Ubuntu Unleashed

lsof

lsof is the Linux/Unix über-tool. I use it most for getting network connection related information from a system, but that's just the beginning for this amazing and 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.



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

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. lsof replaces my need for both netstat and ps entirely. It has everthing I get from those tools and much, much more.

Show Your Network Connections
Show all connections with -i
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)


Show only TCP (works the same for UDP)
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)


-i :port shows all networking related 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)


To show connections to a specific host, use @host
lsof -i@192.168.1.5

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


Show connections based on the host and the port using @host:port
lsof -i@192.168.1.5:22

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


Grepping for "LISTEN" shows what ports your system is waiting for connections on
lsof -i| grep LISTEN

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


Grepping for "ESTABLISHED" shows current active connections
lsof -i| grep ESTABLISHED

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



Working with Users, Processes, and Files

You can also get information on various users, processes, and files on your system using lsof:
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 --


See what files and network connections a 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
syslog-ng 7547 root mem REG 0,0 0 [heap]
syslog-ng 7547 root mem REG 3,3 105435 850412 /lib/libpthread-2.4.so
syslog-ng 7547 root mem REG 3,3 1197180 850396 /lib/libc-2.4.so
syslog-ng 7547 root mem REG 3,3 59868 850413 /lib/libresolv-2.4.so
syslog-ng 7547 root mem REG 3,3 72784 850404 /lib/libnsl-2.4.so
syslog-ng 7547 root mem REG 3,3 32040 850414 /lib/librt-2.4.so
syslog-ng 7547 root mem REG 3,3 126163 850385 /lib/ld-2.4.so
-- snipped --


Pointing to a file shows what's interacting with that file
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


The -p switch lets you see what a given process ID has open, which is good for learning more about unknown processes
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

ps aux | grep Mail

daniel 350 0.0 1.5 405980 31452 ?? S Mon07PM 2:50.28 /Applications/Mail.app



Advanced Usage

Using-a allows you to combine search terms, so the query below says, "show me everything running as daniel connected to 1.1.1.1"
lsof -a -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 you can HUP processes
kill -HUP `lsof -t -c sshd`

You can also use the -t with -u to kill everything a user has open
kill -9 `lsof -t -u daniel`

lsof +L1 shows you all open files that have a link count less than 1, often indicative of a cracker trying to hide something
lsof +L1

(hopefully nothing)



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.


References

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



The original information for this is from: http://dmiessler.com/study/lsof/