add ftp

cspell_dict_bash.txt

@@ -541,4 +541,9 @@ functors
 chrono
 endl
 mutex
-ofstream
+ofstream
+
+PASV
+vsftpd
+ftpusers
+xferlog

docs/source/index.rst

@@ -113,6 +113,7 @@ The organization of this site is as following:
    reference/linux/introduction-to-logs-and-rsyslog
    reference/linux/linux-service
    reference/linux/linux-email-service
+   reference/linux/ftp
 
 
 .. toctree::

docs/source/reference/linux/ftp.md

@@ -0,0 +1,174 @@
+# FTP
+
+File Transfer Protocol
+
+Old and popular protocol to share files. It uses TCP.
+
+Client-server architecture.
+
+ports: 20, 21
+
+20: data port
+21: ftp server command port
+
+The ports of client are usually unprivileged, i.e. a number larger than 1023
+
+## Mode
+
+FTP runs in 2 modes.
+
+### Active Mode
+
+1. Client prepares a random port P1 to receive data
+2. Client sends FTP command PORT P1 to the server port 21 from a random port P2
+3. Server initializes one data channel from its port 20 to port P1 on client
+
+Problem: port P1 is usually behind firewall.
+
+### Passive Mode
+
+1. Client sends PASV command to server port 21 from an arbitrary port P2
+2. Server sends an IP address and a port P3 to client port P2
+3. Client initializes one data channel from another port P1 to port P3 of server
+
+-----------------------------------
+
+There is an additional mode, extended passive mode.
+
+[In FTP, what are the differences between passive and extended passive modes?](in-ftp-what-are-the-differences-between-passive-and-extended-passive-modes)
+
+Passive mode is more common than active mode.
+
+## vsftpd
+
+very secure FTP daemon
+
+Most popular FTP.
+
+### Path
+
+There are generally 3 main config files.
+
+On RedHat/CentOS:
+
+```bash
+/etc/vsftpd/vsftpd.conf # main
+/etc/vsftpd/ftpusers    # list of users DISALLOWED to log in ftp, such as root
+/etc/vsftpd/user_list   # another user list
+                        # either blacklist or whitelist, controlled by main config file
+```
+
+On Ubuntu:
+
+```bash
+/etc/vsftpd.conf
+/etc/ftpusers
+# Not sure where the user_list file is
+```
+
+Shared directory:
+
+* different distribution has different locations
+
+```bash
+/var/ftp/
+/srv/ftp/
+```
+
+Logs:
+
+* some possible locations
+* can be changed in config file
+
+```bash
+/var/log/vsftpd.log
+/var/log/xferlog
+```
+
+Some config snippet:
+
+```bash
+xferlog_enable=YES
+xferlog_std_format=NO
+vsftpd_log_file=/var/log/xferlog
+```
+
+### Config
+
+By default, everyone can access the default shared directory.
+
+Read the man page of specific distribution to get more tuned config.
+
+[Debian vsftpd.conf Man Page](https://manpages.debian.org/stretch/vsftpd/vsftpd.conf.5.en.html)
+
+#### Disable anonymous user
+
+In `/etc/vsftpd/conf`:
+
+```bash
+anonymous_enable=NO
+```
+
+#### Allow anonymous upload
+
+[Allow anonymous upload for Vsftpd?](https://serverfault.com/questions/247096/allow-anonymous-upload-for-vsftpd)
+
+Change config
+
+* In `/etc/vsftpd/conf`:
+
+```bash
+listen=YES
+anonymous_enable=YES
+write_enable=YES
+anon_upload_enable=YES
+anon_mkdir_write_enable=YES
+```
+
+* And restart ftp service.
+
+Change permission of directory used to upload
+
+```bash
+chown -R ftp /var/ftp
+```
+
+Not sure if steps above work,
+but consider SELinux and search around when there is problem.
+
+## FTP User
+
+FTP uses user to manage. Clients must log in as a user.
+
+There are 3 type of users for FTP
+
+1. system user
+    * all the normal users used to login the system
+    * same permission as that user
+    * default directory is the home directory
+    * **all** users of the system can be used to log in FTP, so we need "blacklist" (as mentioned above)
+2. anonymous user
+    * no authentication required
+    * actually a user called "ftp" whose home is "/var/ftp/"
+    * will be created when installing vsftpd
+    * can be disabled
+3. FTP virtual user
+    * FTP-only users
+
+No need to change any config if anonymous user is enough.
+
+## How to Use FTP
+
+GUI is very simple to use.
+
+For CLI use `ftp` command.
+
+## Sample Usage
+
+An FTP directory where people can upload but cannot see the content:
+
+```bash
+cd /var/ftp/
+mkdir dropbox-upload
+chmod 2733 dropbox-upload
+```