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tcp-intercept

This program is designed to intercept TCP connections passing through this box. This can be set up using the TPROXY-target of iptables, together with the appropriate ip route setup. See kernel/Documentation/networking/tproxy.txt for details.

Rationale

The idea is to perform TCP-acceleration on unaware endpoints. This acceleration can consist of using MultiPath-TCP, using another congestion control algorithm, or both.

Other projects exist to simply add MPTCP-support to existing connections on the netfilter-layer (e.g. http://www.ietf.org/mail-archive/web/multipathtcp/current/msg01934.html). This is more performant, since it doesn't maintain any buffers. In order to change the congestion control however, intermediate buffers are needed.

Compiling and installing

If you use a Debian-based system, you can build a .deb file by running dpkg-buildpackage -us -uc, and install it with dpkg -i generated-file.deb. You need to have the dependencies installed before you can build.

Dependencies:

Instructions:

autopoint
autoreconf -i
./configure
make
sudo make install

TCP keepalive support

Inorder to benefit from the TCP-keepalive functionality, you need to enable kernel support. Without this functionality, tcp-intercept has no way of cleaning up connections that hang becasue one of the peers lost connectivity without tcp-inntercept being aware of it. Should you wish to have this feature enabled (default is disabled), please start tcp-intercept with the keepalive option (-k).

You may use the procfs or sysctl interface to configure the kernel parameters.

procfs:

  • /proc/sys/net/ipv4/tcp_keepalive_time
  • /proc/sys/net/ipv4/tcp_keepalive_intvl
  • /proc/sys/net/ipv4/tcp_keepalive_probes

sysctl:

  • net.ipv4.tcp_keepalive_time
  • net.ipv4.tcp_keepalive_intvl
  • net.ipv4.tcp_keepalive_probes

TCP_NODELAY (no Nagel) support

tcp-intercept can disable (optionally) the Nagel-Algorithm used by TCP. TCP by default tries to optimise utilization of the network. In doing so, it buffers data segments (smaller than an MSS), until it reaches the MSS or it receives an ACK message for its sent data segments, and sends them out as one big segment. This way it avoids the frequent sending of small segments that could waste bandwidth. On the contrary, this could have a negative effect on latency/jitter sensitive applications (real time apps). Should you wish to have this feature enabled (default is disabled), please start tcp-intercept with the TCP_NODELAY option (-n).

iptables setup

IP_ROUTE_TABLE_NUMBER=5 # free routing table number to use
FWMARK="0x01/0x01" # Distinguishing FWmark bit to use
LISTEN_PORT=5000 # Port where tcp-intercept is listening on

# First add a custom route table that delivers everything locally
ip rule add fwmark $FWMARK table $IP_ROUTE_TABLE_NUMBER
ip route add local 0.0.0.0/0 dev lo table $IP_ROUTE_TABLE_NUMBER

iptables -t mangle -N tproxy

# If this packet matches an existing, open local socket, deliver it locally
iptables -t mangle -A tproxy -p tcp -m socket -j MARK --set-mark $FWMARK
iptables -t mangle -A tproxy -p tcp -m socket -j RETURN
# If it is a packet with a local destination, don't touch it
iptables -t mangle -A tproxy -p tcp -m addrtype --dst-type LOCAL -j RETURN
# Intercept all other TCP-connections
iptables -t mangle -A tproxy -p tcp -j TPROXY \
                   --on-port $LISTEN_PORT --tproxy-mark $FWMARK

iptables -t mangle -A PREROUTING -j tproxy