Description of the problem
The system loses the network connection because the network card hangs up. The TCP segmentation offload of the NICs is defective and must be deactivated.
Solution
In order to fix the hang up, you can turn off TCP segmentation offloading via the following command:
ethtool -K <interface> tso offThere is currently no fix available or announced that allows offloading TCP segmentation to the NIC without sacrificing performance.
You can ignore the following error message:
Cannot get device udp-fragmentation-offload settings: Operation not supportedOlder ethtool versions try to disable a feature not present in newer kernel versions. Nevertheless, deactivating the TCP segmentation offloading will still work: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=898907
The servers of the SX61 series have the network chip Realtek 8111F on the motherboard, which is unstably operated by the Debian Stretch Linux kernel with the driver r8169. Instead you should install the package r8168-dkms.
Example errors from dmesg
NETDEV WATCHDOG: enp3s0 (r8168): transmit queue 0 timed out
enp3s0: BUG! Tx Ring full when queue awake!
r8168: enp3s0: link up