.09 18:01:43 LOG7[11889:3083742128]: TCP_NODELAY option set on remote
socket
2006.10.09 18:02:45 LOG7[11889:3083742128]: Socket closed on read
2006.10.09 18:02:45 LOG7[11889:3083742128]: SSL write shutdown
2006.10.09 18:02:45 LOG7[11889:3083742128]: SSL alert (write): warning:
close notify
2006.10.09 18:02:45 LOG7[11889:3083742128]: SSL_shutdown retrying
2006.10.09 18:02:45 LOG7[11889:3083742128]: SSL doesn't need to read or write
2006.10.09 18:02:45 LOG6[11889:3083742128]: s_poll_wait timeout: connection
close
2006.10.09 18:02:45 LOG5[11889:3083742128]: Connection closed: 0 bytes sent
to SSL, 405 bytes sent to socket
2006.10.09 18:02:45 LOG7[11889:3083742128]: https finished (0 left)

(and the failed connections leave also no mark in the log, but tcpdump
sees the attempts on the server)

--------------------------------------------------------
A sample of tcpdump's output with incorrect checksum:

18:02:45.150656 IP (tos 0x0, ttl  64, id 11218, offset 0, flags [DF], proto:
TCP (6), length: 77) 192.168.20.18.https > 131.177.254.92.2689: P, cksum
0x5708 (incorrect (-> 0xf1bf), 1035:1072(37) ack 756 win 7504