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NAME
ngrep - network grep
SYNOPSIS
ngrep <-hXViwqpevxlDtT> <-IO pcap_dump
> < -n num > < -d dev
> < -A num > < -s snaplen
> < match expression > < bpf
filter >
DESCRIPTION
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 snoop(1).
OPTIONS
- -h
- Display help/usage information.
- -X
- Treat the match expression as a hexadecimal string. See the
explanation of match expression below.
- -V
- Display version information.
- -i
- Ignore case for the regex expression.
- -w
- Match the regex expression as a word.
- -q
- Be quiet; don't output any information other than packet
headers and their payloads (if relevant).
- -p
- Don't put the interface into promiscuous mode.
- -e
- 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.
- -v
- Invert the match; only display packets that don't match.
- -x
- Dump packet contents as hexadecimal as well as ASCII.
- -l
- Make stdout line buffered.
- -D
- When reading pcap_dump files, replay them at their recorded
time intervals (mimic realtime).
- -t
- Print a timestamp in the form of YYYY/MM/DD HH:MM:SS.UUUUUU
everytime a packet is matched.
- -T
- Print a timestamp in the form of +S.UUUUUU, indicating the
delta between packet matches.
- -s snaplen
- Set the bpf caplen to snaplen (default 65536).
- -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 dumped.
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:
- type
- 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 assumed.
- dir
- 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
direction.
- proto
- 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 numeric format).
- 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)
for details).
- src net net
- True if the IP source address of the packet has a network
number of net.
- 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 (see tcp(4P) 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 mask.
- ip multicast
- True if the packet is an IP multicast packet.
- ip
- 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[0] & 1 != 0' catches all
multicast traffic. The expression `ip[0] & 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[0] 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.
DIAGNOSTICS
Errors from ngrep, libpcap, and the GNU regex
library are all output to stderr.
AUTHOR
Written by Jordan Ritter <jpr5@darkridge.com>.
Examples:
ngrep -qd eth1 'www' tcp port 80
Be quiet, look only at tcp packets with either source
or dest port 80 on interface eth1, look for anything
matching 'www'.
ngrep -qd le0 in-addr port 53
Look at all packets with either source or dest port 53
on interface le0, that match match 'in-addr'. Be quiet.
ngrep 'USER|PASS' tcp port 21
Look only at tcp packets with either source or dest port 21,
look for anything resembling an FTP login.
ngrep -wi 'user|pass' tcp port 21
Look at tcp packets with either source or dest port 21, that
match either 'user' or 'pass' (case insensitively) as a word.
ngrep -wiA 2 'user|pass' tcp port 21
Alternatively, match either 'user' or 'pass' case insensitively,
and dump the next 2 packets following (that match the bpf filter).
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