[Samba] Samba networking problems - A Solution Guide

[jkulow at imap.dentalmail.de]

Hi folks, 
I posted weeks ago reporting my problems with Samba on Redhat 7.3 with 
Win98SE. I got very few replies, it were about 2 or 3, and just one stated it 
could be a networking problem. So it was. 
 
Problm description: 
When copying to a samba share windows breaks with an error message that the 
share is no longer available. Although you can see file parts from both 
systems in the share folder. When you "ok" the message the part vanishes. 
 
Solution: 
The problem is a discrepacy in networking settings for TCP between Win98se and 
Samba or Linux. 
Settings are adjustable in smb.conf for Linux. For windows you can change 
setting with eg. "Dr.TCP" (small executable that does registry updates for 
you). 
 
Targets are to Parameters: 
MTU and RecieveWindow 
 
First check what's currently set via "tcpdump -i eth0 >somelogfile". Change 
eth0 for you networking card if necessary. Search for "mss". Looks like this: 
 
23:20:32.306499 celvin.dentalmail.de.1029 > imap.dentalmail.de.netbios-ssn: S 
133773:133773(0) win 32768 <mss 1460,nop,nop,sackOK> (DF) 
23:20:32.306587 imap.dentalmail.de.netbios-ssn > celvin.dentalmail.de.1029: S 
821427727:821427727(0) ack 133774 win 5840 <mss 1460,nop,nop,sackOK> (DF) 
 
For short: 
windows > linux win 32768 <mss 1460,nop,nop,sackOK> (DF) 
linux > windows win 5840 <mss 1460,nop,nop,sackOK> (DF) 
 
As you can see the Recieve window differs a lot but mss is the same. This was 
just so by accident, I played long time with the settings. But didn't work of 
course. 
The aim should be to make those values identical. 
 
For linux I am not sure where to change the recievewindow so I did it on 
windows. I thought the parameter so_rcvbuf for Samba should work but it had no 
effect. 
 
Now what took me long to understand. MSS is MTU - 40. Thats just per 
definition since the 40 is the lengs of the TCP packet header. 
So to get the right value for windows just add 40 to your mss value, in my 
case 1500 would be correct. 
 
Now it works fine. 
Ok I should increase the recieve window for perfomance, but low traffic is 
better than no traffic ;-) 
 
 
Any comments to that ? 
 
Bye  
 
Jochen Kulow 
cand.med.dent 
 
Erlangen - Germany