Bueno, ahora ya he solucionado el tema de acceso a la shell.
Aquí dejo las salidas que me comentó javi:
~# iptables -t mangle -L -n -v
Chain PREROUTING (policy ACCEPT 89229 packets, 48M bytes)
pkts bytes target prot opt in out source destination
8001 5955K CONNMARK all -- * * 0.0.0.0/0 0.0.0.0/0 CONNMARK restore
480 300K MARK all -- * * 0.0.0.0/0 0.0.0.0/0 MARK match 0x0/0xff MAC 00:23:F8:80:1B:74 MARK set 0x1
243 13228 MARK all -- * * 0.0.0.0/0 0.0.0.0/0 MARK match 0x0/0xff MAC 00:23:F8:D3:B1:7D MARK set 0x2
153 11040 MARK all -- eth3 * 0.0.0.0/0 0.0.0.0/0 MARK match 0x0/0xff MARK set 0x1
7676 5662K CONNMARK all -- * * 0.0.0.0/0 0.0.0.0/0 CONNMARK save
Chain INPUT (policy ACCEPT 41911 packets, 25M bytes)
pkts bytes target prot opt in out source destination
Chain FORWARD (policy ACCEPT 47137 packets, 23M bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 31882 packets, 23M bytes)
pkts bytes target prot opt in out source destination
7078 6071K CONNMARK all -- * * 0.0.0.0/0 0.0.0.0/0 CONNMARK restore
82 54968 MARK all -- * * 0.0.0.0/0 0.0.0.0/0 OWNER GID match 124 MARK set 0x1
Chain POSTROUTING (policy ACCEPT 78972 packets, 46M bytes)
pkts bytes target prot opt in out source destination
Chain EBOX-SHAPER (0 references)
pkts bytes target prot opt in out source destination
Chain EBOX-SHAPER-eth1 (0 references)
pkts bytes target prot opt in out source destination
y
# ip route ls table default
default
nexthop via 192.168.1.1 dev eth0 weight 15
nexthop via 192.168.1.2 dev eth0 weight 15
Quedo a la espera de vuestros comentarios.
Gracias.