- stunnel-users - stunnel.org

Client mode certificate pinning
by Ian Pilcher 11 Oct '25

11 Oct '25

Is $SUBJECT possible? I use stunnel to connect to my old "smart" switches that don't support modern TLS protocols, ciphers, etc. Recently, their existing certificates all expired, and I tried to install new certificates on all of them, only to discover that they won't accept my new CA certificate, because it was signed with a 3072-bit key. I could go through the trouble of generating a separate CA certificate with a 2048-bit key, sign new certificates with that key, etc., etc., but it starts to seem a bit silly at that point. Far better to simply generate a self-signed certificate for each switch and configure stunnel to only accept that particular certificate (i.e. "pin" it). Is this possible with stunnel? If so, how would I go about configuring it to do this? (Search engines are telling me to use "verify = 4", but stunnel(8) says that option is obsolete.) TIA! -- ======================================================================== If your user interface is intuitive in retrospect ... it isn't intuitive ========================================================================

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How to get rid of "No OCSP stapling response received" warning?
by Ian Pilcher 25 Sep '25

25 Sep '25

I use stunnel to connect to my old "smart" switches from modern browsers. Here is a subset of the configuration for one of the switches. [switch1-frontend] sni = frontend:switch1.penurio.us CAfile = /etc/ipa/ca.crt cert = /etc/pki/tls/certs/stunnel/switch1.crt key = /tmp/keys/switch1.key connect = 127.255.255.254:8081 [switch1-backend] client = yes accept = 127.255.255.254:8081 connect = 172.31.4.1:443 verifyChain = yes checkHost = switch1.penurio.us CAfile = /etc/ipa/ca.crt sslVersion = TLSv1 ciphers = DHE-RSA-AES256-SHA options = ALLOW_UNSAFE_LEGACY_RENEGOTIATION securityLevel = 0 OCSPrequire = no OCSPaia = no Everything works, execept that every connection produces a bright red warning in my log: OCSP: No OCSP stapling response received I've read up on OCSP stapling, that this seems to mean that the switch isn't including an OCSP response in its TLS handshake. (Hardly surprising, given its age.) How can I tell stunnel to not request/expect an OCSP response from the switch (or at least to not log the missing OCSP response as a warning)? -- ======================================================================== If your user interface is intuitive in retrospect ... it isn't intuitive ========================================================================

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stunnel FIPS mode 140-2/ Other Modes
by mehmet ozisik 19 Sep '25

19 Sep '25

Hi All, I would like to ask a question about stunnel fips mode. There are lots of question and answers on the internet related with this, but I could not find any answer related with mine. I am compiling with openssl (auto detecting fips) . Here is a part of confgiure output : checking for FIPS_mode_set... yes configure: FIPS mode detected So I am thinking that fips also is being included. Then I try to run stunnel on target platform (in stunnel.conf fips=yes) and it gives below error : Compiled/running with OpenSSL 0.9.8w-fips 23 Apr 2012 Threading:PTHREAD Sockets:POLL,IPv6 SSL:ENGINE,OCSP,FIPS Reading configuration from file stunnel.conf FIPS_mode_set: 2D06906E: error:2D06906E:FIPS routines:FIPS_CHECK_INCORE_FINGERPRINT:fingerprint does not match there are lots of information about this errror on internet. Then when I configure stunnel.conf with fips=no, stunnel is running successfully. I know that fips=yes means that enables FIPS 140-2 mode and I guess my fips canister does not supoort fips 140-2 mode (I do not know which fips mode it has supported). Now my question is coming : When I set fips=no, stunnel also starts with other available fips modes which the canister included? Or it skips running fips mode completely? Plase inform me if anyone has any idea? Regards

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Stunnel, Gmail and Outlook Express
by landjonline＠hotmail.com 01 Sep '25

01 Sep '25

I have Outlook Express and my email addresses are Gmail. Is there a way to get Gmail to work with Outlook Express using Stunnel? Thanks

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Using CAStore on Android (stunnel.conf)
by smf.forum1＠ntlworld.com 31 Jul '25

31 Jul '25

I have build stunnel with a few changes to run in a QThread (Qt 6.9.1) for Android. My Qt App starts stunnel at the start of execution and stops it when my Qt application exits. This all works fine (I need to review and clean up my stunnel changes before I use my solution for real) but I cannot get certification to work, here are two test runs one with verify set to 0 (works) and verify set to 2 (fails). What needs to change in my stunnel.conf file ? Thanks. (obfuscated details) stunnel.conf foreground = yes debug = 6 [mariadb] CAstore = /data/misc/user/0/cacerts-added CApath = /data/misc/user/0/cacerts-added client = yes accept = 127.0.0.1:3307 connect = remote_server:3307 verify = 0 Works: I/stunnel : LOG6[ui]: Initializing inetd mode configuration I/stunnel : LOG5[ui]: stunnel 5.75 on android-mobile-device platform I/stunnel : LOG5[ui]: Compiled/running with OpenSSL 3.1.1 30 May 2023 I/stunnel : LOG5[ui]: Threading:PTHREAD Sockets:SELECT,IPv4 TLS:ENGINE,OCSP,PSK,SNI I/stunnel : LOG6[ui]: Initializing inetd mode configuration I/stunnel : LOG5[ui]: Reading configuration from buffer 0x7280e409b640 I/stunnel : LOG5[ui]: UTF-8 byte order mark not detected I/stunnel : LOG6[ui]: Compression disabled I/stunnel : LOG6[ui]: Initializing service [mariadb] I/stunnel : LOG6[ui]: stunnel default security level set: 2 I/stunnel : LOG6[ui]: Session resumption enabled I/stunnel : LOG6[ui]: Configured trusted server CA: C=AU, ST=Florida, L=York, O=Home, OU=Whiskey, CN=Michael, emailAddress=eric.admin(a)intheether.com W/stunnel : LOG4[ui]: Service [mariadb] needs authentication to prevent MITM attacks I/stunnel : LOG6[ui]: DH initialization skipped: client section I/stunnel : LOG5[ui]: Configuration successful I/stunnel : LOG6[ui]: Service [mariadb] (FD=117) bound to 127.0.0.1:3307 I/stunnel : LOG6[ui]: Accepting new connections I/stunnel : LOG6[per-day]: Executing per-day jobs I/stunnel : LOG6[per-day]: Per-day jobs completed in 0 seconds I/stunnel : LOG5[0]: Service [mariadb] accepted connection from 127.0.0.1:36802 I/stunnel : LOG6[0]: s_connect: connecting 192.168.0.1:3307 I/stunnel : LOG5[0]: s_connect: connected 192.168.0.1:3307 I/stunnel : LOG5[0]: Service [mariadb] connected remote server from 10.0.2.16:58660 I/stunnel : LOG6[0]: SNI: sending servername: remote_server I/stunnel : LOG6[0]: Peer certificate not required I/stunnel : LOG6[0]: Received trusted client CA: C=AU, ST=Florida, L=York, O=Home, OU=Whiskey, CN=Michael, emailAddress=eric.admin(a)intheether.com I/stunnel : LOG6[0]: CERT: Certificate verification disabled I/stunnel : LOG6[0]: CERT: Certificate verification disabled I/stunnel : LOG6[0]: OCSP: Certificate chain verification disabled I/stunnel : LOG6[0]: TLS connected: new session negotiated I/stunnel : LOG6[0]: TLSv1.3 ciphersuite: TLS_AES_256_GCM_SHA384 (256-bit encryption) I/stunnel : LOG6[0]: Peer temporary key: X25519, 253 bits I/stunnel : LOG6[0]: Session id: 90DD6325CCBC47C3F8F560C36F76755B4959E6A8421DD97B751E37FDD1DCCAC7 I/stunnel : LOG6[0]: Session id: 24345E21753FE3B254CA089FB7621C1481B523E72EB9571DDF7A366B5E89E834 stunnel.conf foreground = yes debug = 6 [mariadb] CAstore = /data/misc/user/0/cacerts-added CApath = /data/misc/user/0/cacerts-added client = yes accept = 127.0.0.1:3307 connect = remote_server:3307 verify = 2 Fails: I/stunnel : LOG6[ui]: Initializing inetd mode configuration I/stunnel : LOG5[ui]: stunnel 5.75 on android-mobile-device platform I/stunnel : LOG5[ui]: Compiled/running with OpenSSL 3.1.1 30 May 2023 I/stunnel : LOG5[ui]: Threading:PTHREAD Sockets:SELECT,IPv4 TLS:ENGINE,OCSP,PSK,SNI I/stunnel : LOG6[ui]: Initializing inetd mode configuration I/stunnel : LOG5[ui]: Reading configuration from buffer 0x7280e409b0a0 I/stunnel : LOG5[ui]: UTF-8 byte order mark not detected I/stunnel : LOG6[ui]: Compression disabled I/stunnel : LOG6[ui]: Initializing service [mariadb] I/stunnel : LOG6[ui]: stunnel default security level set: 2 I/stunnel : LOG6[ui]: Session resumption enabled I/stunnel : LOG6[ui]: Configured trusted server CA: C=AU, ST=Florida, L=York, O=Home, OU=Whiskey, CN=Michael, emailAddress=eric.admin(a)intheether.com W/stunnel : LOG4[ui]: Service [mariadb] uses "verifyChain" without subject checks W/stunnel : LOG4[ui]: Use "checkHost" or "checkIP" to restrict trusted certificates I/stunnel : LOG6[ui]: DH initialization skipped: client section I/stunnel : LOG5[ui]: Configuration successful I/stunnel : LOG6[ui]: Service [mariadb] (FD=114) bound to 127.0.0.1:3307 I/stunnel : LOG6[ui]: Accepting new connections I/stunnel : LOG6[per-day]: Executing per-day jobs I/stunnel : LOG6[per-day]: Per-day jobs completed in 0 seconds I/stunnel : LOG5[0]: Service [mariadb] accepted connection from 127.0.0.1:39238 I/stunnel : LOG6[0]: s_connect: connecting 192.168.0.1:3307 I/stunnel : LOG5[0]: s_connect: connected 192.168.0.1:3307 I/stunnel : LOG5[0]: Service [mariadb] connected remote server from 10.0.2.16:59154 I/stunnel : LOG6[0]: SNI: sending servername: remote_server I/stunnel : LOG6[0]: Peer certificate required I/stunnel : LOG6[0]: Received trusted client CA: C=AU, ST=Florida, L=York, O=Home, OU=Whiskey, CN=Michael, emailAddress=eric.admin(a)intheether.com W/stunnel : LOG4[0]: CERT: Pre-verification error: certificate not found in local repository: self-signed certificate W/stunnel : LOG4[0]: Rejected by CERT at depth=0: C=PL, ST=Mazovia Province, L=Warsaw, O=Stunnel Developers, OU=Provisional CA, CN=localhost E/stunnel : LOG3[0]: SSL_connect: tls_post_process_server_certificate@ssl/statem/statem_clnt.c:1890: error:0A000086:SSL routines::certificate verify failed I/stunnel : LOG5[0]: Connection closed/reset: 0 byte(s) sent to TLS, 0 byte(s) sent to socket I/stunnel : LOG6[ui]: Child process 17669 finished with code 0

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Android Stunnel 64bit version
by emxacademy＠gmail.com 18 Jul '25

18 Jul '25

Hello everyone, I've been searching online but still haven’t found a clear or working method to build a 64-bit version of the Android stunnel executable. Since most Android devices now run on 64-bit architecture, the 32-bit version available on the official website no longer works on many phones. If anyone has experience with this or can guide me through the proper way to compile a 64-bit version, I’d really appreciate your help. Thanks in advance!

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Error message when trying to set Stunnel as a start up app
by Edward Smithson 18 Jul '25

18 Jul '25

Hello Stunnel mailing list, We are trying to set Stunnel (5.75) to automatically open as a start up app on Windows 11. When the PC powers up, it gives this error message: stunnel.exe - System Error The code execution cannot proceed because libcrypto-3-x64.dll was not found. Reinstalling the program may fix this problem. We have tried reinstalling, but to no avail. NB, Stunnel works fine if opened manually. This was not an issue with Windows 10. Has anyone else had this problem (and solved it)? Many thanks, Ed Smithson Ed Smithson Reader Services Librarian Nuffield College, Oxford, OX1 1NF +44(0) 1865 278551 [cid:[email protected]]<https://bsky.app/profile/nuffieldlibrary.bsky.social>[cid:[email protected]]<https://www.facebook.com/NuffieldCollegeLibrary>[cid:[email protected]]<https://www.instagram.com/nuffieldlibrary/> [cid:[email protected]]<https://www.nuffield.ox.ac.uk/>

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stunnel with aurora postgreSQL
by Grube, Rolf | Oberender AG 25 Jun '25

25 Jun '25

Hi everybody, Does anybody has already established a connection to AWS aurora postgreSQL with stunnel? I cant establish a connection and it may be an issue with channel binding. My config looks like this: ; Sample stunnel configuration file for Win64 by Michal Trojnara 1998-2025 ; Some options used here may be inadequate for your particular configuration ; This sample file does *not* represent stunnel.conf defaults ; Please consult the manual for detailed description of available options ; ************************************************************************** ; * Global options * ; ************************************************************************** ; Debugging stuff (may be useful for troubleshooting) debug = 7 output = stunnel.log ; Enable FIPS 140-2 mode if needed for compliance fips = yes ; The CNG engine allows to integrate stunnel with the Windows Cryptography API: ; Next Generation (CNG) for authentication with private keys stored in the ; Windows certificate store. It serves as a drop-in replacement for the legacy ; OpenSSL Cryptography API (CAPI) engine. ; https://www.stunnel.org/cng-engine.html ; Each section using this feature also needs the "engineId = cng" option engine = cng ; The pkcs11 engine allows for authentication with cryptographic ; keys isolated in a hardware or software token ; MODULE_PATH specifies the path to the pkcs11 module shared library, ; such as softhsm2-x64.dll or opensc-pkcs11.dll ; IMPORTANT: A 64-bit stunnel requires 64-bit PKCS#11 modules ; Each section using this feature also needs the "engineId = pkcs11" option ;engine = pkcs11 ;engineCtrl = MODULE_PATH:softhsm2-x64.dll ;engineCtrl = PIN:1234 ; ************************************************************************** ; * Service defaults may also be specified in individual service sections * ; ************************************************************************** ; Enable support for the insecure SSLv3 protocol ;options = -NO_SSLv3 ; These options provide additional security at some performance degradation ;options = SINGLE_ECDH_USE ;options = SINGLE_DH_USE ; ************************************************************************** ; * Include all configuration file fragments from the specified folder * ; ************************************************************************** ;include = conf.d ; ************************************************************************** ; * Service definitions (at least one service has to be defined) * ; ************************************************************************** [postgreSQL_TLS] client = yes ;engineId = cng accept = 5433 connect = xxx.rds.amazonaws.com:5432 protocol = pgsql CAfile = C:\cert\xxx-bundle.pem verifyChain = yes checkHost = xxx.rds.amazonaws.com Thanks for any recommendations. Best regards Rolf Grube, MBA Senior Manager <https://www.credly.com/badges/443267d6-0d25-4fc3-9eee-f9654f6f3ee1/public_u rl> Oberender AG Elsenheimerstraße 59 | 80687 München t: +49 89 8207516-0 | m: +49 173 2035 133 <mailto:[email protected]> rolf.grube(a)oberender.com <http://www.oberender.com/> www.oberender.com Vorsitzender des Aufsichtsrats: Dipl.-Volkswirtin Irmtraut Gürkan Vorstand: Jan Hacker (Vorsitzender), Jochen Baierlein Handelsregister: Amtsgericht Bayreuth, HRB 4267 Sitz der Gesellschaft: Bayreuth Diese E-Mail enthält vertrauliche und/oder rechtlich geschützte Informationen. Wenn Sie nicht der richtige Adressat sind oder diese E-Mail irrtümlich erhalten haben, informieren Sie bitte sofort den Absender und vernichten Sie diese Mail. Das unerlaubte Kopieren, sowie die unbefugte Weitergabe dieser Mail sind nicht gestattet.

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Duplicate Received trusted client CA messages
by Sean Burford 18 Jun '25

18 Jun '25

Hi, While configuring stunnel to connect as a client to a TLS server that requires client certificate auth (known as mutual TLS auth), I noticed that each new connection generates lots (up to hundreds) of duplicate logs like: 2025.06.18 10:23:58 LOG6[0]: Received trusted client CA: CN=Example CA 2025.06.18 10:23:58 LOG6[0]: Received trusted client CA: CN=Example CA I expect one log per trusted client CA provided by the server. Openssl s_client -msg shows that the server only sends a small CertificateRequest. Stunnel logs show that only one connection is made. This only happens if the server offers exactly one trusted client CA. If multiple client CA options are offered by the server then a single log entry is recorded for each. What appears to be happening is that OpenSSL’s state machine generates lots of info callbacks (the number depends on latency). This appears to be harmless except for the excessive logging. I’m running stunnel 5.75 against OpenSSL 3.5.0 (compiled with enable-weak-ssl-ciphers, enable-ssl3 and enable-ssl3-method): $ stunnel [ ] Initializing inetd mode configuration [ ] Clients allowed=125 [.] stunnel 5.75 on sparc-sun-solaris2.10 platform [.] Compiled/running with OpenSSL 3.5.0 8 Apr 2025 [.] Threading:PTHREAD Sockets:POLL,IPv6 TLS:ENGINE,OCSP,PSK,SNI [ ] errno: (*(___errno())) … $ openssl version OpenSSL 3.5.0 8 Apr 2025 (Library: OpenSSL 3.5.0 8 Apr 2025) This is probably NOT caused by an OpenSSL 3.5.0 behaviour change, I also see duplicate messages from the public stunnel build running on Windows: [ ] Initializing inetd mode configuration [ ] Running on Windows 6.2 [ ] No limit detected for the number of clients [.] stunnel 5.74 on x64-pc-mingw32-gnu platform [.] Compiled/running with OpenSSL 3.3.2 3 Sep 2024 [.] Threading:WIN32 Sockets:SELECT,IPv6 TLS:ENGINE,FIPS,OCSP,PSK,SNI [ ] errno: (*_errno()) To diagnose this further, I modified stunnel src/ctx.c info_callback() to log the TLS state in callbacks: //#if 0 // s_log(LOG_DEBUG, "state = %x", state); state_string=SSL_state_string_long(ssl); s_log(LOG_DEBUG, "TLS state (where %08X state %08X): %s", where, state, state_string); //#endif This shows that there are many callbacks from where SSL_CB_ACCEPT_EXIT and one final callback from SSL_CB_ACCEPT_LOOP: 2025.06.18 10:23:58 LOG6[0]: Received trusted client CA: CN=Example CA 2025.06.18 10:23:58 LOG7[0]: TLS state (where 00001002 state 00000008): SSLv3/TLS read server certificate request 2025.06.18 10:23:58 LOG6[0]: Received trusted client CA: CN=Example CA 2025.06.18 10:23:58 LOG7[0]: TLS state (where 00001002 state 00000008): SSLv3/TLS read server certificate request 2025.06.18 10:23:58 LOG6[0]: Received trusted client CA: CN=Example CA 2025.06.18 10:23:58 LOG7[0]: TLS state (where 00001002 state 00000008): SSLv3/TLS read server certificate request 2025.06.18 10:23:58 LOG6[0]: Received trusted client CA: CN=Example CA 2025.06.18 10:23:58 LOG7[0]: TLS state (where 00001001 state 00000008): SSLv3/TLS read server certificate request 2025.06.18 10:23:58 LOG6[0]: Received trusted client CA: CN=Example CA 2025.06.18 10:23:58 LOG7[0]: TLS state (connect): SSLv3/TLS read server certificate request Modifying info_callback to look for SSL_CB_LOOP in where suppresses the excess messages: /* log the client certificate request (if received) */ #ifndef SSL3_ST_CR_CERT_REQ_A if(state==TLS_ST_CR_CERT_REQ && *where & SSL_CB_LOOP*) #else if(state==SSL3_ST_CR_CERT_REQ_A && *where & SSL_CB_LOOP*) #endif print_CA_list("Received trusted client CA", SSL_get_client_CA_list(ssl)); Here’s my config: debug=info [outbound-http-to-https-mtls] accept = 127.0.0.1:8447 connect = 127.0.0.1:8445 client = yes ; verify 2 - verify peer certificate chain verify = 2 sni = www.example.com checkHost = www.example.com verifyChain = yes CAfile = /export/home/user/stunnel/trusted-ca-certs.pem ; Authenticate ourselves to peer using a client certificate. cert = /export/home/user/stunnel/user.ecdsa-p256.pem key = /export/home/user/stunnel/user.ecdsa-p256.key -- Thanks, Sean Burford

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[PATCH 0/3] Improve stunnel performance by ~17%
by Edwin Torok 12 Jun '25

12 Jun '25

Hello, Flamegraph profiling on stunnel has shown that most time is NOT spent in encryption/decryption, but in sending/receiving data. Experimental setup: ``` CPU: 2x 18-core Intel Xeon Gold 6354 NIC: Intel E810-XXV, 100 Gbit/s Kernel: 6.14.8-300.fc42.x86_64 x86_64 Mem: 251.32 GiB OS: Fedora Linux 42 openssl version: OpenSSL 3.2.4 11 Feb 2025 (Library: OpenSSL 3.2.4 11 Feb 2025) stunnel: 5.75 from git ``` Network, encryption and memory copy speeds converted into Gbit/s on the above HW: ``` iperf3: 40Gbit/s openssl speed -evp aes-256-gcm: 72Gbit/s perf bench memcpy: 100Gbit/s ``` Under ideal conditions we should be able to achieve at most ~20.5Gbit/s, however I only measured ~7.6Gbit/s. The scripts that I used to perform measurements are included in the attached patch 3. See also related discussion on curl: https://github.com/curl/curl/pull/17548 Enabling readahead, avoiding excessive alloc/free of buffers, increasing buffer sizes, and improving buffer handling can all improve performance. For now I'm sending just the first small patches as proofs of concept, each of them one-liners. I hope you can include these in some form in the next release. Eventually these should probably be exposed as configuration flags in stunnel.conf, I can help implementing that, or I can leave implementing that to you if you prefer. In fact readahead was enabled in previous versions of stunnel, and then disabled again, so I assume you ran into some bugs with it on certain protocols? Increasing the buffer size is not a clear win (as with curl), due to the excessive use of memmove/memset, and that SSL_read/SSL_write only still processes 1 TLS record at a time. I have some further changes that can improve that, but the patches are larger and I haven't finished testing them for correctness yet. Please let me know if you'd want these as patches, or if you'd rather implement them yourself. Edwin Török (3): openssl: enable readahead openssl: disable SSL_MODE_RELEASE_BUFFERS Benchmark scripts src/ctx.c | 5 +- tests/benchmark/benchmark.sh | 120 +++++++++++++++++++++++ tests/benchmark/launch.sh | 185 +++++++++++++++++++++++++++++++++++ 3 files changed, 309 insertions(+), 1 deletion(-) create mode 100644 tests/benchmark/benchmark.sh create mode 100755 tests/benchmark/launch.sh -- 2.43.5

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