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| Linux: NAT slowness/instability |
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Problem:
After introducing a NAT router in the network, separating servers and clients, several odd problems appeared:
- Web browsing sometimes appeared to hang, large images would usually fail.
- FTP browsing would work OK but large file transfers would slow down and usually fail.
- SMB exhibited weird behaviour, like shares suddenly becoming unavailable, read errors and frequent lock-ups.
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Solution:
After going through all the usual troubleshooting of possible firewall problems, we noticed something unusual about the ifconfig output on one of the interfaces:
eth2 Link encap:Ethernet HWaddr 00:05:5D:10:52:BC
inet addr:#.#.#.# Bcast:85.95.37.255 Mask:255.255.255.0
BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:4397856 errors:0 dropped:732 overruns:0 frame:0
TX packets:3597942 errors:0 dropped:0 overruns:0 carrier:3589674
collisions:0 txqueuelen:1000
RX bytes:3756826843 (3582.7 Mb) TX bytes:437463838 (417.1 Mb)
Interrupt:9 Base address:0xcc00
The "carrier" value should normally be 0 or very close to 0. The number would increase by about 1-5 per second during low traffic, high traffic would increment the counter by several hundred per second.
A google for the actual meaning of this value showed nothing, but the name and context suggests some kind of transmission problem. Replacing the cable and trying a different switch port did not have any effect.
We finally replaced the network card, and the problem was solved.
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