Debugging routing and network issues on Linux common approaches. A step by step guide to find out why routing or network service fails

Thursday, 30th November 2023

For system administrators having a Network issue is among the Hell-ish stuff that can happen every now and then. That is especially true in Heterogenous / Hybrid and complicated Network topologies (with missing well crafted documentation), that were build without an initial overview "on the fly".
Such a networking connectivity or routing issues are faced by every novice, mid or even expert system administrators as the Company's Network IT environments are becoming more and more complicated day by day.

When the "Disaster" of being unable to connect two servers or at times  home laptops / PCs to see each other even though on the Physical layer / Transport Layer (Hardware such as external Switches / Routers / Repeaters / Cabling etc.) is Present machines are connected and everything on the 1 Physical Layer from OSI layears is present happens, then it is time to Debug it with some software tools and methods.

To each operating system the tools and methods to test networking connection and routings is a bit different but generally speaking most concepts are pretty much the same across different types of operating systems (Linux ditros / OpenBSD / FreeBSD / Mac OS / Android / iOS / HP-UX / IBM AIX / DOS / Windows etc.).

Debugging network issues across separate operating systems has its variations but in this specific (ideas) are much close to this article. As the goal at that guide will be to point out how to debug network issues on Linux, in future if I have the time or need to debug other OS-es from Linux, I'll try to put an article on how to debug Network issues on Windows when have some time to do it.

Consider to look for the issue following the basic TCP / IP OSI Level model, every system administrator should have idea about it already, it is part of most basic networking courses such as Cisco's CCNA

TCPIP_OSI_model-networking-levels

1. Check what is the Link status of the Interface with ethtool
 

root@freak:~# ethtool eno1
Settings for eno1:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Supported pause frame use: Symmetric
    Supports auto-negotiation: Yes
    Supported FEC modes: Not reported
    Advertised link modes:  10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Advertised pause frame use: Symmetric
    Advertised auto-negotiation: Yes
    Advertised FEC modes: Not reported
    Speed: 100Mb/s
    Duplex: Full
    Auto-negotiation: on
    Port: Twisted Pair
    PHYAD: 1
    Transceiver: internal
    MDI-X: on (auto)
    Supports Wake-on: pumbg
    Wake-on: g
        Current message level: 0x00000007 (7)
                               drv probe link
    Link detected: yes

 

root@freak:~# ethtool eno2
Settings for eno2:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Supported pause frame use: Symmetric
    Supports auto-negotiation: Yes
    Supported FEC modes: Not reported
    Advertised link modes:  10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Advertised pause frame use: Symmetric
    Advertised auto-negotiation: Yes
    Advertised FEC modes: Not reported
    Speed: 1000Mb/s
    Duplex: Full
    Auto-negotiation: on
    Port: Twisted Pair
    PHYAD: 1
    Transceiver: internal
    MDI-X: on (auto)
    Supports Wake-on: pumbg
    Wake-on: g
        Current message level: 0x00000007 (7)
                               drv probe link
    Link detected: yes

 

For example lets check only if Cable of Network card is plugged in and detected to have a network connection to remote node or switch and show the connection speed on which the 'autoneg on' (autonegiation option) of the LAN card has detected the network exat maximum speed:

root@pcfreak:~# ethtool eth0|grep -i 'link detected'; ethtool eth0 |grep 'Speed: '
    Link detected: yes
    Speed: 100Mb/s


1. Check ip command network configuration output

root@freak:~# ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 1000
    link/ether 70:e2:84:13:44:15 brd ff:ff:ff:ff:ff:ff
    altname enp7s0
3: eno2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr1 state UP group default qlen 1000
    link/ether 70:e2:84:13:44:17 brd ff:ff:ff:ff:ff:ff
    altname enp8s0
4: xenbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 70:e2:84:13:44:13 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.7/24 brd 192.168.1.255 scope global dynamic xenbr0
       valid_lft 7361188sec preferred_lft 7361188sec
5: xenbr1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 70:e2:84:13:44:15 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.5/24 brd 192.168.0.255 scope global dynamic xenbr1
       valid_lft 536138sec preferred_lft 536138sec
10: vif2.0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
11: vif2.1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr1 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
12: vif3.0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
13: vif3.1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr1 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
14: vif4.0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
15: vif4.1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr1 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
16: vif5.0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
17: vif5.1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr1 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
18: vif6.0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
19: vif6.1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
30: vif17.0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
31: vif17.1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr1 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
34: vif21.0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr0 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
35: vif21.1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master xenbr1 state UP group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
48: vif25.0-emu: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master xenbr0 state UNKNOWN group default qlen 1000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
49: vif25.1-emu: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master xenbr1 state UNKNOWN group default qlen 1000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
50: vif25.0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq master xenbr0 state DOWN group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
51: vif25.1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq master xenbr1 state DOWN group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
118: vif47.0-emu: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master xenbr0 state UNKNOWN group default qlen 1000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
119: vif47.1-emu: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master xenbr1 state UNKNOWN group default qlen 1000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
120: vif47.0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq master xenbr0 state DOWN group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
121: vif47.1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq master xenbr1 state DOWN group default qlen 2000
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
root@freak:~# 

ip a s (is a also a shortcut command alias) you can enjoy if you have to deal with ip command frequently.

2. Check the status of the interfaces

Old fashioned way is to just do:

/sbin/ifconfig

 

root@freak:~# ifconfig 
eno1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether 70:e2:84:13:44:15  txqueuelen 1000  (Ethernet)
        RX packets 52366502  bytes 10622469320 (9.8 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 242622195  bytes 274688121244 (255.8 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xfb200000-fb27ffff  

eno2: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether 70:e2:84:13:44:17  txqueuelen 1000  (Ethernet)
        RX packets 220995454  bytes 269698276095 (251.1 GiB)
        RX errors 0  dropped 7  overruns 0  frame 0
        TX packets 192319925  bytes 166233773782 (154.8 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xfb100000-fb17ffff  

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 2553  bytes 147410 (143.9 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 2553  bytes 147410 (143.9 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif17.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 14517375  bytes 133226551792 (124.0 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 139688950  bytes 145111993017 (135.1 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif17.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 86113294  bytes 156944058681 (146.1 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 181513904  bytes 267892940821 (249.4 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif2.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 1521875  bytes 88282472 (84.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 152691174  bytes 278372314505 (259.2 GiB)
        TX errors 0  dropped 3 overruns 0  carrier 0  collisions 0

vif2.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 454915  bytes 81069760 (77.3 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 266953989  bytes 425692364876 (396.4 GiB)
        TX errors 0  dropped 26 overruns 0  carrier 0  collisions 0

vif21.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 20043711  bytes 1283926794 (1.1 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 141580485  bytes 277396881113 (258.3 GiB)
        TX errors 0  dropped 3 overruns 0  carrier 0  collisions 0

vif21.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 73004  bytes 3802174 (3.6 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 267151006  bytes 425621892663 (396.3 GiB)
        TX errors 0  dropped 14 overruns 0  carrier 0  collisions 0

vif25.0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif25.1: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif25.0-emu: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 1000  (Ethernet)
        RX packets 2736348  bytes 295661367 (281.9 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 260385509  bytes 265751226663 (247.5 GiB)
        TX errors 0  dropped 200 overruns 0  carrier 0  collisions 0

vif25.1-emu: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 1000  (Ethernet)
        RX packets 145387  bytes 36011655 (34.3 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 370314760  bytes 394725961081 (367.6 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif3.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 55382861  bytes 130042280927 (121.1 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 99040097  bytes 147929196318 (137.7 GiB)
        TX errors 0  dropped 1 overruns 0  carrier 0  collisions 0

vif3.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 5132631  bytes 295493762 (281.8 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 262314199  bytes 425416945203 (396.2 GiB)
        TX errors 0  dropped 16 overruns 0  carrier 0  collisions 0

vif4.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 4902015  bytes 615387539 (586.8 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 149342891  bytes 277802504143 (258.7 GiB)
        TX errors 0  dropped 1 overruns 0  carrier 0  collisions 0

vif4.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 276927  bytes 30720101 (29.2 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 267132395  bytes 425745668273 (396.5 GiB)
        TX errors 0  dropped 14 overruns 0  carrier 0  collisions 0

vif47.0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif47.1: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif47.0-emu: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 1000  (Ethernet)
        RX packets 208745  bytes 20096596 (19.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 110905731  bytes 110723486135 (103.1 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif47.1-emu: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 1000  (Ethernet)
        RX packets 140517  bytes 14596061 (13.9 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 150831959  bytes 162931572456 (151.7 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif5.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 2030528  bytes 363988589 (347.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 152264264  bytes 278131541781 (259.0 GiB)
        TX errors 0  dropped 1 overruns 0  carrier 0  collisions 0

vif5.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 4169244  bytes 1045889687 (997.4 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 263561100  bytes 424894400987 (395.7 GiB)
        TX errors 0  dropped 7 overruns 0  carrier 0  collisions 0

vif6.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 300242  bytes 16210963 (15.4 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 153909576  bytes 278461295620 (259.3 GiB)
        TX errors 0  dropped 2 overruns 0  carrier 0  collisions 0

vif6.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 43  bytes 1932 (1.8 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 154205631  bytes 278481298141 (259.3 GiB)
        TX errors 0  dropped 2 overruns 0  carrier 0  collisions 0

xenbr0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.8  netmask 255.255.255.0  broadcast 192.168.1.255
        ether 70:e2:84:13:44:11  txqueuelen 1000  (Ethernet)
        RX packets 13689902  bytes 923464162 (880.6 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 12072932  bytes 1307055530 (1.2 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

xenbr1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.0.3  netmask 255.255.255.0  broadcast 192.168.0.255
        ether 70:e2:84:13:44:12  txqueuelen 1000  (Ethernet)
        RX packets 626995  bytes 180026901 (171.6 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 12815  bytes 942092 (920.0 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

 

root@freak:~# ifconfig        
eno1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether 70:e2:84:13:44:11  txqueuelen 1000  (Ethernet)
        RX packets 52373358  bytes 10623034427 (9.8 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 242660000  bytes 274734018669 (255.8 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xfb200000-fb27ffff  

eno2: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether 70:e2:84:13:44:12  txqueuelen 1000  (Ethernet)
        RX packets 221197892  bytes 269978137472 (251.4 GiB)
        RX errors 0  dropped 7  overruns 0  frame 0
        TX packets 192573206  bytes 166491370299 (155.0 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xfb100000-fb17ffff  

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 2553  bytes 147410 (143.9 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 2553  bytes 147410 (143.9 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif17.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 14519247  bytes 133248290251 (124.0 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 139708738  bytes 145135168676 (135.1 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif17.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 86206104  bytes 157189755115 (146.3 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 181685983  bytes 268170806613 (249.7 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif2.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 1522072  bytes 88293701 (84.2 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 152712638  bytes 278417240910 (259.2 GiB)
        TX errors 0  dropped 3 overruns 0  carrier 0  collisions 0

vif2.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 454933  bytes 81071616 (77.3 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 267218860  bytes 426217224334 (396.9 GiB)
        TX errors 0  dropped 26 overruns 0  carrier 0  collisions 0

vif21.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 20045530  bytes 1284038375 (1.1 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 141601066  bytes 277441739746 (258.3 GiB)
        TX errors 0  dropped 3 overruns 0  carrier 0  collisions 0

vif21.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 73010  bytes 3802474 (3.6 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 267415889  bytes 426146753845 (396.8 GiB)
        TX errors 0  dropped 14 overruns 0  carrier 0  collisions 0

vif25.0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif25.1: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif25.0-emu: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 1000  (Ethernet)
        RX packets 2736576  bytes 295678097 (281.9 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 260429831  bytes 265797660906 (247.5 GiB)
        TX errors 0  dropped 200 overruns 0  carrier 0  collisions 0

vif25.1-emu: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 1000  (Ethernet)
        RX packets 145425  bytes 36018716 (34.3 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 370770440  bytes 395263409640 (368.1 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif3.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 55392503  bytes 130064444520 (121.1 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 99052116  bytes 147951838129 (137.7 GiB)
        TX errors 0  dropped 1 overruns 0  carrier 0  collisions 0

vif3.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 5133054  bytes 295517366 (281.8 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 262578665  bytes 425941777243 (396.6 GiB)
        TX errors 0  dropped 16 overruns 0  carrier 0  collisions 0

vif4.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 4902949  bytes 615496460 (586.9 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 149363618  bytes 277847322538 (258.7 GiB)
        TX errors 0  dropped 1 overruns 0  carrier 0  collisions 0

vif4.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 276943  bytes 30721141 (29.2 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 267397268  bytes 426270528575 (396.9 GiB)
        TX errors 0  dropped 14 overruns 0  carrier 0  collisions 0

vif47.0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif47.1: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif47.0-emu: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 1000  (Ethernet)
        RX packets 208790  bytes 20100733 (19.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 110950236  bytes 110769932971 (103.1 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif47.1-emu: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 1000  (Ethernet)
        RX packets 140551  bytes 14599509 (13.9 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 151287643  bytes 163469024604 (152.2 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vif5.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 2030676  bytes 363997181 (347.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 152285777  bytes 278176471509 (259.0 GiB)
        TX errors 0  dropped 1 overruns 0  carrier 0  collisions 0

vif5.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 4169387  bytes 1045898303 (997.4 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 263825846  bytes 425419251935 (396.2 GiB)
        TX errors 0  dropped 7 overruns 0  carrier 0  collisions 0

vif6.0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 300266  bytes 16212271 (15.4 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 153931212  bytes 278506234302 (259.3 GiB)
        TX errors 0  dropped 2 overruns 0  carrier 0  collisions 0

vif6.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether fe:ff:ff:ff:ff:ff  txqueuelen 2000  (Ethernet)
        RX packets 43  bytes 1932 (1.8 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 154227291  bytes 278526238467 (259.3 GiB)
        TX errors 0  dropped 2 overruns 0  carrier 0  collisions 0

xenbr0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.8  netmask 255.255.255.0  broadcast 192.168.1.255
        ether 70:e2:84:13:44:11  txqueuelen 1000  (Ethernet)
        RX packets 13690768  bytes 923520126 (880.7 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 12073667  bytes 1307127765 (1.2 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

xenbr1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.0.3  netmask 255.255.255.0  broadcast 192.168.0.255
        ether 70:e2:84:13:44:12  txqueuelen 1000  (Ethernet)
        RX packets 627010  bytes 180028847 (171.6 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 12815  bytes 942092 (920.0 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

 

To see ethernet interfaces that seem up and then do a ifconfig -a to check whether some interfaces are down (e.g. not shown in the simple ifconfig list).
/sbin/ifconfig -a

! Please note that some virtual IP configurations might not appear and noly be visible in an (ip addr show) command.

 

3. Check iproute2 for special rt_tables (Routing Tables) rules
 

By default Linux distributions does not have any additional rules in /etc/iproute2/rt_tables however some Linux router machines, needs to have a multiple Gateways. Perhaps the most elegant way to do multiple routings with Linux is to use iproute2's routing tables rt_tables.

Here is example of an OpenXEN system that has 2 Internet providers attached and routes different traffic via

 

root@freak:~# cat /etc/iproute2/rt_tables
#
# reserved values
#
255    local
254    main
253    default

100    INET1
200     INET2
0    unspec
#
# local
#
#1    inr.ruhep

 

root@freak:~# ip rule list
0:    from all lookup local
32762:    from all to 192.168.1.8 lookup INET2
32763:    from 192.168.1.8 lookup INET2
32764:    from all to 192.168.0.3 lookup INET1
32765:    from 192.168.0.3 lookup INET1
32766:    from all lookup main
32767:    from all lookup default
root@freak:~# 
 

4. Using ip route get to find out traffic route (path)

root@freak:~# ip route get 192.168.0.1
192.168.0.1 via 192.168.0.1 dev xenbr1 src 192.168.0.3 uid 0 
    cache 

 

root@freak:~# /sbin/route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 xenbr0
192.168.0.0     192.168.0.1     255.255.255.0   UG    0      0        0 xenbr1
192.168.0.0     0.0.0.0         255.255.255.0   U     0      0        0 xenbr1
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 xenbr0
root@freak:~# 

root@freak:~# ip route show
default via 192.168.1.1 dev xenbr0 
192.168.0.0/24 via 192.168.0.1 dev xenbr1 
192.168.0.0/24 dev xenbr1 proto kernel scope link src 192.168.0.3 
192.168.1.0/24 dev xenbr0 proto kernel scope link src 192.168.1.8 


If you find that gateway is missing you might want to add it with:

root@freak:~#  ip route add default via 192.168.5.1

If you need to add a speicic network IP range via separate gateways, you can use commands like:

To add routing for 192.168.0.1/24 / 192.168.1.1/24 via 192.168.0.1 and 192.168.1.1

# /sbin/route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.1 dev eth1
# /sbin/route add -net 192.168.0.0 netmask 255.255.255.0 gw 192.168.0.1 dev eth1

 

If you need to delete a configured wrong route with ip command

# ip route del 192.168.1.0/24 via 0.0.0.0 dev eth1
# ip route del 192.168.0.0/24 via 0.0.0.0 dev eth1

5. Use ping (ICMP protocol) the Destionation IP
 

root@freak:~# ping -c 3 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.
64 bytes from 192.168.0.1: icmp_seq=1 ttl=64 time=0.219 ms
64 bytes from 192.168.0.1: icmp_seq=2 ttl=64 time=0.295 ms
64 bytes from 192.168.0.1: icmp_seq=3 ttl=64 time=0.270 ms

— 192.168.0.1 ping statistics —
3 packets transmitted, 3 received, 0% packet loss, time 2048ms
rtt min/avg/max/mdev = 0.219/0.261/0.295/0.031 ms
root@freak:~# ping -c 3 192.168.0.39
PING 192.168.0.39 (192.168.0.39) 56(84) bytes of data.
From 192.168.1.80: icmp_seq=2 Redirect Host(New nexthop: 192.168.0.39)
From 192.168.1.80: icmp_seq=3 Redirect Host(New nexthop: 192.168.0.39)
From 192.168.1.80 icmp_seq=1 Destination Host Unreachable


— 192.168.0.39 ping statistics —
3 packets transmitted, 0 received, +1 errors, 100% packet loss, time 2039ms
pipe 3

 

Note that sometimes you might get 100% traffic loss but still have connection to the destionation in case if the ICMP protocol is filtered for security.

However if you get something like Network is unreachable that is usually an indicator of some routing problem or wrongly configured network netmask.

root@freak:~# ping 192.168.0.5
ping: connect: Network is unreachable

Test network with different packet size. To send 8972 bytes of payload in a Ethernet frame without fragmentation, the following command can be used:

root@pcfreak:~# ping -s 8972 -M do -c 4 freak
PING xen (192.168.1.8) 8972(9000) bytes of data.
ping: local error: message too long, mtu=1500
ping: local error: message too long, mtu=1500
ping: local error: message too long, mtu=1500
^C
— xen ping statistics —
3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2037ms

root@pcfreak:~# 


 -M pmtudisc_opt
           Select Path MTU Discovery strategy.  pmtudisc_option may be either do (prohibit fragmentation, even local one), want (do PMTU discovery, fragment locally when packet size is
           large), or dont (do not set DF flag).

 

root@pcfreak:~# ping -s 8972 -M want -c 4 freak
PING xen (192.168.1.8) 8972(9000) bytes of data.
8980 bytes from xen (192.168.1.5): icmp_seq=1 ttl=64 time=2.18 ms
8980 bytes from xen (192.168.1.5): icmp_seq=2 ttl=64 time=1.90 ms
8980 bytes from xen (192.168.1.5): icmp_seq=3 ttl=64 time=2.10 ms
^C
— xen ping statistics —
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 1.901/2.059/2.178/0.116 ms

root@pcfreak:~# 

  • -M do: prohibit fragmentation
  • -s 8972 8972 bytes of data
  • ICMP header: 8 bytes
  • IP header: 20 bytes (usually, it can be higher)
  • 8980 bytes of bytes is the IP payload
     

These commands can be used to capture for MTU (maximum transmition units) related issues between hosts that are preventing for hosts to properly send traffic between themselves.
A common issue for Linux hosts to be unable to see each other on the same network is caused by Jumbo Frames (MTU 9000) packets enabled on one of the sides and MTU of 1500 on the other side.
Thus it is always a good idea to thoroughully look up all configured MTUs for all LAN Devices on each server.

6. Check traceroute path to host

If there is no PING but ip route get shows routing is properly configured and the routes existing in the Linux machine routing tables, next step is to check the output of traceroute / tracepath / mtr

 

raceroute to 192.168.0.1 (192.168.0.1), 30 hops max, 60 byte packets
 1  pcfreak (192.168.0.1)  0.263 ms  0.166 ms  0.119 ms
root@freak:~# tracepath 192.168.1.1
 1?: [LOCALHOST]                      pmtu 1500
 1:  vivacom-gigabit-router                                0.925ms reached
 1:  vivacom-gigabit-router                                0.835ms reached
     Resume: pmtu 1500 hops 1 back 1 

 

It might be useful to get a frequent output of the command (especially on Linux hosts) where mtr command is not installed with:

 

root@freak:~# watch -n 0.1 traceroute 192.168.0.1

 

root@freak:~# traceroute -4 google.com
traceroute to google.com (172.217.17.110), 30 hops max, 60 byte packets
 1  vivacom-gigabit-router (192.168.1.1)  0.657 ms  1.280 ms  1.647 ms
 2  213.91.190.130 (213.91.190.130)  7.983 ms  8.168 ms  8.097 ms
 3  * * *
 4  * * *
 5  212-39-66-222.ip.btc-net.bg (212.39.66.222)  16.613 ms  16.336 ms  17.151 ms
 6  * * *
 7  142.251.92.65 (142.251.92.65)  18.808 ms  13.246 ms 209.85.254.242 (209.85.254.242)  15.541 ms
 8  142.251.92.3 (142.251.92.3)  14.223 ms 142.251.227.251 (142.251.227.251)  14.507 ms 142.251.92.3 (142.251.92.3)  15.328 ms
 9  ams15s29-in-f14.1e100.net (172.217.17.110)  14.097 ms  14.909 ms 142.251.242.230 (142.251.242.230)  13.481 ms
root@freak:~# 

If you have MTR then you can get plenty of useful additional information such as the Network HOP name or the Country location of the HOP.

 

To get HOP name:

 

root@freak:~# mtr -z google.com

 

To get info on where (which Country) exactly network HOP is located physically:

root@freak:~# mtr -y 2 google.com

 

7. Check iptables INPUT / FORWARD / OUTPUT rules are messing with something
 

# iptables -L -n 

# iptables -t nat -L -n


Ideally you would not have any firewall

# iptables -L -n 

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

# iptables -t nat -L -n
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination         
 


In case if something like firewalld is enabled as a default serviceto provide some modern Linux firewall as Ubuntu and Redhat / CentOS / Fedoras has it often turned on as a service stop and disable the service

# systemctl stop firewalld

# systemctl disable firewalld

 

8. Debug for any possible MAC address duplicates
 

root@pcfrxen:~# arp -an
? (192.168.1.33) at 00:16:3e:59:96:9e [ether] on eth0
? (192.168.1.1) at 18:45:93:c6:d8:00 [ether] on eth1
? (192.168.0.1) at 8c:89:a5:f2:e8:d9 [ether] on eth1
? (192.168.1.1) at 18:45:93:c6:d8:00 [ether] on eth0
? (192.168.1.11) at 7c:0a:3f:89:b6:fa [ether] on eth1
? (192.168.1.17) at <incomplete> on eth0
? (192.168.1.37) at 00:16:3e:ea:05:ce [ether] on eth0
? (192.168.1.80) at 8c:89:a5:f2:e7:d8 [ether] on eth0
? (192.168.1.11) at 7c:0a:3f:89:a5:fa [ether] on eth0
? (192.168.1.30) at 00:16:3e:bb:46:45 [ether] on eth1
? (192.168.0.210) at 00:16:3e:68:d9:55 [ether] on eth1
? (192.168.1.30) at 00:16:3e:bb:46:45 [ether] on eth0
? (192.168.1.18) at 00:16:3e:0d:40:05 [ether] on eth1
? (192.168.0.211) at 00:16:3e:4d:41:05 [ether] on eth1
? (192.168.1.35) at 00:16:3e:d1:8f:77 [ether] on eth0
? (192.168.1.18) at 00:16:3e:0d:43:05 [ether] on eth0
? (192.168.1.28) at 00:16:3e:04:12:1c [ether] on eth1
? (192.168.0.3) at 70:e2:84:13:43:12 [ether] on eth1
? (192.168.0.208) at 00:16:3e:51:de:9c [ether] on eth1
? (192.168.0.241) at 00:16:3e:0d:48:06 [ether] on eth1
? (192.168.1.28) at 00:16:3e:04:12:1c [ether] on eth0
? (192.168.1.33) at 00:16:3e:59:97:8e [ether] on eth1
? (192.168.0.241) at 00:16:3e:0d:45:06 [ether] on eth0
? (192.168.0.209) at 00:16:3e:5c:df:96 [ether] on eth1

root@pcfrxen:~# ip neigh show
192.168.1.33 dev eth0 lladdr 00:16:3e:59:96:9e REACHABLE
192.168.1.1 dev eth1 lladdr 18:45:93:c6:d8:00 STALE
192.168.0.1 dev eth1 lladdr 8c:89:a5:f2:e8:d9 REACHABLE
192.168.1.1 dev eth0 lladdr 18:45:93:c6:d9:01 REACHABLE
192.168.1.11 dev eth1 lladdr 7c:0a:3f:89:a6:fb STALE
192.168.1.17 dev eth0  FAILED
192.168.1.37 dev eth0 lladdr 00:16:3e:ea:06:ce STALE
192.168.1.80 dev eth0 lladdr 8c:89:a5:f2:e8:d9 REACHABLE
192.168.1.11 dev eth0 lladdr 7c:0a:3f:89:a7:fa STALE
192.168.1.30 dev eth1 lladdr 00:16:3e:bb:45:46 STALE
192.168.0.210 dev eth1 lladdr 00:16:3e:68:d8:56 REACHABLE
192.168.1.30 dev eth0 lladdr 00:16:3e:bb:45:46 STALE
192.168.1.18 dev eth1 lladdr 00:16:3e:0d:48:04 STALE
192.168.0.211 dev eth1 lladdr 00:16:3e:4d:40:04 STALE
192.168.1.35 dev eth0 lladdr 00:16:3e:d2:8f:76 STALE
192.168.1.18 dev eth0 lladdr 00:16:3e:0d:48:06 STALE
192.168.1.28 dev eth1 lladdr 00:16:3e:04:11:2c STALE
192.168.0.3 dev eth1 lladdr 70:e2:84:13:44:13 STALE
192.168.0.208 dev eth1 lladdr 00:16:3e:51:de:9c REACHABLE
192.168.0.241 dev eth1 lladdr 00:16:3e:0d:48:07 STALE
192.168.1.28 dev eth0 lladdr 00:16:3e:04:12:1c REACHABLE
192.168.1.33 dev eth1 lladdr 00:16:3e:59:96:9e STALE
192.168.0.241 dev eth0 lladdr 00:16:3e:0d:49:06 STALE
192.168.0.209 dev eth1 lladdr 00:16:3e:5c:dd:97 STALE
root@pcfrxen:~# 


9. Check out with netstat / ss for any irregularities such as high amount of error of faulty ICMP / TCP / UDP network packs

 

For example check out the netstat network stack output

# netstat -s

 

root@pcfrxen:~# netstat -s
Ip:
    Forwarding: 2
    440044929 total packets received
    1032 with invalid addresses
    0 forwarded
    0 incoming packets discarded
    439988902 incoming packets delivered
    396161852 requests sent out
    3 outgoing packets dropped
    100 dropped because of missing route
Icmp:
    1025 ICMP messages received
    540 input ICMP message failed
    ICMP input histogram:
        destination unreachable: 1014
        timeout in transit: 11
    519 ICMP messages sent
    0 ICMP messages failed
    ICMP output histogram:
        destination unreachable: 519
IcmpMsg:
        InType3: 1014
        InType11: 11
        OutType3: 519
Tcp:
    1077237 active connection openings
    1070510 passive connection openings
    1398236 failed connection attempts
    111345 connection resets received
    83 connections established
    438293250 segments received
    508143650 segments sent out
    42567 segments retransmitted
    546 bad segments received
    329039 resets sent
Udp:
    1661295 packets received
    278 packets to unknown port received
    0 packet receive errors
    1545720 packets sent
    0 receive buffer errors
    0 send buffer errors
    IgnoredMulti: 33046
UdpLite:
TcpExt:
    1 invalid SYN cookies received
    1398196 resets received for embryonic SYN_RECV sockets
    1737473 packets pruned from receive queue because of socket buffer overrun
    1118775 TCP sockets finished time wait in fast timer
    638 time wait sockets recycled by time stamp
    656 packetes rejected in established connections because of timestamp
    2218959 delayed acks sent
    2330 delayed acks further delayed because of locked socket
    Quick ack mode was activated 7172 times
    271799723 packet headers predicted
    14917420 acknowledgments not containing data payload received
    171078735 predicted acknowledgments
    52 times recovered from packet loss due to fast retransmit
    TCPSackRecovery: 337
    Detected reordering 1551 times using SACK
    Detected reordering 1501 times using reno fast retransmit
    Detected reordering 61 times using time stamp
    9 congestion windows fully recovered without slow start
    38 congestion windows partially recovered using Hoe heuristic
    TCPDSACKUndo: 241
    104 congestion windows recovered without slow start after partial ack
    TCPLostRetransmit: 11550
    1 timeouts after reno fast retransmit
    TCPSackFailures: 13
    3772 fast retransmits
    2 retransmits in slow start
    TCPTimeouts: 24104
    TCPLossProbes: 101748
    TCPLossProbeRecovery: 134
    TCPSackRecoveryFail: 3
    128989224 packets collapsed in receive queue due to low socket buffer
    TCPBacklogCoalesce: 715034
    TCPDSACKOldSent: 7168
    TCPDSACKOfoSent: 341
    TCPDSACKRecv: 16612
    150689 connections reset due to unexpected data
    27063 connections reset due to early user close
    17 connections aborted due to timeout
    TCPDSACKIgnoredOld: 158
    TCPDSACKIgnoredNoUndo: 13514
    TCPSpuriousRTOs: 9
    TCPSackMerged: 1191
    TCPSackShiftFallback: 1011
    TCPDeferAcceptDrop: 699473
    TCPRcvCoalesce: 3311764
    TCPOFOQueue: 14289375
    TCPOFOMerge: 356
    TCPChallengeACK: 621
    TCPSYNChallenge: 621
    TCPSpuriousRtxHostQueues: 4
    TCPAutoCorking: 1605205
    TCPFromZeroWindowAdv: 132380
    TCPToZeroWindowAdv: 132441
    TCPWantZeroWindowAdv: 1445495
    TCPSynRetrans: 23652
    TCPOrigDataSent: 388992604
    TCPHystartTrainDetect: 69089
    TCPHystartTrainCwnd: 3264904
    TCPHystartDelayDetect: 4
    TCPHystartDelayCwnd: 128
    TCPACKSkippedPAWS: 3
    TCPACKSkippedSeq: 2001
    TCPACKSkippedChallenge: 2
    TCPWinProbe: 123043
    TCPKeepAlive: 4389
    TCPDelivered: 389507445
    TCPAckCompressed: 7343781
    TcpTimeoutRehash: 23311
    TcpDuplicateDataRehash: 8
    TCPDSACKRecvSegs: 17335
IpExt:
    InMcastPkts: 145100
    OutMcastPkts: 9429
    InBcastPkts: 18226
    InOctets: 722933727848
    OutOctets: 759502627470
    InMcastOctets: 58227095
    OutMcastOctets: 3284379
    InBcastOctets: 1756918
    InNoECTPkts: 440286946
    InECT0Pkts: 936

 

  • List all listening established connections to host

# netstat -ltne

  • List all UDP / TCP connections

# netstat -ltua

or if you prefer to do it with the newer and more comprehensive tool ss:
 

  • List all listening TCP connections 

# ss -lt

  • List all listening UDP connections 

# ss -ua

  • Display statistics about recent connections

root@pcfrxen:~# ss -s
Total: 329
TCP:   896 (estab 70, closed 769, orphaned 0, timewait 767)

Transport Total     IP        IPv6
RAW      0         0         0        
UDP      40        36        4        
TCP      127       118       9        
INET      167       154       13       
FRAG      0         0         0 

  • If you need to debug some specific sport or dport filter out the connection you need by port number

# ss -at '( dport = :22 or sport = :22 )'

 

Debug for any possible issues with ICMP unreachable but ports reachable with NMAP / telnet / Netcat
 

# nc 192.168.0.1 -vz

root@pcfrxen:/ # nc 192.168.0.1 80 -vz
pcfreak [192.168.0.1] 80 (http) open


root@pcfrxen:/ # nc 192.168.0.1 5555 -vz
pcfreak [192.168.0.1] 5555 (?) : Connection refused

 

root@pcfrxen:/# telnet 192.168.0.1 3128
Trying 192.168.0.1…
Connected to 192.168.0.1.
Escape character is '^]'.
^]
telnet> quit
Connection closed.

 

root@pcfrxen:/# nmap -sS -P0 192.168.0.1 -p 443 -O
Starting Nmap 7.80 ( https://nmap.org ) at 2023-11-27 19:51 EET
Nmap scan report for pcfreak (192.168.0.1)
Host is up (0.00036s latency).

PORT    STATE SERVICE
443/tcp open  https
MAC Address: 8C:89:A5:F2:E8:D8 (Micro-Star INT'L)
Warning: OSScan results may be unreliable because we could not find at least 1 open and 1 closed port
Aggressive OS guesses: Linux 3.11 (96%), Linux 3.1 (95%), Linux 3.2 (95%), AXIS 210A or 211 Network Camera (Linux 2.6.17) (94%), Linux 2.6.32 (94%), Linux 3.10 (94%), Linux 2.6.18 (93%), Linux 3.2 – 4.9 (93%), ASUS RT-N56U WAP (Linux 3.4) (93%), Linux 3.16 (93%)
No exact OS matches for host (test conditions non-ideal).
Network Distance: 1 hop

OS detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 6.24 seconds
root@pcfrxen:/# 

10. Add static MAC address to Ethernet Interface (if you find a MAC address being wrongly assigned to interface)

Sometimes problems with network unrechability between hosts is caused by wrongly defined MAC addresses on a Switch that did not correspond correctly to the ones assigned on the Linux host.
The easiest resolution here if you don't have access to Switch in work environment is to reassign the default MAC addresses of interfaces to proper MAC addresses, expected by remote router.

 

root@pcfrxen:/#  ​/sbin/ifconfig eth2 hw ether 8c:89:a5:f2:e8:d6

root@pcfrxen:/#  /sbin/ifconfig eth1 hw ether 8c:89:a5:f2:e8:d5

 

root@pcfrxen:/#  ifconfig eth0|grep -i ether
        ether 8c:89:a5:f2:e8:d6 txqueuelen 1000  (Ethernet)

 

11. Check for Network Address Translation (NAT) misconfigurations

If you do use some NAT-ing between Linux host and the remote Network Device you cannot reach, make sure IP Forwarding is enabled (i.e. /etc/sysctl.conf was not mistakenly overwritten by a script or admin for whatever reason).
 

root@server:~# sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 1
root@server:~# sysctl net.ipv4.conf.all.forwarding
net.ipv4.conf.all.forwarding = 1

root@server:~# sysctl net.ipv6.conf.all.forwarding
net.ipv6.conf.all.forwarding = 0

12. Check for Resolving DNS irregularities with /etc/resolv.conf


If network connectivity is okay on TCP / IP , UDP Level but problems with DNS of course, check what you have configured inside /etc/resolv.conf

And if use newer Linux distributions and have resolving managed by systemd check status of resolvectl
 

root@server:~# cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND — YOUR CHANGES WILL BE OVERWRITTEN
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "resolvectl status" to see details about the actual nameservers.

nameserver 127.0.0.1
search pc-freak.net
domain pc-freak.net
nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 109.104.195.2
nameserver 109.104.195.1
nameserver 208.67.222.222
nameserver 208.67.220.220
options timeout:2 rotate

root@pcfreak:~# 

 

root@server:~# resolvectl status
Global
       Protocols: -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
resolv.conf mode: stub

Link 2 (ens3)
    Current Scopes: DNS
         Protocols: +DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
Current DNS Server: 192.168.5.1
       DNS Servers: 192.168.5.1

 

  As seen see, the systemd-resolved service is used to provide domain names resolution and we can modify its configuration file /etc/systemd/resolved.conf to add the DNS server – the following line is set (two DNS servers’ addresses are added):

For example …

DNS=8.8.8.8 

13. Fix problems with wrongly configured Network Speed between hosts

It is not uncommon to have a Switch between two Linux hosts that is set to communicate on a certain maximum amount of Speed but a Linux host is set to communicate or lesser or more of Speed, this might create network issues so in such cases make sure either you use the Auto Negitionation network feature
or set both sides to be communicating on the same amount of network speed.

To turn on auto negotiation for ether interface 

# ethtool -s eth1 speed 1000 duplex full autoneg on


For example to set a Linux network interface to communicate on 1 Gigabit speed and switch off autonegotiation off.

# ethtool -s eth1 speed 1000 duplex full autoneg off

14. Check arp and icmp traffic with tcpdump

On both sides where the IPs can't see each other we can run a tcpdump to check the ARP and ICMP traffic flowing between the hosts.
 

# tcpdump -i eth1 arp or icmp

cpdump: verbose output suppressed, use -v[v]… for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), snapshot length 262144 bytes
15:29:07.001841 IP freak-eth1 > pcfr_hware_local_ip: ICMP echo request, id 13348, seq 65, length 64
15:29:07.001887 IP pcfr_hware_local_ip > freak-eth1: ICMP echo reply, id 13348, seq 65, length 64
15:29:07.598413 ARP, Request who-has pcfr_hware_local_ip tell zabbix-server, length 46
15:29:07.598425 ARP, Reply pcfr_hware_local_ip is-at 8c:89:a5:f2:e8:d8 (oui Unknown), length 28
15:29:07.633055 ARP, Request who-has freak_vivacom_auto_assigned_dhcp_ip tell 192.168.1.1, length 46
15:29:08.025824 IP freak-eth1 > pcfr_hware_local_ip: ICMP echo request, id 13348, seq 66, length 64
15:29:08.025864 IP pcfr_hware_local_ip > freak-eth1: ICMP echo reply, id 13348, seq 66, length 64

 

# tcpdump -i eth1 -vvv

 

If you want to sniff for TCP protocol and specific port and look up for DATA transfered for SMTP you can use something like:

 

# tcpdump -nNxXi eth0 -s 0 proto TCP and port 25​

 

If you need a bit more thorough explanation on what it would do check out my previous article How to catch / track mail server traffic abusers with tcpdump
 

15. Debugging network bridge issues

Having bridge network interface is another brink where things could go totally wrong.
If you have network bridges configured, check out what is the status of the bridge.
 

root@freak:/etc/network# brctl show
bridge name    bridge id        STP enabled    interfaces
xenbr0        8000.70e284134411    yes        eno1
                            vif1.0
                            vif10.0
                            vif16.0
                            vif16.0-emu
                            vif2.0
                            vif3.0
                            vif4.0
                            vif5.0
                            vif6.0
                            vif9.0
                            vif9.0-emu
xenbr1        8000.70e284134412    yes        eno2
                            vif1.1
                            vif10.1
                            vif16.1
                            vif16.1-emu
                            vif2.1
                            vif3.1
                            vif4.1
                            vif5.1
                            vif6.1
                            vif9.1
                            vif9.1-emu


Check out any configurations such as /etc/sysconfig/network-scripts/ifcfg-* are not misconfigured if on Redhat / CentOS / Fedora.
Or if on Debian / Ubuntu and other deb based Linuxes look up for /etc/network/interfaces config problems that might be causing the bridge to misbehave.

For example one bridge network issue, I've experienced recently is related to bridge_ports variable configured as bridge_ports all.
This was causing the second bridge xenbr1 to be unable to see another local network that was directly connected with a cable to it.

The fix was bridge_ports none. Finding out this trivial issue caused by a restored network config from old backup took me days to debug.
As everything seemed on a network level to be perfect just like in Physical layer, same way and on Software level, routings were okay.

Checked everything multiple times and did not see anything irregular. ping was missing and hosts cannot see each other even though having the right netmask and
network configuration in place.

Below is my /etc/network/interfaces configuration with the correct bridge_ports none changed.

root@freak:/etc/network# cat /etc/network/interfaces
auto lo
iface lo inet loopback
 

auto eno1
allow-hotplug eno1
iface eno1 inet manual
dns-nameservers 127.0.0.1 8.8.8.8 8.8.4.4 207.67.222.222 208.67.220.220
auto eno2
allow-hotplug eno2
iface eno2 inet manual
dns-nameservers 127.0.0.1 8.8.8.8 8.8.4.4 207.67.222.222 208.67.220.220

auto xenbr0
allow-hotplug xenbr0
 # Bridge setup
# fetching dhcp ip from 192.168.1.20 (vivacom fiber optics router) routing traffic via 1Gigabit network
 iface xenbr0 inet dhcp
    hwaddress ether 70:e2:84:13:44:11
#    address 192.168.1.5/22
    address 192.168.1.5
    netmask 255.255.252.0
    # address 192.168.1.8 if dhcp takes from vivacom dhcpd
    bridge_ports eno1
    gateway 192.168.1.20
    bridge_stp on
    bridge_waitport 0
    bridge_fd 0
    bridge_ports none
    dns-nameservers 8.8.8.8 8.8.4.4

auto xenbr1
# fetching dhcp ip from pc-freak.net (192.168.0.1) bergon.net routing traffic through it
allow-hotplug xenbr1
 iface xenbr1 inet dhcp
    hwaddress ether 70:e2:84:13:44:11
##    address 192.168.0.3/22
    address 192.168.0.8
    netmask 255.255.252.0
   # address 192.168.0.8 if dhcp takes from vivacom dhcpd (currently mac deleted from vivacom router)
   # address 192.168.0.9 if dhcp takes from pc-freak.net hware host
#    hwaddress ether 70:e2:84:13:44:13
    gateway 192.168.0.1
    bridge_ports eno2
    bridge_stp on
    bridge_waitport 0
    bridge_fd 0
    bridge_ports none
    dns-nameservers 8.8.8.8 8.8.4.4
root@freak:/etc/network# 
 

 

root@freak:/etc/network# brctl showstp xenbr0
xenbr0
 bridge id        8000.70e284134411
 designated root    8000.70e284134411
 root port           0            path cost           0
 max age          20.00            bridge max age          20.00
 hello time           2.00            bridge hello time       2.00
 forward delay          15.00            bridge forward delay      15.00
 ageing time           0.00
 hello timer           1.31            tcn timer           0.00
 topology change timer       0.00            gc timer           0.00
 flags            


eno1 (1)
 port id        8001            state             forwarding
 designated root    8000.70e284134411    path cost          19
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    8001            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif1.0 (2)
 port id        8002            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    8002            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif10.0 (12)
 port id        800c            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    800c            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif16.0 (13)
 port id        800d            state               disabled
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    800d            forward delay timer       0.00
 designated cost       0            hold timer           0.00
 flags            

vif16.0-emu (14)
 port id        800e            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    800e            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif2.0 (4)
 port id        8004            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    8004            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif3.0 (5)
 port id        8005            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    8005            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif4.0 (3)
 port id        8003            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    8003            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif5.0 (6)
 port id        8006            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    8006            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif6.0 (7)
 port id        8007            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    8007            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

vif9.0 (10)
 port id        800a            state               disabled
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    800a            forward delay timer       0.00
 designated cost       0            hold timer           0.00
 flags            

vif9.0-emu (11)
 port id        800b            state             forwarding
 designated root    8000.70e284134411    path cost         100
 designated bridge    8000.70e284134411    message age timer       0.00
 designated port    800b            forward delay timer       0.00
 designated cost       0            hold timer           0.31
 flags            

root@freak:/etc/network# 


Sum it up

We have learned how to debug various routing issues, how to add and remote default gateways, check network reachability with ICMP protocol with ping, traceroute as well check for DNS issues and given some hints how to resolve DNS misconfigurations.
We also learned how to check the configured Network interfaces certain settings and resolve issues caused by Network sides max Speed misconfigurations as well how to track and resolve communication issues caused by wrongly configured MAC addresses.
Further more learned on how to do a basic port and protocol debugging of state of Network packets with netstat and nc and check problems related to iptables Firewall and IP Forwarding misconfigurations.
Finally we learned some basic usage of tcpdump on how to track arp and MAC traffic and look up for a specific TCP / UDP protocol  and its contained data.
There is certainly things this article is missing as the topic of debugging network connectivity issues on Linux is a whole ocean, especially as the complexity of Linux has grown dramatically these days.
I gues it is worthy to mention that unable to see remote network could be caused by wrong VLAN configurations on Linux or even buggy switches and router devices, due to hardware or software,
but I hope this article at least covers the very basics of network debugging and Linux. 

Enjojy 🙂

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