Chapter 10 - Network Applications and Services
Created by : Mr Dk.
2019 / 07 / 09 23:58
@NUAA, Nanjing, Jiangsu, China
又是水得不行的一章... 🤨
10.2 Network Servers
- httpd, apache, apache2：Web servers
- sshd：Secure shell
- postfix, qmail, sendmail：Mail servers
- cupsd：Print server
- nfsd, mountd：Network filesystem daemons
- smbd, nmbd：WIndows file-sharing daemons
- rpcbind：Remote procedure call (RPC) portmap service deamon
网络服务器的共同特征：多进程。
- 至少一个进程监听端口
- 当接收到连接后，监听进程使用
fork()创建新的子进程处理连接 - 同时，监听进程继续监听端口
其中，调用 fork() 给系统带来了较大开销。因此，一些高性能的 TCP 服务器会在启动时创建大量工作进程：worker process。
10.3 Secure Shell (SSH)
支持远程登录、程序执行、文件共享。使用公钥密码体制替代了老旧的、不安全的 telnet 和 rlogin。OpenSSH 是最受欢迎的免费 SSH 实现。
10.3.1 The SSHD Server
运行 sshd 服务需要一个配置文件和密钥：/etc/ssh
- 服务器配置文件：
sshd_config - 客户端配置文件：
ssh_config
服务器配置文件中可以配置监听端口和密钥路径。
Host Keys
SSH version 1 只有 RSA 密钥，SSH version 2 有 RSA 和 DSA 密钥。使用 ssh-keygen 可以生成公私钥对。
10.3.2 The SSH Client
ssh username@host
在 .ssh/known_hosts 中保存了远程服务器的公钥。如果远程服务器的公钥与以前的公钥发生了冲突，比如替换了硬件或操作系统，将会产生警告。将以前的公钥记录删除即可。
SSH File Transfer Clients
scp user1@host1:file user2@host2:dir
sftp 使用 get 和 put 命令，服务器需要运行 sftp-server。
10.4 The inetd and xinetd Daemons
为了避免分别运行每种服务的麻烦，inetd 读取配置文件，监听对应的端口。当接受到连接时，inetd 将连接分配到对应的进程。
10.5 Diagnostic Tools
10.5.1 lsof
lsof 可用于追踪所有打开的文件，也可以查看所有正在监听端口的程序：
$ lsof -i
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
systemd-r 617 systemd-resolve 12u IPv4 14501 0t0 UDP localhost:domain
systemd-r 617 systemd-resolve 13u IPv4 14502 0t0 TCP localhost:domain (LISTEN)
v2ray 734 root 3u IPv6 17736 0t0 TCP *:23591 (LISTEN)
sshd 770 root 3u IPv4 17201 0t0 TCP *:ssh (LISTEN)
sshd 770 root 4u IPv6 17322 0t0 TCP *:ssh (LISTEN)
ssserver 1178 root 4u IPv4 20298 0t0 TCP client-23-254-225-164.hostwindsdns.com:12306 (LISTEN)
ssserver 1178 root 5u IPv4 20299 0t0 UDP client-23-254-225-164.hostwindsdns.com:12306
ssserver 1178 root 8u IPv4 20300 0t0 UDP *:50810
sshd 17879 root 3u IPv4 1012568 0t0 TCP client-23-254-225-164.hostwindsdns.com:ssh->157.0.78.120:65096 (ESTABLISHED)
使用 -n 选项禁止 IP 地址和熟知端口的反向解析：
$ lsof -n -i
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
systemd-r 617 systemd-resolve 12u IPv4 14501 0t0 UDP 127.0.0.53:domain
systemd-r 617 systemd-resolve 13u IPv4 14502 0t0 TCP 127.0.0.53:domain (LISTEN)
v2ray 734 root 3u IPv6 17736 0t0 TCP *:23591 (LISTEN)
sshd 770 root 3u IPv4 17201 0t0 TCP *:ssh (LISTEN)
sshd 770 root 4u IPv6 17322 0t0 TCP *:ssh (LISTEN)
ssserver 1178 root 4u IPv4 20298 0t0 TCP 23.254.225.164:12306 (LISTEN)
ssserver 1178 root 5u IPv4 20299 0t0 UDP 23.254.225.164:12306
ssserver 1178 root 8u IPv4 20300 0t0 UDP *:50810
sshd 17879 root 3u IPv4 1012568 0t0 TCP 23.254.225.164:ssh->157.0.78.120:65096 (ESTABLISHED)
并可以使用各种过滤规则。
10.5.2 tcpdump
10.5.3 netcat
netcat host port
监听一个特定端口：
netcat -l -p port_number
10.5.4 Port Scanning
Network Mapper (Nmap) 程序可以扫描一台机器所有开放的端口：
$ nmap 103.235.46.39
Starting Nmap 7.60 ( https://nmap.org ) at 2019-07-09 13:32 UTC
Nmap scan report for 103.235.46.39
Host is up (0.20s latency).
Not shown: 998 filtered ports
PORT STATE SERVICE
80/tcp open http
443/tcp open https
Nmap done: 1 IP address (1 host up) scanned in 20.10 seconds
10.6 Remote Procedure Call (RPC)
在远程机器上调用函数，并返回一个结果码或消息。RPC 的实现需要将程序编号和传输层端口号映射。
10.9 Sockets: How Processes Communicate with the Network
Socket 是进程通过内核访问网络的接口，表示了用户空间和内核之间的界限，也被用于进程间通信 (inter-process communication, IPC)。有多种不同类型的 socket：
- stream socket：
SOCK_STREAM用于 TCP - datagram sockets：
SOCK_DGRAM用于 UDP
10.10 Unix Domain Sockets
在同一台机器上运行的进程，使用进程间通信进行沟通。进程使用本地 IP 地址 localhost 进行通信，使用的是一种特殊类型的 socket：Unix domain socket。Unix domain socket 实际上没有使用网络，甚至都不需要网络被配置就能使用，也不需要绑定任何的 socket 文件。进程创建的未命名 socket 可以直接通过地址被其它进程共享。
10.10.1 Advantages for Developers
- 便于访问控制
- 不需要进入协议栈，性能更好
10.10.2 Listing Unix Domain Sockets
查看所有的 Unix domain sockets：
$ lsof -U
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
systemd 1 root 14u unix 0xffff965bb6e90c00 0t0 11703 /run/systemd/notify type=DGRAM
systemd 1 root 15u unix 0xffff965bb6e91800 0t0 11704 type=DGRAM
systemd 1 root 16u unix 0xffff965bb6e90000 0t0 11705 type=DGRAM
systemd 1 root 17u unix 0xffff965bb9bd1000 0t0 11706 /run/systemd/private type=STREAM
systemd 1 root 30u unix 0xffff965bb981f000 0t0 11719 /run/udev/control type=SEQPACKET
systemd 1 root 32u unix 0xffff965bb981fc00 0t0 11721 /run/lvm/lvmpolld.socket type=STREAM
systemd 1 root 33u unix 0xffff965bb981e800 0t0 11723 /run/lvm/lvmetad.socket type=STREAM
systemd 1 root 34u unix 0xffff965bb981e000 0t0 11725 /run/systemd/journal/stdout type=STREAM
systemd 1 root 35u unix 0xffff965bb981f800 0t0 11727 /run/systemd/journal/socket type=DGRAM
systemd 1 root 36u unix 0xffff965bbcb19400 0t0 12144 /run/systemd/journal/dev-log type=DGRAM
systemd 1 root 37u unix 0xffff965bb6865000 0t0 12147 /run/systemd/journal/syslog type=DGRAM
systemd 1 root 49u unix 0xffff965bbac4b800 0t0 12774 type=DGRAM
systemd 1 root 51u unix 0xffff965bb684f000 0t0 12843 /run/systemd/journal/stdout type=STREAM
systemd 1 root 52u unix 0xffff965bbcb4e000 0t0 12844 /run/systemd/journal/stdout type=STREAM
systemd 1 root 54u unix 0xffff965bbc927400 0t0 14223 /run/systemd/journal/stdout type=STREAM
systemd 1 root 55u unix 0xffff965bb6877c00 0t0 14380 /run/systemd/journal/stdout type=STREAM
systemd 1 root 56u unix 0xffff965bb684e000 0t0 13459 type=DGRAM
systemd 1 root 57u unix 0xffff965bbca9a400 0t0 15030 /run/uuidd/request type=STREAM
systemd 1 root 58u unix 0xffff965bb684e400 0t0 13460 type=DGRAM
systemd 1 root 59u unix 0xffff965bbcfc9400 0t0 15036 @ISCSIADM_ABSTRACT_NAMESPACE type=STREAM
systemd 1 root 60u unix 0xffff965bb65c5c00 0t0 13531 /run/systemd/journal/stdout type=STREAM
systemd 1 root 61u unix 0xffff965bbab35800 0t0 15046 /run/acpid.socket type=STREAM
systemd 1 root 62u unix 0xffff965bbab35c00 0t0 15058 /var/run/dbus/system_bus_socket type=STREAM
systemd 1 root 63u unix 0xffff965bbab35400 0t0 15069 /run/snapd.socket type=STREAM
systemd 1 root 64u unix 0xffff965bbca9b400 0t0 15071 /run/snapd-snap.socket type=STREAM
systemd 1 root 66u unix 0xffff965bbaba1c00 0t0 15082 /var/lib/lxd/unix.socket type=STREAM
systemd 1 root 68u unix 0xffff965bbc9a1000 0t0 15359 type=STREAM
systemd 1 root 73u unix 0xffff965bbabe3400 0t0 15360 /run/systemd/journal/stdout type=STREAM
systemd 1 root 74u unix 0xffff965bbcb12c00 0t0 15470 /run/systemd/journal/stdout type=STREAM
systemd 1 root 75u unix 0xffff965bbce4e000 0t0 15549 /run/systemd/journal/stdout type=STREAM
systemd 1 root 76u unix 0xffff965bbce4e800 0t0 17309 /run/systemd/journal/stdout type=STREAM
systemd 1 root 78u unix 0xffff965bbcc37800 0t0 15799 /run/systemd/journal/stdout type=STREAM
systemd 1 root 79u unix 0xffff965bbaf47c00 0t0 17077 /run/systemd/journal/stdout type=STREAM
systemd 1 root 80u unix 0xffff965bbcd91000 0t0 18812 /run/systemd/journal/stdout type=STREAM
systemd 1 root 81u unix 0xffff965bbcd43400 0t0 16623 /run/systemd/journal/stdout type=STREAM
systemd 1 root 82u unix 0xffff965bbcdff000 0t0 16628 /run/systemd/journal/stdout type=STREAM
systemd 1 root 84u unix 0xffff965bbccca000 0t0 16630 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 3u unix 0xffff965bb981e000 0t0 11725 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 4u unix 0xffff965bb981f800 0t0 11727 /run/systemd/journal/socket type=DGRAM
systemd-j 410 root 5u unix 0xffff965bbcb19400 0t0 12144 /run/systemd/journal/dev-log type=DGRAM
systemd-j 410 root 14u unix 0xffff965bbcac2800 0t0 12522 type=DGRAM
systemd-j 410 root 16u unix 0xffff965bbc927400 0t0 14223 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 17u unix 0xffff965bb6877c00 0t0 14380 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 18u unix 0xffff965bb684f000 0t0 12843 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 19u unix 0xffff965bbcb4e000 0t0 12844 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 20u unix 0xffff965bb65c5c00 0t0 13531 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 21u unix 0xffff965bbabe3400 0t0 15360 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 22u unix 0xffff965bbcb12c00 0t0 15470 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 23u unix 0xffff965bbce4e000 0t0 15549 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 25u unix 0xffff965bbce4e800 0t0 17309 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 27u unix 0xffff965bbcc37800 0t0 15799 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 28u unix 0xffff965bbaf47c00 0t0 17077 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 29u unix 0xffff965bbcd91000 0t0 18812 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 30u unix 0xffff965bbcd43400 0t0 16623 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 31u unix 0xffff965bbcdff000 0t0 16628 /run/systemd/journal/stdout type=STREAM
systemd-j 410 root 33u unix 0xffff965bbccca000 0t0 16630 /run/systemd/journal/stdout type=STREAM
systemd-u 420 root 1u unix 0xffff965bb684f800 0t0 12404 type=STREAM
systemd-u 420 root 2u unix 0xffff965bb684f800 0t0 12404 type=STREAM
systemd-u 420 root 3u unix 0xffff965bb981f000 0t0 11719 /run/udev/control type=SEQPACKET
systemd-u 420 root 5u unix 0xffff965bbc927c00 0t0 12463 type=DGRAM
systemd-u 420 root 7u unix 0xffff965bbc9a1c00 0t0 12543 type=DGRAM
systemd-u 420 root 8u unix 0xffff965bbcac2400 0t0 12544 type=DGRAM
lvmetad 421 root 1u unix 0xffff965bbcb4e400 0t0 12460 type=STREAM
lvmetad 421 root 2u unix 0xffff965bbcb4e400 0t0 12460 type=STREAM
lvmetad 421 root 3u unix 0xffff965bb981e800 0t0 11723 /run/lvm/lvmetad.socket type=STREAM
systemd-t 519 systemd-timesync 1u unix 0xffff965bb65c5800 0t0 13530 type=STREAM
systemd-t 519 systemd-timesync 2u unix 0xffff965bb65c5800 0t0 13530 type=STREAM
systemd-t 519 systemd-timesync 3u unix 0xffff965bbab47400 0t0 13604 type=DGRAM
systemd-t 519 systemd-timesync 7u unix 0xffff965bbcb12000 0t0 13607 type=DGRAM
systemd-t 519 systemd-timesync 8u unix 0xffff965bbcb12800 0t0 13608 type=DGRAM
systemd-t 519 systemd-timesync 9u unix 0xffff965bbcb13000 0t0 13609 type=DGRAM
systemd-t 519 systemd-timesync 10u unix 0xffff965bbcb13800 0t0 13610 type=DGRAM
systemd-n 599 systemd-network 1u unix 0xffff965bbc927000 0t0 14222 type=STREAM
systemd-n 599 systemd-network 2u unix 0xffff965bbc927000 0t0 14222 type=STREAM
systemd-n 599 systemd-network 4u unix 0xffff965bbcb19800 0t0 14265 type=DGRAM
systemd-n 599 systemd-network 11u unix 0xffff965bbab47800 0t0 14272 type=DGRAM
systemd-n 599 systemd-network 12u unix 0xffff965bbcb18000 0t0 14273 type=DGRAM
systemd-n 599 systemd-network 13u unix 0xffff965bbdb10800 0t0 14274 type=DGRAM
systemd-n 599 systemd-network 14u unix 0xffff965bbcb4f000 0t0 14275 type=DGRAM
systemd-n 599 systemd-network 19u unix 0xffff965bbcfc8800 0t0 15061 type=STREAM
systemd-r 617 systemd-resolve 1u unix 0xffff965bb6876400 0t0 14379 type=STREAM
systemd-r 617 systemd-resolve 2u unix 0xffff965bb6876400 0t0 14379 type=STREAM
systemd-r 617 systemd-resolve 3u unix 0xffff965bb684fc00 0t0 14481 type=DGRAM
systemd-r 617 systemd-resolve 14u unix 0xffff965bbca9a000 0t0 15060 type=STREAM
acpid 725 root 0u unix 0xffff965bbab35800 0t0 15046 /run/acpid.socket type=STREAM
acpid 725 root 3u unix 0xffff965bbabe2000 0t0 15285 type=DGRAM
dbus-daem 726 messagebus 1u unix 0xffff965bbabe2400 0t0 15358 type=STREAM
dbus-daem 726 messagebus 2u unix 0xffff965bbabe2400 0t0 15358 type=STREAM
dbus-daem 726 messagebus 3u unix 0xffff965bbab35c00 0t0 15058 /var/run/dbus/system_bus_socket type=STREAM
dbus-daem 726 messagebus 6u unix 0xffff965bb684ec00 0t0 15382 type=DGRAM
dbus-daem 726 messagebus 8u unix 0xffff965bb6876c00 0t0 15389 type=STREAM
dbus-daem 726 messagebus 9u unix 0xffff965bb6877400 0t0 15390 type=STREAM
dbus-daem 726 messagebus 10u unix 0xffff965bbcfc9000 0t0 15391 /var/run/dbus/system_bus_socket type=STREAM
dbus-daem 726 messagebus 11u unix 0xffff965bbcfc8000 0t0 15392 /var/run/dbus/system_bus_socket type=STREAM
dbus-daem 726 messagebus 12u unix 0xffff965bbca6f400 0t0 15393 /var/run/dbus/system_bus_socket type=STREAM
dbus-daem 726 messagebus 13u unix 0xffff965bba8a2c00 0t0 16957 /var/run/dbus/system_bus_socket type=STREAM
dbus-daem 726 messagebus 14u unix 0xffff965bbae2dc00 0t0 17194 /var/run/dbus/system_bus_socket type=STREAM
dbus-daem 726 messagebus 15u unix 0xffff965bbafa8800 0t0 17213 /var/run/dbus/system_bus_socket type=STREAM
dbus-daem 726 messagebus 16u unix 0xffff965bba8a2800 0t0 17605 /var/run/dbus/system_bus_socket type=STREAM
dbus-daem 726 messagebus 17u unix 0xffff965bba93f800 0t0 17707 /var/run/dbus/system_bus_socket type=STREAM
lxcfs 730 root 1u unix 0xffff965bbcb13c00 0t0 15468 type=STREAM
lxcfs 730 root 2u unix 0xffff965bbcb13c00 0t0 15468 type=STREAM
systemd-l 731 root 1u unix 0xffff965bbcb12400 0t0 15548 type=STREAM
systemd-l 731 root 2u unix 0xffff965bbcb12400 0t0 15548 type=STREAM
systemd-l 731 root 3u unix 0xffff965bbb805000 0t0 17183 type=DGRAM
systemd-l 731 root 12u unix 0xffff965bbae2c400 0t0 17193 type=STREAM
v2ray 734 root 1u unix 0xffff965bbe38b400 0t0 15798 type=STREAM
v2ray 734 root 2u unix 0xffff965bbe38b400 0t0 15798 type=STREAM
cron 739 root 1u unix 0xffff965bbcd42400 0t0 16046 type=STREAM
cron 739 root 2u unix 0xffff965bbcd42400 0t0 16046 type=STREAM
rsyslogd 740 syslog 3u unix 0xffff965bb6865000 0t0 12147 /run/systemd/journal/syslog type=DGRAM
rsyslogd 740 syslog 6u unix 0xffff965bbaf17000 0t0 16934 type=DGRAM
networkd- 741 root 1u unix 0xffff965bbcdff400 0t0 16211 type=STREAM
networkd- 741 root 2u unix 0xffff965bbcdff400 0t0 16211 type=STREAM
networkd- 741 root 3u unix 0xffff965bba8a3400 0t0 17604 type=STREAM
accounts- 743 root 1u unix 0xffff965bbccca800 0t0 16368 type=STREAM
accounts- 743 root 2u unix 0xffff965bbccca800 0t0 16368 type=STREAM
accounts- 743 root 5u unix 0xffff965bba8a3c00 0t0 16956 type=STREAM
sshd 770 root 1u unix 0xffff965bbaef5c00 0t0 17076 type=STREAM
sshd 770 root 2u unix 0xffff965bbaef5c00 0t0 17076 type=STREAM
polkitd 780 root 6u unix 0xffff965bbae2d400 0t0 17212 type=STREAM
unattende 791 root 1u unix 0xffff965bbce4f800 0t0 17307 type=STREAM
unattende 791 root 2u unix 0xffff965bbce4f800 0t0 17307 type=STREAM
unattende 791 root 4u unix 0xffff965bba8e8400 0t0 17706 type=STREAM
systemd 967 root 1u unix 0xffff965bbcd90000 0t0 18804 type=STREAM
systemd 967 root 2u unix 0xffff965bbcd90000 0t0 18804 type=STREAM
systemd 967 root 3u unix 0xffff965bbaf3c800 0t0 18842 type=DGRAM
systemd 967 root 14u unix 0xffff965bbcd90c00 0t0 18880 /run/user/0/systemd/notify type=DGRAM
systemd 967 root 15u unix 0xffff965bbcd90800 0t0 18881 type=DGRAM
systemd 967 root 16u unix 0xffff965bbaf3cc00 0t0 18882 type=DGRAM
systemd 967 root 17u unix 0xffff965bbaf3d800 0t0 18883 /run/user/0/systemd/private type=STREAM
systemd 967 root 22u unix 0xffff965bbaeb1800 0t0 18887 /run/user/0/gnupg/S.gpg-agent.extra type=STREAM
systemd 967 root 23u unix 0xffff965bbcd43800 0t0 18888 /run/user/0/gnupg/S.gpg-agent type=STREAM
systemd 967 root 24u unix 0xffff965bbcd43000 0t0 18889 /run/user/0/gnupg/S.gpg-agent.ssh type=STREAM
systemd 967 root 25u unix 0xffff965bbcd42c00 0t0 18890 /run/user/0/gnupg/S.gpg-agent.browser type=STREAM
systemd 967 root 26u unix 0xffff965bbcd43c00 0t0 18891 /run/user/0/gnupg/S.dirmngr type=STREAM
(sd-pam 974 root 1u unix 0xffff965bbcd90000 0t0 18804 type=STREAM
(sd-pam 974 root 2u unix 0xffff965bbcd90000 0t0 18804 type=STREAM
(sd-pam 974 root 7u unix 0xffff965bbaf3c000 0t0 18820 type=DGRAM
sshd 18762 root 4u unix 0xffff965bba97c800 0t0 1019393 type=DGRAM