TCP Header—Layer 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 23 TCP Header . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 24 TCP Header (continued) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 25 UDP Header—Layer 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 26 Troubleshooting TCP/IP Troubleshooting TCP/IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 28 Troubleshooting TCP/IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 29 Host Configuration Host Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 30 Host Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 31 Boot Linux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 32 Boot Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 33 Determine Addresses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 34 MAC ↔ IP mapping — 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 35 MAC ↔ IP mapping — 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 36 Routing Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 37 Access Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 38 DNS resolver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 39 Checking services provided . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 40 Server Running? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 41 Path Performance: Other tools. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 69 DNS: dig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 119 pathchar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 70 Measuring Throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 71 Measuring Throughput with ttcp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 72 iproute The ip program, iproute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 74 iproute and iptables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 75 Traffic Measurements: Measuring Traffic: dig: Checking forward DNS lookup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 120 dig syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 122 dig: axfr (Zone Transfer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 123 nslookup: an interactive program. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 124 nslookup: reverse lookups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 125 telnet Email: testing with telnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 127 Test the VTC mail server:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 128 netstat -i. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 76 netstat -i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 77 ifconfig, ip. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 78 Traffic measurements: Guide to ip 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 80 Guide to ip 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 81 Packet Capture What is Packet Capture? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 83 When Packet Capture? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 84 Warning: Don’t Get Sacked!. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 85 Testing the VTC pop3 server 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 130 Testing the pop3 server 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 131 pop3 commands: retrieving mail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 132 telnet: Testing Other Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 133 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 134 tcpdump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 86 How to use tcpdump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 87 tcpdump: some options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 88 tcpdump Filters: host and port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 89 tcpdump filters: networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 90 tcpdump filters: protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 91 tcpdump filters: combining. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 92 Writing data to a file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 93 top . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 42 netstat -tua . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 43 lsof: List Open Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 44 ifconfig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 45 route . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 46 References Getting more info using ip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 79 SMTP commands for sending mail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 129 Craig Hunt, TCP/IP Network Administration, Third Edition O’Reilly, April 2002, ISBN: 059600297-1. Chapter 13 covers troubleshooting TCP/IP. Martin A. Brown, Guide to IP Layer Network Administration with Linux, http://linux-ip.net/ Noah Davids, Don’t forget to check your ARP cache, dig: reverse lookup 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 121 http://members.cox.net/~ndav1/self_published/The_ARP_cache.doc Joseph D. Sloan, Network Troubleshooting Tools, O’Reilly, August 2001, ISBN: 059600186-X, TK5105.5 .S557 2001 Cisco’s Troubleshooting Overview at: http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1901.htm 0-1 0-3 0-5 Contents References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 2 Introduction Focus: Basics and Standard Tools. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 3 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 4 Troubleshooting: Learn as you go . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 5 Dcoumentation Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 6 Documentation Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 7 General Troubleshooting Problem Solving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 9 Identify the Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 10 Gather the Facts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 11 Consider Possibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 12 Action Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 13 Implement Action Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 14 Observe Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 15 document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 16 modify action plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 17 TCP/IP OSI—TCP/IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 19 IP Header—Layer 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 20 IP Header . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 21 IP Header (continued) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 22 Cables Connectivity Testing: Cabling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 47 Ping Software tools: Reading a Dumped File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 94 tcpdump: When reading TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 96 ping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 49 Window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 97 Ethereal Ethereal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 99 Ethereal Screenshot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 100 Ethereal Screenshot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 101 You can expand any protocol: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 102 Ethereal 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 103 Good ping, Bad ping? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 50 How to Use ping? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 51 fping: flood ping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 52 hping2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 53 arping: uses ARP requests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 54 traceroute Path Discovery: traceroute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 55 Display Filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 104 Tools → Follow TCP Stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 105 Ntop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 106 Ntop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 107 Switched Networks Port Monitoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 109 How monitor one machine? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 110 Are switched networks secure? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 111 Port Scanning What is a port scanner? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 113 How does nmap identify OS? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 114 Running nmap: Use xnmap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 115 Uses of nmap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 116 DNS troubleshooting DNS troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 118 traceroute: How it Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 56 traceroute Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 57 Measurements Performance Measurements: delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 58 bandwidth and throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 59 Quality of a Link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 60 Throughput: Througput: ping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 61 ping One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 62 Throughput ping 2 remote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 63 Throughput ping 2 remote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 64 Throughput ping 2 remote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 65 Throughput ping 2 remote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 66 Throughput ping 2 remote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 67 Limitations of ping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 68 Copyright Conditions: GNU FDL (see http://www.gnu.org/licenses/fdl.html) HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 95 Nick Urbanik Network Troubleshooting Troubleshooting Tools 0-2 0-4 SNM — ver. 1.4 Network Troubleshooting - p. 1/134 SNM — ver. 1.4 Network Troubleshooting - p. 2/134 Focus: Basics and Standard Tools Solving network problems depends a lot on your understanding Simple tools can tell you what you need to know Example: ping is incredibly useful! Troubleshooting Avoid it by: redundancy documentation training Try quick fixes first simple problems often have big effects: is the power on? is the network cable plugged into the right socket? Is LED flashing? has anything changed recently? Change only one thing at a time test thoroughly after the change Be familiar with the system maintain documentation Be familiar with your tools SNM — ver. 1.4 Network Troubleshooting - p. 4/134 before trouble strikes SNM — ver. 1.4 Network Troubleshooting - p. 3/134 Troubleshooting: Learn as you go Study and be familiar with the normal behaviour of your network Monitoring tools can tell you when things are wrong if you know what things look like when they are right Using tools such as Ethereal can help you understand your network, and TCP / IP— better Documentation Maintain an inventory of equipment and software a list mapping MAC addresses to machines can be very helpful Maintain a change log for each major system, recording: each significant change each problem with the system each entry dated, with name of person who made the entry Two categories of documentation: Configuration information describes the system use system tools to obtain a snapshot, e.g., sysreport in Red Hat Linux Procedural information How to do things SNM ver. 1.4 Network Troubleshooting use—tools that automatically document what you are - p. 6/134 doing, e.g., script SNM — ver. 1.4 Network Troubleshooting - p. 5/134 Documentation Tools Use script: $ script ∼/logs/logfile-$(date +%F-%R).log starts a new shell all you type, all output goes into the file Add comments with # I tried this... Use tee: $ arp -a | tee outfile Use sudo: all commands are recorded in /var/log/secure Use plod from http://bullwinkle.deer-run.com/~hal/plod/ lets you record a worksheet easily Perl, so fine on any platform SNM — ver. 1.4 Network Troubleshooting - p. 7/134 General Troubleshooting SNM — ver. 1.4 Network Troubleshooting - p. 8/134 Problem Solving Solve Problem Define problem Gather facts Consider possibilities based on facts Create action plan Implement action plan Observe results Identify the Problem Problem is reported by a person or by software Often involves communicating with others Somewhat like gathering requirements in software design An iterative process Possible questions to ask: What does not work? What does work? Are the things that do and do not work related? Has the thing that does not work ever worked? When the problem was first noticed? What has changed since the last time it did work? Did anything unusual happen since the last time it worked? When exactly does the problem occur? SNM — ver. Network Can the1.4 problem be reproduced and if so, how Troubleshooting - p. 10/134 can it be reproduced? NO Symptoms stop? YES SNM — ver. 1.4 Document process Network Troubleshooting - p. 9/134 Problem solved Gather the Facts You probably need to find out more about the problem from other sources, including Asking other people: affected users, network administrators, managers, and other key people Network management systems, such as Nagios http://nagios.org/ Tools such as Ethereal, tcpdump, ntop (http://ntop.org/) — see slides starting at §83 Server log files Documentation about your servers and network created by local staff Documentation about software and hardware that are provided by the vendors Consider Possibilities based on Facts Using the information you have gathered, try to eliminate some potential problems from your list. SNM — ver. 1.4 Network Troubleshooting - p. 11/134 SNM — ver. 1.4 Network Troubleshooting - p. 12/134 Create an Action Plan Start with the most likely . . . and those that are easiest to test Plan needs to be methodical Plan to change only one thing at a time You can then understand the cause of the problem Aim to understand the problem so you can learn from it, solve (or prevent) similar problems in the future Aim higher than just removing the symptoms! Implement Action Plan Perform each step carefully Test to see if symptoms go away SNM — ver. 1.4 Network Troubleshooting - p. 13/134 SNM — ver. 1.4 Network Troubleshooting - p. 14/134 Observe Results Gather results as you change each variable Use same techniques you used in slide 11: Check with the key people Check with your tools If Solved: Document Solution Record the problem and its resolution in the documentation you maintain for your company. Ensure others in your team can benefit from the insight you have gained SNM — ver. 1.4 Network Troubleshooting - p. 15/134 SNM — ver. 1.4 Network Troubleshooting - p. 16/134 Otherwise, Modify Action Plan Go back to the steps in slide §13: Modify your action plan, selecting the next most likely action from your list You may have discovered more information in your investigation, so this can help you focus on likely causes. If you have exhausted all the items on your list, and cannot think of what else to do: Get help from the vendor Get help from mailing lists Discuss the problem with your network of colleagues (e.g., the people who are now studying with you, but who move on to work in a similar field!) You could even track me down and ask me! Quite a few of my ex-students do. SNM — ver. 1.4 Network Troubleshooting - p. 17/134 SNM — ver. 1.4 TCP/IP Network Troubleshooting - p. 18/134 OSI—TCP/IP Layer 7 6 5 4 3 2 1 OSI Application Presentation Session Transport Network Data link Physical TCP/IP 0 HTTP, FTP, SMTP Telnet, SSH, SMTP SNMP, NFS, SMB RPC, DNS, OSPF, BGP IP Header—Layer 3 12 16 20 24 28 Padding 31 Bits 4 8 Application 1 2 Words Version IHL Type of Service DF MF Total Length Fragmentation Offset Header Checksum Identification Time to Live Protocol TCP, UDP IP ARP ICMP Transport Internet Network Access 3 4 5 Source Address Destination Address Options (0 to 40 bytes) 5−16 SNM — ver. 1.4 Network Troubleshooting - p. 19/134 SNM — ver. 1.4 Your data starts here Network Troubleshooting - p. 20/134 IP Header Version — this is a 4-bit IP header length field that indicates the version of IP currently used. The current version of IP is 4 (IPv4) but IPv6 is already being implemented experimentally and will be supported on future versions of the IOS. IP Header Length (IHL) — this indicates the datagram header length in 32-bit words. Type of Service (ToS) — ToS specifies how a particular upper-layer protocol would like the current datagram to be handled. Datagrams can be assigned various levels of importance with this field. Total length — this specifies the length of the entire IP packet, including data and header, in bytes. Identification — this field contains an integer that identifies the current datagram. This field is usedTroubleshooting - p. 21/134 to help SNM — ver. 1.4 Network piece together datagram fragments. IP Header (continued) Flags — a flag is a 3-bit field of which the 2 low-order bits control fragmentation. One bit specifies whether the packet can be fragmented; the second bit specifies whether the packet is the last fragment in a series of fragmented packets. Time-to-Live — this field maintains a counter that gradually decrements down to zero, at which point the datagram is discarded. This prevents packets from looping endlessly. Protocol — protocol indicates which upper-layer protocol receives incoming packets after IP processing is complete. Header Checksum — this field helps ensure IP header integrity. Source—Address — this field specifies the sending node.p. 22/134 SNM ver. 1.4 Network Troubleshooting Destination Address — this field specifies the receiving node. Options — this field allows IP to support various options, such as security. Source port and destination port — these fields identify Data — this field contains upper-layer information. the points at which upper-layer source and destination processes receive TCP services. Sequence number — this field usually specifies the number assigned to the first byte of data in the current message. Under certain circumstances, it can also be used to identify an initial sequence number to be used in the upcoming transmission. Acknowledgment number — this field contains the sequence number of the next byte of data the sender of the packet expects to receive. Data offset — this field indicates the number of 32-bit words in the TCP header. Reserved — this field is reserved for future use. SNM — this Network Troubleshooting Flags —ver. 1.4 field carries a variety of control information.p. 24/134 TCP Header—Layer 4 12 16 20 Bits 4 8 TCP Header 24 28 Padding 1 2 Words 3 4 5 header length Source Port Sequence Number Destination Port Acknowledgement Number URG ACK PSH Reserved Checksum SYN FIN RST Window Urgent Pointer Options (0 to 40 bytes) 5−15 SNM — ver. 1.4 Your data starts here Network Troubleshooting - p. 23/134 31 0 TCP Header (continued) Window — this field specifies the size of the sender’s receive window (that is, buffer space available for incoming data). Checksum — this field indicates whether the header was damaged in transit. Urgent pointer — this field points to the first urgent data byte in the packet. Options — this field specifies various TCP options. Data — this field contains upper-layer information. 0 UDP Header—Layer 4 16 31 Destination Port Checksum Your data starts here Bits Source Port Length SNM — ver. 1.4 Network Troubleshooting - p. 25/134 Source and Destination Port fields serve the same functions as they do in the TCP header. Length field specifies the length of the UDP header and data Checksum field allows packet integrity checking. It is optional.ver. 1.4 SNM — Network Troubleshooting - p. 26/134 Troubleshooting TCP/IP Local host YES configuration okay? NO can you ping the YES local router? NO can you reach all routers in path to remote YES Troubleshooting TCP/IP host? remote host configuration okay? NO NO Fix any problems with address, subnet mask, default gateway, DNS entry Fix any router configuration problems, and any LAN or switch problems Fix any router configuration problems, LAN or WAN problems Fix remote host configuration SNM — ver. 1.4 Network Troubleshooting - p. 27/134 SNM — ver. 1.4 Network Troubleshooting - p. 28/134 Troubleshooting TCP/IP Step 1 Host Network Configuration tools ps — information about processes top — dynamic information about processes netstat — show connections and services, routing lsof — list open files ifconfig — shows and changes network interfaces route — shows, changes routing table ip — show, change, set network configuration arp — shows MAC addresses nmap — portscanner: shows open ports, identifies OS First, determine whether your local host is properly configured (for instance, correct subnet mask and default gateway configuration). Step 2 Next, use the ping or traceroute commands to determine whether the routers through which you must communicate can respond. Start with the most local router and progressively ping outwards through the Internet or use traceroute. If you cannot get through a particular node, examine the node configuration and use the various show commands to determine the state of the router (these include show ip route, show ip arp, show running-configuration, and so on.) If you can get to all the routers in the path, check the host SNM — ver. 1.4 configuration at the remote host (or get someone’s - p. 29/134 Network Troubleshooting help to do so), and check its configuration. Step 3 Step 4 SNM — ver. 1.4 Network Troubleshooting - p. 30/134 Checking (and Setting) Host Configuration Solving Boot problems: §32, §33 Determine IP address, netmask, broadcast address: §34 Deterine correct MAC ↔ IP address mapping: §35, §36 Examine routing table: §37 Examine access controls: §38 Examine web proxy settings: check web browser Examine DNS resolver settings: §39 Determine services provided: §40, §41 Determine CPU, memory load conditions (is the server overloaded?) §42 Boot problems: Linux Use grub to interactively boot the computer (see my extensive grub handout: http://ictlab.tyict.vtc.edu.hk/ossi/lab/grub/grub.pdf) Verify that /etc/fstab mounts the correct filesystems Use a rescue disk such as Knoppix or the Red Hat installation CDROM. This gives you full access to the system and repairing boot problems is pretty straightforward. SNM — ver. 1.4 Network Troubleshooting - p. 31/134 SNM — ver. 1.4 Network Troubleshooting - p. 32/134 Boot problems: Windows Use the installation Windows CD to enter the (extremely limited) system repair mode. I believe this is called the recovery console. Use the Linux floppy bootdisk at http://home.eunet.no/~pnordahl/ntpasswd/ to replace the Administrator password Use the bootable Windows CDROM: http://www.nu2.nu/pebuilder/; Gives full access to the NTFS file system. Not as good with Windows as Knoppix is with Linux, but better than another reinstall. takes some time to build. Henry Leung (in A204d) has built some. SNM — ver. 1.4 Network Troubleshooting - p. 33/134 Determine Addresses Linux: On Linux, these commands all show the IP address, MAC address, netmask and broadcast address for all (or the specified) interface. The commands ip and ifconfig are in the directory /sbin; netstat is in /bin. $ $ $ $ $ ip addr ip addr show eth0 ifconfig ifconfig eth0 netstat -i Windows: C:\> ipconfig /all Cisco IOS: these are both privileged commands, as shown by the prompt: SNM — ver. 1.4 Network Troubleshooting - p. 34/134 Router# show running-config Router# show ip interface brief Router# show ip interface MAC ↔ IP mapping — 1 Linux: Windows: MAC ↔ IP mapping — 2 C:\> arp -a $ arp -a $ ip neigh show The lifetime of the ARP cache entries is settable in /proc/sys/net/ipv4/neigh/ interface / gc_stale_time and is normally 60 seconds. Cisco IOS: You may wish to clear the machine with: C:\> arp -d IP address ARP cache on a Windows or clear the entire C:\> arp -d * ARP cache with: Router# show ip arp http://members.cox.net/~ndav1/self_published/The_ARP_cache.doc has a good discussion on troubleshooting ARP. The online book at http://linux-ip.net/ has an excellent chapter on ARP. SNM — ver. 1.4 Network Troubleshooting p. ARP is probably the most dangerously exposed protocol in -a 35/134 LAN, and can easily be subverted by tools such as dsniff and Ettercap. Hardcode important ARP entries to avoid attack. Note that this document: since the Windows ARP cache lives 10 minutes by default, a rather (too?) long time. It can be changed by two registry entries under HKEY_ LOCAL_MACHINE\SYSTEM\CurrentControlSet\ Services\Tcpip\Parameters. SNM — ver. 1.4 Network Troubleshooting - p. 36/134 Routing Table Access Controls Access controls can block access mysteriously unless you think to check for it. Linux: Linux: The commands ip and route are in /sbin, the command netstat is in /bin. $ ip route $ route -n $ netstat -nr There are two main things to check. The iptables command is in the /sbin directory. Windows: $ iptables -L -n C:\> route print C:\> netstat -nr Cisco IOS: Note that Linux and many other POSIX systems implement the tcpwrappers access control in /etc/hosts.allow and /etc/hosts.deny. See man hosts.allow and man hosts.deny. Cisco IOS: Router# show ip route Router# show ip access-list SNM — ver. 1.4 Network Troubleshooting - p. 37/134 SNM — ver. 1.4 Network Troubleshooting - p. 38/134 DNS resolver settings Linux: Checking services provided Linux: The configuration for the resolver is /etc/resolv.conf. This determines what name servers the system will ask. It also tells what domain will be appended to a hostname. The /etc/hosts file is usually the first way hostname ↔ IP address mappings are made, but this can be changed in /etc/nsswitch.conf. To ask the operating system for what it can see there, do: $ getent hosts Linux provides three tools for troubleshooting DNS servers: dig, host and nslookup. Windows: DNS and There are four main ways to check: Verify the processes with ps (see §41) Verify the services that are configured to start when the system boots: $ chkconfig --list | grep on Check that the service is listening on the network interface: $ netstat -tua will show all network connections to this machine. The lsof program can be helpful in diagnosing problems with network services. See §44. Check network connections with See the output of Windows: C:\> ipconfig /all C:\> netstat -a SNM — ver. 1.4 Network Troubleshooting - p. 40/134 See Slides starting at §118 for details of dig and nslookup. Is the network service running on the server? Is the web server running? $ ps aux | grep httpd Is the DHCP server running? $ ps aux | grep dhcpd Is the directory server running? $ ps aux | grep slapd Windows: use the task manager Using ps to See If Server Running for the names of the DNS server the resolver will use. - p. 39/134 SNM — ver. 1.4 Network Troubleshooting Recent versions of Windows provide the program nslookup to help with DNS troubleshooting. Using top to see Resource Hogs The program top shows: load average (the average number of processes that are ready to run, but for which no CPU is available) a load average of 4 or more is “quite high” processes that use the most resources SNM — ver. 1.4 Network Troubleshooting - p. 41/134 SNM — ver. 1.4 Network Troubleshooting - p. 42/134 netstat -tua: See Network Connections netstat -tua shows all network connections, including those listening sudo netstat -tuap shows all network connections, including those listening, and the processes responsible netstat -tul shows all network listeners netstat -t shows only TCP connections that are established netstat -i is like ifconfig, shows info and stats about each interface netstat -nr shows the routing table, like route -n Windows provides netstat also. lsof: List Open Files An amazingly useful tool Available for almost any Unix system lsof -i shows output to Internet and X.25 files, but won’t show connections that have terminated lsof -i@ictlab.tyict.vtc.edu.hk will show only connections to that machine Can monitor progress of an FTP transfer, many, many other applications See manpage, FAQ and quick start guide. Apparently, no equivalent tool available on Windows. SNM — ver. 1.4 Network Troubleshooting - p. 43/134 SNM — ver. 1.4 Network Troubleshooting - p. 44/134 ifconfig ifconfig eth0 — show stats on network interface eth0 sudo ifconfig lo 127.0.0.1 — configure the loopback interface, start it up sudo ifconfig eth0 172.19.233.5 netmask 255.255.255.0 — configure eth0 with IP address 172.19.233.5/24 ifconfig — show all configured network interfaces ifconfig -a — show all interfaces, including those not configured yet. route route -n — print routing table route add 127.0.0.1 — add a route to localhost; should have been done automatically when created device with ifconfig route add -net 172.19.233.0 — add a route to the eth0 configured on previous slide should have been done automatically by ifconfig route add 172.19.64.0 gw 172.19.233.254 — add a static route to network 172.19.64.0 through router 172.19.233.254 route add default gw 172.19.233.253 — add a default route to 172.19.233.253 through eth0 SNM — ver. 1.4 Network Troubleshooting - p. 45/134 SNM — ver. 1.4 Network Troubleshooting - p. 46/134 Connectivity Testing: Cabling Label cables clearly at each end Cable testers ensure wired correctly, check: attenuation length — is it too long? 100BaseT: less than 100m Is the activity light on the interface blinking? Ping SNM — ver. 1.4 Network Troubleshooting - p. 47/134 SNM — ver. 1.4 Network Troubleshooting - p. 48/134 Software tools: ping Most useful check of connectivity Universal If ping hostname, includes a rough check of DNS Sends an ICMP (Internet Control Message Protocol) ECHO_REQUEST Waits for an ICMP ECHO_REPLY Most pings can display round trip time Most pings can allow setting size of packet Can use to make a crude measurement of throughput—see §61 What ping Result is Good, Bad? A steady stream of consistent replies indicates probably okay Usually first reply takes longer due to ARP lookups at each router After that, ARP results are cached ICMP error messages can help understand results: Destination Network Unreachable indicates the host doing ping cannot reach the network Destination Host Unreachable may come from routers further away SNM — ver. 1.4 Network Troubleshooting - p. 49/134 SNM — ver. 1.4 Network Troubleshooting - p. 50/134 How to Use ping? Ensure local host networking is enabled first: ping localhost, local IP address ping a known host on local network ping local and remote interfaces on router ping by IP as well as by hostname if hostname ping fails confirm DNS with dig (or nslookup) — see slide §118 Ping from more than one host fping: flood ping Designed to test a large number of hosts more efficient than ping Used extensively by monitoring software such as mon: http://www.kernel.org/software/mon/, nagios: http://www.nagios.org/ take care not to flood too much! RPMs are available; I built one (a long time ago) and put it on ictlab under ∼ftp/pub/redhat/contrib SNM — ver. 1.4 Network Troubleshooting - p. 51/134 SNM — ver. 1.4 Network Troubleshooting - p. 52/134 hping2: ping anything with anything able to send custom TCP/IP packets and display target replies like ping program does with replies. Can install with $ yum -y install hping2 on Fedora Core 1. See http://www.hping.org/. ICMP arping: uses ARP requests Limited to local network Can work with MAC or IP addresses use to probe for ARP entries in router (very useful!) packet filtering can block ICMP pings, but won’t block ARP requests SNM — ver. 1.4 Network Troubleshooting - p. 53/134 SNM — ver. 1.4 Network Troubleshooting - p. 54/134 Path Discovery: traceroute Sends UDP packets (Microsoft tracert sends ICMP packets) increments Time to Live (TTL) in IP packet header Sends three packets at each TTL records round trip time for each increases TTL until enough to reach destination traceroute: How it Works As IP packets pass through each router, TTL in IP header is decremented Packet is discarded when TTL decrements to 0 ROUTER sends ICMP TIME_EXCEEDED message back to traceroute host When UDP packet reaches destination, gets ICMP PORT_UNREACHABLE, since uses an unused high UDP port SNM — ver. 1.4 Network Troubleshooting - p. 55/134 SNM — ver. 1.4 Network Troubleshooting - p. 56/134 traceroute Limitations Each router has a number of IP addresses but traceroute only shows the one it used get different addresses when run traceroute from other end sometimes route is asymmetric router may be configured to not send ICMP TIME_EXCEEDED messages get stars: * instead of round-trip time in traceroute output Performance Measurements: delay Three sources of delay: transmission delay — time to put signal onto cable or media depends on transmission rate and size of frame propagation delay — time for signal to travel across the media determined by type of media and distance queuing delay — time spent waiting for retransmission in a router SNM — ver. 1.4 Network Troubleshooting - p. 57/134 SNM — ver. 1.4 Network Troubleshooting - p. 58/134 Is Bandwidth == Throughput? bandwidth — the difference between the upper frequency and the lower frequency that a channel can carry measured in Hertz throughput — amount of data that can be sent over link in given time is not the same as bandwidth, which really has no direct meaning with digital information bandwidth is related to throughput by the Shannon-Hartley Theorem; throughput ∝ bandwidth if signal to noise ratio is fixed: Cmax = B log2 1 + SNM — ver. 1.4 Quality of a Link Other measurements needed i.e., for quality of service for multimedia S N bits/sec Network Troubleshooting - p. 59/134 SNM — ver. 1.4 Network Troubleshooting - p. 60/134 ping Roughly = signal power in W, Estimating Throughput = noise power in W. where Cmax B S N = maximum channel capacity, = bandwidth in Hz, Throughput: ping One Remote Host This can be expressed as a simple formula: T P = 16 × Pl − P s bits per second, where tl − t s Pl = size of large packet tl = round-trip time for large packet ts = round-trip time for small packet Here we have: 1100 − 100 T P = 16 × (60 − 30) × 10−3 1000 = 16 × 30 × 10−3 16 × 106 = 30 SNM — ver. 1.4 Network Troubleshooting - p. 62/134 ≈ 530,000 bps Example: measuring throughput between this machine and one remote machine. ping with packet size = 100 bytes, round-trip time = 30ms ping with packet size = 1100 bytes, round-trip time = 60ms So takes 30ms extra (15ms one way) to send additional 1000 bytes, or 8000 bits Throughput is roughly 8000 bits per 15ms, or about 530,000 bits per second A very crude measurement: no account for other traffic, treats all links on path, there and back, as one. Routers sometimes send packets onwards with much higher priority than with which they answer pings. See SNM §68. Network Troubleshooting - p. 61/134 slide — ver. 1.4 Ps = size of small packet Throughput: ping Two Remote Hosts Measure throughput between two remote hosts: may use tools like ping ping two locations with two packet sizes (4 pings altogether, minimum) Many ping programs calculate average ping time: better to make a number of pings, use the average ping time. First ping time may be longer due to the time to get an answer to the arp request May be better to ping once, then start pinging again, and use the average ping time. Example: Address RTT 100 bytes RTT 1100 bytes 5.805 ms 12.823 ms 26.713 ms Network Troubleshooting - p. 63/134 Throughput: ping Two Remote Hosts — 2 Address 205.153.61.1 205.153.60.2 165.166.36.17 RTT 100 bytes 1.380 ms 4.985 ms 8.621 ms RTT 1100 bytes 5.805 ms 12.823 ms 26.713 ms Time difference / 2 (round trip time (RTT) → one way) Divide by size difference in bits: 8000 = 8 × (1100 − 100) Multiply by 1000 (ms → seconds) Mbs = bps/106 Near link 205.153.61.1 205.153.60.2 Far link Time difference Est. Throughput 4.69 Mbps 1.56 Mbps 205.153.60.2 3.413 ms 165.166.36.17 10.254 ms Throughput: ping Two Remote Hosts — 3 Throughput: ping Two Remote Hosts — 4 Pl = large packet size, bytes = 1100 bytes 205.153.61.1 1.380 ms SNM — ver. 1.4 205.153.60.2 4.985 ms 165.166.36.17 8.621 ms SNM — ver. 1.4 Network Troubleshooting - p. 64/134 3.413 ms = (12.823 − 4.985) − (5.805 − 1.380) ms 10.254 ms = (26.713 − 8.621) − (12.823 − 4.985) ms Pl − P s T P = 16 × tf l − tf s − tnl + tns Ps = small packet size, bytes = 100 bytes bits per second tnl = ping time for larger packet to the near link, seconds = 5.805 × 10 −3 seconds where: Pl = large packet size, bytes Ps = small packet size, bytes tnl = ping time for larger packet to the near link, seconds tns = ping time for smaller packet to the near link, seconds tf l = ping time for larger packet to the f ar link, seconds tf s = ping time for smaller packet to the f ar link, seconds tns = ping time for smaller packet to the near link, seconds = 1.380 × 10 −3 seconds tf l = ping time for larger packet to the f ar link, seconds = 12.823 × 10 −3 seconds tf s = ping time for smaller packet to the f ar link, seconds = 4.985 × 10 T P = 16 × = 16 × tf l −3 seconds bits per second bits per second Pl − P s − tf s − tnl + tns 1100 − 100 (12.823 − 4.985 − 5.805 + 1.380) × 10−3 1000 = 16 × 3.413 × 10−3 ≈ 4,687,958 ≈ 4.69 Megabits per second SNM — ver. 1.4 Network Troubleshooting - p. 65/134 SNM — ver. 1.4 Network Troubleshooting - p. 66/134 Throughput: ping Two Remote Hosts — 5 Pl = large packet size, bytes = 1100 bytes Ps = small packet size, bytes = 100 bytes tnl = ping time for larger packet to the near link, seconds = 12.823 × 10 −3 Limitations of measuring with ping Most modern routers give high priority to routing Expecially switching routers, such as the Cisco 6509 in our Campus Many give much lower priority to answering pings The difference can be so great that the ping reply sometimes comes sooner from a more distant router, which according to our formula, indicates a negative throughput! Do not blindly apply this formula! Measurements may not match the kind of traffic created by the application you support The big advantages of these ICMP measurements are: you do not need access to the machines, and you do not need to install any special software on them. SNM — ver. 1.4 Network Troubleshooting - p. 68/134 seconds tns = ping time for smaller packet to the near link, seconds = 4.985 × 10 −3 seconds tf l = ping time for larger packet to the f ar link, seconds = 26.713 × 10 −3 seconds tf s = ping time for smaller packet to the f ar link, seconds = 8.621 × 10 T P = 16 × tf l −3 seconds bits per second bits per second Pl − P s − tf s − tnl + tns 1100 − 100 = 16 × (26.713 − 8.621 − 12.823 + 4.985) × 10−3 1000 = 16 × 10.254 × 10−3 ≈ 1,560,366 ≈ 1.56 Megabits per second SNM — ver. 1.4 Network Troubleshooting - p. 67/134 Path Performance: Other tools Could use a tool like pathchar, bing, clink, pchar, or tmetric that performs this calculation for you Use http://www.google.com/ to locate these tools pathchar is only available in binary form Others in source form, need compile with commands something like this: $ cd bing-1.1.3 $ make $ sudo make install Path measurement with pathchar $ sudo ./pathchar sina.com.hk pathchar to sina.com.hk (202.85.139.140) can’t find path mtu - using 1500 bytes. doing 32 probes at each of 45 sizes (64 to 1500 by 32) 0 localhost (127.0.0.1) | 106 Mb/s, 293 us (698 us), +q 1.18 ms (15.7 KB) 1 172.19.35.246 (172.19.35.246) | 28 Mb/s, 488 us (2.10 ms) 2 192.168.83.2 (192.168.83.2) 3 *1 448 798 2 | 20 Mb/s, 273 us (3.25 ms) 4 cw7204.vtc.edu.hk (202.40.210.220) | 6.8 Mb/s, 521 us (6.04 ms) 5 210.176.123.37 (210.176.123.37) | 52 Mb/s, 20 us (6.31 ms) 6 210.87.254.61 (210.87.254.61) | 136 Mb/s, 116 us (6.63 ms) 7 g5-0-0.wttbr01.imsbiz.com (210.87.254.129) | 33 Mb/s, 0.94 ms (8.88 ms), +q 1.48 ms (6.10 KB) *6 8 iadvantage3-RGE.hkix.net (202.40.161.172) | 164 Mb/s, 45 us (9.04 ms), +q 1.74 ms (35.6 KB) *6 9 v005-m02.hk01.iadvantage.net (202.85.129.53) | ?? b/s, -66 us (8.88 ms) 10 202.85.129.136 (202.85.129.136) | ?? b/s, 459 us (9.79 ms) 11 202.85.139.11 (202.85.139.11) 11 hops, rtt 6.18 ms (9.79 ms), bottleneck 6.8 Mb/s, pipe 9361 bytes SNM — ver. 1.4 SNM — ver. 1.4 Network Troubleshooting - p. 69/134 Network Troubleshooting - p. 70/134 Measuring Throughput May use ftp to transfer a large file, measure time tests whole path problem: affected by disk I / O, xinetd May use a web browser and measure download time Problem: may be affected by caching in the web browser May be affected by caching in web proxies Better: measure using traffic similar to that created by the application. Measuring Throughput with ttcp Use ttcp, not affected by disk I / O Consists of a client and server Need have installed at both ends Part of Red Hat Linux, Cisco IOS Example: first, start receiver on ictlab: $ ttcp -r -s ttcp-r: buflen=8192, nbuf=2048, align=16384/0, port=5001 tcp ttcp-r: socket ttcp-r: accept from 172.19.32.30 ttcp-r: 16777216 bytes in 1.45 real seconds = 11285.88 KB/sec +++ ttcp-r: 9704 I/O calls, msec/call = 0.15, calls/sec = 6684.46 ttcp-r: 0.0user 0.2sys 0:01real 14% 0i+0d 0maxrss 0+2pf 0+0csw SNM — ver. 1.4 Network Troubleshooting - p. 71/134 $ ttcp -t -s ictlab ttcp-t: buflen=8192, nbuf=2048, align=16384/0, port=5001 tcp -> ictlab ttcp-t: socket ttcp-t: connect ttcp-t: 16777216 bytes in 1.45 real seconds = 11335.64 KB/sec +++ ttcp-t: 2048 I/O calls, msec/call = 0.72, calls/sec = 1416.95 SNM — ver. 1.4 Network Troubleshooting - p. 72/134 ttcp-t: 0.0user 0.0sys 0:01real 4% 0i+0d 0maxrss 0+2pf 0+0csw Second, start transmitter on nickpc: The ip program, iproute The ip program in the iproute package provides complete control over TCP / IP networking in a Linux system Provides more networking control facilities than other TCP / IP implementations Supports tunneling in many forms iproute documentation is in two manuals, one for IP routing, the other for tunnelling iproute SNM — ver. 1.4 Network Troubleshooting - p. 73/134 SNM — ver. 1.4 Network Troubleshooting - p. 74/134 iproute and iptables Between these software packages, you can: throttle bandwidth for certain computers throttle bandwidth to certain computers fairly share bandwidth protect your network from DoS attacks protect Internet from your customers multiplex many servers into one, for load balancing or for high availability restrict access to your computers limit access of your users to other hosts do routing based on user id, MAC address, source IP, port, type of service, time of day or content See the Linux Advanced Routing and Traffic Control HOWTO at http://tldp.org for details SNM — ver. 1.4 Network Troubleshooting - p. 75/134 Traffic Measurements: netstat -i The netstat program can show statistics about network interfaces Linux netstat shows lost packets in three categories: errors, drops (queue full: shouldn’t happen!) overruns (last data overwritten by new data before old data was read: shouldn’t happen!) drops and overruns indicate faulty flow control — bad! These values are cumulative (since interface was up) Could put a load on interface to see current condition, with ping -l, to send large number of packets to destination See the difference in values SNM — ver. 1.4 Network Troubleshooting - p. 76/134 Measuring Traffic: netstat -i Here we run netstat -i on ictlab: $ netstat -i Kernel Interface table Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg 3 BMRU eth0 1500 0 407027830 0 0 0 1603191764 0 0 lo 16436 0 2858402 0 0 0 2858402 0 0 0 LRU Traffic measurements: ifconfig, ip ifconfig and ip give more information than netstat -i: $ ifconfig eth0 eth0 Link encap:Ethernet HWaddr 00:00:E2:35:AF:EE inet addr:172.19.64.52 Bcast:172.19.127.255 Mask:255.255.192.0 IPX/Ethernet 802.2 addr:33001601:0000E235AFEE UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:407579600 errors:0 dropped:0 overruns:0 frame:0 TX packets:1605655688 errors:0 dropped:0 overruns:3 carrier:0 collisions:0 txqueuelen:100 RX bytes:3055300191 (2913.7 Mb) TX bytes:2048217058 (1953.3 Mb) Interrupt:18 Base address:0xd000 $ ip -s link list eth0 2: eth0: mtu 1500 qdisc pfifo_fast qlen 100 link/ether 00:00:e2:35:af:ee brd ff:ff:ff:ff:ff:ff RX: bytes packets errors dropped overrun mcast 3058362227 407610495 0 0 0 0 TX: bytes packets errors dropped carrier collsns 2140511920 1605768150 0 0 0 0 Notice that of the 1.6 billion bytes transmitted, there were 3 overuns. Next, blast the path you want to test with packets using ping -l or the spray program, and measure again. SNM — ver. 1.4 Network Troubleshooting - p. 77/134 SNM — ver. 1.4 Network Troubleshooting - p. 78/134 Getting more info using ip The -s (-statistics) option to ip provides statistics. Adding a second gives you even more: $ ip -s -s link list eth0 2: eth0: mtu 1500 qdisc pfifo_fast qlen 100 link/ether 00:00:e2:35:af:ee brd ff:ff:ff:ff:ff:ff RX: bytes packets errors dropped overrun mcast 3070792102 407726727 0 0 0 0 RX errors: length crc frame fifo missed 0 0 0 0 0 TX: bytes packets errors dropped carrier collsns 2445799644 1606151878 0 0 0 0 TX errors: aborted fifo window heartbeat 0 3 0 0 Quick Guide to using ip: set up interface Here we set up a network interface and give it the IP address 192.168.0.1/24: $ ip link set dev eth1 up $ ip addr add 192.168.0.1/24 brd + dev eth1 Two important points: If you do not specify the netmask, a netmask of /32 is assumed brd + means obtain broadcast address by setting the host bits SNM — ver. 1.4 Network Troubleshooting - p. 79/134 SNM — ver. 1.4 Network Troubleshooting - p. 80/134 Quick Guide to using ip: set up routes $ ip route add default dev eth1 via 192.168.0.254 $ ip route add 192.168.1.0/24 via 192.168.0.10 The last adds a static route to another network the first adds the default route. You can omit the device if the network can be reached through a particular interface without any ambiguity I.e., ip is smart enough to figure out which network device to use, though specifying it doesn’t hurt. Packet Capture tcpdump, Ethereal and Ntop SNM — ver. 1.4 Network Troubleshooting - p. 81/134 SNM — ver. 1.4 Network Troubleshooting - p. 82/134 What is Packet Capture? Real time collection of data as it travels over networks Tools called: packet sniffers packet analysers protocol analysers, and sometimes even traffic monitors When Packet Capture? Most powerful technique When need to see what client and server are actually saying to each other When need to analyse type of traffic on network Requires understanding of network protocols to use effectively SNM — ver. 1.4 Network Troubleshooting - p. 83/134 SNM — ver. 1.4 Network Troubleshooting - p. 84/134 Warning: Don’t Get Sacked! Be sure that your boss agrees with you capturing packets on your company’s network People have been sacked for doing this without permission! Some have suffered long lawsuits and criminal records: See http://www.stonehenge.com/merlyn/, and http://www.lightlink.com/spacenka/fors/ for a famous example Do not invade the privacy of others Capturing passwords with insecure protocols such as telnet, ftp, http (that is not encrypted with TLS) is very easy DON’T DO IT! SNM — ver. 1.4 Network Troubleshooting - p. 85/134 tcpdump Available everywhere Windows: http://windump.polito.it/ Syntax also used by other programs (such as Ethereal) Often it is the only tool available, so good to know Works by putting network interface into promiscuous mode normal Ethernet interface will ignore packets not addressed to it in promiscuous mode, will examine all packets that arrive, even those not addressed to it SNM — ver. 1.4 Network Troubleshooting - p. 86/134 How to use tcpdump Can just type its name (as root): $ sudo tcpdump . . . but get a huge amount of data! Can restrict the data collected using a filter A filter may select addresses, protocols, port numbers, . . . -c -w n file tcpdump: some options capture a count of n packets then stop write raw data to file . Very useful — can filter and analyse this later with tcpdump, ethereal or other tools but you cannot see what you are capturing till later! -i -s interface collect from interface instead of lowest numbered network interface bytes collect no more than bytes of data from each packet instead of default 68 bytes -e show link level info, e.g., Ethernet addresses -x gives a hexadecimal dump of packets excluding link level data SNM — ver. 1.4 Network Troubleshooting - p. 87/134 tcpdump Filters: host and port Show all network traffic to and from 192.168.0.1: $ tcpdump host 192.168.0.1 Show packets to 192.168.0.1: $ tcpdump dst 192.168.0.1 Show packets to port 68 on 192.168.0.1: $ tcpdump dst 192.168.0.1 and port 68 -X display ver. 1.4 as well as hexadecimal if have -x option - p. 88/134 ASCII SNM — Network Troubleshooting too Many more options: man tcpdump tcpdump filters: networks Capture traffic to or from 205.153.60/24: $ tcpdump net 172.19.64/18 can specify network as source or destination: $ tcpdump src net 205.153.60/24 $ tcpdump dst net 172.19.64/18 SNM — ver. 1.4 Network Troubleshooting - p. 89/134 SNM — ver. 1.4 Network Troubleshooting - p. 90/134 tcpdump filters: protocol tcpdump ip tcpdump tcp tcpdump ip proto ospf This will catch DNS name lookups, but not zone transfers (which use tcp): tcpdump udp port 53 tcpdump filters: combining This will not work as you might expect: tcpdump host ictlab and udp or arp Instead, need group with parentheses, and quote: tcpdump "host ictlab and (udp or arp)" many more ways of filtering: man tcpdump SNM — ver. 1.4 Network Troubleshooting - p. 91/134 SNM — ver. 1.4 Network Troubleshooting - p. 92/134 Writing data to a file $ sudo tcpdump -c 1000 -w ∼/tmp/tcpdump.pcap tcpdump: listening on eth0 1014 packets received by filter 0 packets dropped by kernel Reading a Dumped File $ tcpdump -nr ∼/tmp/tcpdump.pcap arp 22:32:41.751452 arp who-has 172.19.127.254 tell 172.19.127.29 22:32:41.863173 arp who-has 172.19.64.52 tell 172.19.64.63 22:32:41.863198 arp reply 172.19.64.52 is-at 0:0:e2:35:af:ee 22:32:42.082584 arp who-has 172.19.65.16 tell 172.19.125.229 22:32:43.113655 arp who-has 172.19.123.211 tell 172.19.65.2 22:32:44.635149 arp who-has 172.19.65.16 tell 172.19.127.106 22:32:44.874117 arp who-has 172.19.65.6 tell 172.19.126.174 22:32:45.147178 arp who-has 172.19.65.16 tell 172.19.126.240 22:32:45.209507 arp who-has 172.19.127.254 tell 172.19.125.127 22:32:45.212484 arp who-has 172.19.127.175 tell 172.19.125.127 22:32:45.239445 arp who-has 172.19.127.254 tell 172.19.125.212 22:32:45.455863 arp who-has 172.19.65.16 tell 172.19.126.194 22:32:45.540507 arp who-has 172.19.126.50 (44:30:54:59:43:4d) tell 172.19.65.10 22:32:45.562004 arp who-has 172.19.126.50 tell 172.19.65.2 SNM — ver. 1.4 Network Troubleshooting - p. 93/134 SNM — ver. 1.4 Network Troubleshooting - p. 94/134 HTTP $ tcpdump -nr ∼/tmp/tcpdump.pcap port http 22:43:32.633636 192.168.25.9.14075 > 172.19.64.52.http: S 1015952778:1015952778(0) win 6144 (DF) 22:43:32.633693 172.19.64.52.http > 192.168.25.9.14075: S 1929920485:1929920485(0) ack 1015952779 win 5840 (DF) 22:43:32.635828 192.168.25.9.14075 > 172.19.64.52.http: P 1:590(589) ack 1 win 6144 (DF) 22:43:32.635906 172.19.64.52.http > 192.168.25.9.14075: . ack 590 win 6479 (DF) 22:43:32.636758 172.19.64.52.http > 192.168.25.9.14075: P 1:217(216) ack 590 win 6479 (DF) 22:43:32.636982 172.19.64.52.http > 192.168.25.9.14075: F 217:217(0) ack 590 win 6479 (DF) 22:43:32.639080 192.168.25.9.14075 > 172.19.64.52.http: R 590:590(0) ack 217 win 0 (DF) tcpdump: When reading TCP format: src > dst: flags data-seqno ack window urgent options Flags are some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or a single ‘.’ (no flags). The first time tcpdump sees a tcp ’conversation’, it prints the sequence number from the packet. On subsequent packets of the conversation, the difference between the current packet’s sequence number and this initial sequence number is printed. SNM — ver. 1.4 Network Troubleshooting - p. 95/134 SNM — ver. 1.4 Network Troubleshooting - p. 96/134 Window win nnn specifies data window the sending host will accept in future packets I.e., the maximum number of bytes TCP flow-control: host reduces this number if congested or overloaded will sometimes set to 0 to temporarily halt incoming traffic in this connection Ethereal King of the Packet Analysers! Available for Linux, Unix, Windows SNM — ver. 1.4 Network Troubleshooting - p. 97/134 SNM — ver. 1.4 Network Troubleshooting - p. 98/134 Ethereal Ethereal can read data captured by tcpdump, e.g., $ ethereal -r tcpdump.pca or File → Open Can capture data itself Uses same filter language as tcpdump SNM — ver. 1.4 Network Troubleshooting - p. 99/134 SNM — ver. 1.4 Network Troubleshooting - p. 100/134 You can expand any protocol: If we click on the + next to Bootstrap Protocol, we can see the details of the DHCP Request: SNM — ver. 1.4 Network Troubleshooting - p. 101/134 SNM — ver. 1.4 Network Troubleshooting - p. 102/134 Display Filters Note the box at the bottom of Ethereal for display filters Select only some of the packets captured for display see man ethereal and search for DISPLAY FILTER SYNTAX Different syntax than the syntax for capture filters Example: ip.src==172.19.64.52 and ip.dest==172.19.64.57 SNM — ver. 1.4 Network Troubleshooting - p. 103/134 SNM — ver. 1.4 Network Troubleshooting - p. 104/134 Tools → Follow TCP Stream Can view the contents of an entire TCP stream conversation, in ASCII or in hexadecimal. Be careful not to invade your customers’ privacy. Can use to check if a communications stream is really encrypted Ntop: monitoring data at a point The Ntop program: listens on a network interface puts an Ethernet interface into promiscuous mode and displays statistics through a web interface Shows: percentages of protocols, which machines generate most traffic which traffic is purely local, which traffic comes from outside, which traffic goes from inside to outside of network SNM — ver. 1.4 Network Troubleshooting - p. 105/134 SNM — ver. 1.4 Network Troubleshooting - p. 106/134 Ntop: Installing Installation is pretty easy. On my Fedora Core 1 machine: $ rpmbuild --rebuild ntop-3.0-0.src.rpm $ sudo rpm -Uhv /home/nicku/RPM/RPMS/i386/ntop-3.0-0.i386.rpm $ ls -l /etc/ntop.conf* -rwx------ 1 root root 13203 Apr 27 03:47 /etc/ntop.conf.sample $ sudo cp -a /etc/ntop.conf.sample /etc/ntop.conf $ sudo emacs /etc/ntop.conf & # temporarily comment out the line --daemon $ sudo /usr/bin/ntop @/etc/ntop.conf -A $ sudo service ntop start Switched Networks Using Ethereal, tcpdump, Ntop in a switched network Then open the web browser on http://localhost:3000/ SNM — ver. 1.4 Network Troubleshooting - p. 107/134 SNM — ver. 1.4 Network Troubleshooting - p. 108/134 Port Monitoring: Switched Networks Problem: How monitor one machine? You are asked to check out a server on a switched network: The switch does not support port monitoring (§109), or you do not have administrative access to the switch: what to do? Use a small hub, and use a notebook running the capture software Ethernet Switch a switched network is really a point-to-point network You cannot normally capture the unicast traffic from other hosts on a single switch port How do you use Ethereal, tcpdump or Ntop to monitor traffic between a number of hosts? Solution: many switches support port monitoring, where one port can monitor all traffic on a specified VLAN Cisco 3500XL switches provide the port monitor command: port monitor vlan VLAN1 mini−hub Example: SNM — ver. 1.4 Network Troubleshooting - p. 109/134 SNM — ver. 1.4 Network Troubleshooting - p. 110/134 Are switched networks secure? Is all unicast traffic on one port of a switch private? No, there are tools (dsniff and Ettercap) freely available to automate ARP spoofing and man-in-the-middle attacks, that provide various ways to compromise switch security. Device under test e.g., a server notebook running capture software Port Scanning SNM — ver. 1.4 Network Troubleshooting - p. 111/134 SNM — ver. 1.4 Network Troubleshooting - p. 112/134 What is a port scanner? Sends packets to various ports on a network device Best one available everywhere is nmap can identify the OS of the target machine Do not port scan arbitrary machines in your company’s network without permission! May be interpreted as a cracking attempt How does nmap identify OS? RFCs leave interpretation of some things up to the implementer RFCs do not specify how should work if get contradictory flags, strange sequences of inconsistent packets Most TCP / IP implementations are not complete Every implementation of TCP / IP is different; the “grey areas” are different from one OS to another. nmap sends “strange” packets to the machine, detects how reacts, matches this against a file of OS fingerprints SNM — ver. 1.4 Network Troubleshooting - p. 113/134 SNM — ver. 1.4 Network Troubleshooting - p. 114/134 Running nmap: Use xnmap $ sudo -v $ sudo xnmap & Enter the IP address of machine(s) to identify select other choices from buttons press Start xnmap is simply a way to easily generate command line options to nmap using a graphical interface Uses of nmap Identify the type of a computer that is causing trouble on the network Check what network services a computer is really offering compare with netstat -tua output A cracked computer may be hiding some services with trojaned utilities nmap can help you discover such services SNM — ver. 1.4 Network Troubleshooting - p. 115/134 SNM — ver. 1.4 Network Troubleshooting - p. 116/134 DNS troubleshooting Suspect DNS when get long timeouts before see any response ping name, IP address, see if only IP address works tools on Linux, Unix: dig, nslookup, host tools on Windows: nslookup DNS troubleshooting Troubleshooting DNS Servers SNM — ver. 1.4 Network Troubleshooting - p. 117/134 SNM — ver. 1.4 Network Troubleshooting - p. 118/134 DNS: dig The people who write the most common name server (Bind) promote dig, deprecate nslookup dig output is in form of DNS resource records can copy and paste straight into DNS database files dig: Checking forward DNS lookup $ dig sysadmin.no-ip.com ; <<>> DiG 9.2.1 <<>> sysadmin.no-ip.com ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 23568 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 3 ;; QUESTION SECTION: ;sysadmin.no-ip.com. IN A ;; ANSWER SECTION: sysadmin.no-ip.com. 60 IN A 202.69.77.139 ;; AUTHORITY SECTION: no-ip.com. 60 IN NS nf1.no-ip.com. no-ip.com. 60 IN NS nf2.no-ip.com. no-ip.com. 60 IN NS nf3.no-ip.com. ;; ADDITIONAL SECTION: nf1.no-ip.com. 60 IN A 66.185.166.131 nf2.no-ip.com. 60 IN A 66.185.162.100 nf3.no-ip.com. 60 IN A 216.66.37.10 ;; Query time: 254 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Mon Feb 24 10:55:26 2003 ;; MSG SIZE rcvd: 154 SNM — ver. 1.4 Network Troubleshooting - p. 120/134 SNM — ver. 1.4 Network Troubleshooting - p. 119/134 dig: reverse lookup 1 $ dig -x 202.69.77.139 ; <<>> DiG 9.2.1 <<>> -x 202.69.77.139 ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 22117 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0 ;; QUESTION SECTION: ;;139.77.69.202.in-addr.arpa. IN PTR ;; ANSWER SECTION: 139.77.69.202.in-addr.arpa. 3600 IN PTR 077-139.onebb.com. ;; AUTHORITY SECTION: 77.69.202.in-addr.arpa. 3600 IN NS ns2.onebb.com. 77.69.202.in-addr.arpa. 3600 IN NS ns1.onebb.com. ;; Query time: 310 msec ;; SERVER: 172.19.64.52#53(172.19.64.52) ;; WHEN: Mon Feb 24 11:07:04 2003 ;; MSG SIZE rcvd: 111 dig syntax dig [ options ] [@ server ] name type main option is -x server is the name server to query by default, use first server in /etc/resolv.conf name is what you want to look up type can be: any, a, mx, axfr, soa, etc. default is to get A record(s) SNM — ver. 1.4 Network Troubleshooting - p. 121/134 SNM — ver. 1.4 Network Troubleshooting - p. 122/134 dig: axfr (Zone Transfer) dig can request a complete zone transfer: $ dig @ns tyict.vtc.edu.hk axfr ; <<>> DiG 9.2.2-P3 <<>> @ns tyict.vtc.edu.hk axfr ;; global options: printcmd tyict.vtc.edu.hk. 86400 IN SOA ns.tyict.vtc.edu.hk. nicku.vtc.edu.hk. 2004031000 3600 1800 604800 600 86400 IN NS ns.tyict.vtc.edu.hk. tyict.vtc.edu.hk. 86400 IN NS ns1.tyict.vtc.edu.hk. tyict.vtc.edu.hk. tyict.vtc.edu.hk. 86400 IN NS dns1.vtc.edu.hk. tyict.vtc.edu.hk. 86400 IN NS dns2.vtc.edu.hk. A 172.19.64.92 000081667.tyict.vtc.edu.hk. 86400 IN ... nslookup: an interactive program $ nslookup Note: nslookup is deprecated and may be removed from future releases. Consider using the ‘dig’ or ‘host’ programs instead. Run nslookup with the ‘-sil[ent]’ option to prevent this message from appearing. > sysadmin.no-ip.com Server: 127.0.0.1 Address: 127.0.0.1#53 Non-authoritative answer: Name: sysadmin.no-ip.com Address: 202.69.77.139 result can be copied and pasted as a master file in a DNS server SNM — ver. 1.4 Network Troubleshooting - p. 123/134 SNM — ver. 1.4 Network Troubleshooting - p. 124/134 nslookup: reverse lookups > 202.69.77.139 Server: 127.0.0.1 Address: 127.0.0.1#53 Non-authoritative answer: 139.77.69.202.in-addr.arpa name = 077-139.onebb.com. Authoritative answers can be found from: 77.69.202.in-addr.arpa nameserver = ns1.onebb.com. 77.69.202.in-addr.arpa nameserver = ns2.onebb.com. ns1.onebb.com internet address = 202.180.160.1 ns2.onebb.com internet address = 202.180.161.1 > Telnet: Troubleshooting Email and Other Protocols SNM — ver. 1.4 Network Troubleshooting - p. 125/134 SNM — ver. 1.4 Network Troubleshooting - p. 126/134 Email: testing with telnet Email protocols SMTP, POP3 are text telnet a good tool to test them syntax: $ telnet server portnumber SMTP: port 25 POP3: port 110 Test the VTC mail server: $ telnet smtp.vtc.edu.hk 25 Trying 192.168.79.191... Connected to smtp.vtc.edu.hk (192.168.79.191). Escape character is ’ˆ]’. 220 pandora.vtc.edu.hk ESMTP Mirapoint 3.2.2-GA; Tue, 25 Feb 2003 11:15:30 +0800 (HKT) helo nickpc.tyict.vtc.edu.hk 250 pandora.vtc.edu.hk Hello [172.19.32.30], pleased to meet you mail from: 250 ... Sender ok rcpt to: 250 ... Recipient ok data 354 Enter mail, end with ‘‘.’’ on a line by itself My message body. . 250 AFF21826 Message accepted for delivery quit 221 pandora.vtc.edu.hk closing connection Connection closed by foreign host. SNM — ver. 1.4 Network Troubleshooting - p. 127/134 SNM — ver. 1.4 Network Troubleshooting - p. 128/134 SMTP commands for sending mail helo identify your computer mail from specify sender rcpt to specify receiver data indicates start of message body quit terminate session Testing the VTC pop3 server 1 $ telnet pop.vtc.edu.hk 110 Trying 192.168.79.12... Connected to pop.vtc.edu.hk (192.168.79.12). Escape character is ’ˆ]’. +OK carme.vtc.edu.hk POP3 service (iPlanet Messaging Server 5.2 Patch 1 (built Aug 19 2002)) user nicku +OK Name is a valid mailbox pass password +OK Maildrop ready stat +OK 1 673 Use names, not IP addresses, to specify destination SNM — ver. 1.4 Network Troubleshooting - p. 129/134 SNM — ver. 1.4 Network Troubleshooting - p. 130/134 Testing the pop3 server 2 retr 1 +OK 673 octets Return-path: Received: from pandora.vtc.edu.hk (pandora.vtc.edu.hk [192.168.79.191]) by carme.vtc.edu.hk (iPlanet Messaging Server 5.2 Patch 1 (built Aug 19 2002)) with ESMTP id <0HAU00I35H3HGL@carme.vtc.edu.hk> for nicku@ims-ms-daemon (ORCPT nicku@vtc.edu.hk); Tue, 25 Feb 2003 11:16:29 +0800 (CST) Received: from nickpc.tyict.vtc.edu.hk ([172.19.32.30]) by pandora.vtc.edu.hk (Mirapoint Messaging Server MOS 3.2.2-GA) with SMTP id AFF21826; Tue, 25 Feb 2003 11:16:01 +0800 (HKT) Date: Tue, 25 Feb 2003 11:15:30 +0800 (HKT) From: Nick Urbanik Message-id: <200302250316.AFF21826@pandora.vtc.edu.hk> My message body. . dele 1 +OK message deleted quit +OK Connection closed by foreign host. pop3 commands: retrieving mail See RFC 1939 for easy-to-read details First, must authenticate: user username pass password stat shows number of messages and total size in bytes list list all the message numbers and size in bytes of each message retr messagenum retrieve the message with number messagenum dele messagenum delete the message with message number messagenum quit SNM — ver. 1.4 Network Troubleshooting - p. 132/134 SNM — ver. 1.4 Network Troubleshooting - p. 131/134 telnet: Testing Other Applications Many network protocols are text. telnet can be helpful in checking: IMAP servers: $ telnet hostname 143 Web servers: $ telnet hostname 80 Ftp servers: $ telnet hostname 21 Even ssh (can check version, if responding): $ telnet hostname 22 Conclusion Check the simple things first Be methodical Document what you do Become familiar with common tools Use the tools to become familiar with your network before troubles strike Know what is “normal” Get permission from the boss before using packet sniffing and port scanners SNM — ver. 1.4 Network Troubleshooting - p. 133/134 SNM — ver. 1.4 Network Troubleshooting - p. 134/134