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SSLproxy - transparent SSL/TLS proxy for decrypting and diverting network traffic to other programs for deep SSL inspection Build Status

Copyright (C) 2017-2021, Soner Tari.

Copyright (C) 2009-2019, Daniel Roethlisberger.


SSLproxy is a proxy for SSL/TLS encrypted network connections. It is intended to be used for decrypting and diverting network traffic to other programs, such as UTM services, for deep SSL inspection.

The UTMFW project uses SSLproxy to decyrpt and feed network traffic into its UTM services: Web Filter, POP3 Proxy, SMTP Proxy, and Inline IPS; and also indirectly into Virus Scanner and Spam Filter through those UTM software. Given that most of the Internet traffic is encrypted now, it wouldn't be possible without SSLproxy to deeply inspect most of the network traffic passing through UTMFW.

See this presentation for a summary of SSL interception and potential issues with middleboxes that support it.

Mode of operation

SSLproxy is designed to transparently terminate connections that are redirected to it using a network address translation engine. SSLproxy then terminates SSL/TLS and initiates a new SSL/TLS connection to the original destination address. Packets received on the client side are decrypted and sent to the program listening on a port given in the proxy specification. SSLproxy inserts in the first packet the address and port it is expecting to receive the packets back from the program. Upon receiving the packets back, SSLproxy re-encrypts and sends them to their original destination. The return traffic follows the same path back to the client in reverse order.

Mode of Operation Diagram

This is similar in principle to divert sockets, where the packet filter diverts the packets to a program listening on a divert socket, and after processing the packets the program reinjects them into the kernel. If there is no program listening on that divert socket or the program does not reinject the packets into the kernel, the connection is effectively blocked. In the case of SSLproxy, SSLproxy acts as both the packet filter and the kernel, and the communication occurs over networking sockets.

SSLproxy supports split mode of operation similar to SSLsplit as well. In split mode, packets are not diverted to listening programs, effectively making SSLproxy behave similar to SSLsplit, but not exactly like it, because SSLproxy has certain features non-existent in SSLsplit, such as user authentication and protocol validation. Also, note that the implementation of proxy core in SSLproxy is different from the one in SSLsplit, for example the proxy core in SSLproxy runs lockless, whereas SSLsplit implementation uses thread manager level locks (which does not necessarily make sslproxy run faster than sslsplit). In SSLproxy, split mode can be defined globally or per-proxyspec.

SSLproxy does not automagically redirect any network traffic. To actually implement a proxy, you also need to redirect the traffic to the system running sslproxy. Your options include running sslproxy on a legitimate router, ARP spoofing, ND spoofing, DNS poisoning, deploying a rogue access point (e.g. using hostap mode), physical recabling, malicious VLAN reconfiguration or route injection, /etc/hosts modification and so on.

Proxy specification

For example, given the following proxy specification:

https 8443 up:8080
  • SSLproxy listens for HTTPS connections on
  • Upon receiving a connection from the Client, it decrypts and diverts the packets to a Program listening on The default divert address is, which can be configured by the ua option.
  • After processing the packets, the Program gives them back to SSLproxy listening on a dynamically assigned address, which the Program obtains from the SSLproxy line in the first packet in the connection.
  • Then SSLproxy re-encrypts and sends the packets to the Server.

The response from the Server follows the same path back to the Client in reverse order.

See the SSLsplit documentation for split style proxyspecs.

SSLproxy line

A sample line SSLproxy inserts into the first packet in the connection is the following:

SSLproxy: []:34649,[]:47286,[]:443,s
  • The first IP:port pair is a dynamically assigned address that SSLproxy expects the program send the packets back to it.
  • The second and third IP:port pairs are the actual source and destination addresses of the connection respectively. Since the program receives the packets from SSLproxy, it cannot determine the source and destination addresses of the packets by itself, e.g by asking the NAT engine, hence must rely on the information in the SSLproxy line.
  • The last letter is either s or p, for SSL/TLS encrypted or plain traffic respectively. This information is also important for the program, because it cannot reliably determine if the actual network traffic it is processing was encrypted or not before being diverted to it.

Listening program

The program that packets are diverted to should support this mode of operation. Specifically, it should be able to recognize the SSLproxy address in the first packet, and give the first and subsequent packets back to SSLproxy listening on that address, instead of sending them to the original destination as it normally would.

You can use any software as a listening program as long as it supports this mode of operation. So existing or new software developed in any programming language can be modified to be used with SSLproxy to inspect and/or modify any or all parts of the packets diverted to it.

You can offload the system SSLproxy is running on by diverting packets to remote listening programs too. For example, given the following proxy specification:

https 8443 up:8080 ua: ra:
  • The ua option instructs SSLproxy to divert packets to, instead of as in the previous proxyspec example.
  • The ra option instructs SSLproxy to listen for returned packets from the program on, instead of as in the previous SSLproxy line.

Accordingly, the SSLproxy line now becomes:

SSLproxy: []:34649,[]:47286,[]:443,s

So, the listening program can be running on a machine anywhere in the world. Since the packets between SSLproxy and the listening program are always unencrypted, you should be careful while using such a setup.


Supported protocols

SSLproxy supports plain TCP, plain SSL, HTTP, HTTPS, POP3, POP3S, SMTP, and SMTPS connections over both IPv4 and IPv6. It also has the ability to dynamically upgrade plain TCP to SSL in order to generically support SMTP STARTTLS and similar upgrade mechanisms. Depending on the version of OpenSSL, SSLproxy supports SSL 3.0, TLS 1.0, TLS 1.1, TLS 1.2, and TLS 1.3, and optionally SSL 2.0 as well. SSLproxy supports Server Name Indication (SNI), but not Encrypted SNI in TLS 1.3. It is able to work with RSA, DSA and ECDSA keys and DHE and ECDHE cipher suites.

The following features of SSLproxy are IPv4 only:

  • Divert addresses for listening programs in proxyspecs
  • SSLproxy return addresses dynamically assigned to connections
  • IP addresses in the ua and ra options
  • IP and ethernet addresses of clients in user authentication
  • Target IP and ethernet addresses in mirror logging

OCSP, HPKP, HSTS, Upgrade et al.

SSLproxy implements a number of defences against mechanisms which would normally prevent MitM attacks or make them more difficult. SSLproxy can deny OCSP requests in a generic way. For HTTP and HTTPS connections, SSLproxy mangles headers to prevent server-instructed public key pinning (HPKP), avoid strict transport security restrictions (HSTS), avoid Certificate Transparency enforcement (Expect-CT) and prevent switching to QUIC/SPDY, HTTP/2 or WebSockets (Upgrade, Alternate Protocols). HTTP compression, encodings and keep-alive are disabled to make the logs more readable.

Another reason to disable persistent connections is to reduce file descriptor usage. Accordingly, connections are closed if they remain idle for a certain period of time. The default timeout is 120 seconds, which can be configured by the ConnIdleTimeout option.

Protocol validation

Protocol validation makes sure the traffic handled by a proxyspec is using the protocol specified in that proxyspec. The ValidateProto option can be used to enable global and/or per-proxyspec protocol validation. This feature currently supports HTTP, POP3, and SMTP protocols. If a connection cannot pass protocol validation, then it is terminated.

SSLproxy uses only client requests for protocol validation. However, it also validates SMTP responses until it starts processing the packets from the client. If there is no excessive fragmentation, the first couple of packets in the connection should be enough for validating protocols.


Certificate forging

For SSL and HTTPS connections, SSLproxy generates and signs forged X509v3 certificates on-the-fly, mimicking the original server certificate's subject DN, subjectAltName extension and other characteristics. SSLproxy has the ability to use existing certificates of which the private key is available, instead of generating forged ones. SSLproxy supports NULL-prefix CN certificates but otherwise does not implement exploits against specific certificate verification vulnerabilities in SSL/TLS stacks.

Certificate verification

SSLproxy verifies upstream certificates by default. If the verification fails, the connection is terminated immediately. This is in contrast to SSLsplit, because in order to maximize the chances that a connection can be successfully split, SSLsplit accepts all certificates by default, including self-signed ones. See The Risks of SSL Inspection for the reasons of this difference. You can disable this feature by the VerifyPeer option.

Client certificates

SSLproxy uses the certificate and key from the pemfiles configured by the ClientCert and ClientKey options when the destination requests client certificates. These options can be defined globally and/or per-proxyspec.

Alternatively, you can use the PassSite option to pass through certain destinations requesting client certificates.

User authentication

If the UserAuth option is enabled, SSLproxy requires network users to log in to the system to establish connections to the external network.

SSLproxy determines the user owner of a connection using a users table in an SQLite3 database configured by the UserDBPath option. The users table should be created using the following SQL statement:

   IP             CHAR(45)     PRIMARY KEY     NOT NULL,
   USER           CHAR(31)     NOT NULL,
   ETHER          CHAR(17)     NOT NULL,
   ATIME          INT          NOT NULL,
   DESC           CHAR(50)

SSLproxy does not create this users table or the database file by itself, nor does it log users in or out. So the database file and the users table should already exist at the location pointed to by the UserDBPath option. An external program should log users in and out on the users table. The external program should fill out all the fields in user records, except perhaps for the DESC field, which can be left blank.

When SSLproxy accepts a connection,

  • It searches the client IP address of the connection in the users table. If the client IP address is not in the users table, the connection is redirected to a login page configured by the UserAuthURL option.
  • If SSLproxy finds a user record for the client IP address in the users table, it obtains the ethernet address of the client IP address from the arp cache of the system, and compares it with the value in the user record for that IP address. If the ethernet addresses do not match, the connection is redirected to the login page.
  • If the ethernet addresses match, SSLproxy compares the atime value in the user record with the current system time. If the difference is greater than the value configured by the UserTimeout option, the connection is redirected to the login page.

If the connection passes all these checks, SSLproxy proceeds with establishing the connection.

The atime of the IP address in the users table is updated with the system time while the connection is being terminated. Since this atime update is executed using a privsep command, it is expensive. So, to reduce the frequency of such updates, it is deferred until after the user idle time is more than half of the timeout period.

If a description text is provided in the DESC field, it can be used with the PassSite option to treat the user logged in from different locations, i.e. from different client IP addresses, separately.

If the UserAuth option is enabled, the user owner of the connection is appended at the end of the SSLproxy line, so that the listening program can parse and use this information in its logic and/or logging:

SSLproxy: []:34649,[]:47286,[]:443,s,soner

The user authentication feature is currently available on OpenBSD and Linux only.

User control lists

DivertUsers and PassUsers options can be used to divert, pass through, or block users.

  • If neither DivertUsers nor PassUsers is defined, all users are diverted to listening programs.
  • Connections from users in DivertUsers, if defined, are diverted to listening programs.
  • Connections from users in PassUsers, if defined, are simply passed through to their original destinations. SSLproxy engages the Passthrough mode for that purpose.
  • If both DivertUsers and PassUsers are defined, users not listed in either of the lists are blocked. SSLproxy simply terminates their connections.
  • If no DivertUsers list is defined, only users not listed in PassUsers are diverted to listening programs.

These user control lists can be defined globally or per-proxyspec.

Excluding sites from SSL inspection

PassSite option allows certain SSL sites to be excluded from SSL inspection. If a PassSite matches the SNI or common names in the SSL certificate of a connection, that connection is passed through the proxy without being diverted to the listening program. SSLproxy engages the Passthrough mode for that purpose. For example, sites requiring client authentication can be added as PassSite.

Per-site filters can be defined using client IP addresses, users, and description keywords. If the UserAuth option is disabled, only client IP addresses can be used in PassSite filters. Multiple sites can be defined, one on each line. PassSite rules can search for exact or substring matches.


Logging options include traditional SSLproxy connect and content log files as well as PCAP files and mirroring decrypted traffic to a network interface. Additionally, certificates, master secrets and local process information can be logged.

See the manual pages sslproxy(1) and sslproxy.conf(5) for details on using SSLproxy, setting up the various NAT engines, and for examples.


SSLproxy depends on the OpenSSL, libevent 2.x, libpcap, libnet 1.1.x, and sqlite3 libraries by default. Libpcap and libnet are not needed if the mirroring feature is omitted. Sqlite3 is not needed if the user authentication feature is omitted. The build depends on GNU make and a POSIX.2 environment in PATH. If available, pkg-config is used to locate and configure the dependencies. The optional unit tests depend on the check library. The optional end-to-end tests depend on the TestProxy tool, and are supported only on Linux.

SSLproxy currently supports the following operating systems and NAT mechanisms:

  • FreeBSD: pf rdr and divert-to, ipfw fwd, ipfilter rdr
  • OpenBSD: pf rdr-to and divert-to
  • Linux: netfilter REDIRECT and TPROXY
  • Mac OS X: pf rdr and ipfw fwd

Support for local process information (-i) is currently available on Mac OS X and FreeBSD.

SSL/TLS features and compatibility greatly depend on the version of OpenSSL linked against. For optimal results, use a recent release of OpenSSL or LibreSSL.


With the requirements above available, run:

make test       # optional unit and e2e tests
make sudotest   # optional unit tests requiring privileges
make install    # optional install

Dependencies are autoconfigured using pkg-config. If dependencies are not picked up and fixing PKG_CONFIG_PATH does not help, you can specify their respective locations manually by setting OPENSSL_BASE, LIBEVENT_BASE, LIBPCAP_BASE, LIBNET_BASE, SQLITE_BASE and/or CHECK_BASE to the respective prefixes.

You can override the default install prefix (/usr/local) by setting PREFIX. For more build options and build-time defaults see GNUmakefile and defaults.h.


See the manual pages sslproxy(1) and sslproxy.conf(5) for user documentation. See for release notes listing significant changes between releases and for information on security vulnerability disclosure.


SSLproxy is provided under a 2-clause BSD license. SSLproxy contains components licensed under the MIT and APSL licenses. See LICENSE, LICENSE.contrib and LICENSE.third as well as the respective source file headers for details.


See for the list of contributors.

SSLproxy was inspired by and has been developed based on SSLsplit by Daniel Roethlisberger.

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