Home » Linux network troubleshooting and debugging

Linux network troubleshooting and debugging

Solutons:


I think, general principles of network troubleshooting are:

  1. Find out at what level of TCP/IP stack(or some other stack) occurs the problem.
  2. Understand what is the correct system behavior, and what is deviation from normal system state
  3. Try to express the problem in one sentence or in several words
  4. Using obtained information from buggy system, your own experience and experience of other people(google, various forum, etc.), try to solve the problem until success(or failure)
  5. If you fail, ask other people about help or some advice

As for me, I usually obtain all required information using all needed tools, and try to match this information to my experience. Deciding what level of network stack contains the bug helps to cut off unlikely variants. Using experience of other people helps to solve the problems quickly, but often it leads to situation, that I can solve some problem without its understanding and if this problem occurs again, it’s impossible for me to tackle it again without the Internet.

And in general, I don’t know how I solve network problems. It seems that there is some magic function in my brain named SolveNetworkProblem(information_about_system_state, my_experience, people_experience), which could sometimes return exactly the right answer, and also could sometimes fail(like here TCP dies on a Linux laptop).

I usually use utils from this set for network debugging:

  • ifconfig (or ip link, ip addr) – for obtaining information about network interfaces
  • ping – for validating, if target host is accessible from my machine. ping is also could be used for basic DNS diagnostics – we could ping host by IP-address or by its hostname and then decide if DNS works at all. And then traceroute or tracepath or mtr to look what’s going on on the way there.
  • dig – diagnose everything DNS
  • dmesg | less or dmesg | tail or dmesg | grep -i error – for understanding what the Linux kernel thinks about some trouble.
  • netstat -antp + | grep smth – my most popular usage of netstat command, which shows information about TCP connections. Often I perform some filtering using grep. See also the new ss command (from iproute2 the new standard suite of Linux networking tools) and lsof as in lsof -ai tcp -c some-cmd.
  • telnet <host> <port> – is very useful for communicating with various TCP-services(e.g. on SMTP, HTTP protocols), also we could check general opportunity to connect to some TCP port.
  • iptables-save (on Linux) – to dump the full iptables tables
  • ethtool – get all the network interface card parameters (status of the link, speed, offload parameters…)
  • socat – the swiss army tool to test all network protocols (UDP, multicast, SCTP…). Especially useful (more so than telnet) with a few -d options.
  • iperf – to test bandwidth availability
  • openssl (s_client, ocsp, x509…) to debug all SSL/TLS/PKI issues.
  • wireshark – the powerful tool for capturing and analyzing network traffic, which allows you to analyze and catch many network bugs.
  • iftop – show big users on the network/router.
  • iptstate (on Linux) – current view of the firewall’s connection tracking.
  • arp (or the new (Linux) ip neigh) – show the ARP-table status.
  • route or the newer (on Linux) ip route – show the routing table status.
  • strace (or truss, dtrace or tusc depending on the system) – is useful tool which shows what system calls does the problem process, it also shows error codes(errno) when system calls fails. This information often says enough for understanding the system behavior and solving a problem. Alternatively, using breakpoints on some networking functions in gdb can let you find out when they are made and with which arguments.
  • to investigate firewall issues on Linux: iptables -nvL shows how many packets are matched by each rule (iptables -Z to zero the counters). The LOG target inserted in the firewall chains is useful to see which packets reach them and how they have already been transformed when they get there. To get further NFLOG (associated with ulogd) will log the full packet.

A surprising number of “network problems” boil down to DNS problems of one kind or another. Initial troubleshooting should use ping -n w.x.y.z in order to leave out DNS resolution of a hostname, and just check IP connectivity. After that, use route -n to check the default IP route without DNS resolution.

After verifying IP connectivity, and routing, nslookup, host and dig can yield information. Remember that “locking up” can indicate that DNS timeouts are occuring.

Don’t forget to check existence and contents of /etc/resolv.conf. DHCP clients change that file with every lease, and sometimes they get it wrong, or if disk space is tight, an update might not happen.

Cabling problems can exist. If you have access to the hardware, ensure that the cables are all plugged in and mechanically engaged. If you can see routers or ethernet interfaces, ensure that the link lights are on.

Remotely, you have to depend on ethtool and mii-tool.

[root@flask ~]# ethtool eth0
Settings for eth0:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Speed: 10Mb/s
        Duplex: Half
        Port: MII
        PHYAD: 24
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: g
        Wake-on: d
        Current message level: 0x00000001 (1)
                               drv
        Link detected: yes

“Link detected: yes” is good, but 10Mb/s and Half duplex are not good, as the NIC on that computer can do better. I need to figure out if the NIC is goofed up or the cable is. Another computer plugged into the same router says 100Mb/s, Full duplex.

Related Solutions

Joining bash arguments into single string with spaces

[*] I believe that this does what you want. It will put all the arguments in one string, separated by spaces, with single quotes around all: str="'$*'" $* produces all the scripts arguments separated by the first character of $IFS which, by default, is a space....

AddTransient, AddScoped and AddSingleton Services Differences

TL;DR Transient objects are always different; a new instance is provided to every controller and every service. Scoped objects are the same within a request, but different across different requests. Singleton objects are the same for every object and every...

How to download package not install it with apt-get command?

Use --download-only: sudo apt-get install --download-only pppoe This will download pppoe and any dependencies you need, and place them in /var/cache/apt/archives. That way a subsequent apt-get install pppoe will be able to complete without any extra downloads....

What defines the maximum size for a command single argument?

Answers Definitely not a bug. The parameter which defines the maximum size for one argument is MAX_ARG_STRLEN. There is no documentation for this parameter other than the comments in binfmts.h: /* * These are the maximum length and maximum number of strings...

Bulk rename, change prefix

I'd say the simplest it to just use the rename command which is common on many Linux distributions. There are two common versions of this command so check its man page to find which one you have: ## rename from Perl (common in Debian systems -- Ubuntu, Mint,...

Output from ls has newlines but displays on a single line. Why?

When you pipe the output, ls acts differently. This fact is hidden away in the info documentation: If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is...

mv: Move file only if destination does not exist

mv -vn file1 file2. This command will do what you want. You can skip -v if you want. -v makes it verbose - mv will tell you that it moved file if it moves it(useful, since there is possibility that file will not be moved) -n moves only if file2 does not exist....

Is it possible to store and query JSON in SQLite?

SQLite 3.9 introduced a new extension (JSON1) that allows you to easily work with JSON data . Also, it introduced support for indexes on expressions, which (in my understanding) should allow you to define indexes on your JSON data as well. PostgreSQL has some...

Combining tail && journalctl

You could use: journalctl -u service-name -f -f, --follow Show only the most recent journal entries, and continuously print new entries as they are appended to the journal. Here I've added "service-name" to distinguish this answer from others; you substitute...

how can shellshock be exploited over SSH?

One example where this can be exploited is on servers with an authorized_keys forced command. When adding an entry to ~/.ssh/authorized_keys, you can prefix the line with command="foo" to force foo to be run any time that ssh public key is used. With this...

Why doesn’t the tilde (~) expand inside double quotes?

The reason, because inside double quotes, tilde ~ has no special meaning, it's treated as literal. POSIX defines Double-Quotes as: Enclosing characters in double-quotes ( "" ) shall preserve the literal value of all characters within the double-quotes, with the...

What is GNU Info for?

GNU Info was designed to offer documentation that was comprehensive, hyperlinked, and possible to output to multiple formats. Man pages were available, and they were great at providing printed output. However, they were designed such that each man page had a...

Set systemd service to execute after fstab mount

a CIFS network location is mounted via /etc/fstab to /mnt/ on boot-up. No, it is not. Get this right, and the rest falls into place naturally. The mount is handled by a (generated) systemd mount unit that will be named something like mnt-wibble.mount. You can...

Merge two video clips into one, placing them next to each other

To be honest, using the accepted answer resulted in a lot of dropped frames for me. However, using the hstack filter_complex produced perfectly fluid output: ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack output.mp4 ffmpeg -i input1.mp4 -i input2.mp4...

How portable are /dev/stdin, /dev/stdout and /dev/stderr?

It's been available on Linux back into its prehistory. It is not POSIX, although many actual shells (including AT&T ksh and bash) will simulate it if it's not present in the OS; note that this simulation only works at the shell level (i.e. redirection or...

How can I increase the number of inodes in an ext4 filesystem?

It seems that you have a lot more files than normal expectation. I don't know whether there is a solution to change the inode table size dynamically. I'm afraid that you need to back-up your data, and create new filesystem, and restore your data. To create new...

Why doesn’t cp have a progress bar like wget?

The tradition in unix tools is to display messages only if something goes wrong. I think this is both for design and practical reasons. The design is intended to make it obvious when something goes wrong: you get an error message, and it's not drowned in...