[centos]iptables-save, вывод не согласуется с iptables -t nat -n -L

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собственно, сабж:


[root@cit-ots-3 /]# iptables-save
# Generated by iptables-save v1.4.7 on Mon Nov 14 15:31:05 2011
*nat
:PREROUTING ACCEPT [17335:1118285]
:POSTROUTING ACCEPT [18289:1102226]
:OUTPUT ACCEPT [2655:162595]
COMMIT
# Completed on Mon Nov 14 15:31:05 2011
# Generated by iptables-save v1.4.7 on Mon Nov 14 15:31:05 2011
*mangle
:PREROUTING ACCEPT [734821:777103821]
:INPUT ACCEPT [500191:708450435]
:FORWARD ACCEPT [249058:69751362]
:OUTPUT ACCEPT [354866:249759133]
:POSTROUTING ACCEPT [602083:319381035]
-A POSTROUTING -o virbr1 -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill 
-A POSTROUTING -o virbr0 -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill 
COMMIT
# Completed on Mon Nov 14 15:31:05 2011
# Generated by iptables-save v1.4.7 on Mon Nov 14 15:31:05 2011
*filter
:INPUT ACCEPT [417958:635598238]
:FORWARD ACCEPT [242992:69148201]
:OUTPUT ACCEPT [292844:226141276]
COMMIT
# Completed on Mon Nov 14 15:31:05 2011
[root@cit-ots-3 /]# iptables -t nat -n -L
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
[root@cit-ots-3 /]#

непонятно. Конкретно удивляют строки:

-A POSTROUTING -o virbr1 -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill 
-A POSTROUTING -o virbr0 -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill

Может ли кто дать доступное объяснение или направление поиска?