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

Overview

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. See this presentation for a summary.

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.

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.

For example, given the following proxy specification:

https 127.0.0.1 8443 up:8080

The SSLproxy listens for HTTPS connections on 127.0.0.1:8443. Upon receiving a connection from the Client, it decrypts and diverts the packets to a Program listening on 127.0.0.1:8080. After processing the packets, the Program gives them back to the SSLproxy listening on a dynamically assigned address, which the Program obtains from the first packet in the connection. Then the SSLproxy re-encrypts and sends the packets to the Server. The response from the Server follows the same path to the Client in reverse order.

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 the SSLproxy listening on that address, instead of sending them to the original destination as it normally would.

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

SSLproxy: [127.0.0.1]:34649,[192.168.3.24]:47286,[192.168.111.130]:443,s

The first IP:port pair is a dynamically assigned address that the 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 the SSLproxy, it cannot determine the source and destination addresses of the packets by itself, hence must rely on the information in this 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.

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. SSLproxy fully supports Server Name Indication (SNI) and is able to work with RSA, DSA and ECDSA keys and DHE and ECDHE cipher suites. Depending on the version of OpenSSL, SSLproxy supports SSL 3.0, TLS 1.0, TLS 1.1 and TLS 1.2, and optionally SSL 2.0 as well.

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.

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 changed in a configuration file.

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] (https://insights.sei.cmu.edu/cert/2015/03/the-risks-of-ssl-inspection.html) for the reasons of this difference.

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.

As SSLproxy is based on SSLsplit, this is a modified SSLsplit README file. See the manual page sslproxy(1) for details on using SSLproxy and setting up the various NAT engines.

Requirements

SSLproxy depends on the OpenSSL, libevent 2.x, libpcap and libnet 1.1.x libraries by default; libpcap and libnet are not needed if the mirroring 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.

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.

Installation

With the requirements above available, run:

make
make test       # optional unit 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 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.

Documentation

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

License

SSLsplit is provided under a 2-clause BSD license. SSLsplit 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. The modifications for SSLproxy are licensed under the same terms as SSLsplit.

Credits

See AUTHORS.md for the list of contributors.

SSLsplit was inspired by mitm-ssl by Claes M. Nyberg and sslsniff by Moxie Marlinspike, but shares no source code with them.

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