Linux中iptalbes -t nat -nvL是什么意思

iptables是Linux中的一个防火墙，它常用的有三个表（filter,nat,mangle)，每个表有三个链(input,forward,output)

iptables -t 表名是指定nat表

-nvL 这其实是三个参数，等效于 -n -v -L

-n 不解析主机名和端口名，也就是全部主机和端口都用数字表示

-v 详细信息列表

-L 列表

具体参考 iptables -h

[root@localhost ~]# iptables -h

iptables v1.3.5

Usage: iptables -[AD] chain rule-specification [options]

iptables -[RI] chain rulenum rule-specification [options]

iptables -D chain rulenum [options]

iptables -[LFZ] [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 chainAppend to chain

--delete -D chainDelete 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] List the rules in a chain or all chains

--flush -F [chain] Delete all rules in chain or all chains

--zero-Z [chain] Zero counters in chain or all chains

--new -N chainCreate 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:

--proto -p [!] protoprotocol: 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 tabletable to manipulate (default: `filter')

--verbose -v verbose mode

--line-numbersprint line numbers when listing

--exact -x expand numbers (display exact values)

[!] --fragment -f match second or further fragments only

--modprobe=<command> try to insert modules using this command

--set-counters PKTS BYTES set the counter during insert/append

[!] --version -V print package version.

[root@localhost ~]#

格式为：NVL( string1, replace_with) 功能：如果string1为NULL，则NVL函数返回replace_with的值，否则返回string1的值。 注意事项：string1和replace_with必须为同一数据类型，除非显示的使用TO_CHAR函数。 例：NVL(TO_CHAR(numeric_column), 'some string') 其中numeric_column代指某个数字类型的值。 例：nvl(yanlei777,0) >0 NVL(yanlei777, 0) 的意思是 如果 yanlei777 是NULL， 则取 0值 通过查询获得某个字段的合计值，如果这个值为null将给出一个预设的默认值 例如：select nvl(sum(t.dwxhl),1) rom tb_jhde t 就表示如果sum(t.dwxhl) = NULL 就返回 1 另一个有关的有用方法 declare i integer select nvl(sum(t.dwxhl),1) into i from tb_jhde t where zydm=-1这样就可以把获得的合计值存储到变量i中，如果查询的值为null就把它的值设置为默认的1 orcale中: select nvl(rulescore,0) from zwjc_graderule where rulecode='FWTD' 如果记录中不存在rulecode ='FWTD'的数据.则查不出数据. select nvl(rulescore,0) into rule_score from zwjc_graderule where rulecode='FWTD'会报查不到数据的错 select nvl(sum(rulescore),0) from zwjc_graderule where rulecode='FWTD' 如果记录中不存在rulecode ='FWTD'的数据.还是可以得到一行列名为nvl(rulescore,0),值为0的数据. select nvl(sum(rulescore),0) into rule_score from zwjc_graderule where rulecode='FWTD'不会报错

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