linux - iptables: why only OUTPUT rules are needed for samba clients?

I tried the following iptables rules for samba client and they worked. Please note that policy for INPUT, OUTPUT and FORWARD were all set to DROP

iptables -A OUTPUT -m state --state NEW,ESTABLISHED -p udp --dport 137 -j ACCEPT

iptables -A OUTPUT -m state --state NEW,ESTABLISHED -p udp --dport 138 -j ACCEPT
iptables -A OUTPUT -m state --state NEW,ESTABLISHED -p tcp --dport 139 -j ACCEPT
iptables -A OUTPUT -m state --state NEW,ESTABLISHED -p tcp --dport 445 -j ACCEPT

why we only need OUTPUT rules for samba clients? Why don't we need INPUT rules to open those ports for incoming packets?

An additional question: does the chain names carry any significance of directions internally or are they just mnemonics for easy understanding?

iptables:

-------------
# Generated by iptables-save v1.4.7 on Wed Aug 28 21:18:39 2013
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT DROP [4:284]
-A INPUT -p udp -m udp --dport 177 -j ACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT

-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A INPUT -p tcp -m tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --sport 21 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 20 -m state --state NEW,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --sport 1024:65535 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --sport 1024:65535 --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --sport 1024:65535 --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --sport 80 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --sport 443 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

-A INPUT -p icmp -m icmp --icmp-type 8 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -m icmp --icmp-type 0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --sport 25 -m state --state ESTABLISHED -j ACCEPT
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
-A OUTPUT -p tcp -m tcp --dport 7100 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 6000 -j ACCEPT
-A OUTPUT -p udp -m udp --sport 177 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --sport 22 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT

-A OUTPUT -p tcp -m tcp --sport 20 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --sport 1024:65535 --dport 1024:65535 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --sport 80 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --sport 443 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --sport 1024:65535 --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --sport 1024:65535 --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p icmp -m icmp --icmp-type 0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -p icmp -m icmp --icmp-type 8 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p udp -m state --state NEW,ESTABLISHED -m udp --dport 137 -j ACCEPT

-A OUTPUT -p udp -m state --state NEW,ESTABLISHED -m udp --dport 138 -j ACCEPT
-A OUTPUT -p tcp -m state --state NEW,ESTABLISHED -m tcp --dport 139 -j ACCEPT
-A OUTPUT -p tcp -m state --state NEW,ESTABLISHED -m tcp --dport 445 -j ACCEPT
COMMIT
# Completed on Wed Aug 28 21:18:39 2013

Answer

The default chain names are most definitely involved in the packet flow. There are many diagrams all over the internet showing the various paths a packet might take through the chains, but in general for your scenario traffic from the machine will traverse output, and traffic to the machine will traverse input. They will traverse other chains too, but that doesn't likely matter for the scope of this question.

Also recall that iptables works on a first dispositive match basis (the first match which disposes of a packet, such as by accepting, rejecting or dropping it, causes processing of the chain to stop also). So, none of your input rules after -A INPUT -j REJECT --reject-with icmp-host-prohibited have any effect.

With that said, the reason your samba connections are working is this input rule: -m state --state RELATED,ESTABLISHED -j ACCEPT. That is because when you connect to another samba host, conntrack will record the connection state and this rule will begin accepting the return traffic. I suspect you would find, if you tried to serve something from this box, that nobody could access it.