file: /proc/sys/net/core/netdev_rss_key
variable: net.core.netdev_rss_key
Official reference

RSS (Receive Side Scaling) enabled drivers use a 40 bytes host key that is randomly generated. Some user space might need to gather its content even if drivers do not provide ethtool -x support yet.

myhost:~# cat /proc/sys/net/core/netdev_rss_key 84:50:f4:00:a8:15:d1:a7:e9:7f:1d:60:35:c7:47:25:42:97:74:ca:56:bb:b6:a1:d8: … (52 bytes total)

File contains nul bytes if no driver ever called netdev_rss_key_fill() function. Note: /proc/sys/net/core/netdev_rss_key contains 52 bytes of key, but most drivers only use 40 bytes of it.

myhost:~# ethtool -x eth0 RX flow hash indirection table for eth0 with 8 RX ring(s): 0: 0 1 2 3 4 5 6 7 RSS hash key: 84:50:f4:00:a8:15:d1:a7:e9:7f:1d:60:35:c7:47:25:42:97:74:ca:56:bb:b6:a1:d8:43:e3:c9:0c:fd:17:55:c2:3a:4d:69:ed:f1:42:89