how-to-use-tcpdump
Tcpdump command is a famous network packet analyzing tool that is used to display TCP IP & other network packets being transmitted over the network attached to the system on which tcpdump has been installed. Tcpdump uses libpcap library to capture the network packets & is available on almost all Linux/Unix flavors.
Linux Tcpdump: Filter ipv6 ntp ping packets
Tcpdump: capture DHCP & DHCPv6 packets
20 Advanced Tcpdump Examples On Linux
10 Useful tcpdump command examples
TCPDUMP
README
Tcpdump is one of the best network analysis-tools ever for information security professionals.
Tcpdump is for everyone for hackers and people who have less of TCP/IP understanding.
OPTIONS
Below are some tcpdump options (with useful examples) that will help you working with the tool. They’re very easy to forget and/or confuse with other types of filters, i.e. ethereal, so hopefully this article can serve as a reference for you, as it does me:)
- The first of these is -n, which requests that names are not resolved, resulting in the IPs themselves.
- The second is -X, which displays both hex and ascii content within the packet.
- The final one is -S, which changes the display of sequence numbers to absolute rather than relative.
Show the packet’s contents in both hex and ascii.
tcpdump -X ....
Same as -X, but also shows the ethernet header.
tcpdump -XX
Show the list of available interfaces
tcpdump -D
Line-readable output (for viewing as you save, or sending to other commands)
tcpdump -l
Be less verbose (more quiet) with your output.
tcpdump -q
Give human-readable timestamp output.
tcpdump -t :
Give maximally human-readable timestamp output.
tcpdump -tttt :
Listen on the eth0 interface.
tcpdump -i eth0
Verbose output (more v’s gives more output).
tcpdump -vv
Only get x number of packets and then stop.
tcpdump -c
Define the snaplength (size) of the capture in bytes. Use -s0 to get everything, unless you are intentionally capturing less.
tcpdump -s
Print absolute sequence numbers.
tcpdump -S
Get the ethernet header as well.
tcpdump -e
Decrypt IPSEC traffic by providing an encryption key.
tcpdump -E
For more options, read manual:
-
Find all options here
BASIC USAGE
Display Available Interfaces
tcpdump -D
tcpdump --list-interfaces
Let’s start with a basic command that will get us HTTPS traffic:
tcpdump -nnSX port 443
Find Traffic by IP
tcpdump host 1.1.1.1
Filtering by Source and/or Destination
tcpdump src 1.1.1.1
tcpdump dst 1.0.0.1
Finding Packets by Network
tcpdump net 1.2.3.0/24
Low Output:
tcpdump -nnvvS
Medium Output:
tcpdump -nnvvXS
Heavy Output:
tcpdump -nnvvXSs 1514
Getting Creative
- Expressions are very nice, but the real magic of tcpdump comes from the ability to combine them in creative ways in order to isolate exactly what you’re looking for.
There are three ways to do combination:
AND
and or &&
OR
or or ||
EXCEPT
not or !
Usage Example:
Traffic that’s from 192.168.1.1 AND destined for ports 3389 or 22
tcpdump 'src 192.168.1.1 and (dst port 3389 or 22)'
Advanced
Show me all URG packets:
tcpdump 'tcp[13] & 32 != 0'
Show me all ACK packets:
tcpdump 'tcp[13] & 16 != 0'
Show me all PSH packets:
tcpdump 'tcp[13] & 8 != 0'
Show me all RST packets:
tcpdump 'tcp[13] & 4 != 0'
Show me all SYN packets:
tcpdump 'tcp[13] & 2 != 0'
Show me all FIN packets:
tcpdump 'tcp[13] & 1 != 0'
Show me all SYN-ACK packets:
tcpdump 'tcp[13] = 18'
Show all traffic with both SYN and RST flags set: (that should never happen)
tcpdump 'tcp[13] = 6'
Show all traffic with the “evil bit” set:
tcpdump 'ip[6] & 128 != 0'
Display all IPv6 Traffic:
tcpdump ip6
Print Captured Packets in ASCII
tcpdump -A -i eth0
Display Captured Packets in HEX and ASCII
tcpdump -XX -i eth0
Capture and Save Packets in a File
tcpdump -w 0001.pcap -i eth0
Read Captured Packets File
tcpdump -r 0001.pcap
Capture IP address Packets
tcpdump -n -i eth0
Capture only TCP Packets.
tcpdump -i eth0 tcp
Capture Packet from Specific Port
tcpdump -i eth0 port 22
Capture Packets from source IP
tcpdump -i eth0 src 192.168.0.2
Capture Packets from destination IP
tcpdump -i eth0 dst 50.116.66.139
Capture any packed coming from x.x.x.x
tcpdump -n src host x.x.x.x
Capture any packet coming from or going to x.x.x.x
tcpdump -n host x.x.x.x
Capture any packet going to x.x.x.x
tcpdump -n dst host x.x.x.x
Capture any packed coming from x.x.x.x
tcpdump -n src host x.x.x.x
Capture any packet going to network x.x.x.0/24
tcpdump -n dst net x.x.x.0/24
Capture any packet coming from network x.x.x.0/24
tcpdump -n src net x.x.x.0/24
Capture any packet with destination port x
tcpdump -n dst port x
Capture any packet coming from port x
tcpdump -n src port x
Capture any packets from or to port range x to y
tcpdump -n dst(or src) portrange x-y
Capture any tcp or udp port range x to y
tcpdump -n tcp(or udp) dst(or src) portrange x-y
Capture any packets with dst ip x.x.x.x and port y
tcpdump -n "dst host x.x.x.x and dst port y"
Capture any packets with dst ip x.x.x.x and dst ports x, z
tcpdump -n "dst host x.x.x.x and (dst port x or dst port z)"
Capture ICMP , ARP
tcpdump -v icmp(or arp)
Capture packets on interface eth0 and dump to cap.txt file
tcpdump -i eth0 -w cap.txt
Get Packet Contents with Hex Output
tcpdump -c 1 -X icmp
Show Traffic Related to a Specific Port
tcpdump port 3389
tcpdump src port 1025
Show Traffic of One Protocol
tcpdump icmp
Find Traffic by IP
tcpdump host 1.1.1.1
Filtering by Source and/or Destination
tcpdump src 1.1.1.1
tcpdump dst 1.0.0.1
Finding Packets by Network
tcpdump net 1.2.3.0/24
Get Packet Contents with Hex Output
tcpdump -c 1 -X icmp
Show Traffic Related to a Specific Port
tcpdump port 3389
tcpdump src port 1025
Show Traffic of One Protocol
tcpdump icmp
Show only IP6 Traffic
tcpdump ip6
Find Traffic Using Port Ranges
tcpdump portrange 21-23
Find Traffic Based on Packet Size
tcpdump less 32
tcpdump greater 64
tcpdump <= 128
tcpdump => 128
Reading / Writing Captures to a File (pcap)
tcpdump port 80 -w capture_file
tcpdump -r capture_file
It’s All About the Combinations
Raw Output View
tcpdump -ttnnvvS
Here are some examples of combined commands.
From specific IP and destined for a specific Port
tcpdump -nnvvS src 10.5.2.3 and dst port 3389
From One Network to Another
tcpdump -nvX src net 192.168.0.0/16 and dst net 10.0.0.0/8 or 172.16.0.0/16
Non ICMP Traffic Going to a Specific IP
tcpdump dst 192.168.0.2 and src net and not icmp
Traffic From a Host That Isn’t on a Specific Port
tcpdump -vv src mars and not dst port 22
Isolate TCP RST flags.
tcpdump 'tcp[13] & 4!=0'
tcpdump 'tcp[tcpflags] == tcp-rst'
Isolate TCP SYN flags.
tcpdump 'tcp[13] & 2!=0'
tcpdump 'tcp[tcpflags] == tcp-syn'
Isolate packets that have both the SYN and ACK flags set.
tcpdump 'tcp[13]=18'
Isolate TCP URG flags.
tcpdump 'tcp[13] & 32!=0'
tcpdump 'tcp[tcpflags] == tcp-urg'
Isolate TCP ACK flags.
tcpdump 'tcp[13] & 16!=0'
tcpdump 'tcp[tcpflags] == tcp-ack'
Isolate TCP PSH flags.
tcpdump 'tcp[13] & 8!=0'
tcpdump 'tcp[tcpflags] == tcp-psh'
Isolate TCP FIN flags.
tcpdump 'tcp[13] & 1!=0'
tcpdump 'tcp[tcpflags] == tcp-fin'
Commands that I using almost daily
Both SYN and RST Set
tcpdump 'tcp[13] = 6'
Find HTTP User Agents
tcpdump -vvAls0 | grep 'User-Agent:'
tcpdump -nn -A -s1500 -l | grep "User-Agent:"
By using egrep and multiple matches we can get the User Agent and the Host (or any other header) from the request.
tcpdump -nn -A -s1500 -l | egrep -i 'User-Agent:|Host:'
Capture only HTTP GET and POST packets only packets that match GET.
tcpdump -s 0 -A -vv 'tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x47455420'
tcpdump -s 0 -A -vv 'tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x504f5354'
Extract HTTP Request URL’s
tcpdump -s 0 -v -n -l | egrep -i "POST /|GET /|Host:"
Extract HTTP Passwords in POST Requests
tcpdump -s 0 -A -n -l | egrep -i "POST /|pwd=|passwd=|password=|Host:"
Capture Cookies from Server and from Client
tcpdump -nn -A -s0 -l | egrep -i 'Set-Cookie|Host:|Cookie:'
Capture all ICMP packets
tcpdump -n icmp
Show ICMP Packets that are not ECHO/REPLY (standard ping)
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
Capture SMTP / POP3 Email
tcpdump -nn -l port 25 | grep -i 'MAIL FROM\|RCPT TO'
Troubleshooting NTP Query and Response
tcpdump dst port 123
Capture FTP Credentials and Commands
tcpdump -nn -v port ftp or ftp-data
Rotate Capture Files
tcpdump -w /tmp/capture-%H.pcap -G 3600 -C 200
Capture IPv6 Traffic
tcpdump -nn ip6 proto 6
IPv6 with UDP and reading from a previously saved capture file.
tcpdump -nr ipv6-test.pcap ip6 proto 17
Detect Port Scan in Network Traffic
tcpdump -nn
USAGE EXAMPLE
Example Filter Showing Nmap NSE Script Testing
-
On Target:
nmap -p 80 --script=http-enum.nse targetip -
On Server:
tcpdump -nn port 80 | grep "GET /" GET /w3perl/ HTTP/1.1 GET /w-agora/ HTTP/1.1 GET /way-board/ HTTP/1.1 GET /web800fo/ HTTP/1.1 GET /webaccess/ HTTP/1.1 GET /webadmin/ HTTP/1.1 GET /webAdmin/ HTTP/1.1
Capture Start and End Packets of every non-local host
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
Capture DNS Request and Response
tcpdump -i wlp58s0 -s0 port 53
Capture HTTP data packets
tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
Top Hosts by Packets
tcpdump -nnn -t -c 200 | cut -f 1,2,3,4 -d '.' | sort | uniq -c | sort -nr | head -n 20
Capture all the plaintext passwords
tcpdump port http or port ftp or port smtp or port imap or port pop3 or port telnet -l -A | egrep -i -B5 'pass=|pwd=|log=|login=|user=|username=|pw=|passw=|passwd=|password=|pass:|user:|username:|password:|login:|pass |user '
tcpdump port http or port ftp or port smtp or port imap or port pop3 or port telnet -lA | egrep -i -B5 'pass=|pwd=|log=|login=|user=|username=|pw=|passw=|passwd= |password=|pass:|user:|username:|password:|login:|pass |user '
DHCP Example
tcpdump -v -n port 67 or 68
Cleartext GET Requests
tcpdump -vvAls0 | grep 'GET'
Find HTTP Host Headers
tcpdump -vvAls0 | grep 'Host:'
Find HTTP Cookies
tcpdump -vvAls0 | grep 'Set-Cookie|Host:|Cookie:'
Find SSH Connections
tcpdump 'tcp[(tcp[12]>>2):4] = 0x5353482D'
Find DNS Traffic
tcpdump -vvAs0 port 53
Find FTP Traffic
tcpdump -vvAs0 port ftp or ftp-data
Find NTP Traffic
tcpdump -vvAs0 port 123
Capture SMTP / POP3 Email
tcpdump -nn -l port 25 | grep -i 'MAIL FROM\|RCPT TO'
Line Buffered Mode
tcpdump -i eth0 -s0 -l port 80 | grep 'Server:'
Find traffic with evil bit
tcpdump 'ip[6] & 128 != 0'
Filter on protocol (ICMP) and protocol-specific fields (ICMP type)
Tcpdump: Filter Packets with Tcp Flags
tcpdump -n icmp and ‘icmp[0] != 8 and icmp[0] != 0’
Same command can be used with predefined header field offset (icmptype) and ICMP type field values (icmp-echo and icmp-echoreply):
tcpdump -n icmp and icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply
Filter on TOS field
tcpdump -v -n ip and ip[1]!=0
Filter on TTL field
tcpdump -v ip and 'ip[8]<2'
Filter on TCP flags (SYN/ACK)
tcpdump -n tcp and port 80 and 'tcp[tcpflags] & tcp-syn == tcp-syn'
In the example above, all packets with TCP SYN flag set are captured. Other flags (ACK, for example) might be set also. Packets which have only TCP SYN flags set, can be captured
tcpdump tcp and port 80 and 'tcp[tcpflags] == tcp-syn'
Catch TCP SYN/ACK packets (typically, responses from servers):
tcpdump -n tcp and 'tcp[tcpflags] & (tcp-syn|tcp-ack) == (tcp-syn|tcp-ack)'
tcpdump -n tcp and 'tcp[tcpflags] & tcp-syn == tcp-syn' and 'tcp[tcpflags] & tcp-ack == tcp-ack'
Catch ARP packets
tcpdump -vv -e -nn ether proto 0x0806
Filter on IP packet length
tcpdump -l icmp and '(ip[2:2]>50)' -w - |tcpdump -r - -v ip and '(ip[2:2]<60)'
Remark: due to some bug in tcpdump, the following command doesn’t catch packets as expected:
tcpdump -v -n icmp and '(ip[2:2]>50)' and '(ip[2:2]<60)'
Filter on encapsulated content (ICMP within PPPoE)
tcpdump -v -n icmp
filter
tcpdump -q -i eth0
tcpdump -t -i eth0
tcpdump -A -n -q -i eth0 'port 80'
tcpdump -A -n -q -t -i eth0 'port 80'
Print only useful packets from the HTTP traffic
tcpdump -A -s 0 -q -t -i eth0 'port 80 and ( ((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12:2]&0xf0)>>2)) != 0)'
Dump SIP Traffic
tcpdump -nq -s 0 -A -vvv port 5060 and host 1.2.3.4
Checking packet content
tcpdump -i any -c10 -nn -A port 80
Checking packet content
sudo tcpdump -i any -c10 -nn -A port 80
References & Awesome wikis
Capture ICMP Packets With Tcpdump
Debugging SSH Packets with Tcpdump
Using Tcpdump to Filter DNS Packets
Filtering CDP LLDP packets with Tcpdump
Tcpdump Cheat Sheet (Basic Advanced Examples)
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