SSLproxy/sslsplit.1

.\" SSLsplit - transparent and scalable SSL/TLS interception
.\" Copyright (c) 2009-2014, Daniel Roethlisberger <daniel@roe.ch>
.\" All rights reserved.
.\" http://www.roe.ch/SSLsplit
.\"
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.TH SSLSPLIT 1  "1 April 2012"
.SH NAME
sslsplit \-\- transparent and scalable SSL/TLS interception
.SH SYNOPSIS
.na
.B sslsplit
[\fB-kCKOPZdDgGseumjplLS\fP] \fB-c\fP \fIpem\fP
\fIproxyspecs\fP [...]
.br
.B sslsplit
[\fB-kCKOPZdDgGseumjplLS\fP] \fB-c\fP \fIpem\fP \fB-t\fP \fIdir\fP
\fIproxyspecs\fP [...]
.br
.B sslsplit
[\fB-OPZdDgGseumjplLS\fP] \fB-t\fP \fIdir\fP
\fIproxyspecs\fP [...]
.br
.B sslsplit -E
.br
.B sslsplit -V
.br
.B sslsplit -h
.br
.ad
.SH DESCRIPTION
SSLsplit is a tool for man-in-the-middle attacks against SSL/TLS encrypted
network connections.  Connections are transparently intercepted through a
network address translation engine and redirected to SSLsplit.  SSLsplit
terminates SSL/TLS and initiates a new SSL/TLS connection to the original
destination address, while logging all data transmitted.  SSLsplit is intended
to be useful for network forensics and penetration testing.
.LP
SSLsplit supports plain TCP, plain SSL, HTTP and HTTPS connections over both
IPv4 and IPv6.  For SSL and HTTPS connections, SSLsplit generates and signs
forged X509v3 certificates on-the-fly, based on the original server certificate
subject DN and subjectAltName extension.  SSLsplit fully supports Server Name
Indication (SNI) and is able to work with RSA, DSA and ECDSA keys and DHE and
ECDHE cipher suites.  SSLsplit can also use existing certificates of which the
private key is available, instead of generating forged ones.  SSLsplit supports
NULL-prefix CN certificates and can deny OCSP requests in a generic way.
For HTTP and HTTPS connections, SSLsplit removes response headers
for HPKP in order to prevent public key pinning,
for HSTS to allow the user to accept untrusted certificates,
and Alternate Protocols to prevent switching to QUIC/SPDY.
.LP
SSLsplit supports a number of NAT engines, static forwarding and SNI DNS
lookups to determine the original destination of redirected connections
(see NAT ENGINES and PROXY SPECIFICATIONS below).
.LP
To actually implement an attack, you also need to redirect the traffic to the
system running \fBsslsplit\fP.  Your options include running \fBsslsplit\fP 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.
SSLsplit does not implement the actual traffic redirection.
.SH OPTIONS
.TP
.B \-c \fIpemfile\fP
Use CA certificate from \fIpemfile\fP to sign certificates forged on-the-fly.
If \fIpemfile\fP also contains the matching CA private key, it is also loaded,
otherwise it must be provided with \fB-k\fP.
If \fIpemfile\fP also contains Diffie-Hellman group parameters, they are also
loaded, otherwise they can be provided with \fB-g\fP.
If \fB-t\fP is also given, SSLsplit will only forge a certificate if there is
no matching certificate in the provided certificate directory.
.TP
.B \-C \fIpemfile\fP
Use CA certificates from \fIpemfile\fP as extra certificates in the certificate
chain.  This is needed if the CA given with \fB-k\fP and \fB-c\fP is a sub-CA,
in which case any intermediate CA certificates and the root CA certificate must
be included in the certificate chain.
.TP
.B \-d
Detach from TTY and run as a daemon, logging error messages to syslog instead
of standard error.
.TP
.B \-D
Run in debug mode, log lots of debugging information to standard error.  This
also forces foreground mode and cannot be used with \fB-d\fP.
.TP
.B \-e \fIengine\fP
Use \fIengine\fP as the default NAT engine for \fIproxyspecs\fP without
explicit NAT engine, static destination address or SNI mode.
\fIengine\fP can be any of the NAT engines supported by the system, as
returned by \fB-E\fP.
.TP
.B \-E
List all supported NAT engines available on the system and exit.  See
NAT ENGINES for a list of NAT engines currently supported by SSLsplit.
.TP
.B \-g \fIpemfile\fP
Use Diffie-Hellman group parameters from \fIpemfile\fP for Ephemereal
Diffie-Hellman (EDH/DHE) cipher suites.  If \fB-g\fP is not given, SSLsplit
first tries to load DH parameters from the PEM files given by \fB-K\fP,
\fB-k\fP or \fB-c\fP.  If no DH parameters are found in the key files, built-in
512 or 1024 bit group parameters are automatically used iff a non-RSA private
key is given with \fB-K\fP.
This is because DSA/DSS private keys can by themselves only be used for signing
and thus require DH to exchange an SSL/TLS session key.
If \fB-g\fP is given, the parameters from the given \fIpemfile\fP will always
be used, even with RSA private keys (within the cipher suites available in
OpenSSL).
The \fB-g\fP option is only available if SSLsplit was built against a version
of OpenSSL which supports Diffie-Hellman cipher suites.
.TP
.B \-G \fIcurve\fP
Use the named \fIcurve\fP for Ephemereal Elliptic Curve Diffie-Hellman (EECDH)
cipher suites.  If \fB-G\fP is not given, a default curve (\fBsecp160r2\fP) is
used automatically iff a non-RSA private key is given with \fB-K\fP.
This is because ECDSA/ECDSS private keys can by themselves only be used for
signing and thus require ECDH to exchange an SSL/TLS session key.
If \fB-G\fP is given, the named \fIcurve\fP will always be used, even with RSA
private keys (within the cipher suites available in OpenSSL).
The \fB-G\fP option is only available if SSLsplit was built against a version
of OpenSSL which supports Elliptic Curve Diffie-Hellman cipher suites.
.TP
.B \-h
Display help on usage and exit.
.TP
.B \-j \fIjaildir\fP
Change the root directory to \fIjaildir\fP using chroot(2) after opening files.
Note that this has implications for both \fB-S\fP and for \fBsni\fP
\fIproxyspecs\fP.  The directory given with \fB-S\fP will be relative to
\fIjaildir\fP since the log files cannot be opened before calling chroot(2).
Depending on your operating system, you will need to copy files such as
\fB/etc/resolv.conf\fP to \fIjaildir\fP in order for name resolution to work.
Using \fBsni\fP proxyspecs depends on name resolution.
Some operating systems require special device nodes such as \fB/dev/null\fP
to be present within the jail.  Check your system's documentation for details.
.TP
.B \-k \fIpemfile\fP
Use CA private key from \fIpemfile\fP to sign certificates forged on-the-fly.
If \fIpemfile\fP also contains the matching CA certificate, it is also loaded,
otherwise it must be provided with \fB-c\fP.
If \fIpemfile\fP also contains Diffie-Hellman group parameters, they are also
loaded, otherwise they can be provided with \fB-g\fP.
If \fB-t\fP is also given, SSLsplit will only forge a certificate if there is
no matching certificate in the provided certificate directory.
.TP
.B \-K \fIpemfile\fP
Use private key from \fIpemfile\fP for certificates forged on-the-fly.
If \fB-K\fP is not given, SSLsplit will generate a random 1024-bit RSA key.
.TP
.B \-l \fIlogfile\fP
Log connections to \fIlogfile\fP in a single line per connection format,
including addresses and ports and some HTTP and SSL information, if available.
.TP
.B \-L \fIlogfile\fP
Log connection content to \fIlogfile\fP.  The content log will contain a
parsable log format with transmitted data, prepended with headers identifying
the connection and the data length of each logged segment.
.TP
.B \-O
Deny all Online Certificate Status Protocol (OCSP) requests on all
\fIproxyspecs\fP and for all OCSP servers with an OCSP response of
\fBtryLater\fP, causing OCSP clients to temporarily accept even revoked
certificates.
HTTP requests are being treated as OCSP requests if the method is \fBGET\fP
and the URI contains a syntactically valid OCSPRequest ASN.1 structure
parsable by OpenSSL, or if the method is \fBPOST\fP and the \fBContent-Type\fP
is \fBapplication/ocsp-request\fP.
For this to be effective, SSLsplit must be handling traffic destined to the
port used by the OCSP server.  In particular, SSLsplit must be configured to
receive traffic to all ports used by OCSP servers of targetted certificates
within the \fIcertdir\fP specified by \fB-t\fP.
.TP
.B \-p \fIpidfile\fP
Write the process ID to \fIpidfile\fP and refuse to run if the \fIpidfile\fP
is already in use by another process.
.TP
.B \-P
Passthrough SSL/TLS connections which cannot be split instead of dropping them.
Connections cannot be split if \fB-c\fP and \fB-k\fP are not given and the
site does not match any certificate loaded using \fB-t\fP, or if the connection
to the original server gives SSL/TLS errors.  Specifically, this happens if the
site requests a client certificate.  Passthrough with \fB-P\fP results in
uninterrupted service for the clients, while dropping is the more secure
alternative if unmonitored connections must be prevented.
.TP
.B \-s \fIciphers\fP
Use OpenSSL \fIciphers\fP specification for both server and client SSL/TLS
connections.  If \fB-s\fP is not given, a cipher list of \fBALL:-aNULL\fP is
used.
Normally, SSL/TLS implementations choose the most secure cipher suites, not the
fastest ones.  By specifying an appropriate OpenSSL cipher list, the set of
cipher suites can be limited to fast algorithms, or \fBeNULL\fP cipher suites
can be added.  Note that for connections to be successful, the SSLsplit cipher
suites must include at least one cipher suite supported by both the client and
the server of each connection.
See ciphers(1) for details on how to construct OpenSSL cipher lists.
.TP
.B \-S \fIlogdir\fP
Log connection content to separate log files under \fIlogdir\fP.  For each
connection, a log file will be written, which will contain both directions of
data as transmitted.  Information about the connection will be contained in
the filename only.
If \fB-S\fP is used with \fB-j\fP, \fIlogdir\fP is relative to \fIjaildir\fP.
If \fB-S\fP is used with \fB-u\fP, \fIlogdir\fP must be writable by \fIuser\fP.
.TP
.B \-t \fIcertdir\fP
Use private key, certificate and certificate chain from PEM files in
\fIcertdir\fP for sites matching the respective common names, instead of
using certificates forged on-the-fly.  A single PEM file must contain a
single private key, a single certificate and optionally intermediate and
root CA certificates to use as certificate chain.
If \fB-c\fP and \fB-k\fP are also given, certificates will be forged
on-the-fly for sites matching none of the certificates loaded from
\fIcertdir\fP.
Otherwise, connections matching no certificate will be dropped, or if
\fB-P\fP is given, passed through without splitting SSL/TLS.
.TP
.B \-u
Drop privileges after opening sockets and files by setting the real,
effective and stored user IDs to \fIuser\fP and loading the appropriate
primary and ancillary groups.  If \fB-u\fP is not given, SSLsplit will drop
privileges to the stored UID if EUID != UID (setuid bit scenario), or to
\fBnobody\fP if running with full \fBroot\fP privileges (EUID == UID == 0)
and \fB-S\fP is not used.
.TP
.B \-m
When dropping privileges using \fB-u\fP, override the target primary group
to be set to \fIgroup\fP.
.TP
.B \-V
Display version and compiled features information and exit.
.TP
.B \-Z
Disable SSL/TLS compression on all connections.  This is useful if your
limiting factor is CPU, not network bandwidth.
The \fB-Z\fP option is only available if SSLsplit was built against a version
of OpenSSL which supports disabling compression.
.SH "PROXY SPECIFICATIONS"
Proxy specifications (\fIproxyspecs\fP) consist of the connection type, listen
address and static forward address or address resolution mechanism (NAT engine,
SNI DNS lookup):
.LP
.na
\fBhttps\fP \fIlistenaddr port\fP
[\fInat-engine\fP|\fIfwdaddr port\fP|\fBsni\fP \fIport\fP]
.br
\fBssl\fP   \fIlistenaddr port\fP
[\fInat-engine\fP|\fIfwdaddr port\fP|\fBsni\fP \fIport\fP]
.br
\fBhttp\fP  \fIlistenaddr port\fP
[\fInat-engine\fP|\fIfwdaddr port\fP]
.br
\fBtcp\fP   \fIlistenaddr port\fP
[\fInat-engine\fP|\fIfwdaddr port\fP]
.ad
.TP
\fBhttps\fP
SSL/TLS interception with HTTP protocol decoding, including the removal of
HPKP, HSTS and Alternate Protocol response headers.
.TP
\fBssl\fP
SSL/TLS interception without any lower level protocol decoding; decrypted
connection content is treated as opaque stream of bytes and not modified.
.TP
\fBhttp\fP
Plain TCP connection without SSL/TLS, with HTTP protocol decoding, including
the removal of HPKP, HSTS and Alternate Protocol response headers.
.TP
\fBtcp\fP
Plain TCP connection without SSL/TLS and without any lower level protocol
decoding; decrypted connection content is treated as opaque stream of bytes
and not modified.
.TP
.I listenaddr port
IPv4 or IPv6 address and port or service name to listen on.  This is the
address and port where the NAT engine should redirect connections to.
.TP
.I nat-engine
NAT engine to query for determining the original destination address and port
of transparently redirected connections.
If no engine is given, the default engine is used, unless overridden with
\fB-e\fP.  When using a NAT engine, \fBsslsplit\fP needs to run on the same
system as the NAT rules redirecting the traffic to \fBsslsplit\fP.
See NAT ENGINES for a list of supported NAT engines.
.TP
.I fwdaddr port
Static destination address, IPv4 or IPv6, with port or service name.  When this
is used, connections are forwarded to the given server address and port.
.TP
\fBsni\fP \fIport\fP
Use the Server Name Indication (SNI) hostname sent by the client in the
ClientHello SSL/TLS message to determine the IP address of the server to
connect to.  This only works for \fBssl\fP and \fBhttps\fP \fIproxyspecs\fP and
needs a port or service name as an argument.
Because this requires DNS lookups, it is preferrable to use NAT engine
lookups (see above), except when that is not possible, such as when there is
no supported NAT engine or when running \fBsslsplit\fP on a different system
than the NAT rules redirecting the actual connections.
Note that when using \fB-j\fP with \fBsni\fP, you may need to prepare
\fIjaildir\fP to make name resolution work from within the chroot directory.
.LP
.SH "NAT ENGINES"
SSLsplit currently supports the following NAT engines:
.TP
.B pf
OpenBSD packet filter (pf) \fBrdr\fP/\fBrdr-to\fP NAT redirects, also available
on FreeBSD, NetBSD and Mac OS X 10.7+.
Fully supported on BSD, including IPv6.
Experimental on Mac OS X.
Assuming inbound interface \fBem0\fP, first in old (FreeBSD, Mac OS X),
then in new (OpenBSD 4.7+) syntax:
.LP
.RS
.nf
\fBrdr pass on em0 proto tcp from 2001:db8::/64 to any port  80 \\
         ->       ::1 port 10080\fP
\fBrdr pass on em0 proto tcp from 2001:db8::/64 to any port 443 \\
         ->       ::1 port 10443\fP
\fBrdr pass on em0 proto tcp from  192.0.2.0/24 to any port  80 \\
         -> 127.0.0.1 port 10080\fP
\fBrdr pass on em0 proto tcp from  192.0.2.0/24 to any port 443 \\
         -> 127.0.0.1 port 10443\fP
.fi
.RE
.LP
.RS
.nf
\fBpass in quick on em0 proto tcp from 2001:db8::/64 to any \\
         port  80 rdr-to       ::1 port 10080\fP
\fBpass in quick on em0 proto tcp from 2001:db8::/64 to any \\
         port 443 rdr-to       ::1 port 10443\fP
\fBpass in quick on em0 proto tcp from  192.0.2.0/24 to any \\
         port  80 rdr-to 127.0.0.1 port 10080\fP
\fBpass in quick on em0 proto tcp from  192.0.2.0/24 to any \\
         port 443 rdr-to 127.0.0.1 port 10443\fP
.fi
.RE
.TP
.B ipfw
FreeBSD IP firewall (IPFW) divert sockets, also available on Mac OS X.
Available on FreeBSD and OpenBSD using pf \fBdivert-to\fP.
Fully supported on FreeBSD and OpenBSD, including IPv6.
Only supports IPv4 on Mac OS X due to the ancient version of IPFW included.
First in IPFW, then in pf \fBdivert-to\fP syntax:
.LP
.RS
.nf
\fBipfw add fwd       ::1,10080 tcp from 2001:db8::/64 to any  80\fP
\fBipfw add fwd       ::1,10443 tcp from 2001:db8::/64 to any 443\fP
\fBipfw add fwd 127.0.0.1,10080 tcp from 192.0.2.0/24  to any  80\fP
\fBipfw add fwd 127.0.0.1,10443 tcp from 192.0.2.0/24  to any 443\fP
.fi
.RE
.LP
.RS
.nf
\fBpass in quick on em0 proto tcp from 2001:db8::/64 to any \\
         port  80 divert-to       ::1 port 10080\fP
\fBpass in quick on em0 proto tcp from 2001:db8::/64 to any \\
         port 443 divert-to       ::1 port 10443\fP
\fBpass in quick on em0 proto tcp from  192.0.2.0/24 to any \\
         port  80 divert-to 127.0.0.1 port 10080\fP
\fBpass in quick on em0 proto tcp from  192.0.2.0/24 to any \\
         port 443 divert-to 127.0.0.1 port 10443\fP
.fi
.RE
.TP
.B ipfilter
IPFilter (ipfilter, ipf), available on many systems, including FreeBSD, NetBSD,
Linux and Solaris.
Only supports IPv4 due to limitations in the SIOCGNATL ioctl(2) interface.
Assuming inbound interface \fBbge0\fP:
.LP
.RS
.nf
\fBrdr bge0 0.0.0.0/0 port  80 -> 127.0.0.1 port 10080\fP
\fBrdr bge0 0.0.0.0/0 port 443 -> 127.0.0.1 port 10443\fP
.fi
.RE
.TP
.B netfilter
Linux netfilter using the iptables REDIRECT target.
Only supports IPv4 due to limitations in the SO_ORIGINAL_DST getsockopt(2)
interface.
.LP
.RS
.nf
\fBiptables -t nat -A PREROUTING -s 192.0.2.0/24 \\
         -p tcp --dport  80 \\
         -j REDIRECT --to-ports 10080\fP
\fBiptables -t nat -A PREROUTING -s 192.0.2.0/24 \\
         -p tcp --dport 443 \\
         -j REDIRECT --to-ports 10443\fP
.fi
.RE
.TP
.B tproxy
Linux netfilter using the iptables TPROXY target together with routing
table magic to allow non-local traffic to originate on local sockets.
Fully supported, including IPv6.
.LP
.RS
.nf
\fBip -f inet6 rule add fwmark 1 lookup 100\fP
\fBip -f inet6 route add local default dev lo table 100\fP
\fBip6tables -t mangle -N DIVERT\fP
\fBip6tables -t mangle -A DIVERT -j MARK --set-mark 1\fP
\fBip6tables -t mangle -A DIVERT -j ACCEPT\fP
\fBip6tables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT\fP
\fBip6tables -t mangle -A PREROUTING -s 2001:db8::/64 \\
          -p tcp --dport 80 \\
          -j TPROXY --tproxy-mark 0x1/0x1 --on-port 10080\fP
\fBip6tables -t mangle -A PREROUTING -s 2001:db8::/64 \\
          -p tcp --dport 443 \\
          -j TPROXY --tproxy-mark 0x1/0x1 --on-port 10443\fP
\fBip -f inet rule add fwmark 1 lookup 100\fP
\fBip -f inet route add local default dev lo table 100\fP
\fBiptables -t mangle -N DIVERT\fP
\fBiptables -t mangle -A DIVERT -j MARK --set-mark 1\fP
\fBiptables -t mangle -A DIVERT -j ACCEPT\fP
\fBiptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT\fP
\fBiptables -t mangle -A PREROUTING -s 192.0.2.0/24 \\
         -p tcp --dport 80 \\
         -j TPROXY --tproxy-mark 0x1/0x1 --on-port 10080\fP
\fBiptables -t mangle -A PREROUTING -s 192.0.2.0/24 \\
         -p tcp --dport 443 \\
         -j TPROXY --tproxy-mark 0x1/0x1 --on-port 10443\fP
.fi
.LP
Note that return path filtering (rp_filter) also needs to be disabled on
interfaces which handle TPROXY redirected traffic.
.RE
.SH EXAMPLES
Matching the above NAT engine configuration samples, intercept HTTP and HTTPS
over IPv4 and IPv6 using forged certificates with CA private key \fBca.key\fP
and certificate \fBca.crt\fP, logging connections to \fBconnect.log\fP and
connection data into separate files under \fB/tmp\fP (add \fB-e\fP
\fInat-engine\fP to select the appropriate engine if multiple engines are
available on your system):
.LP
.HS
.nf
\fBsslsplit -k ca.key -c ca.crt -l connect.log -L /tmp \\
         https ::1 10443  https 127.0.0.1 10443 \\
         http  ::1 10080  http  127.0.0.1 10080\fP
.fi
.RE
.LP
Intercepting IMAP/IMAPS using the same settings:
.LP
.HS
.nf
\fBsslsplit -k ca.key -c ca.crt -l connect.log -L /tmp \\
         ssl ::1 10993  ssl 127.0.0.1 10993 \\
         tcp ::1 10143  tcp 127.0.0.1 10143\fP
.fi
.RE
.LP
A more targetted setup, HTTPS only, using certificate/chain/key files from
\fB/path/to/cert.d\fP and statically redirecting to \fBwww.example.org\fP
instead of querying a NAT engine:
.LP
.HS
.nf
\fBsslsplit -t /path/to/cert.d -l connect.log -L /tmp \\
         https ::1       10443 www.example.org 443 \\
         https 127.0.0.1 10443 www.example.org 443\fP
.fi
.RE
.LP
The original example, but using SSL options optimized for speed by disabling
compression and selecting only fast block cipher cipher suites and using a
precomputed private key \fBleaf.key\fP for the forged certificates
(most significant speed increase is gained by choosing fast algorithms and
small keysizes for the CA and leaf private keys; check \fBopenssl speed\fP for
algorithm performance on your system):
.LP
.HS
.nf
\fBsslsplit -Z -s NULL:RC4:AES128 -K leaf.key \\
         -k ca.key -c ca.crt -l connect.log -L /tmp \\
         https ::1 10443  https 127.0.0.1 10443 \\
         http  ::1 10080  http  127.0.0.1 10080\fP
.fi
.RE
.LP
The original example, but running as a daemon under user \fBsslsplit\fP and
writing a PID file:
.LP
.HS
.nf
\fBsslsplit -d -p /var/run/sslsplit.pid -u sslsplit \\
         -k ca.key -c ca.crt -l connect.log -L /tmp \\
         https ::1 10443  https 127.0.0.1 10443 \\
         http  ::1 10080  http  127.0.0.1 10080\fP
.fi
.RE
.LP
To generate a CA private key \fBca.key\fP  and certificate \fBca.crt\fP using
OpenSSL:
.LP
.HS
.nf
\fBcat >x509v3ca.cnf <<'EOF'\fP
[ req ]
distinguished_name = reqdn

[ reqdn ]

[ v3_ca ]
basicConstraints        = CA:TRUE
subjectKeyIdentifier    = hash
authorityKeyIdentifier  = keyid:always,issuer:always
\fBEOF\fP

\fBopenssl genrsa -out ca.key 1024\fP
\fBopenssl req -new -nodes -x509 -sha1 -out ca.crt -key ca.key \\
        -config x509v3ca.cnf -extensions v3_ca \\
        -subj '/O=SSLsplit Root CA/CN=SSLsplit Root CA/' \\
        -set_serial 0 -days 3650\fP
.fi
.SH PERFORMANCE AND SCALABILITY
SSLsplit is able to handle a relatively high number of listeners and
connections due to a multithreaded, event based architecture based on libevent,
taking advantage of platform specific select() replacements such as kqueue.
The main thread handles the listeners and signalling, while a number of worker
threads equal to twice the number of CPU cores is used for handling the actual
connections in separate event bases, including the CPU-intensive SSL/TLS
handling.
.LP
Care has been taken to choose well-performing data structures for caching
certificates and SSL sessions.  Logging is implemented in separate disk writer
threads to ensure that socket event handling threads don't have to block on
disk I/O.
DNS lookups are performed asynchroniously.
SSLsplit uses SSL session caching on both ends to minimize the amount of full
SSL handshakes, but even then, the limiting factor in handling SSL connections
are the actual bignum computations.
.SH "SEE ALSO"
openssl(1), ciphers(1), speed(1),
pf(4), ipfw(8), iptables(8), ip6tables(8), ip(8),
hostapd(8), arpspoof(8), parasite6(8), yersinia(8)
.SH AUTHORS
Daniel Roethlisberger <daniel@roe.ch>
.SH BUGS
Session resumption does not work for SSLv2-only clients.  As a workaround,
clients attempting to resume a session will always be given a new one and thus
require a full handshake on every connection, resulting in degraded performance
with SSLv2 clients.  However, SSLv2-only clients should be rare these days.