lsof is the sysadmin/security über-tool. I use it most for getting network connection related information from a system, but that’s just the beginning for this powerful and too-little-known application. The tool is aptly called lsof because it “lists open files“. And remember, in UNIX just about everything (including a network socket) is a file.

Interestingly, lsof is also the Linux/Unix command with the most switches. It has so many it has to use both minuses and pluses.

usage: [-?abhlnNoOPRstUvV] [+|-c c] [+|-d s] [+D D] [+|-f[cgG]] [-F [f]] [-g [s]] [-i [i]] [+|-L [l]] [+|-M] [-o [o]] [-p s] [+|-r [t]] [-S [t]] [-T [t]] [-u s] [+|-w] [-x [fl]] [--] [names]

As you can see, lsof has a truly staggering number of options. You can use it to get information about devices on your system, what a given user is touching at any given point, or even what files or network connectivity a process is using.

For me, lsof replaces both netstat and ps entirely. It has everything I get from those tools and much, much more. So let’s look at some of its primary capabilities:

It’s important to understand a few key things about how lsof works. Most importantly, when you’re passing options to it, the default behavior is to OR the results. So if you are pulling a list of ports with -i and also a process list with -p you’re by default going to get both results.

Here are a few others like that to keep in mind:

default : without options, lsof lists all open files for active processes

: without options, lists all open files for active processes grouping : it’s possible to group options, e.g. -abC , but you have to watch for which options take parameters

: it’s possible to group options, e.g. , but you have to watch for which options take parameters -a : AND the results (instead of OR)

: AND the results (instead of OR) -l : show the userID instead of the username in the output

: show the userID instead of the username in the output -h : get help

: get help -t : get process IDs only

: get process IDs only -U : get the UNIX socket address

: get the UNIX socket address -F : the output is ready for another command, which can be formatted in various ways, e.g. -F pcfn (for process id, command name, file descriptor, and file name, with a null terminator)

As I said, one of my main usecases for lsof is getting information about how my system is interacting with the network. Here are some staples for getting this info:

Show all connections with -i

Some like to use netstat to get network connections, but I much prefer using lsof for this. The display shows things in a format that’s intuitive to me, and I like knowing that from there I can simply change my syntax and get more information using the same command.

# lsof -i

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME dhcpcd 6061 root 4u IPv4 4510 UDP *:bootpc sshd 7703 root 3u IPv6 6499 TCP *:ssh (LISTEN) sshd 7892 root 3u IPv6 6757 TCP 10.10.1.5:ssh->192.168.1.5:49901 (ESTABLISHED)

Get only IPv6 traffic with -i 6

# lsof -i 6

Show only TCP connections (works the same for UDP)

You can also show only TCP or UDP connections by providing the protocol right after the -i .

# lsof -iTCP

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME sshd 7703 root 3u IPv6 6499 TCP *:ssh (LISTEN) sshd 7892 root 3u IPv6 6757 TCP 10.10.1.5:ssh->192.168.1.5:49901 (ESTABLISHED)

Show networking related to a given port using -i :port

Or you can search by port instead, which is great for figuring out what’s preventing another app from binding to a given port.

# lsof -i :22

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME sshd 7703 root 3u IPv6 6499 TCP *:ssh (LISTEN) sshd 7892 root 3u IPv6 6757 TCP 10.10.1.5:ssh->192.168.1.5:49901 (ESTABLISHED)

Show connections to a specific host using @host

This is quite useful when you’re looking into whether you have open connections with a given host on the network or on the internet.

# lsof -i@172.16.12.5

sshd 7892 root 3u IPv6 6757 TCP 10.10.1.5:ssh->172.16.12.5:49901 (ESTABLISHED)

Show connections based on the host and the port using @host:port

You can also combine the display of host and port.

# lsof -i@172.16.12.5:22

sshd 7892 root 3u IPv6 6757 TCP 10.10.1.5:ssh->192.168.1.5:49901 (ESTABLISHED)

Find listening ports

Find ports that are awaiting connections.

# lsof -i -sTCP:LISTEN

You can also do this by grepping for “LISTEN” as well.

# lsof -i | grep -i LISTEN

iTunes 400 daniel 16u IPv4 0x4575228 0t0 TCP *:daap (LISTEN)

Find established connections

You can also show any connections that are already pinned up.

# lsof -i -sTCP:ESTABLISHED

You can also do this just by searching for “ESTABLISHED” in the output via grep .

# lsof -i | grep -i ESTABLISHED

firefox-b 169 daniel 49u IPv4 0t0 TCP 1.2.3.3:1863->1.2.3.4:http (ESTABLISHED)

You can also get information on various users and what they’re doing on the system, including their activity on the network, their interactions with files, etc.

Show what a given user has open using -u

# lsof -u daniel

-- snipped -- Dock 155 daniel txt REG 14,2 2798436 823208 /usr/lib/libicucore.A.dylib Dock 155 daniel txt REG 14,2 1580212 823126 /usr/lib/libobjc.A.dylib Dock 155 daniel txt REG 14,2 2934184 823498 /usr/lib/libstdc++.6.0.4.dylib Dock 155 daniel txt REG 14,2 132008 823505 /usr/lib/libgcc_s.1.dylib Dock 155 daniel txt REG 14,2 212160 823214 /usr/lib/libauto.dylib -- snipped --

Show what all users are doing except a certain user using -u ^user

# lsof -u ^daniel

-- snipped -- Dock 155 jim txt REG 14,2 2798436 823208 /usr/lib/libicucore.A.dylib Dock 155 jim txt REG 14,2 1580212 823126 /usr/lib/libobjc.A.dylib Dock 155 jim txt REG 14,2 2934184 823498 /usr/lib/libstdc++.6.0.4.dylib Dock 155 jim txt REG 14,2 132008 823505 /usr/lib/libgcc_s.1.dylib Dock 155 jim txt REG 14,2 212160 823214 /usr/lib/libauto.dylib -- snipped --

Kill everything a given user is doing

It’s nice to be able to nuke everything being run by a given user.

# kill -9 `lsof -t -u daniel`

It’s often useful to be able to see what a given program or process is up to, and with lsof you can do this by name or by process ID. Here are a few options:

See what files and network connections a named command is using with -c

# lsof -c syslog-ng

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME syslog-ng 7547 root cwd DIR 3,3 4096 2 / syslog-ng 7547 root rtd DIR 3,3 4096 2 / syslog-ng 7547 root txt REG 3,3 113524 1064970 /usr/sbin/syslog-ng -- snipped --

See what a given process ID has open using -p

# lsof -p 10075

-- snipped -- sshd 10068 root mem REG 3,3 34808 850407 /lib/libnss_files-2.4.so sshd 10068 root mem REG 3,3 34924 850409 /lib/libnss_nis-2.4.so sshd 10068 root mem REG 3,3 26596 850405 /lib/libnss_compat-2.4.so sshd 10068 root mem REG 3,3 200152 509940 /usr/lib/libssl.so.0.9.7 sshd 10068 root mem REG 3,3 46216 510014 /usr/lib/liblber-2.3 sshd 10068 root mem REG 3,3 59868 850413 /lib/libresolv-2.4.so sshd 10068 root mem REG 3,3 1197180 850396 /lib/libc-2.4.so sshd 10068 root mem REG 3,3 22168 850398 /lib/libcrypt-2.4.so sshd 10068 root mem REG 3,3 72784 850404 /lib/libnsl-2.4.so sshd 10068 root mem REG 3,3 70632 850417 /lib/libz.so.1.2.3 sshd 10068 root mem REG 3,3 9992 850416 /lib/libutil-2.4.so -- snipped --

The -t option returns just a PID

# lsof -t -c Mail

350

By looking at a given file or directory you can see what all on the system is interacting with it–including users, processes, etc.

Show everything interacting with a given directory

# lsof /var/log/messages/

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME syslog-ng 7547 root 4w REG 3,3 217309 834024 /var/log/messages

Show everything interacting with a given file

# lsof /home/daniel/firewall_whitelist.txt

Advanced Usage

Similar to tcpdump , the power really shows itself when you start combining queries.

Show me everything daniel is doing connected to 1.1.1.1

# lsof -u daniel -i @1.1.1.1

bkdr 1893 daniel 3u IPv6 3456 TCP 10.10.1.10:1234->1.1.1.1:31337 (ESTABLISHED)

Using the -t and -c options together to HUP processes

# kill -HUP `lsof -t -c sshd`

Show open connections with a port range

# lsof -i @fw.google.com:2150-2180

Conclusion

This primer just scratches the surface of lsof ‘s functionality. For a full reference, run man lsof or check out the online version. I hope this has been useful to you, and as always, comments and corrections are welcomed.

Notes

The lsof man page:

http://www.netadmintools.com/html/lsof.man.html

CREATED: FEBRUARY 2009 | UPDATED: DECEMBER 2015