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Linux Network Troubleshooting tools available in the command line


  • Staff

    Overview

    Here is a list of common network troubleshooting tools that you can use in most linux terminals (debian/ubuntu/redhat/fedora distros).

    IFCONFIG command

    ifconfig (interface configurator) command is use to initialize an interface, assign IP Address to interface and enable or disable interface on demand. With this command you can view IP Address and Hardware / MAC address assign to interface and also MTU (Maximum transmission unit) size.

    # ifconfig
    eth0      Link encap:Ethernet  HWaddr 00:0C:29:28:FD:4C
    inet addr:192.168.50.2  Bcast:192.168.50.255  Mask:255.255.255.0
    inet6 addr: fe80::20c:29ff:fe28:fd4c/64 Scope:Link
    UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
    RX packets:6093 errors:0 dropped:0 overruns:0 frame:0
    TX packets:4824 errors:0 dropped:0 overruns:0 carrier:0
    collisions:0 txqueuelen:1000
    RX bytes:6125302 (5.8 MiB)  TX bytes:536966 (524.3 KiB)
    Interrupt:18 Base address:0x2000
    lo        Link encap:Local Loopback
    inet addr:127.0.0.1  Mask:255.0.0.0
    inet6 addr: ::1/128 Scope:Host
    UP LOOPBACK RUNNING  MTU:16436  Metric:1
    RX packets:8 errors:0 dropped:0 overruns:0 frame:0
    TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
    collisions:0 txqueuelen:0
    RX bytes:480 (480.0 b)  TX bytes:480 (480.0 b)
    

    ifconfig with interface (eth0) command only shows specific interface details like IP Address, MAC Address etc. with -a options will display all available interface details if it is disable also.

    # ifconfig eth0
    eth0      Link encap:Ethernet  HWaddr 00:0C:29:28:FD:4C
    inet addr:192.168.50.2  Bcast:192.168.50.255  Mask:255.255.255.0
    inet6 addr: fe80::20c:29ff:fe28:fd4c/64 Scope:Link
    UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
    RX packets:6119 errors:0 dropped:0 overruns:0 frame:0
    TX packets:4841 errors:0 dropped:0 overruns:0 carrier:0
    collisions:0 txqueuelen:1000
    RX bytes:6127464 (5.8 MiB)  TX bytes:539648 (527.0 KiB)
    Interrupt:18 Base address:0x2000
    

    Assigning IP Address and Gateway

    Assigning an IP Address and Gateway to interface on the fly. The setting will be removed in case of system reboot.

    # ifconfig eth0 192.168.50.5 netmask 255.255.255.0

    Enable or Disable Specific Interface

    To enable or disable specific Interface, we use example command as follows.

    Enable eth0

    # ifup eth0

    Disable eth0

    # ifdown eth0

    Setting MTU Size

    By default MTU size is 1500. We can set required MTU size with below command. Replace XXXX with size.

    # ifconfig eth0 mtu XXXX

    Set Interface in Promiscuous mode

    Network interface only received packets belongs to that particular NIC. If you put interface in promiscuous mode it will received all the packets. This is very useful to capture packets and analyze later. For this you may require superuser access.

    # ifconfig eth0 - promisc

    2. PING Command

    PING (Packet INternet Groper) command is the best way to test connectivity between two nodes. Whether it is Local Area Network (LAN) or Wide Area Network (WAN). Ping use ICMP (Internet Control Message Protocol) to communicate to other devices. You can ping host name of ip address using below command.

    # ping 4.2.2.2
    PING 4.2.2.2 (4.2.2.2) 56(84) bytes of data.
    64 bytes from 4.2.2.2: icmp_seq=1 ttl=44 time=203 ms
    64 bytes from 4.2.2.2: icmp_seq=2 ttl=44 time=201 ms
    64 bytes from 4.2.2.2: icmp_seq=3 ttl=44 time=201 ms
    

    OR

    # ping www.example.com
    PING example.com (50.64.202.120) 56(84) bytes of data.
    64 bytes from 50.64.202.120: icmp_seq=1 ttl=47 time=284 ms
    64 bytes from 50.64.202.120: icmp_seq=2 ttl=47 time=287 ms
    64 bytes from 50.64.202.120: icmp_seq=3 ttl=47 time=285 ms
    

    In Linux ping command keep executing until you interrupt. Ping with -c option exit after N number of request (success or error respond).

    # ping -c 5 www.example.com
    PING example.com (50.64.202.120) 56(84) bytes of data.
    64 bytes from 50.64.202.120: icmp_seq=1 ttl=47 time=285 ms
    64 bytes from 50.64.202.120: icmp_seq=2 ttl=47 time=285 ms
    64 bytes from 50.64.202.120: icmp_seq=3 ttl=47 time=285 ms
    64 bytes from 50.64.202.120: icmp_seq=4 ttl=47 time=285 ms
    64 bytes from 50.64.202.120: icmp_seq=5 ttl=47 time=285 ms
    --- example.com ping statistics ---
    5 packets transmitted, 5 received, 0% packet loss, time 4295ms
    rtt min/avg/max/mdev = 285.062/285.324/285.406/0.599 ms
    

    3. TRACEROUTE Command

    traceroute is a network troubleshooting utility which shows number of hops taken to reach destination also determine packets traveling path. Below we are tracing route to global DNS server IP Address and able to reach destination also shows path of that packet is traveling.

    # traceroute 4.2.2.2
    traceroute to 4.2.2.2 (4.2.2.2), 30 hops max, 60 byte packets
    1  192.168.50.1 (192.168.50.1)  0.217 ms  0.624 ms  0.133 ms
    2  227.18.106.27.mysipl.com (27.106.18.227)  2.343 ms  1.910 ms  1.799 ms
    3  221-231-119-111.mysipl.com (111.119.231.221)  4.334 ms  4.001 ms  5.619 ms
    4  10.0.0.5 (10.0.0.5)  5.386 ms  6.490 ms  6.224 ms
    5  gi0-0-0.dgw1.bom2.pacific.net.in (203.123.129.25)  7.798 ms  7.614 ms  7.378 ms
    6  115.113.165.49.static-mumbai.vsnl.net.in (115.113.165.49)  10.852 ms  5.389 ms  4.322 ms
    7  ix-0-100.tcore1.MLV-Mumbai.as6453.net (180.87.38.5)  5.836 ms  5.590 ms  5.503 ms
    8  if-9-5.tcore1.WYN-Marseille.as6453.net (80.231.217.17)  216.909 ms  198.864 ms  201.737 ms
    9  if-2-2.tcore2.WYN-Marseille.as6453.net (80.231.217.2)  203.305 ms  203.141 ms  202.888 ms
    10  if-5-2.tcore1.WV6-Madrid.as6453.net (80.231.200.6)  200.552 ms  202.463 ms  202.222 ms
    11  if-8-2.tcore2.SV8-Highbridge.as6453.net (80.231.91.26)  205.446 ms  215.885 ms  202.867 ms
    12  if-2-2.tcore1.SV8-Highbridge.as6453.net (80.231.139.2)  202.675 ms  201.540 ms  203.972 ms
    13  if-6-2.tcore1.NJY-Newark.as6453.net (80.231.138.18)  203.732 ms  203.496 ms  202.951 ms
    14  if-2-2.tcore2.NJY-Newark.as6453.net (66.198.70.2)  203.858 ms  203.373 ms  203.208 ms
    15  66.198.111.26 (66.198.111.26)  201.093 ms 63.243.128.25 (63.243.128.25)  206.597 ms 66.198.111.26 (66.198.111.26)  204.178 ms
    16  ae9.edge1.NewYork.Level3.net (4.68.62.185)  205.960 ms  205.740 ms  205.487 ms
    17  vlan51.ebr1.NewYork2.Level3.net (4.69.138.222)  203.867 ms vlan52.ebr2.NewYork2.Level3.net (4.69.138.254)  202.850 ms vlan51.ebr1.NewYork2.Level3.net (4.69.138.222)  202.351 ms
    18  ae-6-6.ebr2.NewYork1.Level3.net (4.69.141.21)  201.771 ms  201.185 ms  201.120 ms
    19  ae-81-81.csw3.NewYork1.Level3.net (4.69.134.74)  202.407 ms  201.479 ms ae-92-92.csw4.NewYork1.Level3.net (4.69.148.46)  208.145 ms
    20  ae-2-70.edge2.NewYork1.Level3.net (4.69.155.80)  200.572 ms ae-4-90.edge2.NewYork1.Level3.net (4.69.155.208)  200.402 ms ae-1-60.edge2.NewYork1.Level3.net (4.69.155.16)  203.573 ms
    21  b.resolvers.Level3.net (4.2.2.2)  199.725 ms  199.190 ms  202.488 ms
    

    4. NETSTAT Command

    Netstat (Network Statistic) command display connection info, routing table information etc. To displays routing table information use option as -r.

    # netstat -r
    Kernel IP routing table
    Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
    192.168.50.0    *               255.255.255.0   U         0 0          0 eth0
    link-local      *               255.255.0.0     U         0 0          0 eth0
    default         192.168.50.1    0.0.0.0         UG        0 0          0 eth0
    

    For more examples of Netstat Command, please read our earlier article on 20 Netstat Command Examples in Linux.

    5. DIG Command

    Dig (domain information groper) query DNS related information like A Record, CNAME, MX Record etc. This command mainly use to troubleshoot DNS related query.

    # dig www.example.com; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.10.rc1.el6 <<>> www.example.com
    ;; global options: +cmd
    ;; Got answer:
    ;; ->>HEADER<
    

    For more examples of Dig Command, please read the article on 10 Linux Dig Commands to Query DNS.

    6. NSLOOKUP Command

    nslookup command also use to find out DNS related query. The following examples shows A Record (IP Address) of example.com.

    # nslookup www.example.com
    Server:         4.2.2.2
    Address:        4.2.2.2#53
    Non-authoritative answer:
    www.example.com canonical name = example.com.
    Name:   example.com
    Address: 50.64.202.120
    For more NSLOOKUP Command, read the article on 8 Linux Nslookup Command Examples.
    ```
    
    # 7. ROUTE Command
    
    route command also shows and manipulate ip routing table. To see default routing table in Linux, type the following command.
    
    ```
    # route
    Kernel IP routing table
    Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
    192.168.50.0    *               255.255.255.0   U     0      0        0 eth0
    link-local      *               255.255.0.0     U     1002   0        0 eth0
    default         192.168.50.1    0.0.0.0         UG    0      0        0 eth0
    ```
    
    Adding, deleting routes and default Gateway with following commands.
    
    ## Route Adding
    
    `# route add -net 10.10.10.0/24 gw 192.168.0.1`
    
    Route Deleting
    
    `# route del -net 10.10.10.0/24 gw 192.168.0.1`
    
    Adding default Gateway
    
    `# route add default gw 192.168.0.1`
    
    # 8. HOST Command
    
    host command to find name to IP or IP to name in IPv4 or IPv6 and also query DNS records.
    
    ```
    # host www.google.com
    www.google.com has address 173.194.38.180
    www.google.com has address 173.194.38.176
    www.google.com has address 173.194.38.177
    www.google.com has address 173.194.38.178
    www.google.com has address 173.194.38.179
    www.google.com has IPv6 address 2404:6800:4003:802::1014
    ```
    
    Using -t option we can find out DNS Resource Records like CNAME, NS, MX, SOA etc.
    
    ```
    # host -t CNAME www.redhat.com
    www.redhat.com is an alias for wildcard.redhat.com.edgekey.net.
    ```
    
    # 9. ARP Command
    
    ARP (Address Resolution Protocol) is useful to view / add the contents of the kernel’s ARP tables. To see default table use the command as.
    
    
    ```
    # arp -e
    Address                  HWtype  HWaddress           Flags Mask            Iface
    192.168.50.1             ether   00:50:56:c0:00:08   C                     eth0
    ```
    
    # 10. ETHTOOL Command
    
    ethtool is a replacement of mii-tool. It is to view, setting speed and duplex of your Network Interface Card (NIC). You can set duplex permanently in /etc/sysconfig/network-scripts/ifcfg-eth0 with ETHTOOL_OPTS variable.
    
    ```
    # ethtool eth0
    Settings for eth0:
    Current message level: 0x00000007 (7)
    Link detected: yes
    ```
    
    # 11. IWCONFIG Command
    
    iwconfig command in Linux is use to configure a wireless network interface. You can see and set the basic Wi-Fi details like SSID channel and encryption. You can refer man page of iwconfig to know more.
    
    `# iwconfig [interface]`
    
    # 12. HOSTNAME Command
    
    hostname is to identify in a network. Execute hostname command to see the hostname of your box. You can set hostname permanently in /etc/sysconfig/network. Need to reboot box once set a proper hostname.
    
    
    ```
    # hostname 
    example.com
    ```

 



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