Netfilter Log Format

Here is a quick reference for the format used by the netfilter log messages.   This is all derived from the source of the netfilter kernel modules (Linux kernel 2.4.2).

Below is a hypothetical log message generated by netfilter. It is based on a real log entry but I have added all possible IP and TCP flags as well as a fragment offset for illustrative purposes.

Apr 16 00:30:45 megahard kernel:

NF: D(I,Priv) IN=eth1

OUT=

MAC=00:80:8c:1e:12:60:00:10:76:00:2f:c2:08:00

SRC=211.251.142.65

DST=203.164.4.223

LEN=60

TOS=0x00

PREC=0x00

TTL=44

ID=31526

CE

DF

MF

FRAG=179

OPT (072728CBA404DFCBA40253CBA4032ECBA403A2CBA4033ECBA40 2C1180746EA18074C52892734A200)

PROTO=TCP

SPT=4515

DPT=111

SEQ=1168094040

ACK=0

WINDOW=32120

RES=0x03

URG

ACK

PSH

RST

SYN

FIN

URGP=0

OPT (020405B40402080A05E3F3C40000000001030300)

The items are explained in sequence:

Apr 16 00:30:45 megahard kernel:	syslog prefix. It is not present if you read log messages from the console.
NF: D(I,Priv)	Enabled with: --log-prefix 'prefix' An arbitrary, user defined log prefix. Including the spaces. A trailing space is necessary to keep the prefix separate from the next token; this is a bug in netfilter.
IN=eth1	Interface the packet was received from. Empty value for locally generated packets.
OUT=	Interface the packet was sent to. Empty value for locally received packets.
MAC= 00:80:8c:1e:12:60: 00:10:76:00:2f:c2: 08:00	Destination MAC=00:80:8c:1e:12:60, Source MAC=00:10:76:00:2f:c2, Type=08:00 (ethernet frame carried an IPv4 datagram)
SRC=211.251.142.65	Source IP address
DST=203.164.4.223	Destination IP address
LEN=60	Total length of IP packet in bytes
TOS=0x00	Type Of Service, “Type” field. Increasingly being replaced by DS and ECN.
PREC=0x00	Type Of Service, “Precedence” field. Increasingly being replaced by DS and ECN.
TTL=44	remaining Time To Live is 44 hops.
ID=31526	Unique ID for this IP datagram, shared by all fragments if fragmented.
CE	Presumably the “ECN CE” flag (Congestion Experienced). This seems to be wrong because according to RFC2481, the CE bit is located in the TOS field.
DF	“Don’t Fragment” flag.
MF	“More Fragments following” flag.
FRAG=179	Fragment offset in units of “8-bytes”. In this case the byte offset for data in this packet is 179*8=1432 bytes.
OPT (0727..A200)	Enabled with: --log-ip-options IP options. This variable length field is rarely used. Certain IP options, f.e. source routing, are often disallowed by netadmins. Even harmless options like “Record Route” may only be allowed if the transport protocol is ICMP, or not at all.
PROTO=TCP	Protocol name or number. Netfilter uses names for TCP, UDP, ICMP, AH and ESP. Other protocols are identified by number. A list is in your /etc/protocols.  A complete list is in the file protocol-numbers
SPT=4515	Source port (TCP and UDP).  A list of port numbers is in your /etc/services.  A complete list is in the file port-numbers
DPT=111	Destination port (TCP and UDP).  See SPT above.
SEQ=1168094040	Enabled with: --log-tcp-sequence Receive Sequence number. By cleverly chosing this number, a cryptographic “cookie” can be implemented while still satisfying TCP protocol requirements. These “SYN-cookies” defeat some types of SYN-flooding DoS attacks and should be enabled on all systems running public TCP servers. echo 1 > /proc/sys/net/ipv4/tcp_syncookies
ACK=0	Same as the Receive Sequence number, but for the other end of the TCP connection.
WINDOW=32120	The TCP Receive Window size. This may be scaled by bit-shifting left by a number of bits specified in the “Window Scale” TCP option. If the host supports ECN, then the TCP Receive Window size will also be controlled by that.
RES=0x03	Reserved bits. The ECN flags “CWR” and “ECNE” will show up in the two least significant bits of this field.
URG	Urgent flag.   See URGP below.
ACK	Acknowledgement flag.
PSH	Push flag.
RST	RST (Reset) flag.
SYN	SYN flag, only exchanged at TCP connection establishment.
FIN	FIN flag, only exchanged at TCP disconnection.
URGP=0	The Urgent Pointer allows for urgent, “out of band” data transfer. Unfortunately not all protocol implementations agree, so this facility is hardly ever used.
OPT (020405...300)	enabled with: --log-tcp-options TCP options. This variable length field gets a lot of use. Important options include: Window Scaling, Selective Acknowledgement and Explicit Congestion Notification.
	Unfortunately the rule number in the chain which matched the packet is for architectural reasons not available in netfilter logs. You will have to “cook your own” by using the user-prefix feature.

More interesting files, such as multicast-addresses, can be found in http://www.iana.org/protocols/.

Issues with netfilter log format

Protocol Header Information

IP Header Format as defined in RFC-791:

0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
IP Version				Hdr.Length				TOS / DS,ECN								Total Length
Identification																–	DF	MF	Fragment Offset
Time To Live								Protocol Number								Header Checksum
32 bit Source Address
32 bit Destination Address
Options (0 to 10 Words of 32 Bits)
IP Payload (including header of heigher protocol)

The header of an IP packet consists of 5 or more words of 32 bits (4 bytes) each.  The minimum header length (no options) is therefore 20 bytes.  The Version field for the shown type of packet is 4 = IPv4 (Internet Protocol version 4).  The header Length field is the header length in 32bit words, this would be 5 without options, and at most 15 with options.  The Total Length is in bytes and includes the header.  Data length can then be calculated from the supplied values.

TOS / DS / ECN:   This field has had an unstable history.  This is briefly explained in RFC2481, section 19 (near the end).

	0	1	2	3	4	5	6	7
TOS	Precedence			Type				–
DS,ECN	DS Codepoint						ECT	CE

Many sites are starting to implement Differentiated Services DS [RFC2474] in their routers. DS uses code-points which are stored in bits 0 to 5 of the old TOS field. The content and meaning of this field can change at network boundaries.

If the host is ECN [RFC2481] capable and the payload is a TCP packet, then up to two flag bits will be needed in the old TOS field. Bit 6 becomes the ECT (ECN-capable Transport) flag, and Bit 7 becomes the CE (Congestion Experienced) flag.

IP datagrams can be fragmented if the link layer cannot fit it into a single link layer data unit. The fragment offset is specified in units of 8-bytes, thus allowing the available 13 bits to cover the necessary values for up to 64K of data.

IP packets usually carry a higher level protocol such as TCP.  In the case of TCP, the PROTO field would be set to 6 and the  TCP Protocol Data Unit (PDU)  is carried in the IP Payload field of the packet.  See below.

TCP Header Format (as defined in RFC-793):

The header of a TCP packet consists of 5 or more words of 32 bits (4 bytes) each.  The minimum header length (no options) is therefore 20 bytes.  The Data Offset field is the header length in 32bit words, this would be 5 without options, and at most 15 with options.

Explicit Congestion Notification (ECN) [RFC2481] adds 2 new flags to the TCP header: Congestion Window Reduced (CWR) and ECN-Echo (ECNE). ECN also requires 1 or 2 additional flags in the IP header.

Commonly, the TCP header will carry options related to enhancements of the TCP protocol. Important options are Window Scaling, Selective Acknowledgement (SACK) [RFC2018, RFC2883] and Explicit Congestion Notification (ECN) [RFC2481].

TCP data payload length is the IP payload length minus the TCP header length.

TCP packets usually carry an application level data stream, f.e. HTTP, FTP, Telnet, SSH, etc.

UDP Header format (as defined in RFC-768):

0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Source Port																Destination Port
Total Length																Checksum (optional)
UDP Payload

The header of a UDP packet consists of 2 words of 32 bits (4 bytes) each.  The header length is therefore always 8 bytes.  The Total Length field includes the UDP header and is measured in bytes.

UDP packets usually carry an application level datagram as their payload, f.e. DNS, NTP, NFS, etc