Why can't I add any iptables rule?

Question

Why can't I add any iptables rule?

root@ROUTER:~# iptables -L INPUT -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
6934 685K ACCEPT 0 -- any any anywhere anywhere state RELATED,ESTABLISHED
0 0 DROP udp -- ppp0 any anywhere anywhere udp dpt:route
0 0 DROP udp -- br0 any anywhere anywhere udp dpt:route
0 0 ACCEPT udp -- any any anywhere anywhere udp dpt:route
2 120 logaccept tcp -- any any anywhere ROUTER tcp dpt:22
0 0 DROP icmp -- ppp0 any anywhere anywhere
0 0 DROP igmp -- any any anywhere anywhere
5 350 ACCEPT 0 -- lo any anywhere anywhere state NEW
568 35947 logaccept 0 -- br0 any anywhere anywhere state NEW
339 38020 DROP 0 -- any any anywhere anywhere

Then I try to add a rule:

root@ROUTER:~# /usr/sbin/iptables -I INPUT -i ppp0 -m multiport --dport 21,22,23,53,67,80,443 -j DROP

But it didn't got added to the list:

root@ROUTER:~# iptables -L INPUT -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
6992 690K ACCEPT 0 -- any any anywhere anywhere state RELATED,ESTABLISHED
0 0 DROP udp -- ppp0 any anywhere anywhere udp dpt:route
0 0 DROP udp -- br0 any anywhere anywhere udp dpt:route
0 0 ACCEPT udp -- any any anywhere anywhere udp dpt:route
2 120 logaccept tcp -- any any anywhere ROUTER tcp dpt:22
0 0 DROP icmp -- ppp0 any anywhere anywhere
0 0 DROP igmp -- any any anywhere anywhere
5 350 ACCEPT 0 -- lo any anywhere anywhere state NEW
569 36007 logaccept 0 -- br0 any anywhere anywhere state NEW
343 38428 DROP 0 -- any any anywhere anywhere

I try to add a rule again:

root@ROUTER:~# /usr/sbin/iptables -I INPUT -i ppp0 --dport 21 -j DROP

And see if there is any changes:

root@ROUTER:~# iptables -L INPUT -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
7142 704K ACCEPT 0 -- any any anywhere anywhere state RELATED,ESTABLISHED
0 0 DROP udp -- ppp0 any anywhere anywhere udp dpt:route
0 0 DROP udp -- br0 any anywhere anywhere udp dpt:route
0 0 ACCEPT udp -- any any anywhere anywhere udp dpt:route
2 120 logaccept tcp -- any any anywhere ROUTER tcp dpt:22
0 0 DROP icmp -- ppp0 any anywhere anywhere
0 0 DROP igmp -- any any anywhere anywhere
5 350 ACCEPT 0 -- lo any anywhere anywhere state NEW
574 36309 logaccept 0 -- br0 any anywhere anywhere state NEW
346 38780 DROP 0 -- any any anywhere anywhere

But there isn't.. :D

Version information:

root@ROUTER:~# iptables -V
iptables v1.3.7

WHY?

Firmware: DD-WRT v24-sp2 (08/07/10) std

Answer 1

iptables -I INPUT -i ppp0 --dport 21 -j DROP is syntactically wrong (it lacks of protocol specification), therefore we can assume it just silently fails. This one is correct:

iptables -I INPUT -i ppp0 --dport 21 -j DROP -p tcp

so give it a try instead.