SYN and SYN+ACK Packets

TCP packets are the backbone of network communication, and two of the most significant types are SYN and SYN+ACK.
Huginn Net analyzes the behavior and characteristics of these packets to infer the operating system (OS) of the client or server, and the geographic or logical distance (network hop count).

TCP Signature

For TCP traffic, signature layout is as follows:

sig = ver:ittl:olen:mss:wsize,scale:olayout:quirks:pclass
Key Description
ver Signature for IPv4 ('4'), IPv6 ('6'), or both ('*').
ittl Initial TTL used by the OS. Almost all operating systems use 64, 128, or 255; ancient versions of Windows sometimes used 32, and several obscure systems sometimes resort to odd values such as 60.
olen Length of IPv4 options or IPv6 extension headers.
mss Maximum segment size, if specified in TCP options. Special value of '*' can be used to denote that MSS varies depending on the parameters of sender's network link, and should not be a part of the signature. In this case, MSS will be used to guess the type of network hookup according to the [mtu] rules.
wsize Window size. Can be expressed as a fixed value, but many operating systems set it to a multiple of MSS or MTU, or a multiple of some random integer. P0f automatically detects these cases, and allows notation such as 'mss*4', 'mtu*4', or '%8192' to be used. Wildcard ('*') is possible too.
scale Window scaling factor, if specified in TCP options. Fixed value or '*'.
olayout Comma-delimited layout and ordering of TCP options, if any. This is one of the most valuable TCP fingerprinting signals. See supported values below.
quirks Comma-delimited properties and quirks observed in IP or TCP headers. See supported values below.
pclass Payload size classification: '0' for zero, '+' for non-zero, '*' for any. The packets we fingerprint right now normally have no payloads, but some corner cases exist.

TCP Options (olayout)

Supported values for the olayout field:

Value Description
eol+n Explicit end of options, followed by n bytes of padding
nop No-op option
mss Maximum segment size
ws Window scaling
sok Selective ACK permitted
sack Selective ACK (should not be seen)
ts Timestamp
?n Unknown option ID n

Quirks

Supported values for the quirks field:

Value Description
df "Don't fragment" set (probably PMTUD); ignored for IPv6
id+ DF set but IPID non-zero; ignored for IPv6
id- DF not set but IPID is zero; ignored for IPv6
ecn Explicit congestion notification support
0+ "Must be zero" field not zero; ignored for IPv6
flow Non-zero IPv6 flow ID; ignored for IPv4
seq- Sequence number is zero
ack+ ACK number is non-zero, but ACK flag not set
ack- ACK number is zero, but ACK flag set
uptr+ URG pointer is non-zero, but URG flag not set
urgf+ URG flag used
pushf+ PUSH flag used
ts1- Own timestamp specified as zero
ts2+ Non-zero peer timestamp on initial SYN
opt+ Trailing non-zero data in options segment
exws Excessive window scaling factor (> 14)
bad Malformed TCP options

SYN Packet

Initiate a TCP connection. Sent by the client to a server.

SYN Packet Analyzed 

[TCP SYN] 1.2.3.4:1524 → 4.3.2.1:80
  OS:     Windows XP
  Dist:   8
  Params: none
  Sig:    4:120+8:0:1452:65535,0:mss,nop,nop,sok:df,id+:0

SYN Key Fields

  • OS: The identified operating system matched from the database signature.

  • Dist: The distance in network hops to the server.

  • Params: Optional TCP parameters.

  • Sig: The TCP signature showing packet structure (ver:ittl:olen:mss:wsize,scale:olayout:quirks:pclass).

SYN+ACK Packet

Sent by the server in response to a SYN, signaling acknowledgment and readiness to establish the connection.

SYN+ACK Packet Analyzed 

[TCP SYN+ACK] 4.3.2.1:80 → 1.2.3.4:1524
  OS:     Linux 3.x
  Dist:   0
  Params: none
  Sig:    4:64+0:0:1460:mss*10,0:mss,nop,nop,sok:df:0

SYN+ACK Key Fields

  • OS: The identified operating system matched from the database signature.

  • Dist: The distance in network hops (always 0 for server responses).

  • Params: Optional TCP parameters.

  • Sig: The TCP signature showing packet structure (ver:ittl:olen:mss:wsize,scale:olayout:quirks:pclass).
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