Linux: NAT slowness/instability

[ATC]

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.

[ATC]

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.