Network (mainly Linux) notes

A memorandum of what I did not understand when studying on Linux (ubuntu18.04) regarding networks I will leave it here.

netns(network namespace)

--PC You can always build an independent network Example: Create a network called hellorold with sudo ip netns add helloworld

--sudo ip netns exec helloworld bash starts bash on helloworld. Exit with exit --sudo ip netns delete Delete hello world with hello world.

--Use veth as follows to connect network namespaces

--Example: sudo ip link add ns1-veth0 type veth peer name ns2-veth0 (connect ns1 and ns2)

Also, since it is necessary to change the network state from down to up, do as follows. - sudo ip netns exec ns1 ip link set ns1-veth0 up - sudo ip netns exec ns2 ip link set ns2-veth0 up

--Set a default route when communicating to a location other than the same network --By making the route to the router the default route, you can send a request to the router for an IP address that is not in the routing table. --If you still cannot, the following settings are insufficient --sudo ip netns exec (router name) sysctl net.ipv4.ip_forward = 1 This shows if it can act as an ipv4 router

--A routing table is also required for communication between routers. Therefore, the local one is added to the table by static routing that adds the routing table by sending a command. Global ones do dynamic routing (protocols such as BGP, OSPF, etc. are used)


--Synonymous with switching hub


――A port is like a room in an apartment. It becomes a 16-bit numerical value. --The port value is determined by the application. --0 ~ 1023: System port --1024 ~ 49151: User port, or register port --49152 ~ 65535: Dynamic port or private port

--nc -ulnv ip (address) (port number) Start as a server with. "-U: UDP communication" "-L: Act as a server" "-N: Prevent IP addresses from being resolved by DNS" "-V: To display commands in detail" --nc -u ip (address) (port number) Launched as a client


--When connecting, check the connection with a 3-way handshake. --Check the flags such as SYN, ACK with the 6-bit control bit in the header. --SYN: Synchronize the sequence numbers with each other by sending only packets with this bit. In other words, manage the order of data --ACK: Recognized as ACK when the corresponding bit is set

--P is included in the control bit flag when sending a string

Application layer


--The port number used is TCP 80

You can issue an http request in the form of

--Also, if the server also has python3

You can start the server like this. At this time, the page displayed at the time of access will be index.html on the current directory.


--The DNS server is often an external one. For example, google's ( --The resolver on your PC decides which DNS to use.

--Locally, for linux, there is a list for DNS resolution in the "/ etc / hosts" file.

--The port used is UDP port 53


――As a role --IP address allocation --Creating a routing table --Specify the name server used for name resolution I do

--The communication method is server / client method. In many cases, the router is mainly responsible for the function of the DHCP server.

--The port used is UDP port 67


--Translate between local address and global address.

--Used on linux with iptables command

Socket programming in python

socket (): Specifies what kind of communication is performed on the socket bind (): Specify the IP address and port number to listen for the connection listen (): Start listening for a connection accept (): Process the connected client (wait for connection in this function) send () / recv (): receive byte sequence close (): close the connection


-- Loopback address. Used when you want to communicate with yourself on the network

--tcpdump: Capture (sniffing) tcp / ip data flowing on your computer

--Without the routing table, communication to the outside is not possible. --To see the routing table, "ip route show" --A default route is required to communicate to the outside --The default route is the routing entry used when it does not match other destinations.

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