Introduction
ngrepgrep‘s common features, applying them to the network layer. ngrep is a pcap-aware tool that will allow you to specify extended regular or hexadecimal expressions to match against data payloads of packets. It currently recognizes IPv4/6, TCP, UDP, ICMPv4/6, IGMP and Raw across Ethernet, PPP, SLIP, FDDI, Token Ring and null interfaces, and understands BPF (Berkeley Packet Filter) logic in the same fashion as more common packet sniffing tools, such as tcpdump and snoop. You can print live networking packets to stdout, redirect (>) the contents to a file, or pipe (|) to another utility.
Installation
ngrep is intended to be used alongside your standard *nix command-line tooling. Thus, most package repositories are sufficiently up-to-date.
On Ubuntu use apt-get:
$ sudo apt-get install ngrep
On CentOS, use yum:
$ sudo yum install ngrep
In the following examples, it is assumed that br0 is the used network interface (unless otherwise stated).
Packet Sniffing
Monitor all interfaces and protocols for a string match of “HTTP“. The -q flag will quiet the output by printing only packet headers and relevant payloads:
$ sudo ngrep -q 'HTTP'
Use the -t flag to print a timestamp along with the matched information. Use -T to print the time elapsed between successive matches:
$ sudo ngrep -qt 'HTTP'
Monitor all activity crossing source or destination port 25 (SMTP):
$ sudo ngrep -d any port 25
Monitor any network-based syslog traffic for the occurrence of the word “error“. ngrep knows how to convert service port names (on Linux, located in /etc/services) to port numbers:
$ sudo ngrep -d any 'error' port syslog
Monitor any traffic crossing source or destination port 21 (FTP), looking case-insensitively for the words “user or pass“, matched as word-expressions (the match term(s) must have non-alphanumeric, delimiting characters surrounding them):
$ sudo ngrep -wi -d any 'user|pass' port 21
Monitor all traffic not going over port 22 (SSH):
$ sudo ngrep not port 22 | strings -n 8
Monitor all traffic coming from a certain host:
$ sudo ngrep host 192.168.1.111
Capture network traffic incoming/outgoing to/from br0 interface and show the DNS (UDP/53) querys and responses:
$ sudo ngrep -l -q -d br0 -i "" udp and port 53
ngrep‘s syntax is similar to that of tcpdump:
$ sudo ngrep port 80 and src host 192.168.1.111 and dst host 192.168.1.1
Now let’s look for people misusing bandwidth:
$ sudo ngrep -i 'game*|chat|recipe' -W byline > bad_user.txt
To monitor current email transactions and print the addresses:
$ sudo ngrep -i 'rcpt to|mail from' tcp port smtp
Common BPF filters
The BPF specifies a rich syntax for filtering network packets based on information such as IP address. Matches all headers containing the string “HTTP” sent to or from the IP address starting with “192.168“:
$ sudo ngrep -q 'HTTP' 'host 192.168'
Will do as above, but instead match a destination host:
$ sudo ngrep -q 'HTTP' 'dst host 192.168'
Will do as above, but instead match a source host:
$ sudo ngrep -q 'HTTP' 'src host 192.168'
IP protocol:
$ sudo ngrep -q 'HTTP' 'tcp'
$ sudo ngrep -q 'HTTP' 'udp'
$ sudo ngrep -q 'HTTP' 'icmp'
Port number:
$ sudo ngrep -q 'HTTP' 'port 80'
Debugging HTTP interactions
In certain scenarios it is desirous to see how web browsers communicate with web servers, and to inspect the “HTTP” headers and possibly cookie values that they are exchanging:
$ sudo ngrep port 80
To see which files your browser is requesting any packet on the network which consists of “GET” followed by any characters but ending in “HTTP/1.0” or “HTTP/1.1“:
$ sudo ngrep -q '^GET .* HTTP/1.[01]'
Match only requests going to port 80:
$ sudo ngrep -q '^GET .* HTTP/1.[01]' 'port 80'
Match only requests going to the destination “1it.click“:
$ sudo ngrep -q '^GET .* HTTP/1.[01]' 'host 1it.click'
You can use regex such as ‘.*‘ in the search string:
$ sudo ngrep -d any "domain-.*.com" port 80
-W byline mode tells ngrep to respect embedded line feeds when they occur. You’ll note from the output above that there is still a trailing dot (.) on each line, which is the carriage-return portion of the CRLF { Carriage Return (ASCII 13, \r) Line Feed (ASCII 10, \n) } pair:
$ sudo ngrep -W byline port 80
Capture network traffic incoming/outgoing to/from br0 interface and show parameters following HTTP (TCP/80) “GET or POST” methods
$ sudo ngrep -l -q -d br0 -i "^GET |^POST " tcp and port 80
Capture network traffic incoming/outgoing to/from br0 interface and show the HTTP (TCP/80) “User-Agent: ” string
$ sudo ngrep -l -q -d br0 -i "User-Agent: " tcp and port 80
In the above command :
a) tcp and port 80 – is the BPF filter, that sniffs only TCP packet with port number 80
b) The -d option specifies the interface to sniff ( br0 in this case)
c) "User-Agent: " is the string to search for all packets that have that string are displayed
Monitor specific traffic:
$ sudo ngrep -t '^(GET|POST|HEAD) ' 'dst host 216.58.210.14 and tcp and dst port 80'
Then send a header request to a specific URL:
$ curl -I google.com
Or, break the response by newlines:
$ sudo ngrep -t '^(GET|POST|HEAD) ' 'dst host 216.58.210.14 and tcp and dst port 80' -W byline
Processing PCAP dump files, looking for patterns
Timestamp all traffic on port 53 (DNS) on all devices (if the box has multiple devices) and send the output to a pcap file specified by the -O switch:
$ sudo ngrep -O /tmp/dns.dump -d any -T port domain
Now we have a PCAP dump file, and so let’s search for the letter ‘m‘ for some patterns:
$ sudo ngrep -w 'm' -I /tmp/dns.dump
Here we’ve added -t which means print the absolute timestamp on the packet, and -D which means replay the packets by the time interval at which they were recorded. The latter is a neat little feature for observing the traffic at the rates/times they originally seen, though in this example it’s not terribly effective as there is only one packet being matched.
$ sudo ngrep -tD ns3 -I /tmp/dns.dump
$ sudo ngrep -I /tmp/dns.dump port 80
There’s no port 80 traffic in the dump, so of course the BPF filter yields us no results.
Debugging MySQL
Show the query and results of SELECT queries going to your MySQL server:
$ sudo ngrep -d br0 -i 'select' port 3306
Show the query and results of all queries going to your MySQL server:
If the following MySQL query returns the following:
$ sudo mysql -h 127.0.0.1 -B -e 'select * from user;' mysql
Watch the traffic:
# sudo ngrep -d lo -wi "" port 3306
Note: The command example below show another example of 
