Hi,
I have a linux router setup with 3 Ethernet cards routing between 3
networks. Everything works fine, except for a few strange quirks. From
the router itself (running 2.1.122) or other 2.1.x boxes (running 2.1.122
or 2.1.125) I am unable to telnet into a couple of ISDN routers that we
have. The tcpdump trace shows the following:
18:06:58.446681 2.1.122.router.my.net.4290 > isdn.router.my.net.telnet: S
3219896362:3219896362(0) win 32120 <mss 1460,sackOK,timestamp
29064260[|tcp]> (DF)
18:07:01.445020 2.1.122.router.my.net.4290 > isdn.router.my.net.telnet: S
3219896362:3219896362(0) win 32120 <mss 1460,sackOK,timestamp
29064560[|tcp]> (DF)
18:07:03.444993 arp who-has isdn.router.my.net tell 2.1.122.router.my.net
18:07:03.447627 arp reply isdn.router.my.net is-at 0:20:8a:4:65:3c
18:07:07.445006 2.1.122.router.my.net.4290 > isdn.router.my.net.telnet: S
3219896362:3219896362(0) win 32120 <mss 1460,sackOK,timestamp
29065160[|tcp]> (DF)
..... -> continues trying until times out.
However, under 2.0.35 or from other OSes, I can telnet through the router
to these TAs Fine. From 2.0.35 I see:
18:06:23.100185 2.0.35.host.my.net.17103 > isdn.router.my.net.telnet: S
591560757:591560757(0) win 512 <mss 1460>
18:06:23.103485 isdn.router.my.net.telnet > 2.0.35.host.my.net.17103: S
1257242624:1257242624(0) ack 591560758 win 1024 <mss 1446>
18:06:23.106869 2.0.35.host.my.net.17103 > isdn.router.my.net.telnet: .
ack 1 win 31744 (DF)
.... -> successful telnet session
and it does not matter which of the networks I am on, the behaviour is
always the same - works for 2.0.35, fails for 2.1.x. I can telnet and
make other tcp connections fine under 2.1.x to various machines, but not
these ISDN boxes.
My theory is that 2.1.x is putting some extra options in the TCP packet
header (as seen in the tcpdump trace above) which are unsupported on the
TAs and therefore the packages are dropped? My networking options has
most options turned on, but not TCP syn cookies, so I don't think it
anything to do with that. How do I stop this behavoir?
Another strange behavoir I get sometimes is that when first instantiating
a TCP connection (e.g. by ftp or telnet) to a remote host, sometimes I get
an ICMP Host unreachable message back from the linux router. The next
time I try, and for several subsequent attempts everything works fine.
This was never seen under our previous configuration (which had a
dedicated router instead of a linux box), and is hard to reproduce
reliably. I don't believe it is an external network issue.
I am wondering if these are bugs / quirks / features of the 2.1.x routing
/ networking implementation?
Can I fix these problems easily? Any help would be greatly appreciated.
Best Regards,
Tom.
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