Demonstrations of ttysnoop, the Linux eBPF/bcc version.
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ttysnoop watches a tty or pts device, and prints the same output that is
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appearing on that device. It can be used to mirror the output from a shell
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session, or the system console.
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Let's snoop /dev/pts/2:
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# ./ttysnoop 2
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<screen clears>
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date
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Sun Oct 16 01:28:47 UTC 2016
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# uname -a
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Linux 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
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# df -h
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Filesystem Size Used Avail Use% Mounted on
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udev 7.4G 0 7.4G 0% /dev
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tmpfs 1.5G 89M 1.4G 6% /run
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/dev/xvda1 7.8G 4.5G 3.3G 59% /
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tmpfs 7.4G 0 7.4G 0% /dev/shm
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tmpfs 5.0M 0 5.0M 0% /run/lock
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tmpfs 7.4G 0 7.4G 0% /sys/fs/cgroup
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tmpfs 250M 0 250M 0% /run/shm
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/dev/md0 160G 20G 141G 13% /mnt
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tmpfs 1.5G 0 1.5G 0% /run/user/0
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# ^C
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What we're seeing is another shell session. The first line was "date" without
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the shell prompt ("#") because we began tracing after the prompt was printed.
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The other commands appeared, keystroke by keystroke, as the user was typing
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them. Spooky!
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Remember to Ctrl-C to exit ttysnoop.
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To figure out which pts device number to use, you can check your own with "ps"
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and other's with "w". For example:
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# ps -p $$
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PID TTY TIME CMD
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9605 pts/1 00:00:00 bash
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# w
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01:26:37 up 9 days, 35 min, 2 users, load average: 0.22, 0.22, 0.15
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USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
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root pts/1 100.127.65.241 00:39 2.00s 0.33s 0.33s -bash
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root pts/2 100.127.65.241 00:40 16.00s 1.06s 1.06s -bash
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So I'm pts/1, and there's another session that's pts/2.
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This can also snoop tty devices using their full path. Eg, snooping the system
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console:
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# ./ttysnoop /dev/console
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Oct 16 01:32:06 bgregg-xenial-bpf-i-xxx kernel: [780087.407428] bash (9888): drop_caches: 1
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Oct 16 01:32:38 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied
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Oct 16 01:33:32 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied
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Oct 16 01:34:26 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied
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^C
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Neat!
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USAGE:
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# ./ttysnoop.py -h
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usage: ttysnoop.py [-h] [-C] device
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Snoop output from a pts or tty device, eg, a shell
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positional arguments:
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device path to a tty device (eg, /dev/tty0) or pts number
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optional arguments:
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-h, --help show this help message and exit
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-C, --noclear don't clear the screen
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examples:
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./ttysnoop /dev/pts/2 # snoop output from /dev/pts/2
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./ttysnoop 2 # snoop output from /dev/pts/2 (shortcut)
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./ttysnoop /dev/console # snoop output from the system console
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./ttysnoop /dev/tty0 # snoop output from /dev/tty0
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