Summarize methods to disable host key checking while connecting ssh.

Environment and Prerequisite

• Linux
• SSH(OpenSSH)

Host Key Checking

• When we connect to remote server by using ssh, client compares client’s saved key and remote server’s key to authenticate server.
• This process prevents from Man-in-the-middle attack.

Disable Host Key Checking When Using SSH

• Remote host’s key will be added if there is no added key in ~/.ssh/known_hosts.
• Connect to remote server even though saved key is different from server’s key.

1. Disable when using ssh command

• Use StrictHostKeyChecking=no option.

ssh -o StrictHostKeyChecking=no [DOMAIN_OR_IP]

2. Edit ssh config file

~/.ssh/config

• Add option to specific host like below.

Host twpower-private-server
    HostName [IP ADDRESS]
    StrictHostKeyChecking no
    Port 22
    User [USERNAME]
    IdentityFile [IDENTITY KEY FILE]

/etc/ssh/ssh_config

• There is * in config file. So this disables host key checking to all hosts.
• /etc/ssh/ssh_config defines all the default settings for the client utilities for all users on that system.

Host *
    StrictHostKeyChecking no

Reference

• https://askubuntu.com/questions/87449/how-to-disable-strict-host-key-checking-in-ssh
• https://en.wikibooks.org/wiki/OpenSSH/Client_Configuration_Files#~/.ssh/known_hosts

파이썬(Python) List Comprehensions에 대해 알아보자.

환경

• Linux
• Python 3.X.X

파이썬(Python) List Comprehensions

List Comprehensions

results=[]
for i in range(10):
    results.append(i*2)

results=[x*2 for x in range(10)]

• List Comprehensions: 파이썬에서 리스트 자료구조를 조금 더 간결하게 만들 수 있도록 도와주는 기능입니다.
• 리스트를 새로 생성할 때 각각의 원소가 어떠한 연산의 결과이거나 조건을 만족하는 부분집합을 만들 때 사용할 수 있습니다.

형태

list_name = [element_format for clause]

• [ ]로 감싸고 제일 앞에 추가할 원소의 형태를 넣으며 그 다음에 for문을 넣습니다.
• 아래에 나오지만 for문 중첩도 가능합니다.

예제

제곱

results=[]
for i in range(10):
    results.append(i**2)

results=[x**2 for x in range(10)]

튜플 만들기

results=[]
for x in [1,3,5]:
    for y in [2,4,6]:
        results.append((x,y))

results=[(x,y) for x in [1,3,5] for y in [2,4,6]]

함수와 함께 사용하기

from math import pi

results=[]
for i in range(1, 6):
    results.append(str(round(pi, i)))

from math import pi
results=[str(round(pi, i)) for i in range(1, 6)]

중첩

results=[]
for x in [1,2]:
    for y in [3,4]:
        for z in [5,6]:
            results.append((x,y,z))

results=[(x,y,z) for x in [1,2] for y in [3,4] for z in [5,6]]

조건문과 함께 사용하기

results=[]
for x in range(20):
    if x%2 == 0:
        results.append(x)

results=[x for x in range(20) if x%2 == 0]

참고자료

• https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions

Practice Python List Comprehensions

Environment and Prerequisite

• Linux
• Python 3.X.X

Python List Comprehensions

List Comprehensions

results=[]
for i in range(10):
    results.append(i*2)

results=[x*2 for x in range(10)]

• List Comprehensions: List comprehensions provide a concise way to create lists. It can make list much simpler.
• It can be used when you make new list which is result of each element’s operation or subset elements of some conditions.

Format

list_name = [element_format for clause]

• Cover with brackets([ ]), add element format in the front and add for clause in the end.
• As like below example, we can also use nested for loop.

Examples

Square

results=[]
for i in range(10):
    results.append(i**2)

results=[x**2 for x in range(10)]

Make tuple

results=[]
for x in [1,3,5]:
    for y in [2,4,6]:
        results.append((x,y))

results=[(x,y) for x in [1,3,5] for y in [2,4,6]]

Use with function

from math import pi

results=[]
for i in range(1, 6):
    results.append(str(round(pi, i)))

from math import pi
results=[str(round(pi, i)) for i in range(1, 6)]

Nested loop

results=[]
for x in [1,2]:
    for y in [3,4]:
        for z in [5,6]:
            results.append((x,y,z))

results=[(x,y,z) for x in [1,2] for y in [3,4] for z in [5,6]]

Use with if statement

results=[]
for x in range(20):
    if x%2 == 0:
        results.append(x)

results=[x for x in range(20) if x%2 == 0]

Reference

• https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions

systemd service의 Type 옵션 중에서 simple과 oneshot의 차이를 정리해보자.

환경

• Linux
• systemd

목표

• 평소에 모호했던 systemd service의 Type 옵션중 하나인 simple과 oneshot의 차이에 대해 직접 정리해본다.
• 이 포스트에서 systemd나 service에 대해서는 정리하지 않습니다.
• Reference를 참고해 작성하였습니다.

systemd service type

simple

[Service]
Type=simple

• 메인 프로세스가 시작되면 바로 systemd unit이 정상적으로 실행되었다고 판단하고 active 상태로 바꾼다. 실행된 메인 프로세스가 끝날때까지 기다려주지 않고 연결된 다음 systemd unit을 실행한다. 만약 네트워크를 다 올리는 작업같이 기다렸다가 다음 systemd unit을 실행해야하는 상황이면 다른 옵션과 함께 사용하던가 아래 oneshot을 이용해야 한다.
• ExecStart=를 하나만 가질 수 있다.
• 프로세스가 실행되면 active 상태가 되며 다 끝났을때 inactive가 됩니다.

oneshot

[Service]
Type=oneshot

• 메인 프로세스가 시작되면 상태를 activating으로 바꾸고 끝나때까지 기다리며 해당 메인 프로세스가 끝나야지만 다음 systemd unit으로 넘어간다.
• ExecStart=를 여러개 가질 수 있다.
• 프로세스가 실행되면 activating 상태가 되며 프로세스가 다 끝나고 inactive 상태로 바뀐다. 만약, RemainAfterExit=true 옵션을 사용하면 프로세스가 다 끝났을때 inactive가 아닌 active 상태가 된다.(실행되고 있는 프로세스는 없을수도 있다!)

둘의 공통점

• forking된 프로세스에 대해서는 신경쓰지 않으며 자식 프로세스를 생성하고 싶다면 forking 옵션을 사용해야한다.

참고자료

• https://stackoverflow.com/questions/39032100/what-is-the-difference-between-systemds-oneshot-and-simple-service-types/39050387#39050387
• https://www.freedesktop.org/software/systemd/man/systemd.service.html
• https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files

Summarize difference between simple and oneshot in systemd service’s Type option

Environment and Prerequisite

• Linux
• systemd

Goal

• Summarize by myself about difference between simple and oneshot in systemd service’s Type option.
• This post does not explain about systemd or service
• I referred to many things on Reference.

systemd service type

simple

[Service]
Type=simple

• It changes its state to active because its option considers process started successfully just after starting the main process. It does not wait until the main process exits and moves to next systemd unit. If you are in condition like wait for the network up and then run the next systemd unit, you rather use oneshot option or use other options.
• It can only has one ExecStart=.
• State will be changed to active after running process and it will be changed to inactive after finish of it.

oneshot

[Service]
Type=oneshot

• It waits until the main process exits in activating state after running the main process. It moves to next systemd unit after the main process exits.
• It can have multiple ExecStart=.
• State will be changed to activating after running process and it will be changed to inactive after finish of it. If use RemainAfterExit=true option, then state will be changed to active not inactive(even though there is no running process!)

common things between two

• Both don’t care about forking child processes. Use Type=forking instead.

Reference

• https://stackoverflow.com/questions/39032100/what-is-the-difference-between-systemds-oneshot-and-simple-service-types/39050387#39050387
• https://www.freedesktop.org/software/systemd/man/systemd.service.html
• https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files