packetlevel.ch

common Filters

IP Filters

ip[0] & 0x0f	low nibble: header length in 4octet words. should be 5
ip[1]	type of service/QoS/DiffServ
ip[2:2]	total length of datagram in octets
ip[4:2]	IP ID number
ip[6] & 0x80	reserved bit (possibly used for ECN)
ip[6] & 0x40	DF bit
ip[6] & 0x20	MF bit
ip[6:2] & 0x1fff	fragment offset (number of 8octet blocks)
ip[8]	ttl
ip[9]	protocol
ip[10:2]	header checksum
ip[12:4]	source IP
ip[16:4]	destination IP
Samples
(ip[12:4] = ip[16:4])	Src IP = Dest IP (land attack)
ip[0] & 0xf0	high nibble: IP version. almost always 4
(ip[0] & 0xf0 != 0x40)	IP versions !=4
(ip[0:1] & 0x0f > 5)	IP with options set
(ip[19] = 0xff)	Broadcasts to x.x.x.255
(ip[19] = 0x00)	Broadcasts to x.x.x.0
(ip and ip[1] & 0xfc == 0xb8)	search for EF in DSCP
(ip and ip[1] & 0xfc == 0x28)	search for AF11 in DSCP
(ip and ip[1] & 0xfc != 0x00)	search for DCSP Packets != 0
(ip[6] & 0x20 != 0) && (ip[6:2] & 0x1fff = 0)	initial fragments
(ip[6] & 0x20 != 0) && (ip[6:2] & 0x1fff != 0)	intervening fragments
(ip[6] & 0x20 = 0) && (ip[6:2] & 0x1fff != 0)	terminal fragments
(ip[0] & 0x0f) != 5	has ip options (or is truncated, or is just some sort of freak...)
ip[8] < 5	short TTL value
ip[6] = 32	MF set
iip[2:2] > 999	IP Packet greater then 999

ICMP Filters

icmp[0]	type
icmp[1]	code
icmp[2:2]	checksum
Samples
icmp[0]=0x#	all Packets with ICMP Type
icmp[0]=0x# and icmp[1]=0x#	all Packets with ICMP Type X and Code = Y
icmp[0]=8	ICMP Request Messages
icmp[8]=0	ICMP Request Replay
icmp[0]=0x11	ICMP Address Mask Request
icmp[0]=0x12	ICMP Address Mask Replay
icmp[0]=11 and icmp[1]=0	ICMP Time Exeedet
icmp[0]=3 and icmp[1]=4	ICMP Time Exeedet
icmp[0]=8 and ip[2:2] > 64	Large ICMP Packets

TCP Filters

tcp[0:2]	source port
tcp[2:2]	destination port
tcp[4:4]	sequence number
tcp[8:4]	ack number
tcp[12]	header length
tcp[13]	tcp flags
	---- --S- 0000 0010 = 0x02 normal syn
	---A --S- 0001 0010 = 0x12 normal syn-ack
	---A ---- 0001 0000 = 0x10 normal ack
	--UA P--- 0011 1000 = 0x38 psh-urg-ack. interactive stuff like ssh
	---A -R-- 0001 0100 = 0x14 rst-ack. it happens.
	---- --SF 0000 0011 = 0x03 syn-fin scan
	--U- P--F 0010 1001 = 0x29 urg-psh-fin. nmap fingerprint packet
	-Y-- ---- 0100 0000 = 0x40 anything >= 0x40 has a reserved bit set
	XY-- ---- 1100 0000 = 0xC0 both reserved bits set
	XYUA PRSF 1111 1111 = 0xFF FULL_XMAS scan
tcp[14:2]	window size
tcp[16:2]	checksumt
tcp[18:2]	urgent pointer
Samples
tcp[13] = 0x02	is SYN. nothing else.
(tcp[13] & 0x02) != 0	contains SYN. we don't care what else...
(tcp[13] & 0x03) = 3	is some kind of SYN-FIN. realy Bad
	winnuke (not tested)
tcp[20:4] = 0x47455420	GET in request

UDP Filters

protocols

Routing Protocols

ether Filters

ether[20:2] == 0x2000	CDP pakets
ether[12:2] == 0x0806	ARP pakets

IPv6

ip6	filters native IPv6 traffic (including ICMPv6)
icmp6	filters native ICMPv6 traffic
proto ipv6	filters tunneled IPv6-in-IPv4 traffic
TCP
ip6 and (ip6[6] == 0x06)	IPv6 TCP
ip6 and (ip6[6] == 0x06) and (ip6[53] == 0x02)	IPv6 TCP Syn
ip6 and (ip6[6] == 0x06) and (ip6[53] == 0x10)	IPv6 TCP ACK
ip6 and (ip6[6] == 0x06) and (ip6[53] == 0x12)	IPv6 TCP Syn/ACK
UDP
ip6 and (ip6[6] == 0x11)	IPv6 TCP
ICMP
(ip6[6] == 0x3a)	ICMP v6
(ip6[6] == 0x3a) and (ip6[40] == 0x01)	ipv6 and type 1 Dest Unreachable
(ip6[6] == 0x3a) and (ip6[40] == 0x02)	ipv6 and type 2 Packet too big
(ip6[6] == 0x3a) and (ip6[40] == 0x03)	ipv6 and type 3 Time Exeedet
(ip6[6] == 0x3a) and (ip6[40] == 0x04)	ipv6 and type 4 Parameter Problem
(ip6[6] == 0x3a) and (ip6[40] == 0x80)	ipv6 and type 128 Echo Request
(ip6[6] == 0x3a) and (ip6[40] == 0x81)	ipv6 and type 129 Echo Reply
(ip6[6] == 0x3a) and (ip6[40] == 0x86)	ipv6 and type 133 Router Solicitation
(ip6[6] == 0x3a) and (ip6[40] == 0x87)	ipv6 and type 134 Router Advertisement
(ip6[6] == 0x3a) and (ip6[40] == 0x88)	ipv6 and type 135 Neighbor Solicitation
(ip6[6] == 0x3a) and (ip6[40] == 0x89)	ipv6 and type 136 Neighbor Advertisement

MY Filters

tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

IP broadcast or multicast packets that were not sent via ethernet broadcast or multicast:

More Infos

byte_offsets.txt
tcpdump.filters