NGREPSection: User Manuals (8)
Updated: December 2001
Return to Main Contents
ngrep - network grep
ngrep <-hXViwqpevxlDtT> <-IO pcap_dump > <
-n num > < -d dev > < -A num
> < -s snaplen > < -S limitlen >
< match expression > < bpf filter >
ngrep strives to provide most of GNU grep's common features, applying them to
the network layer. ngrep is a pcap-aware tool that will allow you to specify
extended regular expressions to match against data payloads of packets. It
currently recognizes TCP, UDP and ICMP across Ethernet, PPP, SLIP, FDDI and null
interfaces, and understands bpf filter logic in the same fashion as more common
packet sniffing tools, such as tcpdump(8) and
- Display help/usage information.
- Treat the match expression as a hexadecimal string. See the explanation of
match expression below.
- Display version information.
- Ignore case for the regex expression.
- Match the regex expression as a word.
- Be quiet; don't output any information other than packet headers and their
payloads (if relevant).
- Don't put the interface into promiscuous mode.
- Show empty packets. Normally empty packets are discarded because they have
no payload to search. If specified, empty packets will be shown, regardless of
the specified regex expression.
- Invert the match; only display packets that don't match.
- Dump packet contents as hexadecimal as well as ASCII.
- Make stdout line buffered.
- When reading pcap_dump files, replay them at their recorded time intervals
- Print a timestamp in the form of YYYY/MM/DD HH:MM:SS.UUUUUU everytime a
packet is matched.
- Print a timestamp in the form of +S.UUUUUU, indicating the delta between
- -s snaplen
- Set the bpf caplen to snaplen (default 65536).
- -S limitlen
- Set the upper limit on the size of packets that ngrep will look at. Useful
for looking at only the first N bytes of packets without changing the BPF
- -I pcap_dump
- Input file pcap_dump into ngrep. Works with any pcap-compatible dump file
format. This option is useful for searching for a wide range of different
patterns over the same packet stream.
- -O pcap_dump
- Output matched packets to a pcap-compatible dump file. This feature does
not interfere with normal output to stdout.
- -n num
- Match only num packets total, then exit.
- -d dev
- By default ngrep will select a default interface to listen on. Use this
option to force ngrep to listen on interface dev.
- -A num
- Dump num packets of trailing context after matching a packet.
- match expression
- A match expression is either an extended regular expression, or if the
-X option is specified, a string signifying a hexadecimal value. An
extended regular expression follows the rules as implemented by the GNU
regex library. Hexadecimal expressions can optionally be preceded
by `0x'. E.g., `DEADBEEF', `0xDEADBEEF'.
- bpf filter
- Selects a filter that specifies what packets will be dumped. If no bpf
filter is given, all IP packets seen on the selected interface will be
dumped. Otherwise, only packets for which bpf filter is `true' will be
The bpf filter consists of one or more primitives. Primitives
usually consist of an id (name or number) preceded by one or more
qualifiers. There are three different kinds of qualifier:
- qualifiers say what kind of thing the id name or number refers to.
Possible types are host, net and port. E.g., `host
blort', `net 1.2.3', `port 80'. If there is no type qualifier, host is
- qualifiers specify a particular transfer direction to and/or from
id. Possible directions are src, dst, src or dst
and src and dst. E.g., `src foo', `dst net 1.2.3', `src or dst
port ftp-data'. If there is no dir qualifier, src or dst is assumed.
For `null' link layers (i.e. point to point protocols such as slip) the
inbound and outbound qualifiers can be used to specify a desired
- qualifiers are restricted to ip-only protocols. Possible protos are:
tcp , udp and icmp. e.g., `udp src foo' or `tcp port 21'.
If there is no proto qualifier, all protocols consistent with the type are
assumed. E.g., `src foo' means `ip and ((tcp or udp) src foo)', `net bar'
means `ip and (net bar)', and `port 53' means `ip and ((tcp or udp) port 53)'.
In addition to the above, there are some special `primitive' keywords that
don't follow the pattern: gateway, broadcast, less,
greater and arithmetic expressions. All of these are described below.
More complex filter expressions are built up by using the words and,
or and not to combine primitives. E.g., `host blort and not port
ftp and not port ftp-data'. To save typing, identical qualifier lists can be
omitted. E.g., `tcp dst port ftp or ftp-data or domain' is exactly the same as
`tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
Allowable primitives are:
- dst host host
- True if the IP destination field of the packet is host, which may
be either an address or a name.
- src host host
- True if the IP source field of the packet is host.
- host host
- True if either the IP source or destination of the packet is host.
Any of the above host expressions can be prepended with the keywords,
ip, arp, or rarp as in:
ip host host
which is equivalent to:
- ether dst ehost
- True if the ethernet destination address is ehost. Ehost may
be either a name from /etc/ethers or a number (see ethers(3N) for
- ether src ehost
- True if the ethernet source address is ehost.
- ether host ehost
- True if either the ethernet source or destination address is ehost.
- gateway host
- True if the packet used host as a gateway. I.e., the ethernet
source or destination address was host but neither the IP source nor
the IP destination was host. Host must be a name and must be
found in both /etc/hosts and /etc/ethers. (An equivalent expression is
ether host ehost and not host host
which can be used with either names or numbers for host / ehost.)
- dst net net
- True if the IP destination address of the packet has a network number of
net. Net may be either a name from /etc/networks or a network
number (see networks(4)
- src net net
- True if the IP source address of the packet has a network number of
- net net
- True if either the IP source or destination address of the packet has a
network number of net.
- net net mask mask
- True if the IP address matches net with the specific netmask. May
be qualified with src or dst.
- net net/len
- True if the IP address matches net a netmask len bits wide.
May be qualified with src or dst.
- dst port port
- True if the packet is ip/tcp or ip/udp and has a destination port value of
port. The port can be a number or a name used in /etc/services
and udp(4P)). If a
name is used, both the port number and protocol are checked. If a number or
ambiguous name is used, only the port number is checked (e.g., dst port
513 will print both tcp/login traffic and udp/who traffic, and port
domain will print both tcp/domain and udp/domain traffic).
- src port port
- True if the packet has a source port value of port.
- port port
- True if either the source or destination port of the packet is
port. Any of the above port expressions can be prepended with the
keywords, tcp or udp, as in:
tcp src port port
which matches only tcp packets whose source port is port.
- less length
- True if the packet has a length less than or equal to length. This
is equivalent to:
len <= length.
- greater length
- True if the packet has a length greater than or equal to length.
This is equivalent to:
len >= length.
- ip proto protocol
- True if the packet is an ip packet (see ip(4P)) of protocol
type protocol. Protocol can be a number or one of the names
tcp, udp or icmp. Note that the identifiers tcp
and udp are also keywords and must be escaped via backslash (\), which
is \\ in the C-shell.
- ip broadcast
- True if the packet is an IP broadcast packet. It checks for both the
all-zeroes and all-ones broadcast conventions, and looks up the local subnet
- ip multicast
- True if the packet is an IP multicast packet.
- Abbreviation for:
ether proto ip
- tcp, udp, icmp
- Abbreviations for:
ip proto p
where p is one of the above protocols.
- expr relop expr
- True if the relation holds, where relop is one of >, <,
>=, <=, =, !=, and expr is an arithmetic expression composed of
integer constants (expressed in standard C syntax), the normal binary
operators [+, -, *, /, &, |], a length operator, and special packet data
accessors. To access data inside the packet, use the following syntax:
proto [ expr : size ]
Proto is one of ip, tcp, udp or icmp, and indicates
the protocol layer for the index operation. The byte offset, relative to the
indicated protocol layer, is given by expr. Size is optional and
indicates the number of bytes in the field of interest; it can be either one,
two, or four, and defaults to one. The length operator, indicated by the
keyword len, gives the length of the packet.
For example, `ether & 1 != 0' catches all multicast traffic.
The expression `ip & 0xf != 5' catches all IP packets with
options. The expression `ip[6:2] & 0x1fff = 0' catches only
unfragmented datagrams and frag zero of fragmented datagrams. This check is
implicitly applied to the tcp and udp index operations. For
instance, tcp always means the first byte of the TCP header,
and never means the first byte of an intervening fragment.
Primitives may be combined using:
- A parenthesized group of primitives and operators (parentheses are special
to the Shell and must be escaped).
- Negation (`!' or `not').
- Concatenation (`&&' or `and').
- Alternation (`||' or `or').
Negation has highest precedence. Alternation and concatenation have equal
precedence and associate left to right. Note that explicit and tokens,
not juxtaposition, are now required for concatenation.
If an identifier is given without a keyword, the most recent keyword is
assumed. For example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )
Expression arguments can be passed to ngrep as either a single argument or as
multiple arguments, whichever is more convenient. Generally, if the expression
contains Shell metacharacters, it is easier to pass it as a single, quoted
argument. Multiple arguments are concatenated with spaces before being parsed.
Errors from ngrep, libpcap, and the GNU regex library are all
output to stderr.
Written by Jordan Ritter <email@example.com>.
Send bug reports to the author.
ALL YOUR BASE ARE BELONG TO US.
This document was created by man2html, using the manual
Time: 02:27:05 GMT, August 10, 2003