Posts Tagged ‘网络编程’

Unix网络编程 第13章 Daemon Processes and the inetd Superserver 笔记

# The syslogd daemon runs in an infinite loop that calls select, waiting for any one of its three descriptors to be readable. it reads the log message and does what the configuration file says to do with that message. If the daemon receives the SIGHUP signal, it rereads its configuration file. So, what are the three descriptors that the select system call is waiting for ?  1. A unix domain socket is created and bound to the pathname /var/run/log (/dev/log on some systems). 2. A udp socket is created and bound to port 514(the syslog service). 3. The pathname /devklog is opened. Any error messages from within the kernel appears as input on this device. Newer implementation disable the creation of the UDP socket, unless specified by the administrator, as allowing anyone to send UDP datagrams to this port opens the system up to denial-of-service attacks, where some one could fill up the filesystem.

# syslog函数的%m specification表示当前errno对应的error message.

# syslog函数的level和facility是为了配置如何处理各种log.配置文件是/etc/syslog.conf.

# logger命令可以产生log message。于是可以在shell脚本里使用logger.

# The purpose of the second fork is to guarantee that the daemon cannot automatically acquire a controlling terminal should it open a terminal device in the future. When a session leader without a controlling terminal opens a terminal device(that is not currently some other session's controlling terminal), the termianl becomes the controlling terminal of the session leader. But by calling fork a second time, we guarantee that the second child is no longer a session leader, so it cfannot acquire a controlling terminal. We must ignore SIGHUP because when the session leader terminates(the first child), all processes in the session(our second child)receive the SIGHUP signal.

# daemon通常把当前工作目录设为 / .如果不这样的话就会有可能使得不能unmount某些文件系统。

# On linux, /var/log/message is where the system send all LOG_USER messages after connecting from the same machine(e.g. localhost). Page370.

# 早期的Unix系统,早于4.3BSD.有很多服务像ftp, telnet, rlogin, tftp等都是以daemon的形式运行。每一个都要在进程表里占一个位置(each one took a slot in the process table).但是每个daemon大多数时间都在睡眠状态。从4.3BSD开始引入了inetd.

# inetd的配置对于UDP的wait_flag必须是wait.因为UDP socket只有一个.如果不wait话,parent存在可能性先于child进程得到CPU。而udp socket缓冲中的数据还未来得及读出。这样,inetd的select又返回这个socket可读。wait_flag的wait的意思就是要wait到fork出的子进程结束。而tcp socket会在accept返回的时候给子进程一个connected socket.父进程可以立即得到CPU执行select判断listenning socket是否可读。

# xinetd的配置采用每个服务一个配置文件.而inetd用一个monolithic configuration file.

# On a Berkely-derived kernel the timeout for a tcp connect is normally 75秒.

Be the first to comment - What do you think?  Posted by zausiu - December 27, 2011 at 00:59

Categories: Network, Tech Articles, Unix / Linux   Tags: ,

Unix网络编程 第11章 Name and Address Conversions 笔记

# gethostbyname 和 gethostbyaddr 用来在 IPv4 地址和 hostname 之间转换. getservbyport 和 getservbyname 则是与服务相关。gethostbyname出错时不设errno而是设h_errno,并有hstrerror()函数。

# FQDN的全称是: Fully Qualified domain name. 技术上说必须以点号(period)终止.

# AAAA 被称为 "quad A" rcord, 给出了从hostname到Ipv6地址的映射。 PTR用来把IP地址到hostname.

# Entries in the DNS are known as Resource Records(RRs).

# 一个点分十进制(dotted-decimal)IPv4的地址前加 0::ffff:就是 IPv6的字符串形式。

# 与getpeername对应的函数不是gethostname而是getsockname.

# getaddrinfo函数的host参数指定为dotted-decimal IPv4或 IPv6 hex string,会使得只有IPv4或IPv6的addrinfo返回。

# 不给UDP套接字设置SO_REUSEADDR选项。We do not set the SO_REUSEADDR socket option for the UDP socket because this socket option can allow multiple sockets to bind the same UDP port on hosts that support multicasting. Since there is nothing like TCP's TIME_WAIT state for a UDP socket, there is no need to set this socket option when the server is started.

# 一般情况下,同端口的不同协议对应同样的服务。但也有例外。对于端口514,which is the rsh service with TCP, but the syslog service with UDP.

# gethostbyaddr的第一个参数是char* addr,而其实它并非指向一个char* 事实上指向in_addr结构体。

# getaddrinfo好复杂呀!hint的ai_flags设置了AI_CONONNAME成员得到host的canonical name.

# port 53 是domain service的端口号.

# 如果设置了IPV6_V6ONLY.那么一个来自ipv4 client的连接会被拒绝。

# POSIX says that specifying AF_UNSPEC will return addresses that can be used with any protocol family that can be used with the hostname and service name.

# POSIX specification also implies that if the AI_PASSIVE flag is specified without a hostname, then the IPv6 wildcard address(IN6ADDR_ANY_INIT or 0::0) should be returned as a sockaddr_in6 structure, along with the IPv4 wildcard address(INADDR_ANY or, which is returned as a sockaddr_in structure.

# An ipv6 server socket can handle both ipv4 and ipv6 on a dual-stack host. Refer to page319 in UNP for details.

Be the first to comment - What do you think?  Posted by zausiu - December 26, 2011 at 20:48

Categories: Network, Tech Articles, Unix / Linux   Tags: ,

Unix 网络编程 第十章 SCTP Client/Server Example


1. 什么是 head-of-line blocking.

Head-of-line blocking occurs when a TCP segment is lost and a subsequent TCP segment arrives out of order. That subsequent segment is held until the first TCP segment is retransmitted and arrives at the receiver.

2. 怎么更改SCTP连接的stream的数量。

SCTP连接的streams的数量是在association的握手之前协商好的。对于FreeBSD的KAME实现,SCTP的outbound streams默认为10。这个值可以用setsocket函数更改。与SCTP_INITMSG scoket option相关,设置struct sctp_initmsg结构体。

也可以用sendmsg函数发送ancillary数据来到达同样的目标。但发送ancillary data只对one-to-many形式的sctp socket有效。

3. 怎么结束一个SCTP连接。

可以设置sctp_sndrcvinfo结构的sinfo_flags值的MSG_EOF flag来关闭一个sctp连接gracefully. This flag forces an association to shut down after the message being sent is acknowledged.

还可以给sinfo_flags设置 MSG_ABORT。这样就会立即发送一个ABORT给peer端。任何还没来得及发送出的数据会被丢弃。

Be the first to comment - What do you think?  Posted by zausiu - December 19, 2011 at 19:25

Categories: Network, Tech Articles, Unix / Linux   Tags: ,

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