Network problem determination: AIX tools for a system administrator: Part 1, The basics of network troubleshooting

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1 Network problem determination: AIX tools for a system administrator: Part 1, The basics of network troubleshooting Martin Wicks (wicksy@wicksy.com) Freelance UNIX consultant 711 Consultancy Limited 20 July 2010 This two-part series discusses the tools that can assist the systems administrator in diagnosing TCP/IP network-related issues on AIX. This first installment details testing connectivity to a host, getting useful network-related information about a host, and analyzing data sent to and from a host. View more content in this series As an IBM AIX systems administrator, it's inevitable that at some point in your role you will encounter a problem that's linked to or directly caused by an issue within the LAN or WAN. In such instances, its good practice to make an initial diagnosis of the problem before engaging the help of a suitable network administrator to help identify root cause, or at least give the administrator a general direction in which to start his or her investigation. Frequently used acronyms BIND: Berkeley Internet Name Domain DNS: Domain Name System LAN: Local area network RFC: Request for Comments TCP/IP: Transmission Control Protocol/Internet Protocol WAN: Wide area network Once engaged, you may be required to assist in the analysis, so it's essential that you come armed with a relevant diagnostic toolkit. This article provides you with a set of commands available on AIX, many of which are also available on other flavors of UNIX, which can help you to troubleshoot TCP/IP network-related issues. For the purposes of this article, the target host system used in all sample commands and output is called testhost. Copyright IBM Corporation 2010 Trademarks Page 1 of 12

2 developerworks ibm.com/developerworks/ Is anybody there? The first step in diagnosing any network-related issue is to verify whether the target host is running. You can use ping to test whether a host is reachable across a network (see Listing 1). This command sends an Internet Control Message Protocol (ICMP) echo request packet to the host and waits for an echo reply. A successful ping means that: Your host has an active network adapter that can be used to send out the request. The target host is running and has an active network adapter configured with the IP address that you used. Name resolution is working for that host if a name was used rather than an IP address. There is a route from your host to the target and back. No firewalls on the route between hosts or running on either host are blocking ICMP traffic. The output from a successful ping can also be useful in helping to determine network latency, as it reports on the time taken to receive the echo reply. Long response times are likely to mean poor performance for any applications that exchange data with the target host. Listing 1. Pinging a responsive host ping testhost PING testhost: ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=253 time=0 ms 64 bytes from : icmp_seq=1 ttl=253 time=0 ms 64 bytes from : icmp_seq=2 ttl=253 time=0 ms 64 bytes from : icmp_seq=3 ttl=253 time=0 ms ----testhost PING Statistics packets transmitted, 4 packets received, 0% packet loss round-trip min/avg/max = 0/0/0 ms If no echo reply is received, then one or more of the conditions described above hasn't been met and the ping fails (see Listing 2). A ping fails when the number of packets received is less than the number sent and packet loss is greater than 0 percent. Listing 2. Pinging an unresponsive host ping testhost PING testhost.testdomain.com: ( ): 56 data bytes ----testhost.testdomain.com PING Statistics packets transmitted, 0 packets received, 100% packet loss If the ping was unsuccessful, you can check whether the adapter used to send the request is up by using ifconfig. You can use the ifconfig command to display the status of an individual adapter (for example, en1 shown in Listing 3) or all adapters using the -a switch, also shown in Listing 3. You should ensure that the adapter used to send packets out to your host is showing as UP and RUNNING. If it's not, then you need to investigate further. Page 2 of 12

3 ibm.com/developerworks/ developerworks Listing 3. Displaying network adapter status ifconfig en1 en1: flags=7e080863,40<up,broadcast,notrailers,running,simplex,multicast,grouprt,64bit, CHECKSUM_OFFLOAD,CHECKSUM_SUPPORT,PSEG> inet netmask 0xffffff00 broadcast tcp_sendspace tcp_recvspace ifconfig -a en2: flags=7e080863,40<up,broadcast,notrailers,running,simplex,multicast,grouprt,64bit, CHECKSUM_OFFLOAD,CHECKSUM_SUPPORT,PSEG> inet netmask 0xffffff80 broadcast en1: flags=7e080863,40<up,broadcast,notrailers,running,simplex,multicast,grouprt,64bit, CHECKSUM_OFFLOAD,CHECKSUM_SUPPORT,PSEG> inet netmask 0xffffff00 broadcast tcp_sendspace tcp_recvspace en0: flags=7e080822,10<broadcast,notrailers,simplex,multicast,grouprt,64bit, CHECKSUM_OFFLOAD,CHECKSUM_SUPPORT,PSEG> lo0: flags=e08084b<up,broadcast,loopback,running,simplex,multicast,grouprt,64bit> inet netmask 0xff broadcast inet6 ::1/0 tcp_sendspace tcp_recvspace You can display Ethernet statistics on an adapter with entstat (see Listing 4). The example shown uses the -d switch to display all statistics, including device-specific statistics, for the adapter en2. This command can also be useful for telling you the link status (up or down) and media speed (for example, 100Mbps Full Duplex). The media speed is useful if you need to verify the setting based on the link partner and network that the adapter is connected to, as a speed or duplex mismatch can cause problems. Listing 4. Displaying Ethernet statistics for a network adapter entstat -d en ETHERNET STATISTICS (en2) : Device Type: 10/100/1000 Base-TX PCI-X Adapter ( ) Hardware Address: 00:02:55:d3:37:be Elapsed Time: 114 days 22 hours 48 minutes 20 seconds Transmit Statistics: Receive Statistics: Packets: Packets: Bytes: Bytes: Interrupts: 0 Interrupts: Transmit Errors: 0 Receive Errors: 0 Packets Dropped: 0 Packets Dropped: 0 Bad Packets: 0 Max Packets on S/W Transmit Queue: 109 S/W Transmit Queue Overflow: 0 Current S/W+H/W Transmit Queue Length: 0 Broadcast Packets: 442 Broadcast Packets: Multicast Packets: 0 Multicast Packets: 349 No Carrier Sense: 0 CRC Errors: 0 DMA Underrun: 0 DMA Overrun: 0 Lost CTS Errors: 0 Alignment Errors: 0 Max Collision Errors: 0 No Resource Errors: 0 Late Collision Errors: 0 Receive Collision Errors: 0 Deferred: 0 Packet Too Short Errors: 0 SQE Test: 0 Packet Too Long Errors: 0 Timeout Errors: 0 Packets Discarded by Adapter: 0 Single Collision Count: 0 Receiver Start Count: 0 Page 3 of 12

4 developerworks ibm.com/developerworks/ Multiple Collision Count: 0 Current HW Transmit Queue Length: 0 General Statistics: No mbuf Errors: 0 Adapter Reset Count: 0 Adapter Data Rate: 200 Driver Flags: Up Broadcast Running Simplex 64BitSupport ChecksumOffload PrivateSegment DataRateSet 10/100/1000 Base-TX PCI-X Adapter ( ) Specific Statistics: Link Status: Up Media Speed Selected: 100 Mbps Full Duplex Media Speed Running: 100 Mbps Full Duplex PCI Mode: PCI-X ( ) PCI Bus Width: 64-bit Jumbo Frames: Disabled TCP Segmentation Offload: Enabled TCP Segmentation Offload Packets Transmitted: TCP Segmentation Offload Packet Errors: 0 Transmit and Receive Flow Control Status: Disabled Transmit and Receive Flow Control Threshold (High): Transmit and Receive Flow Control Threshold (Low): Transmit and Receive Storage Allocation (TX/RX): 16/48 If the adapter is up, you can establish whether the route from your host to the target is correct by using route get (see Listing 5). If there's no route at all, then ping will inform you, but if there is, you will need to establish what it is to verify with the network administrator that it's correct. Based on the information in the routing table that your host uses, route get will tell you the gateway the packets will be routed to when leaving your host on the way to the target. Listing 5. Getting routing table information for a host route get testhost route to: testhost destination: mask: gateway: interface: en2 interf addr: myhost flags: <UP,GATEWAY,DONE,PRCLONING> recvpipe sendpipe ssthresh rtt,msec rttvar hopcount mtu expire If the route is correct, then you can use traceroute to determine the exact route that packets will take across the network to the target host. The output of a successful traceroute (see Listing 6) shows each router the packets travel through to reach the target host along with the minimum, average, and maximum response time taken to get to that router. Page 4 of 12

5 ibm.com/developerworks/ developerworks Listing 6. Tracing a successful route to a host traceroute testhost trying to get source for testhost source should be traceroute to testhost ( ) from ( ), 30 hops max outgoing MTU = ( ) 1 ms 0 ms 0 ms ( ) 0 ms 0 ms 0 ms 3 testhost ( ) 1 ms 1 ms 1 ms An unsuccessful traceroute (see Listing 7) has asterisks (*) in the time fields, as they cannot be determined because the probe to the next router timed out. The example also shows the use of the -n switch, which prints numeric host addresses, thereby avoiding name lookup and resolution and speeding up the trace. Listing 7. Tracing an unsuccessful route to a host traceroute -n testhost traceroute testhost trying to get source for testhost source should be traceroute to from , 30 hops max outgoing MTU = ms 0 ms 0 ms ms 1 ms 1 ms ms 2 ms 2 ms 4 * * * 5 * * * 6 * * * Services running at the application layer of a TCP/IP network listen on one or more ports that are used to exchange data between clients and the host server as managed by the transport layer. If a valid route exists to the host, and it's responding to pings but the application service is failing to respond, then you can check connectivity to the relevant ports using telnet. The telnet command, used in its basic form, establishes a terminal connection to a host. However, you can also use it to establish a connection to a specific port on the host (the default being 23, the telnet service). For a list of standard ports, look in /etc/services. If the connection is successful, a message indicating the telnet escape sequence is shown (see Listing 8). You need to enter this key sequence (typically, Control-]) to escape back to a telnet> prompt and enter quit to return to a shell prompt. Listing 8. Testing port 80 (HTTP) on a host (successful) telnet testhost 80 Trying... Connected to testhost. Escape character is '^]'. ^] telnet> quit Connection closed. Page 5 of 12

6 developerworks ibm.com/developerworks/ Depending on the type of connection you're making, the remote service you're connecting to may generate a message similar to Listing 9. Listing 9. Testing port 25 (SMTP) on a host (successful) telnet testhost 25 Trying... Connected to testhost. Escape character is '^]'. 220 testhost.testdomain.com ESMTP Sendmail Wed, 10 Feb :52:28 GMT ^] telnet> quit Connection closed. If the connection fails, then either a connection timeout or a connection refused message will be displayed (see Listing 10). This message can mean that the service on the target host isn't running (and therefore nothing is listening on the port), or that a firewall running on the host (or somewhere en route) is blocking connections to the port. Listing 10. Testing port 515 (remote printing) on a host (unsuccessful) telnet testhost 515 Trying... telnet: Unable to connect to remote host: Connection timed out Do I know you? When using a host name in an application or any of the diagnostic commands covered in this article, it's imperative that the host name can be resolved to an IP address. An IP address is what the Internet layer of a TCP/IP network uses when handling data packets. A host name must resolve through one of the name-resolution services specified in /etc/irs.conf and /etc/netsvc.conf. The hosts record determines the order the name resolution is performed. Only local and BIND/DNS resolution is covered here; the remaining options are outside the scope of this article. When local is specified, the /etc/hosts file is used to resolve host names. So, check to see whether there's an entry for the target host (see Listing 11). Listing 11. Looking for a host in /etc/hosts grep testhost /etc/hosts testhost testhost.testdomain.com aixserver If you specify bind or dns, then DNS is used to resolve host names, and you can use nslookup to check whether the host name resolves (see Listing 12). Page 6 of 12

7 ibm.com/developerworks/ developerworks Listing 12. Resolving a host name via DNS nslookup testhost Server: testdns.testdomain.com Address: Name: testhost.testdomain.com Address: A more powerful DNS interrogation tool is dig. This command has a much richer set of options and arguments than nslookup. The latter has an interactive mode that provides the additional functionality. So, for more complex queries particularly where the output will be parsed by a script dig is preferred (see Listing 13). Listing 13. Reverse lookup of an IP address in DNS dig -x ; <<>> DiG <<>> -x ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ; in-addr.arpa. IN PTR ;; ANSWER SECTION: in-addr.arpa IN PTR testhost.testdomain.com. ;; Query time: 11 msec ;; SERVER: ( ) ;; WHEN: Fri Feb 12 13:28: ;; MSG SIZE rcvd: 82 A host uses the Address Resolution Protocol (ARP) table or arp cache to keep track of the media access control (MAC) address of other network devices alongside their IP addresses. The link layer of a TCP/IP network uses a device's MAC address, so the ARP table is used to translate a MAC address to an IP address and back. If your host has communicated successfully with another host, it's likely that there is an entry in the ARP table. You can use arp to display the entry for a particular host if one exists (see Listing 14). Listing 14. Displaying a host entry in the ARP table arp testhost testhost ( ) at 0:c:29:44:90:28 [ethernet] stored in bucket 0 You can also display the entire table using the -a switch (see Listing 15). The -n switch specifies that host name-to-ip address resolution shouldn't be performed. Listing 15. Displaying the contents of the ARP table arp -an? ( ) at 00:c:29:44:90:28 [ethernet] stored in bucket 0 Page 7 of 12

8 developerworks ibm.com/developerworks/? ( ) at 0:10:db:27:d9:8 [ethernet] stored in bucket 4? ( ) at 0:1b:78:59:88:d8 [ethernet] stored in bucket 4? ( ) at 0:11:25:a6:20:78 [ethernet] stored in bucket 14? ( ) at 0:1b:78:57:a:d0 [ethernet] stored in bucket 14? ( ) at 0:0:c:7:ac:0 [ethernet] stored in bucket 15? ( ) at 0:d:65:e2:4c:c2 [ethernet] stored in bucket 18? ( ) at 0:11:25:a6:d7:9a [ethernet] stored in bucket 24 bucket: 0 contains: 1 entries bucket: 1 contains: 0 entries bucket: 2 contains: 0 entries bucket: 3 contains: 0 entries bucket: 4 contains: 2 entries..... There are 8 entries in the arp table. Can you hear me? To establish a connection, TCP uses a three-way handshake. A client initiates a connection to a host (and a specific port) by sending a SYN synchronize packet. After successfully receiving it, the host responds with a SYN-ACK acknowledgement. If the client successfully receives this acknowledgement, the client completes the handshake with an ACK acknowledgement. All of this assumes that the host server is listening on the specified port, that a route exists from the client to the host and back again, and that no firewalls are blocking this kind of traffic. You can use netstat to display existing connections from your host to other hosts and the current state of each. Using the command with the -a switch (show the state of all sockets) and the -n switch (show addresses numerically, avoiding lookup), you can pipe the output to a suitable grep to look for connections in a particular state (for example, ESTABLISHED for post-handshake, active connections) or connections to a particular host or port. Listing 16 shows all connections and their state to a particular IP address (two connections to , both fully established), to a particular port at a particular IP address (to at port 22), and all fully established connections to any host respectively. Listing 16. Displaying the status of connections to hosts netstat -an grep tcp ESTABLISHED tcp ESTABLISHED netstat -an grep tcp ESTABLISHED netstat -an grep ESTABLISHED tcp ESTABLISHED tcp ESTABLISHED tcp ESTABLISHED tcp ESTABLISHED tcp ESTABLISHED tcp ESTABLISHED tcp ESTABLISHED tcp ESTABLISHED Page 8 of 12

9 ibm.com/developerworks/ developerworks You can monitor outgoing data sent from a particular adapter using tcpdump, which displays the content of each packet as it is sent. The command takes various options to allow you to display more or less of the packet either in descriptive or raw form and allows a number of Boolean expressions to filter the type of data you want to see. For example, monitoring packets on adapter en2, you can show only data being sent to a specific host (see Listing 17). Listing 17. Display packets destined for a specific host tcpdump -i en2 dst host testhost tcpdump: listening on en2 10:08: myhost > testhost.22: P : (48) ack win (DF) 10:08: myhost > testhost.22: P 1:49(48) ack 48 win (DF) 10:08: myhost > testhost.22:. ack 96 win (DF) 10:08: myhost > testhost.22: P : (48) ack win (DF) ^C 53 packets received by filter 0 packets dropped by kernel You can show only packets coming from a specific host (see Listing 18). Listing 18. Display packets sent by a specific host tcpdump -i en2 src host testhost tcpdump: listening on en2 10:10: testhost.22 > myhost.46183:. ack 130 win (DF) 10:10: testhost.22 > myhost.46183: F 529:529(0) ack 225 win (DF) 10:10: testhost > myhost: icmp: echo reply 10:10: testhost > myhost: icmp: echo reply ^C 102 packets received by filter 0 packets dropped by kernel You can show only packets sent to or coming from a specific port (see Listing 19). Listing 19. Display packets destined for or sent by a specific host on a specific port tcpdump -i en2 host testhost port 22 12:15: myhost > testhost.22:. ack win (DF) 12:15: myhost > testhost.22: P 145:193(48) ack 192 win (DF) 12:15: testhost.22 > myhost.47216: P 192:240(48) ack 193 win (DF) 12:15: myhost > testhost.22: P 193:241(48) ack 240 win (DF) 12:15: testhost.22 > myhost.47216: P 240:288(48) ack 241 win (DF) 12:15: myhost > testhost.22:. ack 288 win (DF) 12:15: myhost > testhost.22: P 241:289(48) ack 288 win (DF) Page 9 of 12

10 developerworks ibm.com/developerworks/ 12:15: testhost.22 > myhost.47216: P 288:336(48) ack 289 win (DF) 12:15: testhost.22 > myhost.47216: P 336:432(96) ack 289 win (DF) ^C 145 packets received by filter 0 packets dropped by kernel You can stop the trace by pressing Control-C. The tcpdump command is much more feature rich than the simple examples shown here, so I recommend that you familiarize yourself with its man pages. As you can see from the output in these three examples, traffic is shown with: A timestamp Source Host.Source Port Destination Host.Destination Port Packet flags Other packet information You can use the command to establish whether traffic is leaving your host destined for the target host and whether traffic is making its way back. If no inbound traffic appears, it may be that the host isn't responding or there's no valid route from your host to the target or vice versa. If a particular service (TCP port) isn't responding or a firewall is blocking packets of the type you are sending, you will typically see an R in the packet flags field, indicating that the connection has been reset. For more information on the exact layout and format of a TCP packet, refer to RFC 793: Transmission Control Protocol. Depending on the nature of the problem, it is sometimes good practice to run a tcpdump for a period of time while capturing packet information to a file using the -w switch. Once you feel you have captured enough data, press Control-C to stop the trace. At this point, you can process the file using the -r option to read the packet data captured. You can then use the vast array of switches, options, and Boolean arguments to analyze the data. Listing 20 shows an example of this process. Listing 20. Capture packet data to a file and analyze it tcpdump -w /var/tmp/tcpdump.out -i en1 tcpdump: listening on en1 ^C 305 packets received by filter 0 packets dropped by kernel tcpdump -r /var/tmp/tcpdump.out host testhost 13:10: testhost.22 > myhost.47216: P :790352(48) ack win (DF) 13:10: myhost > testhost.22: P :135297(48) ack win (DF) 13:10: testhost.22 > myhost.47216: P :790416(64) ack win (DF) 13:10: myhost > testhost.22:. ack win (DF) 13:11: myhost > testhost: icmp: echo request (DF) 13:11: testhost > myhost: icmp: echo reply (DF) Page 10 of 12

11 ibm.com/developerworks/ developerworks tcpdump -r /var/tmp/tcpdump.out not port 22 13:11: myhost > testhost: icmp: echo request (DF) 13:11: testhost > myhost: icmp: echo reply (DF) 13:11: arp who-has tell :11: :2:16:9e:20:a 1:80:c2:0:0: : aecb e f00 13:11: oraclehost.testdomain.com.2175 > myhost.tnslsnr: P 502:591(89) ack 421 win :11: oraclehost.testdomain.com.2175 > myhost.tnslsnr: P 591:606(15) ack 548 win :11: netbios-ns > netbios-ns: udp 50 13:11: netbios-ns > netbios-ns: udp 50 Conclusion This article covered some of the AIX tools you can use to test connectivity to a host, extract useful network-related information about a host, and analyze data sent to and from a host. In the next article, you'll get under the covers to see what is really going on when your host has problems communicating with another. The article will conclude with a step-by-step guide to logical problem diagnosis when encountering network-related issues. RELATED TOPICS: RFC 793 Transmission Control Protocol UNIX network performance analysis Series: Optimizing AIX 5L performance Page 11 of 12

12 developerworks ibm.com/developerworks/ About the author Martin Wicks Martin Wicks is a freelance UNIX consultant living and working in London, England. He has been working on UNIX systems for more than 15 years and for many years before that on IBM mainframes. He is a major film buff who enjoys snowboarding and mountain biking. You can reach Martin at wicksy@wicksy.com. Copyright IBM Corporation 2010 ( Trademarks ( Page 12 of 12