Seguramente te falte en el kernel el modulo de NAT
Te pasteo la config del router de mi curre:
#
# Networking options
#
CONFIG_PACKET=y
CONFIG_PACKET_MMAP=y
# CONFIG_NETLINK_DEV is not set
CONFIG_NETFILTER=y
# CONFIG_NETFILTER_DEBUG is not set
CONFIG_FILTER=y
CONFIG_UNIX=y
CONFIG_INET=y
# CONFIG_IP_MULTICAST is not set
CONFIG_IP_ADVANCED_ROUTER=y
# CONFIG_IP_MULTIPLE_TABLES is not set
# CONFIG_IP_ROUTE_MULTIPATH is not set
# CONFIG_IP_ROUTE_TOS is not set
# CONFIG_IP_ROUTE_VERBOSE is not set
# CONFIG_IP_ROUTE_LARGE_TABLES is not set
# CONFIG_IP_PNP is not set
# CONFIG_NET_IPIP is not set
# CONFIG_NET_IPGRE is not set
# CONFIG_ARPD is not set
# CONFIG_INET_ECN is not set
# CONFIG_SYN_COOKIES is not set
#
# IP: Netfilter Configuration
#
CONFIG_IP_NF_CONNTRACK=y
CONFIG_IP_NF_FTP=y
CONFIG_IP_NF_IRC=y
# CONFIG_IP_NF_QUEUE is not set
CONFIG_IP_NF_IPTABLES=y
# CONFIG_IP_NF_MATCH_LIMIT is not set
CONFIG_IP_NF_MATCH_MAC=y
# CONFIG_IP_NF_MATCH_PKTTYPE is not set
# CONFIG_IP_NF_MATCH_MARK is not set
CONFIG_IP_NF_MATCH_MULTIPORT=y
# CONFIG_IP_NF_MATCH_TOS is not set
# CONFIG_IP_NF_MATCH_ECN is not set
# CONFIG_IP_NF_MATCH_DSCP is not set
# CONFIG_IP_NF_MATCH_AH_ESP is not set
# CONFIG_IP_NF_MATCH_LENGTH is not set
# CONFIG_IP_NF_MATCH_TTL is not set
# CONFIG_IP_NF_MATCH_TCPMSS is not set
# CONFIG_IP_NF_MATCH_HELPER is not set
# CONFIG_IP_NF_MATCH_STATE is not set
# CONFIG_IP_NF_MATCH_CONNTRACK is not set
# CONFIG_IP_NF_MATCH_UNCLEAN is not set
# CONFIG_IP_NF_MATCH_OWNER is not set
CONFIG_IP_NF_FILTER=y
CONFIG_IP_NF_TARGET_REJECT=m
CONFIG_IP_NF_TARGET_MIRROR=m
CONFIG_IP_NF_NAT=y
CONFIG_IP_NF_NAT_NEEDED=y
CONFIG_IP_NF_TARGET_MASQUERADE=y
CONFIG_IP_NF_TARGET_REDIRECT=y
# CONFIG_IP_NF_NAT_LOCAL is not set
# CONFIG_IP_NF_NAT_SNMP_BASIC is not set
CONFIG_IP_NF_NAT_IRC=y
CONFIG_IP_NF_NAT_FTP=y
# CONFIG_IP_NF_MANGLE is not set
# CONFIG_IP_NF_TARGET_LOG is not set
# CONFIG_IP_NF_TARGET_ULOG is not set
# CONFIG_IP_NF_TARGET_TCPMSS is not set
# CONFIG_IP_NF_ARPTABLES is not set
# CONFIG_IPV6 is not set
# CONFIG_KHTTPD is not set
# CONFIG_ATM is not set
# CONFIG_VLAN_8021Q is not set
# CONFIG_IPX is not set
# CONFIG_ATALK is not set