Esses são os sinalizadores da interface. Eles estão documentados na netdevice(7)
man-page. Abaixo está a parte relevante (reordenada alfabeticamente):
IFF_ALLMULTI Receive all multicast packets.
IFF_AUTOMEDIA Auto media selection active.
IFF_BROADCAST Valid broadcast address set.
IFF_DEBUG Internal debugging flag.
IFF_DORMANT Driver signals dormant (since Linux 2.6.17)
IFF_DYNAMIC The addresses are lost when the interface goes down.
IFF_ECHO Echo sent packets (since Linux 2.6.25)
IFF_LOOPBACK Interface is a loopback interface.
IFF_LOWER_UP Driver signals L1 up (since Linux 2.6.17)
IFF_MASTER Master of a load balancing bundle.
IFF_MULTICAST Supports multicast
IFF_NOARP No arp protocol, L2 destination address not set.
IFF_NOTRAILERS Avoid use of trailers.
IFF_POINTOPOINT Interface is a point-to-point link.
IFF_PORTSEL Is able to select media type via ifmap.
IFF_PROMISC Interface is in promiscuous mode.
IFF_RUNNING Resources allocated.
IFF_SLAVE Slave of a load balancing bundle.
IFF_UP Interface is running.
Portanto, LOWER_UP
significa que há um sinal no nível físico (ou seja, algo ativo está conectado à interface de rede). NO-CARRIER
, é exatamente o oposto: nenhum sinal é detectado no nível físico.