ip IP
arp arp
tcp tcp
udp udp
icmp icmp
ip multicast IP Multicast
ether multicast Ethernet Multicast
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
udp[0:2] source port
udp[2:2] destination port
udp[4:2] datagram length
udp[6:2] UDP checksum
protocols
ip[9] == 8 EGP
ip[9] == 9 IGP
ip[9] == 88 EIRGP
ip[9] == 50 ESP
ip[9] == 51 AH
ip[9] == 89 OSPF
ip[9] == 124 ISIS
other, see /etc/protocols
Routing Protocols
(udp and port 520) or (host 224.0.0.9) RIP 1 + 2
tcp and port 179 BGP
ip[9] == 8 EGP
ip[9] == 9 IGP
ip[9] == 88 EIRGP
ip[9] == 89 OSPF
ip[9] == 124 ISIS
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
Basic syntax :
==============
Filtering hosts :
-----------------
- Match any traffic involving 192.168.1.1 as destination or source
# tcpdump -i eth1 host 192.168.1.1
- As soure only
# tcpdump -i eth1 src host 192.168.1.1
- As destination only
# tcpdump -i eth1 dst host 192.168.1.1
Filtering ports :
-----------------
- Match any traffic involving port 25 as source or destination
# tcpdump -i eth1 port 25
- Source
# tcpdump -i eth1 src port 25
- Destination
# tcpdump -i eth1 dst port 25
Network filtering :
-------------------
# tcpdump -i eth1 net 192.168
# tcpdump -i eth1 src net 192.168
# tcpdump -i eth1 dst net 192.168
Protocol filtering :
--------------------
# tcpdump -i eth1 arp
# tcpdump -i eth1 ip
# tcpdump -i eth1 tcp
# tcpdump -i eth1 udp
# tcpdump -i eth1 icmp
Let's combine expressions :
---------------------------
Negation : ! or "not" (without the quotes)
Concatanate : && or "and"
Alternate : || or "or"
- This rule will match any TCP traffic on port 80 (web) with 192.168.1.254 or 192.168.1.200 as destination host
# tcpdump -i eth1 '((tcp) and (port 80) and ((dst host 192.168.1.254) or (dst host 192.168.1.200)))'
- Will match any ICMP traffic involving the destination with physical/MAC address 00:01:02:03:04:05
# tcpdump -i eth1 '((icmp) and ((ether dst host 00:01:02:03:04:05)))'
- Will match any traffic for the destination network 192.168 except destination host 192.168.1.200
# tcpdump -i eth1 '((tcp) and ((dst net 192.168) and (not dst host 192.168.1.200)))'
Advanced header filtering :
===========================
Before we continue, we need to know how to filter out info from headers
proto[x:y] : will start filtering from byte x for y bytes. ip[2:2] would filter bytes 3 and 4 (first byte begins by 0)
proto[x:y] & z = 0 : will match bits set to 0 when applying mask z to proto[x:y]
proto[x:y] & z !=0 : some bits are set when applying mask z to proto[x:y]
proto[x:y] & z = z : every bits are set to z when applying mask z to proto[x:y]
proto[x:y] = z : p[x:y] has exactly the bits set to z
Operators : >, <, >=, <=, =, !=
This may not be clear in the first place but you'll find examples below involving these.
Of course, it is important to know what the protocol headers look like before diving into more advanced filters.
IP header
---------
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding | <-- optional
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DATA ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
I'll consider we are only working with the IPv4 protocol suite for these examples.
In an ideal world, every field would fit inside one byte. This is not the case, of course.
Are IP options set ?
--------------------
Let's say we want to know if the IP header has options set. We can't just try to filter out the 21st byte
because if no options are set, data start at the 21st byte. We know a "normal" header is usually 20 bytes
(160 bits) long. With options set, the header is longer than that. The IP header has the header
length field which we will filter here to know if the header is longer than 20 bytes.
+-+-+-+-+-+-+-+-+
|Version| IHL |
+-+-+-+-+-+-+-+-+
Usually the first byte has a value of 01000101 in binary.
Anyhow, we need to divide the first byte in half...
0100 = 4 in decimal. This is the IP version.
0101 = 5 in decimal. This is the number of blocks of 32 bits in the headers. 5 x 32 bits = 160 bits or 20 bytes.
The second half of the first byte would be bigger than 5 if the header had IP options set.
We have two ways of dealing with that kind of filters.
1. Either try to match a value bigger than 01000101. This would trigger matches for IPv4 traffic with IP options set,
but ALSO any IPv6 traffic !
In decimal 01000101 equals 69.
Let's recap how to calculate in decimal.
0 : 0 \
1 : 2^6 = 64 \ First field (IP version)
0 : 0 /
0 : 0 /
-
0 : 0 \
1 : 2^2 = 4 \ Second field (Header length)
0 : 0 /
1 : 2^0 = 1 /
64 + 4 + 1 = 69
The first field in the IP header would usually have a decimal value of 69.
If we had IP options set, we would probably have 01000110 (IPv4 = 4 + header = 6), which in decimal equals 70.
This rule should do the job :
# tcpdump -i eth1 'ip[0] > 69'
Somehow, the proper way is to mask the first half/field of the first byte, because as mentionned earlier,
this filter would match any IPv6 traffic.
2. The proper way : masking the first half of the byte
0100 0101 : 1st byte originally
0000 1111 : mask (0x0f in hex or 15 in decimal). 0 will mask the values while 1 will keep the values intact.
=========
0000 0101 : final result
The correct filter :
# tcpdump -i eth1 'ip[0] & 15 > 5'
or
# tcpdump -i eth1 'ip[0] & 0x0f > 5'
DF bit (don't fragment) set ?
-----------------------------
Let's now trying to know if we have fragmentation occuring, which is not desirable. Fragmentation occurs
when a the MTU of the sender is bigger than the path MTU on the path to destination.
Fragmentation info can be found in the 7th and 8th byte of the IP header.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Bit 0: reserved, must be zero
Bit 1: (DF) 0 = May Fragment, 1 = Don't Fragment.
Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.
The fragment offset field is only used when fragmentation occurs.
If we want to match the DF bit (don't fragment bit, to avoid IP fragmentation) :
The 7th byte would have a value of :
01000000 or 64 in decimal
# tcpdump -i eth1 'ip[6] = 64'
Matching fragmentation ?
------------------------
- Matching MF (more fragment set) ? This would match the fragmented datagrams but wouldn't match the last
fragment (which has the 2nd bit set to 0).
# tcpdump -i eth1 'ip[6] = 32'
The last fragment have the first 3 bits set to 0... but has data in the fragment offset field.
- Matching the fragments and the last fragments
# tcpdump -i eth1 '((ip[6:2] > 0) and (not ip[6] = 64))'
A bit of explanations :
"ip[6:2] > 0" would return anything with a value of at least 1
We don't want datagrams with the DF bit set though.. the reason of the "not ip[6] = 64"
If you want to test fragmentation use something like :
ping -M want -s 3000 192.168.1.1
Matching datagrams with low TTL
-------------------------------
The TTL field is located in the 9th byte and fits perfectly into 1 byte.
The maximum decimal value of the TTL field is thus 255 (11111111 in binary).
This can be verified :
$ ping -M want -s 3000 -t 256 192.168.1.200
ping: ttl 256 out of range
+-+-+-+-+-+-+-+-+
| Time to Live |
+-+-+-+-+-+-+-+-+
We can try to find if someone on our network is using traceroute by using something like this on the gateway :
# tcpdump -i eth1 'ip[8] < 5'
Matching packets longer than X bytes
------------------------------------
Where X is 600 bytes
# tcpdump -i eth1 'ip[2:2] > 600'
More IP filtering
-----------------
We could imagine filtering source and destination addresses directly in decimal addressing.
We could also match the protocol by filtering the 10th byte.
It would be pointless anyhow, because tcpdump makes it already easy to filter out that kind of info.
TCP header
----------
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |C|E|U|A|P|R|S|F| |
| Offset| Res. |W|C|R|C|S|S|Y|I| Window |
| | |R|E|G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- Matching any TCP traffic with a source port > 1024
# tcpdump -i eth1 'tcp[0:2] > 1024'
- Matching TCP traffic with particular flag combinations
The flags are defined in the 14th byte of the TCP header.
+-+-+-+-+-+-+-+-+
|C|E|U|A|P|R|S|F|
|W|C|R|C|S|S|Y|I|
|R|E|G|K|H|T|N|N|
+-+-+-+-+-+-+-+-+
In the TCP 3-way handshakes, the exchange between hosts goes like this :
1. Source sends SYN
2. Destination answers with SYN, ACK
3. Source sends ACK
- If we want to match packets with only the SYN flag set, the 14th byte would have a binary
value of 00000010 which equals 2 in decimal.
# tcpdump -i eth1 'tcp[13] = 2'
- Matching SYN, ACK (00010010 or 18 in decimal)
# tcpdump -i eth1 'tcp[13] = 18'
- Matching either SYN only or SYN-ACK datagrams
# tcpdump -i eth1 'tcp[13] & 2 = 2'
We used a mask here. It will returns anything with the ACK bit set (thus the SYN-ACK combination as well)
Let's assume the following examples (SYN-ACK)
00010010 : SYN-ACK packet
00000010 : mask (2 in decimal)
--------
00000010 : result (2 in decimal)
Every bits of the mask match !
- Matching PSH-ACK packets
# tcpdump -i eth1 'tcp[13] = 24'
- Matching any combination containing FIN (FIN usually always comes with an ACK so we either
need to use a mask or match the combination ACK-FIN)
# tcpdump -i eth1 'tcp[13] & 1 = 1'
- Matching RST flag
# tcpdump -i eth1 'tcp[13] & 4 = 4'
By looking at the TCP state machine diagram (http://www.wains.be/pub/networking/tcp_state_machine.jpg)
we can find the different flag combinations we may want to analyze.
Ideally, a socket in ACK_WAIT mode should not have to send a RST. It means the 3 way handshake has not completed.
We may want to analyze that kind of traffic.
Matching SMTP data :
--------------------
I will make a filter that will match any packet containing the "MAIL" command from SMTP exchanges.
I use something like http://www.easycalculation.com/ascii-hex.php to convert values from ASCII to hexadecimal.
"MAIL" in hex is 0x4d41494c
The rule would be :
# tcpdump -i eth1 '((port 25) and (tcp[20:4] = 0x4d41494c))'
It will check the bytes 21 to 24. "MAIL" is 4 bytes/32 bits long..
This rule would not match packets with IP options set.
This is an example of packet (a spam, of course) :
# tshark -V -i eth0 '((port 25) and (tcp[20:4] = 0x4d41494c))'
Capturing on eth0
Frame 1 (92 bytes on wire, 92 bytes captured)
Arrival Time: Sep 25, 2007 00:06:10.875424000
[Time delta from previous packet: 0.000000000 seconds]
[Time since reference or first frame: 0.000000000 seconds]
Frame Number: 1
Packet Length: 92 bytes
Capture Length: 92 bytes
[Frame is marked: False]
[Protocols in frame: eth:ip:tcp:smtp]
Ethernet II, Src: Cisco_X (00:11:5c:X), Dst: 3Com_X (00:04:75:X)
Destination: 3Com_X (00:04:75:X)
Address: 3Com_X (00:04:75:X)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
Source: Cisco_X (00:11:5c:X)
Address: Cisco_X (00:11:5c:X)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
Type: IP (0x0800)
Internet Protocol, Src: 62.163.X (62.163.X), Dst: 192.168.X (192.168.X)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 78
Identification: 0x4078 (16504)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 118
Protocol: TCP (0x06)
Header checksum: 0x08cb [correct]
[Good: True]
[Bad : False]
Source: 62.163.X (62.163.X)
Destination: 192.168.X (192.168.XX)
Transmission Control Protocol, Src Port: 4760 (4760), Dst Port: smtp (25), Seq: 0, Ack: 0, Len: 38
Source port: 4760 (4760)
Destination port: smtp (25)
Sequence number: 0 (relative sequence number)
[Next sequence number: 38 (relative sequence number)]
Acknowledgement number: 0 (relative ack number)
Header length: 20 bytes
Flags: 0x18 (PSH, ACK)
0... .... = Congestion Window Reduced (CWR): Not set
.0.. .... = ECN-Echo: Not set
..0. .... = Urgent: Not set
...1 .... = Acknowledgment: Set
.... 1... = Push: Set
.... .0.. = Reset: Not set
.... ..0. = Syn: Not set
.... ...0 = Fin: Not set
Window size: 17375
Checksum: 0x6320 [correct]
[Good Checksum: True]
[Bad Checksum: False]
Simple Mail Transfer Protocol
Command: MAIL FROM:
Command: MAIL
Request parameter: FROM:
Matching HTTP data :
--------------------
Let's make a filter that will find any packets containing GET requests
The HTTP request will begin by :
GET / HTTP/1.1\r\n (16 bytes counting the carriage return but not the backslashes !)
If no IP options are set.. the GET command will use the byte 20, 21 and 22
Tcpdump is only able to match data size of either 1, 2 or 4 bytes, we will take the following ASCII
character following the GET command (a space)
"GET " in hex : 47455420
# tcpdump -i eth1 'tcp[20:4] = 0x47455420'
Matching other interesting TCP things :
---------------------------------------
SSH connection (on any port) :
We will be looking for the reply given by the SSH server.
OpenSSH usually replies with something like "SSH-2.0-OpenSSH_3.6.1p2".
The first 4 bytes (SSH-) have an hex value of 0x5353482D.
# tcpdump -i eth1 'tcp[(tcp[12]>>2):4] = 0x5353482D'
If we want to find any connection made to older version of OpenSSH (version 1, which are insecure and subject to MITM attacks) :
The reply from the server would be something like "SSH-1.99.."
# tcpdump -i eth1 '(tcp[(tcp[12]>>2):4] = 0x5353482D) and (tcp[((tcp[12]>>2)+4):2] = 0x312E)'
UDP header
----------
0 7 8 15 16 23 24 31
+--------+--------+--------+--------+
| Source | Destination |
| Port | Port |
+--------+--------+--------+--------+
| | |
| Length | Checksum |
+--------+--------+--------+--------+
| |
| DATA ... |
+-----------------------------------+
Nothing really interesting here.
If we want to filter ports we would use something like :
# tcpdump -i eth1 udp dst port 53
ICMP header
-----------
See different ICMP messages :
http://img292.imageshack.us/my.php?image=icmpmm6.gif
We will usually filter the type (1 byte) and code (1 byte) of the ICMP messages.
Here are common ICMP types :
0 Echo Reply [RFC792]
3 Destination Unreachable [RFC792]
4 Source Quench [RFC792]
5 Redirect [RFC792]
8 Echo [RFC792]
11 Time Exceeded [RFC792]
We may want to filter ICMP messages type 4, these kind of messages are sent in case of congestion of the network.
# tcpdump -i eth1 'icmp[0] = 4'
If we want to find the ICMP echo replies only, having an ID of 500. By looking at the image with all the ICMP packet description
we see the ICMP echo reply have the ID spread across the 5th and 6th byte. For some reason, we have to filter out with the value in hex.
# tcpdump -i eth0 '(icmp[0] = 0) and (icmp[4:2] = 0x1f4)'
TOS (Dec) TOS (Hex) TOS (Bin) TOS Precedence (Bin) TOS Precedence (Dec) TOS Precedence Name TOS Delay flag TOS Throughput flag TOS Reliability flag DSCP (Bin) DSCP (Hex) DSCP (Dec) DSCP/PHB Class
0 0x00 00000000 000 0 Routine 0 0 0 000000 0x00 0 none
32 0x20 00100000 001 1 Priority 0 0 0 001000 0x08 8 cs1
40 0x28 00101000 001 1 Priority 0 1 0 001010 0x0A 10 af11
48 0x30 00110000 001 1 Priority 1 0 0 001100 0x0C 12 af12
56 0x38 00111000 001 1 Priority 1 1 0 001110 0x0E 14 af13
64 0x40 01000000 010 2 Immediate 0 0 0 010000 0x10 16 cs2
72 0x48 01001000 010 2 Immediate 0 1 0 010010 0x12 18 af21
80 0x50 01010000 010 2 Immediate 1 0 0 010100 0x14 20 af22
88 0x58 01011000 010 2 Immediate 1 1 0 010110 0x16 22 af23
96 0x60 01100000 011 3 Flash 0 0 0 011000 0x18 24 cs3
104 0x68 01101000 011 3 Flash 0 1 0 011010 0x1A 26 af31
112 0x70 01110000 011 3 Flash 1 0 0 011100 0x1C 28 af32
120 0x78 01111000 011 3 Flash 1 1 0 011110 0x1E 30 af33
128 0x80 10000000 100 4 FlashOverride 0 0 0 100000 0x20 32 cs4
136 0x88 10001000 100 4 FlashOverride 0 1 0 100010 0x22 34 af41
144 0x90 10010000 100 4 FlashOverride 1 0 0 100100 0x24 36 af42
152 0x98 10011000 100 4 FlashOverride 1 1 0 100110 0x26 38 af43
160 0xA0 10100000 101 5 Critical 0 0 0 101000 0x28 40 cs5
184 0xB8 10111000 101 5 Critical 1 1 0 101110 0x2E 46 ef
192 0xC0 11000000 110 6 InterNetworkControl 0 0 0 110000 0x30 48 cs6
224 0xE0 11100000 111 7 NetworkControl 0 0 0 111000 0x38 56 cs7
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