file:///C|/mynapster/Downloads/warez/tcpip/ch10_08.htm specified by S, and ruleset 4. The mailer definition for smtp in our sample configuration defines two rulesets for S - 11 and 31. [23] The first ruleset is used for rewriting the sender address in the "envelope" and the second is used to rewrite the sender address in the message header. [23] Many versions of sendmail define only one ruleset each for S and R. Based on the information in Figure 10.4 and in the S field of the smtp mailer, we know that the rulesets that process the message header sender address are 3, 1, 31 and 4. So we run sendmail with the -bt option and enter 3,1,31,4 craig at the command prompt. This command processes the sender address through each of these rulesets in succession. We also know that the envelope sender address is processed by rulesets 3, 1, 11, and 4. To test that, we enter 3,1,11,4 craig. The results of these tests are exactly the same as those shown in the example above. The value of the M macro rewrites the hostname in the message sender address just as we wanted. The hostname in the envelope sender address is not rewritten. Usually this is acceptable. However, we want to create exactly the same configuration as in the m4 example. The FEATURE(masquerade_envelope) command used in the m4 example causes the envelope sender address to be rewritten. Therefore, we want this configuration to also rewrite it. The only difference between how the message and envelope addresses are processed is that one goes through ruleset 31 and the other goes through ruleset 11. The tests show that both rulesets call ruleset 51 and then ruleset 61. They diverge at that point because ruleset 31 calls ruleset 93 and ruleset 11 calls ruleset 94. The tests also show that ruleset 93 provides the address rewrite that we want for the message sender address, while the envelope sender address is not processed in the manner we desire by ruleset 94. The test.cf code for rulesets 94, 11, and 31 is shown below: ################################################################### ### Ruleset 94 -- convert envelope names to masquerade form ### ################################################################### S94 #R$+ $@ $>93 $1 R$* < @ *LOCAL* > $* $: $1 < @ $j . > $2 # # envelope sender rewriting # S11 R$+ $: $>51 $1 sender/recipient common R$* :; <@> $@ list:; special case R$* $: $>61 $1 qualify unqual'ed names R$+ $: $>94 $1 do masquerading # # header sender and masquerading header recipient rewriting # S31 R$+ $: $>51 $1 sender/recipient common R:; <@> $@ list:; special case file:///C|/mynapster/Downloads/warez/tcpip/ch10_08.htm (7 of 10) [2001-10-15 09:18:44] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. file:///C|/mynapster/Downloads/warez/tcpip/ch10_08.htm R$* <@> $* $@ $1 <@> $2 pass null host through R< @ $* > $* $@ < @ $1 > $2 pass route-addr through R$* $: $>61 $1 qualify unqual'ed names R$+ $: $>93 $1 do masquerading Clearly, ruleset 94 does not do what we want and ruleset 93 does. A quick inspection of ruleset 94 shows that it does not contain a single reference to macro M. Yet the comment on the line in ruleset 11 that calls it indicates that ruleset 94 should "do masquerading." The first line of ruleset 94 calls ruleset 93, but it is commented out. Our solution is to uncomment the first line of ruleset 94 so that it now calls ruleset 93, which is the ruleset that really does the masquerade processing. Debugging a sendmail.cf file is more of an art than a science. Deciding to edit the first line of ruleset 94 to call ruleset 93 is little more than a hunch. The only way to verify the hunch is through testing. We run sendmail -bt -Ctest.cf again to test the addresses craig, craig@peanut, and craig@localhost through rulesets 3, 1, 11, and 4. All tests run successfully, rewriting the various input addresses into craig@nuts.com. We then retest by sending mail via sendmail -v -t -Ctest.cf. Only when all of these tests run successfully do we really believe in our hunch and move on to the next task, which is to rewrite the user part of the email address into the user's first and last names. 10.8.2 Using Key Files in sendmail The last feature we added to the m4 source file was FEATURE(genericstable), which adds a database process to the configuration that we use to convert the user portion of the email address from the user's login name to the user's first and last names. To do the same thing here, create a text file of login names and first and last names and build a database with makemap. [24] [24] See the m4 section for more information about makemap. # cat realnames dan Dan.Scribner tyler Tyler.McCafferty pat Pat.Stover willy Bill.Wright craig Craig.Hunt # makemap dbm realnames < realnames Once the database is created, define it for sendmail. Use the K command to do this. To use the database that we have just built, insert the following lines into the Local Information section of the sendmail.cf file: # define a database to map login names to firstname.lastname Krealnames dbm /etc/realnames The K command defines realnames as the internal sendmail name of this database. Further, it identifies that this is a database of type dbm and that the path to the database is /etc/realnames. sendmail adds the correct filename extensions to the pathname depending on the type of the database, so you don't need to worry about it. file:///C|/mynapster/Downloads/warez/tcpip/ch10_08.htm (8 of 10) [2001-10-15 09:18:44] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. file:///C|/mynapster/Downloads/warez/tcpip/ch10_08.htm Finally, we add a new rule that uses the database to rewrite addresses. We add it to ruleset 11 and ruleset 31 immediately after the lines in those rulesets that call ruleset 93. This way, our new rule gets the address as soon as ruleset 93 finishes processing it. # when masquerading convert login name to firstname.lastname R$-<@$M.>$* $:$(realnames $1 $)<@$2.>$3 user=>first.last This rule is designed to process the output of ruleset 93, which rewrites the hostname portion of the address. Addresses that meet the criteria to have the hostname part rewritten are also the addresses for which we want to rewrite the user part. Look at the output of ruleset 93 from the earlier test. That address, craig<@nuts.com.>, matches the pattern $-<@$M.>$*. The address has exactly one token (craig) before the literal <@, followed by the value of M (nuts.com), the literal .> and zero tokens. The transformation part of this rule takes the first token ($1) from the input address and uses it as the key to the realnames database, as indicated by the $:$(realnames $1 $) syntax. For the sample address craig<@nuts.com>, $1 is craig. When used as an index into the database realnames shown at the beginning of this section, it returns Craig.Hunt. This returned value is prepended to the literal <@, the value of indefinite token $2, the literal .>, and the value of $3, as indicated by the <@$2.>$3 part of the transformation. The effect of this new rule is to convert the username to the user's real first and last names. After adding the new rule to rulesets 11 and 31, a test yields the following results: # sendmail -bt -Ctest.cf ADDRESS TEST MODE (ruleset 3 NOT automatically invoked) Enter <ruleset> <address> > 3,1,11,4 craig rewrite: ruleset 3 input: craig rewrite: ruleset 96 input: craig rewrite: ruleset 96 returns: craig rewrite: ruleset 3 returns: craig rewrite: ruleset 1 input: craig rewrite: ruleset 1 returns: craig rewrite: ruleset 11 input: craig rewrite: ruleset 51 input: craig rewrite: ruleset 51 returns: craig rewrite: ruleset 61 input: craig rewrite: ruleset 61 returns: craig < @ *LOCAL* > rewrite: ruleset 93 input: craig < @ *LOCAL* > rewrite: ruleset 93 returns: craig < @ nuts . com . > rewrite: ruleset 11 returns: Craig . Hunt < @ nuts . com . > rewrite: ruleset 4 input: Craig . Hunt < @ nuts . com . > rewrite: ruleset 4 returns: Craig . Hunt @ nuts . com > 3,1,31,4 craig rewrite: ruleset 3 input: craig rewrite: ruleset 96 input: craig rewrite: ruleset 96 returns: craig rewrite: ruleset 3 returns: craig rewrite: ruleset 1 input: craig file:///C|/mynapster/Downloads/warez/tcpip/ch10_08.htm (9 of 10) [2001-10-15 09:18:44] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. file:///C|/mynapster/Downloads/warez/tcpip/ch10_08.htm rewrite: ruleset 1 returns: craig rewrite: ruleset 31 input: craig rewrite: ruleset 51 input: craig rewrite: ruleset 51 returns: craig rewrite: ruleset 61 input: craig rewrite: ruleset 61 returns: craig < @ *LOCAL* > rewrite: ruleset 93 input: craig < @ *LOCAL* > rewrite: ruleset 93 returns: craig < @ nuts . com . > rewrite: ruleset 31 returns: Craig . Hunt < @ nuts . com . > rewrite: ruleset 4 input: Craig . Hunt < @ nuts . com . > rewrite: ruleset 4 returns: Craig . Hunt @ nuts . com > ^D If the tests do not give the results you want, make sure that you have correctly entered the new rewrite rules and that you have correctly built the database. If sendmail complains that it can't lock the database file, you need to download a more recent release of sendmail V8. The following error message could also be displayed: test.cf: line 116: readcf: map realnames: class dbm not available This indicates that your system does not support dbm databases. Change the database type on the K command line to hash and rerun sendmail -bt. If it complains again, try it with btree. When you find a type of database that your sendmail likes, rerun makemap using that database type. If your sendmail doesn't support any database type, see Appendix E for information on re-compiling sendmail with database support. Note that all of the changes made directly to the sendmail.cf file in the second half of this chapter (masquerading the sender address, masquerading the envelope address and converting usernames) were handled by just three lines in the m4 source file. These examples were used to demonstrate how to use the sendmail test tools. If you really need to make a new, custom configuration, use m4. It is easiest to maintain and enhance the sendmail configuration through the m4 source file. Previous: 10.7 Modifying a sendmail.cf File TCP/IP Network Administration Next: 10.9 Summary 10.7 Modifying a sendmail.cf File Book Index 10.9 Summary [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] file:///C|/mynapster/Downloads/warez/tcpip/ch10_08.htm (10 of 10) [2001-10-15 09:18:44] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 10] 10.9 Summary Previous: 10.8 Testing sendmail.cf Chapter 10 sendmail Next: 11. Troubleshooting TCP/IP 10.9 Summary sendmail sends and receives SMTP mail, processes mail aliases, and interfaces between user mail agents and mail delivery agents. sendmail is started as a daemon at boot time to process incoming SMTP mail. sendmail aliases are defined in the /etc/aliases file. The rules for interfacing between user agents and mail delivery agents can be complex. sendmail uses the sendmail.cf file to define these rules. Configuring the sendmail.cf file is the most difficult part of setting up a sendmail server. The file uses a very terse command syntax that is hard to read. Sample sendmail.cf files are available to simplify this task. Most systems come with a sample file and others are available with the sendmail V8 software distribution. The sendmail V.8 sample files must first be processed by the m4 macro processor. Once the proper sample file is available very little of it needs to be changed. Almost all of the changes needed to complete the configuration occur at the beginning of the file and are used to define information about the local system, such as the hostname and the name of the mail relay host. sendmail provides an interactive testing tool that is used to check the configuration before it is installed. This chapter concludes our study of TCP/IP servers configuration, our last configuration task. In the next chapter we begin to look at the ongoing tasks that are part of running a network once it has been installed and configured. We begin this discussion with troubleshooting. Previous: 10.8 Testing sendmail.cf TCP/IP Network Administration Next: 11. Troubleshooting TCP/IP 10.8 Testing sendmail.cf Book Index 11. Troubleshooting TCP/IP [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] file:///C|/mynapster/Downloads/warez/tcpip/ch10_09.htm [2001-10-15 09:18:44] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 11] Troubleshooting TCP/IP Previous: 10.9 Summary Chapter 11 Next: 11.2 Diagnostic Tools 11. Troubleshooting TCP/IP Contents: Approaching a Problem Diagnostic Tools Testing Basic Connectivity Troubleshooting Network Access Checking Routing Checking Name Service Analyzing Protocol Problems Protocol Case Study Simple Network Management Protocol Summary Network administration tasks fall into two very different categories: configuration and troubleshooting. Configuration tasks prepare for the expected; they require detailed knowledge of command syntax, but are usually simple and predictable. Once a system is properly configured, there is rarely any reason to change it. The configuration process is repeated each time a new operating system release is installed, but with very few changes. In contrast, network troubleshooting deals with the unexpected. Troubleshooting frequently requires knowledge that is conceptual rather than detailed. Network problems are usually unique and sometimes difficult to resolve. Troubleshooting is an important part of maintaining a stable, reliable network service. In this chapter, we discuss the tools you will use to ensure that the network is in good running condition. However, good tools are not enough. No troubleshooting tool is effective if applied haphazardly. Effective troubleshooting requires a methodical approach to the problem, and a basic understanding of how the network works. We'll start our discussion by looking at ways to approach a network problem. 11.1 Approaching a Problem To approach a problem properly, you need a basic understanding of TCP/IP. The first few chapters of file:///C|/mynapster/Downloads/warez/tcpip/ch11_01.htm (1 of 4) [2001-10-15 09:18:45] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 11] Troubleshooting TCP/IP this book discuss the basics of TCP/IP and provide enough background information to troubleshoot most network problems. Knowledge of how TCP/IP routes data through the network, between individual hosts, and between the layers in the protocol stack, is important for understanding a network problem. But detailed knowledge of each protocol usually isn't necessary. When you need these details, look them up in a definitive reference - don't try to recall them from memory. Not all TCP/IP problems are alike, and not all problems can be approached in the same manner. But the key to solving any problem is understanding what the problem is. This is not as easy as it may seem. The "surface" problem is sometimes misleading, and the "real" problem is frequently obscured by many layers of software. Once you understand the true nature of the problem, the solution to the problem is often obvious. First, gather detailed information about exactly what's happening. When a user reports a problem, talk to her. Find out which application failed. What is the remote host's name and IP address? What is the user's hostname and address? What error message was displayed? If possible, verify the problem by having the user run the application while you talk her through it. If possible, duplicate the problem on your own system. Testing from the user's system, and other systems, find out: ● Does the problem occur in other applications on the user's host, or is only one application having trouble? If only one application is involved, the application may be misconfigured or disabled on the remote host. Because of security concerns, many systems disable some services. ● Does the problem occur with only one remote host, all remote hosts, or only certain "groups" of remote hosts? If only one remote host is involved, the problem could easily be with that host. If all remote hosts are involved, the problem is probably with the user's system (particularly if no other hosts on your local network are experiencing the same problem). If only hosts on certain subnets or external networks are involved, the problem may be related to routing. ● Does the problem occur on other local systems? Make sure you check other systems on the same subnet. If the problem only occurs on the user's host, concentrate testing on that system. If the problem affects every system on a subnet, concentrate on the router for that subnet. Once you know the symptoms of the problem, visualize each protocol and device that handles the data. Visualizing the problem will help you avoid oversimplification, and keep you from assuming that you know the cause even before you start testing. Using your TCP/IP knowledge, narrow your attack to the most likely causes of the problem, but keep an open mind. 11.1.1 Troubleshooting Hints Below we offer several useful troubleshooting hints. They are not part of a troubleshooting methodology - just good ideas to keep in mind. ● Approach problems methodically. Allow the information gathered from each test to guide your file:///C|/mynapster/Downloads/warez/tcpip/ch11_01.htm (2 of 4) [2001-10-15 09:18:45] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 11] Troubleshooting TCP/IP testing. Don't jump on a hunch into another test scenario without ensuring that you can pick up your original scenario where you left off. ● Work carefully through the problem, dividing it into manageable pieces. Test each piece before moving on to the next. For example, when testing a network connection, test each part of the network until you find the problem. ● Keep good records of the tests you have completed and their results. Keep a historical record of the problem in case it reappears. ● Keep an open mind. Don't assume too much about the cause of the problem. Some people believe their network is always at fault, while others assume the remote end is always the problem. Some are so sure they know the cause of a problem that they ignore the evidence of the tests. Don't fall into these traps. Test each possibility and base your actions on the evidence of the tests. ● Be aware of security barriers. Security firewalls sometimes block ping, traceroute, and even ICMP error messages. If problems seem to cluster around a specific remote site, find out if they have a firewall. ● Pay attention to error messages. Error messages are often vague, but they frequently contain important hints for solving the problem. ● Duplicate the reported problem yourself. Don't rely too heavily on the user's problem report. The user has probably only seen this problem from the application level. If necessary, obtain the user's data files to duplicate the problem. Even if you cannot duplicate the problem, log the details of the reported problem for your records. ● Most problems are caused by human error. You can prevent some of these errors by providing information and training on network configuration and usage. ● Keep your users informed. This reduces the number of duplicated trouble reports, and the duplication of effort when several system administrators work on the same problem without knowing others are already working on it. If you're lucky, someone may have seen the problem before and have a helpful suggestion about how to resolve it. ● Don't speculate about the cause of the problem while talking to the user. Save your speculations for discussions with your networking colleagues. Your speculations may be accepted by the user as gospel, and become rumors. These rumors can cause users to avoid using legitimate network services and may undermine confidence in your network. Users want solutions to their problems; they're not interested in speculative techno-babble. ● Stick to a few simple troubleshooting tools. For most TCP/IP software problems, the tools discussed in this chapter are sufficient. Just learning how to use a new tool is often more time- consuming than solving the problem with an old familiar tool. ● Thoroughly test the problem at your end of the network before locating the owner of the remote system to coordinate testing with him. The greatest difficulty of network troubleshooting is that you do not always control the systems at both ends of the network. In many cases, you may not even know who does control the remote system. [1] The more information you have about your end, the simpler the job will be when you have to contact the remote administrator. [1] Chapter 13, Internet Information Resources explains how to find out who is responsible for a remote network ● Don't neglect the obvious. A loose or damaged cable is always a possible problem. Check file:///C|/mynapster/Downloads/warez/tcpip/ch11_01.htm (3 of 4) [2001-10-15 09:18:45] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 11] Troubleshooting TCP/IP plugs, connectors, cables, and switches. Small things can cause big problems. Previous: 10.9 Summary TCP/IP Network Administration Next: 11.2 Diagnostic Tools 10.9 Summary Book Index 11.2 Diagnostic Tools [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] file:///C|/mynapster/Downloads/warez/tcpip/ch11_01.htm (4 of 4) [2001-10-15 09:18:45] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 11] 11.2 Diagnostic Tools Previous: 11.1 Approaching a Problem Chapter 11 Troubleshooting TCP/IP Next: 11.3 Testing Basic Connectivity 11.2 Diagnostic Tools Because most problems have a simple cause, developing a clear idea of the problem often provides the solution. Unfortunately, this is not always true, so in this section we begin to discuss the tools that can help you attack the most intractable problems. Many diagnostic tools are available, ranging from commercial systems with specialized hardware and software that may cost thousands of dollars, to free software that is available from the Internet. Many software tools are provided with your UNIX system. You should also keep some hardware tools handy. To maintain the network's equipment and wiring you need some simple hand tools. A pair of needle- nose pliers and a few screwdrivers may be sufficient, but you may also need specialized tools. For example, attaching RJ45 connectors to Unshielded Twisted Pair (UTP) cable requires special crimping tools. It is usually easiest to buy a ready-made network maintenance toolkit from your cable vendor. A full-featured cable tester is also useful. Modern cable testers are small hand-held units with a keypad and LCD display that test both thinnet or UTP cable. Tests are selected from the keyboard and results are displayed on the LCD screen. It is not necessary to interpret the results because the unit does that for you and displays the error condition in a simple text message. For example, a cable test might produce the message "Short at 74 feet." This tells you that the cable is shorted 74 feet away from the tester. What could be simpler? The proper test tools make it easier to locate, and therefore fix, cable problems. A laptop computer can be a most useful piece of test equipment when properly configured. Install TCP/IP software on the laptop. Take it to the location where the user reports a network problem. Disconnect the Ethernet cable from the back of the user's system and attach it to the laptop. Configure the laptop with an appropriate address for the user's subnet and reboot it. Then ping various systems on the network and attach to one of the user's servers. If everything works, the fault is probably in the user's computer. The user trusts this test because it demonstrates something she does every day. She will have more confidence in the laptop than an unidentifiable piece of test equipment displaying the message "No faults found." If the test fails, the fault is probably in the network equipment or wiring. That's the time to bring out the cable tester. Another advantage of using a laptop as a piece of test equipment is its inherent versatility. It runs a wide variety of test, diagnostic, and management software. Install UNIX on the laptop and run the file:///C|/mynapster/Downloads/warez/tcpip/ch11_02.htm (1 of 3) [2001-10-15 09:18:46] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... underlying cause Previous: 11.2 Diagnostic Tools 11.2 Diagnostic Tools TCP/IP Network Administration Book Index Next: 11.4 Troubleshooting Network Access 11.4 Troubleshooting Network Access [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark file:///C|/mynapster/Downloads/warez/tcpip/ch11_03.htm... purchase PDF Split-Merge on www.verypdf.com to remove this watermark file:///C|/mynapster/Downloads/warez/tcpip/ch11_05.htm (5 of 6) [2001-10-15 09:18:49] [Chapter 11] 11.5 Checking Routing Previous: 11.4 Troubleshooting Network Access 11.4 Troubleshooting Network Access TCP/IP Network Administration Next: 11.6 Checking Name Service Book Index 11.6 Checking Name Service [ Library Home | DNS & BIND | TCP/IP. .. securely stored in a wire closet The network components in the user's office should be sufficiently isolated from the network so that damage to those components does not damage the entire network The new network will solve your collision problem and reduce the amount of hardware troubleshooting you are called upon to do Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark file:///C|/mynapster/Downloads/warez/tcpip/ch11_04.htm... network hardware problem If you see these conditions when communicating over great distances on a wide area network, there is nothing to worry about TCP/IP was designed to deal with Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark file:///C|/mynapster/Downloads/warez/tcpip/ch11_03.htm (3 of 4) [2001-10-15 09:18:46] [Chapter 11] 11.3 Testing Basic Connectivity unreliable networks,... High output errors could indicate a saturated local network or a bad physical connection between the host and the network High input errors could indicate that the network is saturated, the local host is overloaded, or there is a physical network problem Tools, such as ping statistics or a cable tester, can help you determine if it is a physical network problem Evaluating the collision rate can help... back together through a router or a bridge? No If you have an old network that is finally reaching saturation, it is time to install a new network built on a more robust technology A shared media network, a network where everyone is on the same cable (in this example, a coaxial cable Ethernet) is an accident waiting to happen Design a network that a user cannot bring down by merely disconnecting his... diagnostic tool Previous: 11.1 Approaching a Problem 11.1 Approaching a Problem TCP/IP Network Administration Book Index Next: 11.3 Testing Basic Connectivity 11.3 Testing Basic Connectivity [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark file:///C|/mynapster/Downloads/warez/tcpip/ch11_02.htm... Troubleshooting Network Access 11.4.4.1 Network hardware problems Some of the tests discussed in this section can show a network hardware problem If a hardware problem is indicated, contact the people responsible for the hardware If the problem appears to be in a leased telephone line, contact the telephone company If the problem appears to be in a wide area network, contact the management of that network. .. device Previous: 11.3 Testing Basic Connectivity 11.3 Testing Basic Connectivity TCP/IP Network Administration Book Index Next: 11.5 Checking Routing 11.5 Checking Routing [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark file:///C|/mynapster/Downloads/warez/tcpip/ch11_04.htm... 11.4 Troubleshooting Network Access Previous: 11.3 Testing Basic Connectivity Chapter 11 Troubleshooting TCP/IP Next: 11.5 Checking Routing 11.4 Troubleshooting Network Access The "no answer" and "cannot connect" errors indicate a problem in the lower layers of the network protocols If the preliminary tests point to this type of problem, concentrate your testing on routing and on the network interface . Testing sendmail.cf TCP/IP Network Administration Next: 11. Troubleshooting TCP/IP 10.8 Testing sendmail.cf Book Index 11. Troubleshooting TCP/IP [ Library. Diagnostic Tools TCP/IP Network Administration Next: 11.4 Troubleshooting Network Access 11.2 Diagnostic Tools Book Index 11.4 Troubleshooting Network Access