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Real-time Scanning of Other Terminals with Strace

by Tsuji DaishiroMay 7th, 2020

Too Long; Didn't Read

Real-time Scanning of Other Terminals with Strace is a tool that makes it easy to see the terminals of other users logged in on the server. It's incredible simple and very useful and can be used to trace a system call. For example, if the current directory isand there is an empty file namedunder, the result of stracing is shown below. The basic flow of the process is as follows: Retrieving the PID of the ttyty process's read-load process as follows.

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featured image - Real-time Scanning of Other Terminals with Strace

Introduction

Do you ever want to see other people's terminals on a Linux server? I have.Of course, having the terminal physically shown is one way, but it's a bit of a hassle. You can also use the

script

command, but it is a little more laborious because you have to ask the shower to execute the

script

command.

So I've created a tool that makes it easy to see the terminals of other users logged in on the server! This tool wraps

strace

. It is called

ttycopy

. It's incredible simple and very useful.

https://github.com/d-tsuji/ttycopy

strace

It is well known that

strace

command can be used to trace a system call. Before introducing the workings of

ttycopy

, let's review how to check a system call with

strace

. For example, if the current directory is

/tmp

and there is an empty file named

sample.txt

under

/tmp

, the result of stracing is shown below. In other words, the situation is as follows.

[root@localhost tmp]# pwd
/tmp
[root@localhost tmp]# ls
sample.txt

We can check with

strace

what kind of system call is being called when you call

ls

[root@localhost tmp]# strace ls
execve("/usr/bin/ls", ["ls"], [/* 27 vars */]) = 0
brk(NULL)                               = 0xaf7000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f2afb9f4000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=84100, ...}) = 0
mmap(NULL, 84100, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f2afb9df000
close(3)                                = 0
open("/lib64/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320i\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=155784, ...}) = 0
mmap(NULL, 2255184, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f2afb5ad000
mprotect(0x7f2afb5d1000, 2093056, PROT_NONE) = 0
mmap(0x7f2afb7d0000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x23000) = 0x7f2afb7d0000
mmap(0x7f2afb7d2000, 6480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f2afb7d2000

...

mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f288e995000
write(1, "sample.txt\n", 11sample.txt
)            = 11
close(1)                                = 0
munmap(0x7f288e995000, 4096)            = 0
close(2)                                = 0
exit_group(0)                           = ?

Even if we take one operation to get the result of the

ls

command as described above, we can see that many system calls are made behind the scenes.

ttycopy

I would like to tell you how this is being processing. The basic flow of the process is as follows.

Check the PID of the tty's login process
Retrieving read system calls to PID using the strace command
Print a formatted string of the read system call output by strace

1.Check the PID of the tty's login process

The shell implementation is as follows.

pid=`ps fauwwx | grep sshd.*${tty} | grep -v grep | sed -e 's/^[a-zA-Z0-9]\+[ \n\r\f\t]\+\([0-9]\+\).*/\1/'`

This captures the PID using a regular expression with a string of the following process tree that can be retrieved when grep is done. You can use regular expressions to make it look neat.

[tsuji@localhost ~]$ ps fauwwx | grep sshd.
root      1531  0.0  0.1  82568  6236 ?        Ss   15:29   0:00 /usr/sbin/sshd -D
root      2830  0.0  0.2 149824  8916 ?        Ss   15:29   0:01  \_ sshd: root@pts/0
root     13315  0.0  0.2 158980 10280 ?        Ss   18:58   0:00  \_ sshd: root@notty
root     14956  0.0  0.2 154512  9352 ?        Ss   20:10   0:00  \_ sshd: tsuji [priv]
tsuji    14959  0.0  0.1 154512  4092 ?        S    20:10   0:00      \_ sshd: tsuji@pts/1
tsuji    15012  0.0  0.0 112672  2268 pts/1    S+   20:11   0:00              \_ grep --color=auto sshd.

2.Retrieving read system calls to PID using the strace command

The shell implementation is as follows.

strace -e read -s16384 -q -xx -p ${pid} 2>&1

What we're doing is using trace to retrieve the read system call issued by the PID associated with the tty we just retrieved. Because various other system calls are called besides the read system call, only the read system call is extracted with the option

-e read

of trace.

The read system call was the system call defined below, and the results that can be obtained with starce are also as follows.

#include <unistd.h>

ssize_t read(int fd, void *buf, size_t count);

For a user who is logged in at another terminal, execute the

ls

command in the

/tmp

directory.

straing terminal

# strace -p 14959 -e read
read(0, "l", 1)                         = 1
read(0, "s", 1)                         = 1
read(0, "\r", 1)                        = 1
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=2985, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---

We can only get results for read system calls that are being read from the standard input as shown above. The standard input is the file descriptor number 0.

3.Print a formatted string of the read system call output by strace

The goal is to display the contents of the second argument of the read system call obtained in step 2. The shell implementation is as follows.

sed -une "s/^read([0-9]\+, \"\(.*\)\".*/echo -n \$'\1'/p" | bash

First, the first part

sed -une "s/^read([0-9]\+, \"\(.*\)\".*/echo -n \$'\1'/p"

. It captures the second argument of the read system call in a regular expression and gives it the strings

echo -n` and `$''

.

Therefore, it is possible to restore the commands executed in the scanning terminal in real time.

The string output to the source terminal

[tsuji@localhost tmp]$ ls
sample.txt
[tsuji@localhost tmp]$

Strace the terminal string to be output after processing.

echo -n $'\x00\x00\x00\x10\x97\xcb\x9f\x29\xb7\x68\xee\xb5\x30\x84\xee\xad\x9d\x73\x52\x01\x17\x63\x30\x62\xe3\xe2\x7e\x61\x26\x64\x5b\x73\x93\x64\x96\xfd'
echo -n $'l'
echo -n $'\x00\x00\x00\x10\x70\x94\x30\xff\x6a\xfa\xbf\xfb\x1a\x1c\x23\x95\x85\x7d\x8f\xa4\xb7\x55\x42\xa4\x8e\x6e\x3e\x3e\xcd\xe4\xd1\xf7\xfb\x05\x5c\xb8'
echo -n $'s'
echo -n $'\x00\x00\x00\x10\x2d\xda\xa7\x40\xa5\xbc\x6a\xf4\x3f\x42\xce\x89\x6d\x3b\x6a\xe1\x16\xe6\x2e\x14\x55\x0d\xdd\x2b\xd8\x9c\xe0\x81\x84\x7d\x8d\x9d'
echo -n $'\r\n'
echo -n $'sample.txt\r\n'
echo -n $'\x1b\x5d\x30\x3b\x74\x73\x75\x6a\x69\x40\x6c\x6f\x63\x61\x6c\x68\x6f\x73\x74\x3a\x2f\x74\x6d\x70\x07\x5b\x74\x73\x75\x6a\x69\x40\x6c\x6f\x63\x61\x6c\x68\x6f\x73\x74\x20\x74\x6d\x70\x5d\x24\x20'

The

-n

option of

echo -n

is to avoid line breaks when interpreting. the

$''

form of Bash has the following effect. excerpt from the Man page

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:

ref: https://linux.die.net/man/1/bash

This can be used to expand to the meaning that the string holds.

All that's left to do is to let the shell interpret the