Filter stdin of process executed with socat client

My question is: is there a way to pre-process the stdin of a command executed via socat client.

Context: I am launching a reverse shell with socat using this command:

$> socat -T30 "openssl-connect:api.hoposhell.com:10000,verify=0,keepalive" "exec:$SHELL,pty,stderr,setsid"

In order to keep the connection alive, my server sends the ACK ASCII character (0x06) every 10 seconds. However, this has side effects in some applications and some ^F characters appears.

In bash or zsh it is possible to use pipes with sed like below:

$> echo "rick roll" | sed 's/r//g'
ick oll

Is there a way to do something similar with socat? (e.g. use cat between the socket read and the executed command)

Note:

• this is a follow up to this question Keep alive SSL client connection with socat
• I would be happy with other ASCII character (other than 0x06) if that allows to circumvent the issue
• I am using bash and zsh under linux and macos (Big Sur)

Edit: simple equivalent example

Let us say I listen to the reverse shell with (machine 1)

1> socat -d -d TCP4-LISTEN:4242 STDOUT

and connect a reverse shell with (machine 2)

2> socat TCP4:localhost:4242 SYSTEM:/bin/bash,pty,setsid,stderr

I can now use a shell from computer 1 (executed on computer 2) Now I want that everytime I send character 0x06 from 1 to 2, is is ignored.

Now, if I replace by:

2>  socat TCP4:localhost:4243 SYSTEM:"/usr/bin/sed -u 's/a/b/g' | bash -i",pty,setsid,stderr

Now I should get a reverse shell when all the "a" become "b" as below (on machine 1). But I get two issues:

• when I type "a", the "a" is still shown (twice) as below (I pressed "la" + ENTER)

bash-3.2$ la
la
lb
bash: lb: command not found
bash-3.2$ 

• when my server sends some "a" characters they still appear as is

bash-3.2$ aaaaaaaaaa
aaaaaaaaaa
bbbbbbbbbb
bash: bbbbbbbbbb: command not found
bash-3.2$ 

My objective is to get respectively:

bash-3.2$ lb
bash: lb: command not found
bash-3.2$ bbbbbbbbbb
bash: bbbbbbbbbb: command not found

The post Sed with port forwarding in socat recommends using a command similar to:

socat TCP-LISTEN:8002,fork SYSTEM:/path/to/replace.sh

The file replace.sh should have the execution permission.

An example for changing all a to b:

#!/bin/bash
sed -u 's/a/b/g'

For sed, it is important to use -u (unbuffered).