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I am having a really strange problem with my Redhat box (from 7.2 to 9 I am currently using). When I am downloading something (using maximum bandwidth of my 64K ISDN dialup), it is very difficult to open new connections. DNS resolver often times out, web pages are loading extremely slow and ssh/telnet connections are unusable. If I start a second download, it begins with extremely low speed and only after some minutes it reaches the 50% of my bandwidth. A third connection is almost impossible. Not to say that with 2 simultaneous downloads, I cannot browse at all.
Using ping to a nearby host reveals an unusual high roundtrip (more than 8 seconds) compared to 500ms average roundtrip of another box plugged on the same ISDN TA under the same conditions.
There is a packet loss starting from 100%, if I interrupt ping in less than 8 seconds and going very low (less then 1%) if I leave ping running for a while. I guess this reflects the 8 seconds round trip of packets transmitted every 1 second, since there is a standard loss of 7 to 8 packets.
I am using an ISDN TA called NetMod (netmod.intracom.gr), using either serial or USB interface, which I am pretty sure that has nothing to do with the problem, because:
1. It is widely (~100%) used in Greece without similar problems
2. I have used it flawless it in the past with a couple Linux boxes (Slakware 3.x/Redhat 7.2)
3. I have tried 3 of these TAs on my box with the same problem
4. Windows and FreeBSD boxes connected the same pieces work fine
My first assumption that it was a serial port problem proved wrong. The situation remains the same with my new USB ISDN TA (same brand).
My Linux box is a Compaq Armada E500 Laptop (P3 500, 192MB RAM, 6G IBM 2" HD, 3COM PCMCIA NIC).
Currently running Redhat 9 (Kernel 2.4.20-8), but I had the problem since the first installation of 7.2 and all the intermediate upgrades (7.3 & 8). I have also used a 2.4.19 Kernel from kernel.org compiled by me (I used Redhat config as a starting point), without better luck.
I have not tried another distribution, nor a format/reinstall (is there any point to do it?).
This problem is really frustrating, because I have no idea what causes it. Any help would be much appriciated.
Either your DNS server is too many hops away, or you have a pcmcia hardware problem. 3com pcmcia cards fry at a moments notice. They like to grab network configuration settings and hold them when they are fried. Check your hardware first.