MTU or Fragmentation Problems on 7.0?

[Ian Smith]

On Sun, 25 Jan 2009, Len Gross wrote:
 > The following configuration works fine _until_ I make a change in MTU
 > setting on the link between FreeBSD1 and FreeBSD2
 > 
 > Internet
 >                                    |
 > Router                      x.x.x.x
 >                          192.168.0.1/16
 >                                    |
 > FreeBSD #1       192.168.0.202 /16
 >   6.3                  192.168.1.1/ 24
 >                                    |
 > FreeBSD #2        192.168.1.2/24					
 >   7.0                   192.168.1.5/24
 > 	                  |
 > FreeBSD #3        192.168.5.2/24
 > 7.0
 > 
 > All connections are Ethernet.
 > 
 > If I change the MTU on 192.168.1.1 to 1450  and the corresponding MTU
 > on 192.168.1.2 to 1450, then Web Browsing on FreeBSD2 continues to
 > work, BUT browsing on FreeBSD3  "fails"  (mostly.)
 > 
 > On FreeBSD 3
 > Ping and nslookup work fine from FreeBSD3
 > I can get to Google but virtually no other web sites
 > Using tcpdump there is lots of unusual stuff, some relating to
 > fragmentation ICMP?

Do any of these machines have a firewall rule blocking ICMP?  You want 
to be sure at least icmptypes 3,11 are flowing freely to/from FreeBSD3, 
as well as pings (icmptypes 0,8) which are apparently permitted.

cheers, Ian

 > If I put a Web Proxy on FreeBSD 1, everything works fine.
 > 
 > I have tried putting mtu = 1450 using route change on all the routes,
 > but that didn't help.
 > When I did this I verified all routes had 1450 mtu via netstat ?arW
 > 
 > So I am unsure if this is a FreeBSD bug, a "internet" fragmentation issue or ???
 > Amongst the strangest things is that FreeBSD 2 is unaffected; Firefox
 > runs fine there
 > 
 > (There was a thread in October about mtu issues in 7.0 but it didn't
 > seem to help my problem.)
 > (I run 1450 MTU to support testing of an experimental protocol., but
 > all the above is with straight out of the box FreeBSD.)
 > 
 > -- Len