8.3. IP Keywords

8.3.1. ttl

The ttl keyword is used to check for a specific IP time-to-live value in the header of a packet. The format is:

ttl:<number>;

For example:

ttl:10;

ttl uses an unsigned 8-bit integer.

At the end of the ttl keyword you can enter the value on which you want to match. The Time-to-live value determines the maximal amount of time a packet can be in the Internet-system. If this field is set to 0, then the packet has to be destroyed. The time-to-live is based on hop count. Each hop/router the packet passes subtracts one from the packet TTL counter. The purpose of this mechanism is to limit the existence of packets so that packets can not end up in infinite routing loops.

Example of the ttl keyword in a rule:

alert ip $EXTERNAL_NET any -> $HOME_NET any (msg:"IP Packet With TTL 0"; ttl:0; classtype:misc-activity; sid:1; rev:1;)

8.3.2. ipopts

With the ipopts keyword you can check if a specific IP option is set. Ipopts has to be used at the beginning of a rule. You can only match on one option per rule. There are several options on which can be matched. These are:

IP Option

Description

rr

Record Route

eol

End of List

nop

No Op

ts

Time Stamp

sec

IP Security

esec

IP Extended Security

lsrr

Loose Source Routing

ssrr

Strict Source Routing

satid

Stream Identifier

any

any IP options are set

Format of the ipopts keyword:

ipopts: <name>;

For example:

ipopts: ts;

Example of ipopts in a rule:

alert ip $EXTERNAL_NET any -> $HOME_NET any (msg:"IP Packet with timestamp option"; ipopts:ts; classtype:misc-activity; sid:2; rev:1;)

8.3.3. sameip

Every packet has a source IP-address and a destination IP-address. It can be that the source IP is the same as the destination IP. With the sameip keyword you can check if the IP address of the source is the same as the IP address of the destination. The format of the sameip keyword is:

sameip;

Example of sameip in a rule:

alert ip any any -> any any (msg:"IP Packet with the same source and destination IP"; sameip; classtype:bad-unknown; sid:3; rev:1;)

8.3.4. ip_proto

With the ip_proto keyword you can match on the IP protocol in the packet-header. You can use the name or the number of the protocol. You can match for example on the following protocols:

 1     ICMP        Internet Control Message
 6     TCP         Transmission Control Protocol
17     UDP         User Datagram
47     GRE         General Routing Encapsulation
50     ESP         Encap Security Payload for IPv6
51     AH          Authentication Header for Ipv6
58     IPv6-ICMP   ICMP for Ipv6

For the complete list of protocols and their numbers see http://en.wikipedia.org/wiki/List_of_IP_protocol_numbers

Example of ip_proto in a rule:

alert ip any any -> any any (msg:"IP Packet with protocol 1"; ip_proto:1; classtype:bad-unknown; sid:5; rev:1;)

The named variant of that example would be:

ip_proto:ICMP;

8.3.5. ipv4.hdr

Sticky buffer to match on content contained within an IPv4 header.

Example rule:

alert ip any any -> any any (msg:"IPv4 header keyword example"; ipv4.hdr; content:"|06|"; offset:9; depth:1; sid:1; rev:1;)

This example looks if byte 10 of IPv4 header has value 06, which indicates that the IPv4 protocol is TCP.

8.3.6. ipv6.hdr

Sticky buffer to match on content contained within an IPv6 header.

Example rule:

alert ip any any -> any any (msg:"IPv6 header keyword example"; ipv6.hdr; content:"|06|"; offset:6; depth:1; sid:1; rev:1;)

This example looks if byte 7 of IP64 header has value 06, which indicates that the IPv6 protocol is TCP.

8.3.7. id

With the id keyword, you can match on a specific IP ID value. The ID identifies each packet sent by a host and increments usually with one with each packet that is being send. The IP ID is used as a fragment identification number. Each packet has an IP ID, and when the packet becomes fragmented, all fragments of this packet have the same ID. In this way, the receiver of the packet knows which fragments belong to the same packet. (IP ID does not take care of the order, in that case offset is used. It clarifies the order of the fragments.)

Format of id:

id:<number>;

Example of id in a rule:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"id keyword example"; id:1; content:"content|3a 20|"; fast_pattern; classtype:misc-activity; sid:12; rev:1;)

8.3.8. geoip

The geoip keyword enables matching on the source, destination or source and destination IPv4 addresses of network traffic, and to see to which country it belongs. To be able to do this, Suricata uses the GeoIP2 API of MaxMind.

The syntax of geoip:

geoip: src,RU;
geoip: both,CN,RU;
geoip: dst,CN,RU,IR;
geoip: both,US,CA,UK;
geoip: any,CN,IR;

Option

Description

both

Both source and destination have to match with the given geoip(s)

any

Either the source or the destination has to match with the given geoip(s).

dest

The destination matches with the given geoip.

src

The source matches with the given geoip.

geoip currently only supports IPv4. As it uses the GeoIP2 API of MaxMind, libmaxminddb must be compiled in. You must download and install the GeoIP2 or GeoLite2 database editions desired. Visit the MaxMind site at https://dev.maxmind.com/geoip/geolite2-free-geolocation-data for details.

You must also supply the location of the GeoIP2 or GeoLite2 database file on the local system in the YAML-file configuration (for example):

geoip-database: /usr/local/share/GeoIP/GeoLite2-Country.mmdb

8.3.9. fragbits (IP fragmentation)

With the fragbits keyword, you can check if the fragmentation and reserved bits are set in the IP header. The fragbits keyword should be placed at the beginning of a rule. Fragbits is used to modify the fragmentation mechanism. During routing of messages from one Internet module to the other, it can occur that a packet is bigger than the maximal packet size a network can process. In that case, a packet can be send in fragments. This maximum of the packet size is called Maximal Transmit Unit (MTU).

You can match on the following bits:

M - More Fragments
D - Do not Fragment
R - Reserved Bit

Matching on this bits can be more specified with the following modifiers:

+         match on the specified bits, plus any others
*         match if any of the specified bits are set
!         match if the specified bits are not set

Format:

fragbits:[*+!]<[MDR]>;

Example of fragbits in a rule:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"fragbits keyword example non-fragmented packet with fragment offset>0"; fragbits:M; fragoffset:>0; classtype:bad-unknown; sid:123; rev:1;)

8.3.10. fragoffset

With the fragoffset keyword you can match on specific decimal values of the IP fragment offset field. If you would like to check the first fragments of a session, you have to combine fragoffset 0 with the More Fragment option. The fragmentation offset field is convenient for reassembly. The id is used to determine which fragments belong to which packet and the fragmentation offset field clarifies the order of the fragments.

You can use the following modifiers:

<       match if the value is smaller than the specified value
>       match if the value is greater than the specified value
!       match if the specified value is not present

Format of fragoffset:

fragoffset:[!|<|>]<number>;

Example of fragoffset in a rule:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"fragoffset keyword example invalid non-fragmented packet with fragment offset>0"; fragbits:M; fragoffset:>0; classtype:bad-unknown; sid:13; rev:1;)

8.3.11. tos

The tos keyword can match on specific decimal values of the IP header TOS field. The tos keyword can have a value from 0 - 255. This field of the IP header has been updated by rfc2474 to include functionality for Differentiated services. Note that the value of the field has been defined with the right-most 2 bits having the value 0. When specifying a value for tos, ensure that the value follows this.

E.g, instead of specifying the decimal value 34 (hex 22), right shift twice and use decimal 136 (hex 88).

You can specify hexadecimal values with a leading x, e.g, x88.

Format of tos:

tos:[!]<number>;

Example of tos in a rule:

alert ip any any -> any any (msg:"tos keyword example tos value 8"; flow:established; tos:8; classtype:not-suspicious; sid:123; rev:1;)

Example of tos with a negated value:

alert ip any any -> any any (msg:"tos keyword example with negated content"; flow:established,to_server; tos:!8; classtype:bad-unknown; sid:14; rev:1;)

8.4. TCP keywords

8.4.1. tcp.flags

The tcp.flags keyword checks for specific TCP flag bits.

The following flag bits may be checked:

Flag

Description

F

FIN - Finish

S

SYN - Synchronize sequence numbers

R

RST - Reset

P

PSH - Push

A

ACK - Acknowledgment

U

URG - Urgent

C

CWR - Congestion Window Reduced

E

ECE - ECN-Echo

0

No TCP Flags Set

The following modifiers can be set to change the match criteria:

Modifier

Description

+

match on the bits, plus any others

*

match if any of the bits are set

!

match if the bits are not set

To handle writing rules for session initiation packets such as ECN where a SYN packet is sent with CWR and ECE flags set, an option mask may be used by appending a comma and masked values. For example, a rule that checks for a SYN flag, regardless of the values of the reserved bits is tcp.flags:S,CE;

Format of tcp.flags:

tcp.flags:[modifier]<test flags>[,<ignore flags>];
tcp.flags:[!|*|+]<FSRPAUCE0>[,<FSRPAUCE>];

Example:

 alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"Example tcp.flags sig"; \
:example-rule-emphasis:`tcp.flags:FPU,CE;` classtype:misc-activity; sid:1; rev:1;)

It is also possible to use the tcp.flags content as a fast_pattern by using the prefilter keyword. For more information on prefilter usage see Prefiltering Keywords

Example:

 alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"Example tcp.flags sig"; \
:example-rule-emphasis:`tcp.flags:FPU,CE; prefilter;` classtype:misc-activity; sid:1; rev:1;)

8.4.2. seq

The seq keyword can be used in a signature to check for a specific TCP sequence number. A sequence number is a number that is generated practically at random by both endpoints of a TCP-connection. The client and the server both create a sequence number, which increases with one with every byte that they send. So this sequence number is different for both sides. This sequence number has to be acknowledged by both sides of the connection. Through sequence numbers, TCP handles acknowledgement, order and retransmission. Its number increases with every data-byte the sender has send. The seq helps keeping track of to what place in a data-stream a byte belongs. If the SYN flag is set at 1, than the sequence number of the first byte of the data is this number plus 1 (so, 2).

Example:

seq:0;

Example of seq in a signature:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"GPL SCAN NULL"; flow:stateless; ack:0; flags:0; seq:0; reference:arachnids,4; classtype:attempted-recon; sid:2100623; rev:7;)

Example of seq in a packet (Wireshark):

../_images/Wireshark_seq.png

8.4.3. ack

The ack is the acknowledgement of the receipt of all previous (data)-bytes send by the other side of the TCP-connection. In most occasions every packet of a TCP connection has an ACK flag after the first SYN and a ack-number which increases with the receipt of every new data-byte. The ack keyword can be used in a signature to check for a specific TCP acknowledgement number.

Format of ack:

ack:1;

Example of ack in a signature:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"GPL SCAN NULL"; flow:stateless; ack:0; flags:0; seq:0; reference:arachnids,4; classtype:attempted-recon; sid:2100623; rev:7;)

Example of ack in a packet (Wireshark):

../_images/Wireshark_ack.png

8.4.4. window

The window keyword is used to check for a specific TCP window size. The TCP window size is a mechanism that has control of the data-flow. The window is set by the receiver (receiver advertised window size) and indicates the amount of bytes that can be received. This amount of data has to be acknowledged by the receiver first, before the sender can send the same amount of new data. This mechanism is used to prevent the receiver from being overflowed by data. The value of the window size is limited and can be 2 to 65.535 bytes. To make more use of your bandwidth you can use a bigger TCP-window.

The format of the window keyword:

window:[!]<number>;

Example of window in a rule:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"GPL DELETED typot trojan traffic"; flow:stateless; flags:S,12; window:55808; reference:mcafee,100406; classtype:trojan-activity; sid:2182; rev:8;)

8.4.5. tcp.mss

Match on the TCP MSS option value. Will not match if the option is not present.

tcp.mss uses an unsigned 16-bit integer.

The format of the keyword:

tcp.mss:<min>-<max>;
tcp.mss:[<|>]<number>;
tcp.mss:<value>;

Example rule:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (flow:stateless; flags:S,12; tcp.mss:<536; sid:1234; rev:5;)

8.4.6. tcp.hdr

Sticky buffer to match on the whole TCP header.

Example rule:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (flags:S,12; tcp.hdr; content:"|02 04|"; offset:20; byte_test:2,<,536,0,big,relative; sid:1234; rev:5;)

This example starts looking after the fixed portion of the header, so into the variable sized options. There it will look for the MSS option (type 2, option len 4) and using a byte_test determine if the value of the option is lower than 536. The tcp.mss option will be more efficient, so this keyword is meant to be used in cases where no specific keyword is available.

8.5. UDP keywords

8.5.1. udp.hdr

Sticky buffer to match on the whole UDP header.

Example rule:

alert udp any any -> any any (udp.hdr; content:"|00 08|"; offset:4; depth:2; sid:1234; rev:5;)

This example matches on the length field of the UDP header. In this case the length of 8 means that there is no payload. This can also be matched using dsize:0;.

8.6. ICMP keywords

ICMP (Internet Control Message Protocol) is a part of IP. IP at itself is not reliable when it comes to delivering data (datagram). ICMP gives feedback in case problems occur. It does not prevent problems from happening, but helps in understanding what went wrong and where. If reliability is necessary, protocols that use IP have to take care of reliability themselves. In different situations ICMP messages will be send. For instance when the destination is unreachable, if there is not enough buffer-capacity to forward the data, or when a datagram is send fragmented when it should not be, etcetera. More can be found in the list with message-types.

There are four important contents of a ICMP message on which can be matched with corresponding ICMP-keywords. These are: the type, the code, the id and the sequence of a message.

8.6.1. itype

The itype keyword is for matching on a specific ICMP type (number). ICMP has several kinds of messages and uses codes to clarify those messages. The different messages are distinct by different names, but more important by numeric values. For more information see the table with message-types and codes.

itype uses an unsigned 8-bit integer.

The format of the itype keyword:

itype:min<>max;
itype:[<|>]<number>;

Example This example looks for an ICMP type greater than 10:

itype:>10;

Example of the itype keyword in a signature:

alert icmp $EXTERNAL_NET any -> $HOME_NET any (msg:"GPL SCAN Broadscan Smurf Scanner"; dsize:4; icmp_id:0; icmp_seq:0; itype:8; classtype:attempted-recon; sid:2100478; rev:4;)

The following lists all ICMP types known at the time of writing. A recent table can be found at the website of IANA

ICMP Type

Name

0

Echo Reply

3

Destination Unreachable

4

Source Quench

5

Redirect

6

Alternate Host Address

8

Echo

9

Router Advertisement

10

Router Solicitation

11

Time Exceeded

12

Parameter Problem

13

Timestamp

14

Timestamp Reply

15

Information Request

16

Information Reply

17

Address Mask Request

18

Address Mask Reply

30

Traceroute

31

Datagram Conversion Error

32

Mobile Host Redirect

33

IPv6 Where-Are-You

34

IPv6 I-Am-Here

35

Mobile Registration Request

36

Mobile Registration Reply

37

Domain Name Request

38

Domain Name Reply

39

SKIP

40

Photuris

41

Experimental mobility protocols such as Seamoby

8.6.2. icode

With the icode keyword you can match on a specific ICMP code. The code of a ICMP message clarifies the message. Together with the ICMP-type it indicates with what kind of problem you are dealing with. A code has a different purpose with every ICMP-type.

icode uses an unsigned 8-bit integer.

The format of the icode keyword:

icode:min<>max;
icode:[<|>]<number>;

Example: This example looks for an ICMP code greater than 5:

icode:>5;

Example of the icode keyword in a rule:

alert icmp $HOME_NET any -> $EXTERNAL_NET any (msg:"GPL MISC Time-To-Live Exceeded in Transit"; icode:0; itype:11; classtype:misc-activity; sid:2100449; rev:7;)

The following lists the meaning of all ICMP types. When a code is not listed, only type 0 is defined and has the meaning of the ICMP code, in the table above. A recent table can be found at the website of IANA

ICMP Code

ICMP Type

Description

3

0

Net Unreachable

1

Host Unreachable

2

Protocol Unreachable

3

Port Unreachable

4

Fragmentation Needed and Don't Fragment was Set

5

Source Route Failed

6

Destination Network Unknown

7

Destination Host Unknown

8

Source Host Isolated

9

Communication with Destination Network is Administratively Prohibited

10

Communication with Destination Host is Administratively Prohibited

11

Destination Network Unreachable for Type of Service

12

Destination Host Unreachable for Type of Service

13

Communication Administratively Prohibited

14

Host Precedence Violation

15

Precedence cutoff in effect

5

0

Redirect Datagram for the Network (or subnet)

1

Redirect Datagram for the Host

2

Redirect Datagram for the Type of Service and Network

3

Redirect Datagram for the Type of Service and Host

9

0

Normal router advertisement

16

Doesn't route common traffic

11

0

Time to Live exceeded in Transit

1

Fragment Reassembly Time Exceeded

12

0

Pointer indicates the error

1

Missing a Required Option

2

Bad Length

40

0

Bad SPI

1

Authentication Failed

2

Decompression Failed

3

Decryption Failed

4

Need Authentication

5

Need Authorization

8.6.3. icmp_id

With the icmp_id keyword you can match on specific ICMP id-values. Every ICMP-packet gets an id when it is being send. At the moment the receiver has received the packet, it will send a reply using the same id so the sender will recognize it and connects it with the correct ICMP-request.

Format of the icmp_id keyword:

icmp_id:<number>;

Example: This example looks for an ICMP ID of 0:

icmp_id:0;

Example of the icmp_id keyword in a rule:

alert icmp $EXTERNAL_NET any -> $HOME_NET any (msg:"GPL SCAN Broadscan Smurf Scanner"; dsize:4; icmp_id:0; icmp_seq:0; itype:8; classtype:attempted-recon; sid:2100478; rev:4;)

8.6.4. icmp_seq

You can use the icmp_seq keyword to check for a ICMP sequence number. ICMP messages all have sequence numbers. This can be useful (together with the id) for checking which reply message belongs to which request message.

Format of the icmp_seq keyword:

icmp_seq:<number>;

Example: This example looks for an ICMP Sequence of 0:

icmp_seq:0;

Example of icmp_seq in a rule:

alert icmp $EXTERNAL_NET any -> $HOME_NET any (msg:"GPL SCAN Broadscan Smurf Scanner"; dsize:4; icmp_id:0; icmp_seq:0; itype:8; classtype:attempted-recon; sid:2100478; rev:4;)

8.6.5. icmpv4.hdr

Sticky buffer to match on the whole ICMPv4 header.

8.6.6. icmpv6.hdr

Sticky buffer to match on the whole ICMPv6 header.

8.6.7. icmpv6.mtu

Match on the ICMPv6 MTU optional value. Will not match if the MTU is not present.

icmpv6.mtu uses an unsigned 32-bit integer.

The format of the keyword:

icmpv6.mtu:<min>-<max>;
icmpv6.mtu:[<|>]<number>;
icmpv6.mtu:<value>;

Example rule:

alert ip $EXTERNAL_NET any -> $HOME_NET any (icmpv6.mtu:<1280; sid:1234; rev:5;)