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|Contents||Bulletin||Scripting in shell and Perl||Network troubleshooting||History||Humor|
|Linux Network Troubleshooting||Recommended Links||Redhat Network Configuration||Suse 10 network configuration|
|Ethernet Protocol||ifconfig||ethtool||How to change IP address in RHEL||netstat||Xinetd||Remote Syslog|
|route command||Linux Routing||DNS||nslookup||hostname||Changing hostname||Procedure for installing Qlogic QLE2460 cards|
|NFS||vsftp||pure ftpd||rsync||NTP||RHEL NTP configuration||Troubleshooting NTP on Red Hat Linux|
|Telnet Protocol||VNC on Linux||SSH||Autonegotiation||Samba||Sendmail on RHEL||Postfix|
|TCP Performance Tuning||Bonding Multiple Network Interfaces||Linux multipath||InfiniBand||Installing Mellanox InfiniBand Driver on RHEL 6.5||Setting up a basic infiniband network||Troubleshooting InfiniBand connection issues using OFED tools|
|USB to Ethernet Adapter in Linux||USB to wireless adapter||Network Manager overwrites resolv.conf||Tips||Admin Horror Stories||Humor||Etc|
15 May 2000
This document addresses communication issues that generate about a third of the support calls coming into the TCP/IP group at Novell Technical Support. We recommend that anyone who is implementing TCP/IP in a NetWare 5.x environment read and understand the information presented here.
This article is divided into two parts: understanding the concepts behind IP routing, and troubleshooting common TCP/IP problems. A follow-up article will explain some of the TCP/IP tools that are available for use in troubleshooting problems in a TCP/IP environment.
The majority of connectivity issues involve problems with routing table entries. Every packet being processed by a TCP/IP host has a source and destination IP address. Upon receiving each packet, the IP protocol examines the destination address of the packet, compares it with entries in its local routing table, and then decides what action to take:
The TCP/IP routing table can maintain four different types of routes, listed below in the order that they are searched for a match:
IP compares the destination IP address of the packet that it is processing with the entries in the table. If IP finds that a host entry exists and matches the destination IP address, it will forward the packet to the next hop associated with that host entry. Host entries are usually found in routing tables when ICMP (Internet Control Message Protocol) has added the entry because of the pathMTU algorithm, or from an "ICMP redirect" call. To check this, load the TCPCON utility at the server console prompt and look at the IP Routing Table option to verify if the protocol associated with that route is ICMP.
IP has three classes of addresses: Class A, Class B and Class C. Each class contains a default subnet mask (for instance, Class A has 255.0.0.0. as a default subnet) until a class of addresses is broken into extra networks (i.e., subnetted). However, once the network is subnetted, the IP address will not have the default subnet mask.
So if IP doesn't find a host entry, but does find a subnet entry that matches the packet's destination IP address, IP will forward the packet to the next hop associated with that subnet entry. Subnet entries exist when RIP2 (Routing Internet Protocol v2), OSPF (Open Shortest Path First), or static entries have been added to the routing table through a non-default subnet mask.
If IP doesn't find a subnet entry in the TCP/IP routing table but does find a network entry that matches the destination IP address, IP will forward the packet to the next hop associated with that network entry. (Customers running in default NetWare TCP/IP mode will have network entries.)
Finally, if IP doesn't find a network entry, but does find that a default route entry exists, IP will forward the packet to the next hop associated with that default entry. The default route is most commonly inserted as a static route through NetWare's server console INETCFG utility. However, the route may also be learned via RIP or OSPF. Failure to at least have a default route can often lead to communication problems on the network.
If an IP packet match has not been found in the TCP/IP routing table at this stage, the packet is simply dropped and an ICMP "destination unreachable" message is triggered to notify the sender that the host or network is unreachable.
When a TCP/IP communication problem occurs, the most common reason is that a route entry doesn't exist for the network or host with which you are trying to communicate. When this is the case, you can either add a route entry or try to figure out why the route is missing.
When troubleshooting any networking problem, it is helpful to take a logical approach. Some questions to ask are:
Troubleshooting a problem "from the bottom up" is often a good way to quickly isolate what's wrong and come up with a solution. The "bottom up" approach from an IP routing perspective is to start by verifying that the problem is not related to the physical layer (cabling, hubs, switches, and so on) or ARP (Address Resolution Protocol). Next, you ensure that the IP routing table is functioning correctly. Finally, you check to see whether the problem is at a generic TCP/UDP or application level.
To better understand the TCP/IP troubleshooting scenarios covered in this article, we'll use a small example network to illustrate some of the most common IP problems. This example network is shown in Figure 1.
Figure 1: Example network for TCP/IP troubleshooting scenarios.
In this network, Workstation 1 accesses the Internet/WAN through a NetWare server which contains two network adapters, each with its own IP address: 188.8.131.52 and 184.108.40.206. Workstation 2 accesses the Internet/WAN through the Internet Router with the IP address of 220.127.116.11. The NetWare server also communicates to the Internet/WAN through the Internet Router, as well as the Unix box (whose IP address is 18.104.22.168), which also communicates to the Internet/WAN through the Internet Router (22.214.171.124). The Internet Router's IP internet address is 126.96.36.199.
It's also important that you understand the terms "local host" and "remote host" in an IP network environment:
From the point of view of Workstation 1 in Figure 1, the NetWare server is considered a local host because its network adapter is attached to the same IP subnet as Workstation 1. Workstation 2, whose IP subnet address is different than that of Workstation 1, can be considered a remote host.
The following scenarios, which represent six of the most common IP problems, use the example network in Figure 1 as a reference. The most common solutions are given for each of these problems. While this is not a comprehensive list of solutions, they cover most of the routing issues that customers face.
Symptom: The user cannot PING from Workstation 1 (188.8.131.52) to the local segment side of the NetWare server (184.108.40.206).
Solutions: If two nodes on the same subnet cannot PING each other successfully, you can use the "ARP _A" command at a Windows workstation to check the ARP table entries. The -A parameter displays the ARP entries by interrogating the current protocol data. If more than one network adapter uses the Address Resolution Protocol, you'll see entries for each ARP table.
You can also use the TCPCON utility on the NetWare server to view the IP Address Translations Table. Select the Protocol Information | IP | IP Address Translation options, and see if the computers have the correct MAC addresses listed for each other.
Note: You can use the IPConfig utility (for Windows NT), the WINIPCFG utility (for Windows 95/98), or type CONFIG <Enter> at the NetWare server console to determine a host's MAC address (displayed as Node Address).
Symptom: The user can PING from Workstation 1 (220.127.116.11) to the local segment side of the NetWare server (18.104.22.168), but not from Workstation 1 to the other side of the NetWare server (22.214.171.124).Solutions:
In this scenario, Workstation 1 would need to configure as its default router the IP address of the server's network adapter that is local to the workstation. The IP address would be 126.96.36.199. This implies that any packets that Workstation 1 will transmit to any remote hosts will be sent through this IP address.
The best way to verify that TCP/IP has been loaded with forwarding enabled is through the TCPCON utility. Load TCPCON at the server console. You will see the "IP Forwarded: numbers" entry in the lower left-hand corner of the top window. If this entry has numbers after it (even if it is 0), then this server is configured as an IP router. If this entry has DISABLED after the statistic, it is not set to gateway mode. To enable this, load the INETCFG utility at the server console, select the Protocols entry, the TCP/IP entry, and then ensure that the "IP Packet Forwarding" parameter is set to ENABLED. (See TID #10013002 for more details.)
If this entry shows a non-zero value, increase the minimum packet receive buffers setting for the server. To do this in MONITOR, select the Server Parameters | Communications options, then select the Minimum Packet Receive Buffers entry and double it. Note that the changes won't take effect until you restart the server.
Symptom: From Workstation 1 (188.8.131.52) the user can ping both IP addresses that are bound to the network adapters in the NetWare server (184.108.40.206 and 220.127.116.11), but cannot ping the Internet Router (18.104.22.168).Solutions:
To fix this problem, insert a static route entry at the IP Router. On a NetWare server, this can be done using INETCFG by selecting Protocols| TCPIP| Static Routes. This entry tells the IP Router that in order to get to the 22.214.171.124 subnet, packets must go through the 126.96.36.199 gateway, which is the IP address of the NetWare server for the segment local to the IP Router. This implies that any time the Internet Router has a packet destined for 188.8.131.52, it will send it to the 184.108.40.206 gateway.
From Workstation 1 (220.127.116.11), the user can PING both IP addresses that are bound to the network adaptersin the NetWare server (18.104.22.168 and 22.214.171.124), and the Internet Router (126.96.36.199), but cannot PING Workstation 2 (188.8.131.52).Solutions:
As described in Scenario 2, the workstation must have its default router or gateway set in order to reply or send packets to segments other than its local segment gateway. (See TID #10018660 for information on configuring and troubleshooting client issues on Windows 95/98 and NT.)
From Workstation 1 (184.108.40.206), the user can ping both IP addresses that are bound to the network adapters in the NetWare server (220.127.116.11 and 18.104.22.168) and the Internet Router (22.214.171.124), but cannot PING the UNIX box (126.96.36.199).Solutions:
route add net 188.8.131.52 184.108.40.206 1
(For more information on the route command for UNIX, refer to the documentation that comes with your UNIX software.)
From Workstation 1 (220.127.116.11), the user can ping both IP addresses that are bound to the network adapters in the NetWare server (18.104.22.168 and 22.214.171.124) and the Internet Router (126.96.36.199). The user can also ping Workstation 2 (188.8.131.52) and the UNIX box (184.108.40.206), but cannot PING past the Internet Router.Solutions:
To troubleshoot this problem, you first need to understand the network layout. Having the layout in mind will enable you to identify other routers in the network that should be advertising the route. You can use LAN traces to verify whether or not these other routers are advertising the missing network, and if so, with the proper parameters, such as hop count. In some cases, invalid hop counts may be advertised and the routes are being dropped accordingly.
In the next column, we'll look into an extension of this troubleshooting scenario dealing with subnets and a couple of the more common problems that users face with subnetting.
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