Linux firewall状态查看和设置修改

本文介绍如何在CentOS中使用iptables以及在Ubuntu中使用ufw进行防火墙配置。主要内容包括iptables的基本命令如查看规则、增加规则及重启服务等;ufw的安装与基本使用方法,例如启用、关闭防火墙,查看防火墙状态,以及如何开启或禁用特定端口和服务。

(1)centos – iptables

http://blog.youkuaiyun.com/huaishuming/article/details/41518881
http://blog.youkuaiyun.com/bslzl/article/details/7937899/
首先查看当前规则:
iptables –list
增加:
iptables -A INPUT -s xxx.xx.xxx.xxx -j ACCEPT
重启服务
service iptables restart

(2)ubuntu – ufw

安装方法
sudo apt-get install ufw
使用方法
1 启用
sudo ufw enable
sudo ufw default deny
作用:开启了防火墙并随系统启动同时关闭所有外部对本机的访问(本机访问外部正常)。
2 关闭
sudo ufw disable
2 查看防火墙状态
sudo ufw status
3 开启/禁用相应端口或服务举例
sudo ufw allow 80 允许外部访问80端口
sudo ufw delete allow 80 禁止外部访问80 端口
sudo ufw allow from 192.168.1.1 允许此IP访问所有的本机端口
sudo ufw deny smtp 禁止外部访问smtp服务
sudo ufw delete allow smtp 删除上面建立的某条规则
sudo ufw deny proto tcp from 10.0.0.0/8 to 192.168.0.1 port 22 要拒绝所有的TCP流量从10.0.0.0/8 到192.168.0.1地址的22端口
可以允许所有RFC1918网络(局域网/无线局域网的)访问这个主机(/8,/16,/12是一种网络分级):
sudo ufw allow from 10.0.0.0/8
sudo ufw allow from 172.16.0.0/12
sudo ufw allow from 192.168.0.0/16
推荐设置
sudo apt-get install ufw
sudo ufw enable
sudo ufw default deny
这样设置已经很安全,如果有特殊需要,可以使用sudo ufw allow开启相应服务。

### USACO 2016 January Contest Subsequences Summing to Sevens Problem Solution and Explanation In this problem from the USACO contest, one is tasked with finding the size of the largest contiguous subsequence where the sum of elements (IDs) within that subsequence is divisible by seven. The input consists of an array representing cow IDs, and the goal is to determine how many cows are part of the longest sequence meeting these criteria; if no valid sequences exist, zero should be returned. To solve this challenge efficiently without checking all possible subsequences explicitly—which would lead to poor performance—a more sophisticated approach using prefix sums modulo 7 can be applied[^1]. By maintaining a record of seen remainders when dividing cumulative totals up until each point in the list by 7 along with their earliest occurrence index, it becomes feasible to identify qualifying segments quickly whenever another instance of any remainder reappears later on during iteration through the dataset[^2]. For implementation purposes: - Initialize variables `max_length` set initially at 0 for tracking maximum length found so far. - Use dictionary or similar structure named `remainder_positions`, starting off only knowing position `-1` maps to remainder `0`. - Iterate over given numbers while updating current_sum % 7 as you go. - Check whether updated value already exists inside your tracker (`remainder_positions`). If yes, compare distance between now versus stored location against max_length variable's content—update accordingly if greater than previous best result noted down previously. - Finally add entry into mapping table linking latest encountered modulus outcome back towards its corresponding spot within enumeration process just completed successfully after loop ends normally. Below shows Python code implementing described logic effectively handling edge cases gracefully too: ```python def find_largest_subsequence_divisible_by_seven(cow_ids): max_length = 0 remainder_positions = {0: -1} current_sum = 0 for i, id_value in enumerate(cow_ids): current_sum += id_value mod_result = current_sum % 7 if mod_result not in remainder_positions: remainder_positions[mod_result] = i else: start_index = remainder_positions[mod_result] segment_size = i - start_index if segment_size > max_length: max_length = segment_size return max_length ```
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