How Do You DiagnOSe Mac No-access
Understanding Internet Addressing
When you connect to the Internet, you are assigned a unique public IP address, such as 91.102.154.153 for IPv4 or 2000:ecd2:a463:6f14:89f4:1d16:225b:4459 for IPv6. You can verify this information by visiting https://test-ipv6.com/. However, explaining and managing these addresses, as well as MAC addresses like 6d:ab:3c:c2:c0:72, can be challenging for those without technical expertise. This also does not provide any historical data, which is crucial when troubleshooting past issues.
Navigating the World Wide Web
When you want to visit a website like https://nikolaus.com, your computer first contacts a DNS server to convert the host portion (nikolaus) in combination with the Top Level Domain (com) of the URL into an IP address, such as 218.72.132.132. Additionally, your computer and browser transmit their specifications with each web request, for example: Mozilla/5.0 (Windows NT x.y; Win64; x64; rv:10.0) Gecko/20100101 Firefox/10.0
The Significance of Default Gateways
The default gateway, typically assigned automatically through DHCP, is where your computer forwards all its traffic. It is usually assigned an address like 10.213.130.62 (although they typically end in .1 or .254 based on the scope size). For IPv6 connectivity, detailed instructions can be found at how-to-fix-ipv6-connectivity/, or you can use the following commands to check on Mac or Linux:
IPv4 Routes and the Host IPv4 Route Table (inc. VPN)
netstat -rn -f inet | egrep -i "default|0/1|128.0/1"
0/1 172.18.12.193 UGScg utun3 default 10.213.130.62 UGScg en0 128.0/1 172.18.12.193 UGSc utun3
Note: We are not just looking for the default but also for any VPN that overrides the public v4 address space.
IPv6 Routes and the Host IPv6 Route Table (inc. VPN)
netstat -rn -f inet6 | egrep -i "default|2000::/3"
If you have IPv6 active the above should return at least one route (as per below) via a known interface such as “en0 " on a Mac.
default fe80:6c74:32cd:411c:4ac2%en0 UGcg en0 default fe80::%utun0 UGcIg utun0 default fe80::%utun1 UGcIg utun1 default fe80::%utun2 UGcIg utun2 2000::/3 utun3 USc utun3
Note: We are not just looking for the default but also for any VPN that overrides the public v6 address space.
Debugging DHCP for both IPv4 and IPv6
To get a look at the low level DHCP configuration (Mac/Linux):
ipconfig getpacket en0
...
domain_name_server (ip_mult): {24.224.122.252, 60.125.29.114}
end (none):
...
So, in the above we are not getting IPv6 DNS servers from the DHCPv4 reply but…
ipconfig getv6packet en0
DHCPv6 REPLY (7) Transaction ID 0x80940b Length 76
Options[4] = {
CLIENTID (1) Length 14: DUID LLT HW 1 Time 668691856 Addr 6d:ab:3c:c2:c0:72
DNS_SERVERS (23) Length 32: 2606:4700:4700::1111, 2001:4860:4860::8844
DOMAIN_LIST (24) Length 0: Invalid
SERVERID (2) Length 10: DUID LL HW 1 Addr 3f:22:90:90:64:a7
}
Resolving Issues with Wired and Wireless Connections
Whether you are using a wired or wireless (Wi-Fi) connection at the physical and data layer, troubleshooting and fixing any issues that may arise is crucial.
Troubleshooting Tips for Apple macOS / OSX
No matter the version of OSX/macOS you are currently using, whether it’s 10.12.3, 11.3.1, or 12.3.9, there are a variety of tools available for troubleshooting. However, these manual actions and scripts do not provide a series of correlated values over time. This is where automated remote troubleshooting becomes essential, especially for teams that embrace remote work and Work From Anywhere (WFA).
Utilizing Built-in Scripts for Assistance
One extremely useful tool on OSX/macOS is the sudo wdutil info command, which provides a dump of current wireless settings to the CLI, and can be configured to generate specific logs for troubleshooting. Additionally, the sysdiagnose tool can be used to generate a comprehensive range of logs, although much of it is point-in-time data related to wireless, similar to the wdutil command.
To run the sysdiagnose tool in the background and write logs to /var/tmp/<blah>.tar.gz, use the command sudo nohup /usr/bin/sysdiagnose -u &. If you prefer to run it interactively, use sudo /usr/bin/sysdiagnose, but be aware of the large file sizes, approximately 300MB.
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