原文：Packet Filtering HOWTO
Packet Filtering HOWTO 讀後筆記I
7. Using iptables
Extensions to iptables: New Matches
iptables具備可擴充性(extensible)，這表示我們可以在kernel或是iptable上新增新的功能。目前kernel和iptables已經具備許多的標準擴充功能，您可以在kernel/net/netfilter目錄底下看到許多的xt_XXX.c。在iptables也會有相對應的extensions，在extensions/libxt_XXX.c，iptables的extensions會被build成shared library，需要使用-m將extensions載入。如果要取得extensions('-p'的說明，只要在後面加上-h或是--help即可。
# iptables -p tcp --help
iptables v1.4.12
Usage: iptables -[ACD] chain rule-specification [options]
iptables -I chain [rulenum] rule-specification [options]
iptables -R chain rulenum rule-specification [options]
iptables -D chain rulenum [options]
iptables -[LS] [chain [rulenum]] [options]
iptables -[FZ] [chain] [options]
iptables -[NX] chain
iptables -E old-chain-name new-chain-name
iptables -P chain target [options]
iptables -h (print this help information)
Commands:
Either long or short options are allowed.
--append -A chain Append to chain
--check -C chain Check for the existence of a rule
--delete -D chain Delete matching rule from chain
--delete -D chain rulenum
Delete rule rulenum (1 = first) from chain
--insert -I chain [rulenum]
Insert in chain as rulenum (default 1=first)
--replace -R chain rulenum
Replace rule rulenum (1 = first) in chain
--list -L [chain [rulenum]]
List the rules in a chain or all chains
--list-rules -S [chain [rulenum]]
Print the rules in a chain or all chains
--flush -F [chain] Delete all rules in chain or all chains
--zero -Z [chain [rulenum]]
Zero counters in chain or all chains
--new -N chain Create a new user-defined chain
--delete-chain
-X [chain] Delete a user-defined chain
--policy -P chain target
Change policy on chain to target
--rename-chain
-E old-chain new-chain
Change chain name, (moving any references)
Options:
--ipv4 -4 Nothing (line is ignored by ip6tables-restore)
--ipv6 -6 Error (line is ignored by iptables-restore)
[!] --proto -p proto protocol: by number or name, eg. `tcp'
[!] --source -s address[/mask][...]
source specification
[!] --destination -d address[/mask][...]
destination specification
[!] --in-interface -i input name[+]
network interface name ([+] for wildcard)
--jump -j target
target for rule (may load target extension)
--goto -g chain
jump to chain with no return
--match -m match
extended match (may load extension)
--numeric -n numeric output of addresses and ports
[!] --out-interface -o output name[+]
network interface name ([+] for wildcard)
--table -t table table to manipulate (default: `filter')
--verbose -v verbose mode
--line-numbers print line numbers when listing
--exact -x expand numbers (display exact values)
[!] --fragment -f match second or further fragments only
--modprobe= try to insert modules using this command
--set-counters PKTS BYTES set the counter during insert/append
[!] --version -V print package version.
tcp match options:
[!] --tcp-flags mask comp match when TCP flags & mask == comp
(Flags: SYN ACK FIN RST URG PSH ALL NONE)
[!] --syn match when only SYN flag set
(equivalent to --tcp-flags SYN,RST,ACK,FIN SYN)
[!] --source-port port[:port]
--sport ...
match source port(s)
[!] --destination-port port[:port]
--dport ...
match destination port(s)
[!] --tcp-option number match if TCP option set
Other Match Extensions
mac 這個module必須用'-m mac'或'--match mac'載入，用於比對source MAC，只能用於PREOUTING和INPUT這兩個chain。
limit 這個module必須用'-m limit'或'--match limit'載入，用於限制match的速率。--limit單位可以是/second、/minute、/hour或是/day，如5/s，每秒5個。--limit-burstnumber to match in a burst。用下圖來描述這兩者的涵義
limit-burst就是水桶的容量，limit就是水桶流出的流量，當packet裝滿水桶，後面的就會被丟棄，隨著時間過去，水桶會流出lmit個packet，就是可以再允許limit流進水桶。
7.4 Target Specifications
何謂target？what to do to the packets which match our tests. This is called a rule's target.(當match我們的條件之後，所要採取的動作就稱為rule的target)。target也可以是User-defined chains。
iptables是first match，packet進入INPUT這個chain後，會由第一條rule開始比對，如果比對成功並且有明確的target(ACCRPT/DROP)就會執行丟棄/同意packet並且終止比對，當target是另外一個chain時，就會jump到該chain中繼續進行比對，如果都沒有match，就會再回到原來的chain中繼續往下比對。
# iptables -A INPUT -p icmp
# iptables -A INPUT -p icmp -j LOG
# iptables -A INPUT -p icmp -j ACCEPT
# iptables -L -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
0 0 icmp -- any any anywhere anywhere
0 0 LOG icmp -- any any anywhere anywhere
LOG level warning
0 0 ACCEPT icmp -- any any anywhere anywhere
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
# ping 127.0.0.1 -c 1
PING 127.0.0.1 (127.0.0.1): 56 data bytes
[ 6975.682678] IN=lo OUT= MAC=00:00:00:00:00:00:00:00:00:00:00:00:08:00 SRC=127.
0.0.1 DST=127.0.0.1 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 C
ODE=0 ID=4908 SEQ=0
[ 6975.697694] IN=lo OUT= MAC=00:00:00:00:00:00:00:00:00:00:00:00:08:00 SRC=127.
0.0.1 DST=127.0.0.1 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=49768 PROTO=ICMP TYPE=0
CODE=0 ID=4908 SEQ=0
64 bytes from 127.0.0.1: seq=0 ttl=64 time=34.397 ms
--- 127.0.0.1 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 34.397/34.397/34.397 ms
# iptables -L -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
2 168 icmp -- any any anywhere anywhere
2 168 LOG icmp -- any any anywhere anywhere
LOG level warning
2 168 ACCEPT icmp -- any any anywhere anywhere
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 2 packets, 168 bytes)
pkts bytes target prot opt in out source destination
Extensions to iptables: New Targets
還有LOG、REJECT、RETURN和QUEUE等target。詳細用法請自行參考該page。
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