Shulou(Shulou.com)06/02 Report--

Today, on a whim, I studied the ftp protocol and built a ftp server in linux, mainly studying the active and passive modes of ftp.

Let's briefly talk about ftp installation and configuration.

Ftp server: 192.168.3.14 linux vsftpd

Ftp client: windows 7

Check if ftp is installed

Run yum install vsftpd-y directly without installation

Modify ftp configuration file

Vim / etc/vsftpd/vsftpd.conf

Whether chroot_local_user=YES-- restricts all users to the home directory, YES is enabled

Anonymous_enable=NO-Anonymous access is not allowed

Whether chroot_list_enable=YES-- starts the list of restricted users YES is enabled

Whether chroot_list_file=/etc/vsftpd/chroot_list-- is restricted to the list of users under the home directory

Save with the above changes

Create a user authentication file chroot_list under the / etc/vsftpd directory and add local system users for ftp authentication

[root@jw vsftpd] # cat chroot_list-- add local operating system users to authenticate one account per line

Ftpuser

Ftptest

Change password: passwd ftptest

Password: niceman

Restart the service and see if port 21 is listening

Use lsof to see if the port is listening

Now start to visit ftp; Note, before access, please turn off selinux or temporarily close the firewall, specific in-depth ftp server configuration, please check the documents of Baidu, do not repeat.

Install wireshark

Yum install wireshark-y

Bag grab command: tshark-ni eth0-R "ftp"

It can be found that ftp is transmitted in clear text.

Tshark-ni eth0-R "tcp.dstport eq 21"-- grab packet control port

Next, let's talk about two modes of FTP.

Active Mode of FTP

Active FTP:

Command connection: client is greater than port 1023-> server port 21

Data connection: client is greater than 1023 port server 21 port

Data connection: client is greater than port 1023-> server > port 1023

The client still uses a port greater than 1023 to link to the FTP21 command control port that the server listens to, the server opens a random port greater than 1023 for data transmission to the client, and the passive mode server no longer uses 20 data port for transmission.

Note: windows uses IE or explorer to open the input address ftp://ip access, whether active or passive mode depends on the configuration here, see screenshot; IE--Internet option-Advanced

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