I have been having a problem with PPP under linux for several
months. It has followed me up the 2.1 chain, to 2.1.current. I have found
a workaround, but I dislike it and am hoping someone can confirm whether
this is a bug or my ineptitude (or a bug with some other third-party
system).
The setup:
My Linux, currently 2.1.89, RedHat 5.0, is 24/7 dialin client using
PPP-2.3.3. The remote end is a Linux-2.0.33 with same PPP package. I have
one w95 client connected to my linux via ethernet, and I am masquerading
it. Everything is/has been working fine (except for the problem I'm about
to describe). MTU/MRU on both dialin server and my linux is 596. The
dialin server is connected via ethernet to a Cisco 2501.
The problem:
The w95 client can not negotiate proper TCP sessions with a handful of
sites on the Internet. An example is "www.metacrawler.com". The first
half-dozen packets or so are exchanged, but the connection simply hangs
forever after that. Turning on/off pmtu with
/proc/sys/net/ipv4/ip_no_pmtu_disc does NOT solve the problem.
The workaround:
If I log into the dialin server and change the MTU on it's ppp interface
(corresponding to my connection) to 1500, the masqueraded host can reach
the sites. Only the setting of 1500 allows the session to work; I have
experimented with other values and the problem does not go away. Because
this is a slow link, I'd like to use a smaller MRU.
tcpdump output:
Session with MTU 596/MRU 596, to www.metacrawler.com from masqueraded host,
hanging.
18:59:43.263087 208.152.24.33.61694 > 206.253.196.50.http:
S 8434373:8434373(0) win 8192 <mss 1460> (DF)
18:59:43.403087 206.253.196.50.http > 192.168.2.2.1197:
S 2377011980:2377011980(0) ack 8434374 win 8760 <mss 1460> (DF)
18:59:43.403087 208.152.24.33.61694 > 206.253.196.50.http:
. ack 2377011981 win 8760 (DF)
18:59:43.583087 208.152.24.33.61694 > 206.253.196.50.http:
P 0:234(234) ack 1 win 8760 (DF)
18:59:43.833087 206.253.196.50.http > 192.168.2.2.1197:
. ack 235 win 8760 (DF)
(sits forever right here)
5 packets received by filter
0 packets dropped by kernel
Session with MTU 596/MRU 1500, to www.metacrawler.com from masqeuraded host,
hanging.
19:04:41.953087 208.152.24.33.61716 > 206.253.196.50.http:
S 8733304:8733304(0) win 8192 <mss 1460> (DF)
19:04:42.083087 206.253.196.50.http > 192.168.2.2.1221:
S 2323311837:2323311837(0) ack 8733305 win 8760 <mss 1460> (DF)
19:04:42.083087 208.152.24.33.61716 > 206.253.196.50.http:
. ack 2323311838 win 8760 (DF)
19:04:42.123087 208.152.24.33.61716 > 206.253.196.50.http:
P 0:374(374) ack 1 win 8760 (DF)
19:04:42.483087 206.253.196.50.http > 192.168.2.2.1221:
P 1:396(395) ack 375 win 8760 (DF)
19:04:42.523087 206.253.196.50.http > 192.168.2.2.1221:
P 396:438(42) ack 375 win 8760 (DF)
19:04:42.523087 208.152.24.33.61716 > 206.253.196.50.http:
. ack 438 win 8323 (DF)
19:04:42.883087 206.253.196.50.http > 192.168.2.2.1221:
P 438:482(44) ack 375 win 8760 (DF)
19:04:43.033087 208.152.24.33.61716 > 206.253.196.50.http:
. ack 482 win 8279 (DF)
19:04:43.523087 206.253.196.50.http > 192.168.2.2.1221:
P 482:522(40) ack 375 win 8760 (DF)
19:04:43.613087 206.253.196.50.http > 192.168.2.2.1221:
P 522:566(44) ack 375 win 8760 (DF)
19:04:43.613087 208.152.24.33.61716 > 206.253.196.50.http:
. ack 566 win 8195 (DF)
(session continues normally here)
12 packets received by filter
0 packets dropped by kernel
Thanks for any advice,
-bp
-- B. James Phillippe <bryan@terran.org> Linux Software Engineer, WGT Inc. http://earth.terran.org/~bryan
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