-extern struct nf_nat_protocol nf_nat_protocol_tcp;
-extern struct nf_nat_protocol nf_nat_protocol_udp;
-extern struct nf_nat_protocol nf_nat_protocol_icmp;
-extern struct nf_nat_protocol nf_nat_unknown_protocol;
+extern const struct nf_nat_protocol nf_nat_protocol_tcp;
+extern const struct nf_nat_protocol nf_nat_protocol_udp;
+extern const struct nf_nat_protocol nf_nat_protocol_icmp;
+extern const struct nf_nat_protocol nf_nat_unknown_protocol;