code-scan starred graftcp

Introduction

graftcp can redirect the TCP connection made by the given program [application, script, shell, etc.] to SOCKS5 or HTTP proxy.

Compared with tsocks, proxychains or proxychains-ng, graftcp is not using the LD_PRELOAD trick which only work for dynamically linked programs, e.g., applications built by Go can not be hook by proxychains-ng. graftcp can trace or modify any given program's connect by ptrace(2), so it is workable for any program. The principle will be explained in this paragraph of how does it work.

Installation

Install from source

graftcp runs on Linux. Building graftcp-local requires Go installed.

git clone https://github.com/hmgle/graftcp.git
cd graftcp
make

After make finishes, you'll be able to use local/graftcp-local and ./graftcp. Optionally, you can also install them to system:

sudo make install
# Enable and activate systemed unit 
sudo make install_systemd

Install from binary package

Download the Debian or Arch Linux package from https://github.com/hmgle/graftcp/releases and install.

Usage

graftcp-local:

$ local/graftcp-local -h
Usage of local/graftcp-local:
  -config string
        Path to the configuration file
  -http_proxy string
        http proxy address, e.g.: 127.0.0.1:8080
  -listen string
        Listen address (default ":2233")
  -logfile string
        Write logs to file
  -loglevel value
        Log level (0-6) (default 1)
  -pipepath string
        Pipe path for graftcp to send address info (default "/tmp/graftcplocal.fifo")
  -select_proxy_mode string
        Set the mode for select a proxy [auto | random | only_http_proxy | only_socks5] (default "auto")
  -service string
        Control the system service: ["start" "stop" "restart" "install" "uninstall"]
  -socks5 string
        SOCKS5 address (default "127.0.0.1:1080")
  -syslog
        Send logs to the local system logger (Eventlog on Windows, syslog on Unix)

graftcp:

$ graftcp -h
Usage: graftcp [options] prog [prog-args]

Options:
  -c --conf-file=<config-file-path>
                    Specify configuration file
  -a --local-addr=<graftcp-local-IP-addr>
                    graftcp-local's IP address. Default: localhost
  -p --local-port=<graftcp-local-port>
                    Which port is graftcp-local listening? Default: 2233
  -f --local-fifo=<fifo-path>
                    Path of fifo to communicate with graftcp-local.
                    Default: /tmp/graftcplocal.fifo
  -b --blackip-file=<black-ip-file-path>
                    The IP in black-ip-file will connect direct
  -w --whiteip-file=<white-ip-file-path>
                    Only redirect the connect that destination ip in the
                    white-ip-file to SOCKS5
  -n --not-ignore-local
                    Connecting to local is not changed by default, this
                    option will redirect it to SOCKS5
  -V --version
                    Show version
  -h --help
                    Display this help and exit

mgraftcp: Combined graftcp-local and graftcp (mgraftcp = graftcp-local + graftcp). mgraftcp can be used to replace graftcp without running graftcp-local.

Usage: mgraftcp [-hn] [-b value] [--enable-debug-log] [--http_proxy value] [--select_proxy_mode value] \
    [--socks5 value] [--socks5_password value] [--socks5_username value] [--version] [-w value] prog [prog-args]
 -b, --blackip-file=value
                The IP in black-ip-file will connect direct
     --enable-debug-log
                enable debug log
 -h, --help     Display this help and exit
     --http_proxy=value
                http proxy address, e.g.: 127.0.0.1:8080
 -n, --not-ignore-local
                Connecting to local is not changed by default, this option
                will redirect it to SOCKS5
     --select_proxy_mode=value
                Set the mode for select a proxy [auto | random |
                only_http_proxy | only_socks5 | direct] [auto]
     --socks5=value
                SOCKS5 address [127.0.0.1:1080]
     --socks5_password=value
                SOCKS5 password
     --socks5_username=value
                SOCKS5 username
     --version  Print the mgraftcp version information
 -w, --whiteip-file=value
                Only redirect the connect that destination ip in the
                white-ip-file to SOCKS5

Configuration

graftcp-local and mgraftcp look for config file in following order:

File provided as a --config argument
$(the path of the executeable)/graftcp-local.conf
/etc/graftcp-local/graftcp-local.conf

Demo

Assume you are running the SOCKS5 proxy with the default IP address: "localhost:1080". Start the graftcp-local first:

Install the Go package from golang.org (now is blocked by the GFW) via graftcp:

./graftcp go get -v golang.org/x/net/proxy

Open Chromium / Chrome / Firefox browser via graftcp, then all the requests from this browser will redirect to the SOCKS5 proxy:

./graftcp chromium-browser

Launch Bash / Zsh / Fish via graftcp, then all the TCP traffic generated by the command in this shell will redirect to the SOCKS5 proxy:

% ./graftcp bash
$ wget https://www.google.com

How does it work?

To achieve the goal of redirecting the TCP connection of a app to another destination address and the app itself is not aware of it, these conditions are probably required:

fork(2) a new process and trace it using ptrace(2), execve(2) to run the app. Every connect(2) syscall will be intercepted, then get the destination address argument and send it to graftcp-local via pipe.
Modify the destination address argument of connect(2) to graftcp-local's address, and restart the stopped syscall. After the syscall returns successfully, the app thought it has connected the original destination address, but in face it connected the graftcp-local, so we named it "graft".
graftcp-local establish a SOCKS5 connection based on the information of app's original destination address, then redirect the requests from the app to the SOCKS5 proxy.

Someone may have a question here: since we can modify the arguments of a syscall, modify the app's write(2) / send(2) buf argument, attach the original destination information to the write buffer, isn't it simpler? The answer is that cannot be done. Because attach data to the buffer of the tracked child process, it may case a buffer overflow, causing crash or overwrite other data.
In addition, as the execve(2) will detach and unmap all shared memory, we also cannot add extra data to the write buffer of traced app by sharing memory, so we send the original destination address via pipe.

The simple sketch is as follows:

+---------------+             +---------+         +--------+         +------+
|   graftcp     |  dest host  |         |         |        |         |      |
|   (tracer)    +---PIPE----->|         |         |        |         |      |
|      ^        |  info       |         |         |        |         |      |
|      | ptrace |             |         |         |        |         |      |
|      v        |             |         |         |        |         |      |
|  +---------+  |             |         |         |        |         |      |
|  |         |  |  connect    |         | connect |        | connect |      |
|  |         +--------------->| graftcp +-------->| SOCKS5 +-------->| dest |
|  |         |  |             | -local  |         |  or    |         | host |
|  |  app    |  |  req        |         |  req    | HTTP   |  req    |      |
|  |(tracee) +--------------->|         +-------->| proxy  +-------->|      |
|  |         |  |             |         |         |        |         |      |
|  |         |  |  resp       |         |  resp   |        |  resp   |      |
|  |         |<---------------+         |<--------+        |<--------+      |
|  +---------+  |             |         |         |        |         |      |
+---------------+             +---------+         +--------+         +------+

FAQ and Tips

What are some ways to redirect TCP connections?

The main ones are: global way, environment variables setting way, and programs selection way.

Global way: e.g., use iptables + RedSocks to convert the system's traffic that match certain rules into SOCKS5 traffic. The pros is that it is globally effective; the cons is that all traffic that satisfies the rule is redirected, and the scope of influence is large.

Environment variable setting: some programs will read the proxy-related environment variables to determine whether to convert their own traffic to the corresponding proxy protocol traffic, such as curl will read http_proxy, ftp_proxy, all_proxy Environment variables and decide which proxy traffic to convert based on the request URL scheme. This way is effective only if the program itself implements the traffic conversion function, so it is very limited.

programs selection way: this way can only perform redirection for specified programs, such as tsocks or proxychains. As mentioned earlier, they were using the LD_PRELOAD hijacking dynamic library function, and the default static link compiled program such as Go is invalid. graftcp improves this by being able to redirect TCP connections from any program.

Will `graftcp` redirect the connection to the SOCKS5 proxy if the target address is localhost?

No. By default, graftcp ignore the connections to localhost. If you want to redirect all addresses, you can use the -n option. If you want to ignore more addresses, you can add them to the blacklist IP file; if you want to redirect only certain IP addresses, you can add them to the whitelist IP file. Use graftcp --help to get more information.

I am suffering a DNS cache poisoning attack, does `graftcp` handle DNS requests?

No. graftcp currently only handles TCP connections. dnscrypt-proxy or ChinaDNS may help you.

The `clone(2)`'s argument has a flag `CLONE_UNTRACED` to avoid being traced, how does `graftcp` do forced tracing?

graftcp will intercept the clone(2) syscall, and clearing the CLONE_UNTRACED flag, so the tracked child process could not escape the fate of being tracked. In addition, this CLONE_UNTRACED flag is intended for the kernel, and user space program should not set it.

Linux provides a way to limit the ptrace(2): set the value of /proc/sys/kernel/yama/ptrace_scope. If ptrace(2) is invalid, check if the default value has been modified.

Does it support macOS?

No. macOS's ptrace(2) is useless. ~~However, it can also be achieved theoretically by referring to DTrace~~. See issue 12. Anyone try it? 😁

TODO

ARM/Linux Support
i386/Linux Support

Acknowledgements and References

maybe, proxychains and proxychains-ng for inspiration
strace
uthash
service
dlog

LICENSE

Released under the terms of the GNU General Public License, version 3