TCP MSS Issue Post: 302882229

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Special Forums IP Networking TCP MSS Issue Post 302882229 by denis4inet on Monday 6th of January 2014 02:02:03 AM

01-06-2014

Registered User

TCP MSS Issue

Gretings! I have developed simple utility using Berkly sockets interface:
So, what do I?
$ ifconfig lo mtu 1500
$ make
//run netcat
$ ./target 127.0.0.1 5555 1460 1000000 #mss: 1460, loop sleep: 1s
On other virtual terminal:
$ netcat -l 127.0.0.1 -p 5555
On third virtual terminal:
# tcpdump -i lo -n -t

Code:

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 65535 bytes
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [S], seq 453033050, win 29200, options [mss 1460,sackOK,TS val 53605642 ecr 0,nop,wscale 7], length 0
IP 127.0.0.1.5555 > 127.0.0.1.54707: Flags [S.], seq 1575152534, ack 453033051, win 28960, options [mss 1460,sackOK,TS val 53605642 ecr 53605642,nop,wscale 7], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [.], ack 1, win 229, options [nop,nop,TS val 53605642 ecr 53605642], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [.], seq 1:4345, ack 1, win 229, options [nop,nop,TS val 53605642 ecr 53605642], length 4344
IP 127.0.0.1.5555 > 127.0.0.1.54707: Flags [.], ack 4345, win 295, options [nop,nop,TS val 53605642 ecr 53605642], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [P.], seq 4345:4381, ack 1, win 229, options [nop,nop,TS val 53605642 ecr 53605642], length 36
IP 127.0.0.1.5555 > 127.0.0.1.54707: Flags [.], ack 4381, win 295, options [nop,nop,TS val 53605642 ecr 53605642], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [.], seq 4381:8725, ack 1, win 229, options [nop,nop,TS val 53607142 ecr 53605642], length 4344
IP 127.0.0.1.5555 > 127.0.0.1.54707: Flags [.], ack 8725, win 346, options [nop,nop,TS val 53607142 ecr 53607142], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [P.], seq 8725:8761, ack 1, win 229, options [nop,nop,TS val 53607142 ecr 53605642], length 36
IP 127.0.0.1.5555 > 127.0.0.1.54707: Flags [.], ack 8761, win 346, options [nop,nop,TS val 53607142 ecr 53607142], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [.], seq 8761:13105, ack 1, win 229, options [nop,nop,TS val 53608642 ecr 53607142], length 4344
IP 127.0.0.1.5555 > 127.0.0.1.54707: Flags [.], ack 13105, win 346, options [nop,nop,TS val 53608642 ecr 53608642], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [P.], seq 13105:13141, ack 1, win 229, options [nop,nop,TS val 53608642 ecr 53607142], length 36
IP 127.0.0.1.5555 > 127.0.0.1.54707: Flags [.], ack 13141, win 346, options [nop,nop,TS val 53608642 ecr 53608642], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [F.], seq 13141, ack 1, win 229, options [nop,nop,TS val 53609460 ecr 53608642], length 0
IP 127.0.0.1.5555 > 127.0.0.1.54707: Flags [F.], seq 1, ack 13142, win 355, options [nop,nop,TS val 53609460 ecr 53609460], length 0
IP 127.0.0.1.54707 > 127.0.0.1.5555: Flags [.], ack 2, win 229, options [nop,nop,TS val 53609460 ecr 53609460], length 0

Question: Why TCP segment size has set to 1460, but tcp segments real size is: 4344 + 36 = 3*mss?

My OS: Linux 3.12.6-1-ARCH x86_64 GNU/Linux

Thanks in advance!

---------- Post updated 06-01-14 at 02:02 AM ---------- Previous update was 05-01-14 at 11:51 AM ----------

I have disable Generic\TCP Segmentation offload with

Code:

# ethtool -K lo gso off
# ethtool -K lo tso off

Solved!

denis4inet

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LEARN ABOUT DEBIAN

synos

synos(1)							Mail Avenger 0.8.3							  synos(1)

NAME

       synos - guess operating system from TCP SYN fingerprint

SYNOPSIS

       synos [--mtu mtu] [--db path] syn-fingerprint

DESCRIPTION

       synos takes a SYN fingerprint, in the format described for the CLIENT_SYNFP environment variable in the avenger(1) man page, and outputs a
       guess as to the type of the client operating system.  synos makes use of the OpenBSD SYN fingerprint database (which is also repackaged
       with Mail Avenger).

   OPTIONS
       --mtu val
	   Certain operating systems set the initial TCP window size based on the maximum transmission unit, or MTU, of the network.  For such
	   operating systems, synos usually checks the window size using both the client's MSS option plus 40 bytes (for TCP and IP headers), or a
	   hard-coded MTU, which defaults to 1,500 bytes.  If either value works, the fingerprint is considered to match the operating system.
	   You can change the value 1,500 by specifying this option.  A value of 0 tells synos to use only the value derived from the MSS option.

       --db file
	   Specifies an alternate location for the SYN fingerprint database.

FILES

       /usr/local/share/pf.os
	   Default location of SYN fingerprint database.

SEE ALSO

       avenger(1), asmtpd(8)

       The Mail Avenger home page: <http://www.mailavenger.org/>.

       The OpenBSD home page: <http://www.openbsd.org/>.

BUGS

       The operating system type is determined by heuristics that are not always reliable.  Moreover, not all operating systems can be
       distinguished.  The database may not even contain a client's particular operating system and version.

       It is not hard to fool synos deliberately by changing TCP socket options or injecting raw packets onto the network.

AUTHOR

       David Mazieres

Mail Avenger 0.8.3						    2012-04-05								  synos(1)