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    SubjectTransparent proxy support in 2.4 - revisited
    A few months ago, I asked on this list why the transparent proxy feature
    (CONFIG_IP_TRANSPARENT_PROXY) that was supported in Linux 2.2 is no longer
    supported in Linux 2.4:

    > > I am writing a transparent-proxy-like application, that needs to be able to
    > > bind a TCP socket with a non-local address (i.e., the proxy contacts the
    > > origin-server, in the local network, pretending to be the original client.
    > > The reply will get back to the proxy because it acts as the default
    > > gateway, and the kernel needs to pass that reply to the socket).

    I included an example code, where bind() on a foreign address would work (if
    I set ip_nonlocal_bind sysctl to 1), but a later connect() on that socket
    would not.

    Andrey Savochkin wrote a useful reply, on how to make the connect work():

    > To make a custom kernel where you can use non-local addresses more freely,
    > find source address checks in ip_route_output_slow() and get rid of all of
    > them except considering
    > MULTICAST(saddr) || BADCLASS(saddr) || ZERONET(saddr) ||
    > saddr == htonl(INADDR_BROADCAST)
    > as invalid.

    I did that, and indeed now connect() works, and sends out (when considering
    TCP) the appropriate SYN packet.

    Unfortunately, that's not enough. When the return SYN-ACK packet carrying a
    non-local destination address is received (in practice, the transparent
    proxy machine is acting as a default gateway to the other machine), this
    packet is either ignored or forwarded out (depending on ip_forward), but is
    never accepted as a local packet and transfered to the appropriate socket
    as it should.
    To do that, some cache of open sockets will need to be consulted for
    incoming packets that are not destined to local addresses (whether or not
    ip_forward is enabled). This was done in kernel 2.2, where I found
    CONFIG_IP_TRANSPARENT_PROXY-related changes in several files (af_inet.c,
    icmp.c, ip_forward.c, ip_fw.c, ip_input.c, raw.c, route.c, syncookies.c,
    tcp_ipv4.c and udp.c).

    As I'm not an experienced Linux kernel hacker, and I'm barely familar with
    Linux's networking code, so I hesitate to attempt such a feat (resurrecting
    the transparent proxy code) myself, so I was wondering if anyone else had any
    plans of doing that?

    If not, does anyone know of a reason why this feature was abandoned on the
    change to Linux 2.4: Was it a deliberate decision (e.g., because of performance
    reasons, because it blurred the distinction between layer 2 and 3 when routing,
    etc.) or simply a matter of priorities (e.g., adding transparent proxy support
    would take up valuable kernel programmer time and was not deemed important


    Nadav Har'El | Monday, Sep 3 2001, 15 Elul 5761 |-----------------------------------------
    Phone: +972-53-245868, ICQ 13349191 |It's fortunate I have back luck - without |it I would have no luck at all!
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