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Newer page: version 4 Last edited on Wednesday, July 6, 2005 2:06:18 pm by IanMcDonald
Older page: version 3 Last edited on Wednesday, July 6, 2005 1:44:43 pm by JimSansing Revert
@@ -5,9 +5,9 @@
 * Modern networks have functions to help people use them, such as domain name servers, which translate between network addresses and human readable host names. But when troubleshooting network problems, these servers may be inaccessible. So use the most basic method of communication for testing access and collecting status information. The commands to find the network addresses that will be needed are in the procedures below. 
  
 * Networks may be slow, or services unavailable. There is usually a timeout set for networks commands, which may have to run to completion before a program can continue. So, patience is required for troubleshooting network problems. 
  
-* Network protocols expect things to go wrong, so retries are built in. But during troubleshooting, this is not always a good thing. If possible, override the retry value with a count parameter. Otherwise, it may be necessary to interrupt a program that is not responding. 
+* Network protocols expect things to go wrong, so retries are built in most of the time . But during troubleshooting, this is not always a good thing. If possible, override the retry value with a count parameter. Otherwise, it may be necessary to interrupt a program that is not responding. 
  
 !!Parts list 
  
 * Application software: The local host is point A. However, there are many points of failure before data can leave the local host. 
@@ -24,10 +24,10 @@
  - /etc/HOSTNAME 
  - /etc/services 
  
  * API layer: The connection between a client application and the network software is made through a "socket". On the client side, the Application Program Interface to the network software is usually not a problem. However, it can happen that as many connections as are allowed by a single program are in use, so no more connections can be made. This can be checked using the netstat command described below. 
- * TCP/UDP layer: The Transmission Control Protocol is what the network software uses to make sure that all data sent is received. The User Datagram Protocol is similar, but it is called "unreliable" because the receiving host does confirm that data was received. Both use network ports to identify a "session" between two hosts. When a session is active, that pair of ports cannot be used between the same two hosts for another session. Active sessions can be displayed using the netstat command described below.  
- * IP layer: The Internet Protocol is what the network software uses to determine how to establish a session between two hosts. It uses network addresses to identify the hosts and a route table to know where to send or "route" the data. The network address of the local host is usually in the /etc/hosts table. The route table can be displayed using the netstat command described below. 
+ * TCP/UDP layer: The Transmission Control Protocol is what the network software uses to make sure that all data sent is received. The User Datagram Protocol is similar, but it is called "unreliable" because the receiving host does not confirm that data was received. Both use network ports to identify a "session" between two hosts. When a session is active, that pair of ports cannot be used between the same two hosts for another session. Active sessions can be displayed using the netstat command described below.  
+ * IP layer: The Internet Protocol is what the network software uses to determine how to establish a session between two hosts. It uses network addresses to identify the hosts and a route table to know where to send or "route" the data. The network address of the local host is usually in the /etc/hosts table if a fixed address is used . The route table can be displayed using the netstat command described below. 
  
 * Interface and Local Network 
  * Files: 
  - /etc/sysconfig/network 
@@ -36,17 +36,17 @@
  * Hardware/Driver: The cable that connects the local host to the network is attached to a piece of hardware that can get data from one end of the cable to the other. There are many such connections between the local host and the server host, but usually problems with those connections must be handled by others. The driver is simply the software that is used to control the hardware.%%% The configuration of the hardware and its driver are found in the file, /etc/sysconfig/network. The driver is started and stopped using the /etc/init.d/network command. The status of the hardware and its driver can be displayed using the ifconfig command described below. 
  - Serial port/PPP: This hardware is the slowest network connection. The cable is connected to the serial port on the local host and to a modem at the other end. The modem is connected to a telephone jack. Often modems are built into the local host, and it is connected directly to the telephone jack. The Point-to-Point Protocol is used to send data between the local host and the Internet Service Provider's (ISP) modem. The ISP then has a faster connection to the network. 
  - NIC card/Ethernet driver: A Network Interface Card uses software for faster networks. Most NICs use the Ethernet protocol, although there are several others available. The Ethernet protocol is used to send data between the local host and a router, which is connected to the network. On home networks, the cable from the NIC is usually connected to a cable modem or a Digital Subscriber Line (DSL) modem, which is connected to a router at the cable television or telephone company office. 
  - Wifi card/Wifi driver: There is no cable connected to a Wifi card. However, the Wifi card communicates with an "access point" which usually has an Ethernet NIC that is connected the same as a local host Ethernet NIC. 
- - Home networks usually have a router between the local hosts and the cable or DSL modem. It is possible to plug more than one local host into the router, but most network use a hub or a switch to allow a single router connector or "port" to be used. There may be an application available to display the status of the router. The same may be true for switches. However, hubs usually only display a little bit of status information on the front panel.%%% These devices usually have very little control capability, and are reset by powering them off and then back on. It is advisable to keep them physically close together, and if they are all plugged into a single power strip, then resetting them simultaneously is as simple as turning off and on the main power strip switch. 
+ - Home networks usually have a router between the local hosts and the cable or DSL modem. It is possible to plug more than one local host into the router, but most networks use a hub or a switch to allow a single router connector or "port" to be used. There may be an application available to display the status of the router. The same may be true for switches. However, hubs usually only display a little bit of status information on the front panel.%%% These devices usually have very little control capability, and are reset by powering them off and then back on. It is advisable to keep them physically close together, and if they are all plugged into a single power strip, then resetting them simultaneously is as simple as turning off and on the main power strip switch. 
  
 * Network 
  * Files: 
  - /etc/hosts 
  - /etc/resolv.conf 
  
  * Connection to network: The ISP provides the hardware and software to get access to any host connected to the world wide web, if everything is working correctly. To simplify making connections, domain name servers translate between network addresses and host names. For example, by issuing the ''host'' command, the host names can have their addresses displayed. The address of the ISP's domain name servers is in the file, /etc/resolv.conf. Any hosts defined in the file, /etc/hosts, will be used without requesting them to be translated by the domain name servers. So, it is a good idea to use this table for hosts on the local network, and even a few of the ISP hosts. The names in the /etc/hosts file do not have to match the actual name of the host defined. So instead of using the real domain server name of, ns02.someplace.st.us02.comcast.net, the name dns1 could be used. 
- * Network path: Under the TCP/IP protocol, data is broken into "packets" of 500 to 1,500 bytes . The protocol is designed to choose any path that is available for each individual packet. This means that while some packets may be lost, there is usually a way for the entire message to reach its destination. However, sometimes there is a point of failure that it is impossible to bypass. There are several tools, described below, to determine if the path to the server is available. 
+ * Network path: Under the TCP/IP protocol, data is broken into "packets" of various sizes . The protocol is designed to choose any path that is available for each individual packet. This means that while some packets may be lost, there is usually a way for the entire message to reach its destination. However, sometimes there is a point of failure that it is impossible to bypass. There are several tools, described below, to determine if the path to the server is available. 
  
 * Destination host 
  * Files: 
  - /etc/ssh/sshd_config 
@@ -78,9 +78,9 @@
  
 * Testing commands 
  
  * ping: This is a way of testing whether a host and its NIC are active. Using ''ping -c <number>'' will limit the number of attempts. Otherwise use Ctrl-C to break out of it. Because ping has been misused and abused, many routers and firewalls are configured to discard it. So, just because there is no response, it does not mean that there is a network problem. But if there is a response, then the problem is probably a configuration or application issue. 
- * traceroute: This sets the distance that packets will travel through the network to one hop, and then gradually increases it. Each router or host that receives packets returns an error message that the traceroute program can use to determine that device's identity. This way, the list of routers along the path to the destination can be seen. Typically, if there is no response, three asterisks are displayed. If this continues, then it is probably where the problem is. Unfortunately, this is another utility that has been misused and abused, so many devices do not respond to the traceroute packets. Sometimes, the list will continue after one or more lines of asterisks. But usually, the packets are not being allowed to continue along the network path. 
+ * traceroute: This sets the distance that packets will travel through the network to one hop, and then gradually increases it. Each router or host that receives packets returns an error message that the traceroute program can use to determine that device's identity. This way, the list of routers along the path to the destination can be seen. Typically, if there is no response, three asterisks are displayed. If this continues, then it is probably where the problem is. Unfortunately, this is another utility that has been misused and abused, so many devices do not respond to the traceroute packets. Sometimes, the list will continue after one or more lines of asterisks. But usually, the packets are not being allowed to continue along the network path. It can be useful to use ''traceroute -n'' as then it does not try to resolve names
  * telnet/ssh/ftp: An attempt to make a telnet, ssh, or ftp connection will usually at least result in a message indicating access denied if the host isaccessible through the network. So this method can be used where ping and traceroute are disallowed. 
  
 !!Procedure 
  
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