1Demonstrations of ttysnoop, the Linux eBPF/bcc version. 2 3 4ttysnoop watches a tty or pts device, and prints the same output that is 5appearing on that device. It can be used to mirror the output from a shell 6session, or the system console. 7 8Let's snoop /dev/pts/2: 9 10# ./ttysnoop 2 11<screen clears> 12date 13Sun Oct 16 01:28:47 UTC 2016 14# uname -a 15Linux bgregg-xenial-bpf-i-xxx 4.8.0-rc4-virtual #1 SMP Wed Aug 31 22:54:37 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux 16# df -h 17Filesystem Size Used Avail Use% Mounted on 18udev 7.4G 0 7.4G 0% /dev 19tmpfs 1.5G 89M 1.4G 6% /run 20/dev/xvda1 7.8G 4.5G 3.3G 59% / 21tmpfs 7.4G 0 7.4G 0% /dev/shm 22tmpfs 5.0M 0 5.0M 0% /run/lock 23tmpfs 7.4G 0 7.4G 0% /sys/fs/cgroup 24tmpfs 250M 0 250M 0% /run/shm 25/dev/md0 160G 20G 141G 13% /mnt 26tmpfs 1.5G 0 1.5G 0% /run/user/0 27# ^C 28 29What we're seeing is another shell session. The first line was "date" without 30the shell prompt ("#") because we began tracing after the prompt was printed. 31The other commands appeared, keystroke by keystroke, as the user was typing 32them. Spooky! 33 34Remember to Ctrl-C to exit ttysnoop. 35 36 37To figure out which pts device number to use, you can check your own with "ps" 38and other's with "w". For example: 39 40# ps -p $$ 41 PID TTY TIME CMD 42 9605 pts/1 00:00:00 bash 43# w 44 01:26:37 up 9 days, 35 min, 2 users, load average: 0.22, 0.22, 0.15 45USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT 46root pts/1 100.127.65.241 00:39 2.00s 0.33s 0.33s -bash 47root pts/2 100.127.65.241 00:40 16.00s 1.06s 1.06s -bash 48 49So I'm pts/1, and there's another session that's pts/2. 50 51 52This can also snoop tty devices using their full path. Eg, snooping the system 53console: 54 55# ./ttysnoop /dev/console 56Oct 16 01:32:06 bgregg-xenial-bpf-i-xxx kernel: [780087.407428] bash (9888): drop_caches: 1 57Oct 16 01:32:38 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied 58Oct 16 01:33:32 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied 59Oct 16 01:34:26 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied 60^C 61 62Neat! 63 64 65USAGE: 66 67# ./ttysnoop.py -h 68usage: ttysnoop.py [-h] [-C] device 69 70Snoop output from a pts or tty device, eg, a shell 71 72positional arguments: 73 device path to a tty device (eg, /dev/tty0) or pts number 74 75optional arguments: 76 -h, --help show this help message and exit 77 -C, --noclear don't clear the screen 78 79examples: 80 ./ttysnoop /dev/pts/2 # snoop output from /dev/pts/2 81 ./ttysnoop 2 # snoop output from /dev/pts/2 (shortcut) 82 ./ttysnoop /dev/console # snoop output from the system console 83 ./ttysnoop /dev/tty0 # snoop output from /dev/tty0 84