[Background]
As per below technical nodes, there is some improvement done in DHCP.
RHEL 6.7 Technical Notes
Enhancement
- BZ#1058674
With this update, the dhcpd daemon is able to handle dhcp option 97 - Client Machine Identifier (pxe-client-id),
so it is now possible to statically allocate an IP address for a particular client based on its identifier, sent in option 97.
example:
host pixi { option pxe-client-id 0 00:11:22:33:44:
[Problem]
In previous DHCP package (dhcp-4.1.1-43.P1) in RHEL6.6, we always received same IP address for adding new network interface with in existing network interface in case pxe calls through my Linux system reboot.
In RHEL 6.10, the DHCP package(dhcp-4.1.1-61.P1) always received different IP address for adding new network interface with in existing network interface.
It seems different IP is assigned due to this identifier. So, we are checking a options to disable sending of pxe-client-id in pxe client configuration.
[question]
Is there any way to disable or enable option 97(pxe-client-ID) in dhcp.
It seems same thing has been reported in RHEL7 link.
https://access.redhat.com/solutions/2158491
Related
The closest thing to a solution I've found is using Get-NetConnectionProfile to return all active interfaces, which works fine when there's only an active physical interface and the VPN itself. However, this would not work if the user's machine has 2 active physical interfaces (e.g Wi-Fi + Ethernet) along with the VPN.
Ideally, I'd like a solution that works similarly to "ifconfig -v" in MacOS, which tells you the effective interface for a virtual interface:
Unfortunately it seems there is no sure-fire way to get the underlying physical adapter for a VPN using a Windows API. Short of involving a packet sniffer such as Wireshark, the best solution I found involves parsing the output of two PowerShell commands: Get-NetAdapter and Get-NetRoute.
With the information from these commands, I can know which interfaces are virtual and which ones are physical, and I can rank the physical interfaces by 3 different criteria (in case of tie, we move on to the next criteria):
Sorting the physical interfaces by the interface metric + the route metric to the default gateway (0.0.0.0).
Wired connections over wireless ones (PhysicalMediaType=802.3).
Prioritizing faster adapters.
With this logic all the VPNs I tested appear to reflect the expected network interface, although some VPNs let you force traffic through a particular physical adapter in which case obviously this all goes out the window.
First,
You can install wireshark or some other traffic monitoring tool and capture the relevant packets (filtering using openvVPN protocol or port etc.)
Second,
As far as I know there is no hard linking between the virtual network interface and the regular (ethernet, WIFI etc.) interface, at least not in OpenVPN (there are diffrent VPN protocols). The openVPN packets will be routed to the remote server using you OS routing table.
This way if your ethernet interface is your primary default gateway, and it gets unplugged, your VPN service will be able to recover, since it will have a route to your remote VPN-server address using your WIFI interface.
I have a machine with CoreOS 1800(or 1855) installed onto disk, and with following systemd-networkd config (there is only one network interface in the machine):
$ cat /etc/systemd/network/zz-default.network
[Network]
DHCP=yes
[DHCP]
ClientIdentifier=mac
UseMTU=true
UseDomains=true
Another notable thing is that this machine is also configured with PXE boot but PXE server will reject boot so it will finally boot from disk.
When I restart the machine, there will be two DHCP IPs allocated for it, I confirmed it by checking /var/lib/dhcpd.leases in DHCP server:
lease 100.79.223.152 {
starts 5 2018/09/28 02:34:00; ends 6 2018/09/29 02:33:59; tstp 6 2018/09/29 02:33:59; cltt 5 2018/09/28 02:34:00;
binding state active; next binding state free; rewind binding state free;
hardware ethernet 08:9e:01:d9:28:64;
option agent.circuit-id 0:5:8:b9:1:0:29;
}
lease 100.79.223.150 {
starts 5 2018/09/28 02:34:29; ends 6 2018/09/29 02:34:28; tstp 6 2018/09/29 02:34:28; cltt 5 2018/09/28 02:34:29;
binding state active; next binding state free; rewind binding state free;
hardware ethernet 08:9e:01:d9:28:64; uid "001010236001331(d";
option agent.circuit-id 0:5:8:b9:1:0:29;
}
The lease record 100.79.223.152 is requested by the PXE loader, though rejected by DHCP server.
The lease record 100.79.223.150 is requested by systemd-networkd of
CoreOS. (I can confirm it by running systemctl restart systemd-networkd and watch the leases file)
All seems fine, but the PXE lease record 100.79.223.152 cause other problem (when really PXE boot the machine and deploy another OS to it then it will get the 100.79.223.152 instead of 150, then cause other private problem).
If I install other OS which does not use systemd-networkd, then the reboot only cause 1 lease record.
You can see the lease 100.79.223.150 has a field uid "001010236001331(d", which means let DHCP server allocate IP by the uid (client identifier), currently it is actually same content of mac address, just be printed as octet.
This is the root cause of two IPs.
To prevent this two IP problem, I've tried to set deny duplicates in /etc/dhcp/dhcpd.conf in DHCP server, but nothing changes.
I was wandering that if it is possible to tell systemd-networkd not to send uid (client identifier). According to source of systemd, it is intentionally implemented to "always send client identifier",
given such condition, how can I prevent systemd-networkd from sending client identifier?
EDIT 2019/02/17: I found that I misunderstood the meaning of deny duplicates, it does not help this problem.
I remembered I had ever tested another option first but not works.
ignore-client-uids on;
The ignore-client-uids statement
ignore-client-uids flag;
If the ignore-client-uids statement is present and has a value of true
or on, the UID for clients will not be recorded. If this statement is
not present or has a value of false or off, then client UIDs will be
recorded.
https://www.isc.org/wp-content/uploads/2017/08/dhcp43.html
The DHCP server version is isc-dhcpd-4.2.4
EDIT 2019-03-12: I had some mistaken and found something, so answered this question myself. Simple answer is ignore-client-uids true; on server side works well, ClientIdentifier=mac on client side does not work well.
Have you tried setting the client identifier to (empty)?
$ cat /etc/systemd/network/zz-default.network
[Network]
DHCP=yes
[DHCP]
ClientIdentifier=
UseMTU=true
UseDomains=true
After many times of experiments, I found that only ignore-client-uids true; works constantly, all mystery disappeared., when you set it, you can confirm that no uid "....." appear in /var/lib/dhcp/dhcpd.leases` file, the server completely ignore the client identifier sent from client and just use MAC to determine how to allocate IP.
If you insist on using ClientIdentifier=mac, you can take a look at what I found:
specifying ClientIdentifier=mac (on client *.network) does let me get same IP as before. The reason why I said it does not work is probably because I have another NIC which also enabled DHCP by default hence caused a new IP.
/lib/systemd/network/zz-default.network
[Network]
DHCP=yes
[DHCP]
UseMTU=true
UseDomains=true
After I change above file to
[Network]
DHCP=no
I got only 1 and same IP as before.
The client identifier will be a string "\0x1" + MAC, you can confirm it grep uid "..." in /var/lib/dhcp/dhcpd.leasesfile, e.g.,uid "001304TDD210272"`, for any non-printable char it will be encoded as 3 digits Octal such as 304. Some client automatically generate an client identifier like this "\0x1" + "MAAS" + MAC ...
The most unfortunate thing is: once a client send client identifier, for the same MAC, if the client send anther request WITHOUT client identifier, it will get new IP.
Considering DDNS, for same MAC, the DHCP request with and without client identifier are treated as different client when DHCP server composing DNS update request for it. Simply speaking,
for DHCP request without client identifier -> server send DDNS request with a hash of the MAC -> DNS server: OK
for DHCP request with client identifier -> server send DDNS request with a hash of the client identifier -> DNS server: rejected due to the hash is not same, for security.
That is all I found, hope it helpful.
You can also check if there is configuration under the /run/.../systemd/network/*.network, I had the same issue because of netplan putting a configuration network file in the /run which is applied instead of the etc or lib one.
The solution in this case is to add the dhcp-identifier: mac in the the netplan yml configuration
I'd like to check the option to debug my kernel driver installed over remote physical machine (since I don't have firewire cables). Reading the relevant documentation, I haven't seen any limitation about remote physical debugging medium, so I deduced both firewire cables and ip over wireless network should work.
I thought that lldb remote connection using kdp-remote <machine-ip> would do the trick, but I don't get any response.
From remote VM however, it succeed even though the VM can be located on remote physical machine.
My boot-args configuration are keepsyms=1 debug=0x144 -v
We figured out the problem in the comments (item 2 below), but for posterity, here's a list of things to check if xnu kernel debugging isn't working:
The target machine must have a physical ethernet port which is connected via PCIe or Thunderbolt, or you must use a direct firewire connection (optionally via Thunderbolt). USB to ethernet adapters will not work on the target end. The client machine is less fussy, you can use wifi or USB-ethernet there.
The protocol uses UDP, not TCP. Have you got a firewall running on your lldb machine which might be blocking UDP packets? (You could try sending udp packets from target to client with the nc (netcat) tool while the machine is not crashed.)
Is the ARP entry correct on the client machine? arp <target ip> should yield the target interface's MAC address.
The crashed Mac will NOT respond to pings, only to KDP packets via UDP. So not getting pings back doesn't mean anything.
As far as I'm aware the machine won't request a new DHCP lease when it crashes, so that shouldn't be the problem, but you can always try setting a static IP address just to be sure.
Did you reboot after setting the boot-args? They only take effect on a fresh boot.
If SIP is active, you can only set nvram variables from the recovery environment from OS X/macOS 10.11 onwards. You can run nvram boot-args to verify that the settings stuck.
My personal recommendation is to use FireWire for kernel debugging if possible, it seems to be the fastest and most reliable in my experience.
Is there a way to find out the IP address of a device that is directly connected to a specific ethernet interface? I.e. given one host, one wired ethernet connection and one second host connected to this wired connection, which layer or protocol below IP could be used to find this out.
I would also be comfortable with a Windows-only solution using some Windows-API function or callback.
(I know that the real way to do this would probably via DHCP, but this is about discovering a legacy device.)
Mmh ... there are many ways.
I answer another network discovery question, and I write a little getting started.
Some tcpip stacks reply to icmp broadcasts.
So you can try a PING to your network broadcast address.
For example, you have ip 192.168.1.1 and subnet 255.255.255.0
ping 192.168.1.255
stop the ping after 5 seconds
watch the devices replies : arp -a
Note : on step 3. you get the lists of the MAC-to-IP cached entries, so there are also the hosts in your subnet you exchange data to in the last minutes, even if they don't reply to icmp_get.
Note (2) : now I am on linux. I am not sure, but it can be windows doesn't reply to icm_get via broadcast.
Is it the only one device attached to your pc ?
Is it a router or another simple pc ?
To use DHCP, you'd have to run a DHCP server on the primary and a client on the secondary; the primary could then query the server to find out what address it handed out. Probably overkill.
I can't help you with Windows directly. On Unix, the "arp" command will tell you what IP addresses are known to be attached to the local ethernet segment. Windows will have this same information (since it's a core part of the IP/Ethernet interface) but I don't know how you get at it.
Of course, the networking stack will only know about the other host if it has previously seen traffic from it. You may have to first send a broadcast packet on the interface to elicit some sort of response and thus populate the local ARP table.
Windows 7 has the arp command within it.
arp -a should show you the static and dynamic type interfaces connected to your system.
Your Best Approach is to install Wireshark, reboot the device wait for the TCP/UDP stream , broadcasts will announce the IP address for both Ethernet ports
This is especially useful when the device connected does not have DHCP Client enabled, then you can go from there.
You can also get information from directly connected networking devices, such as network switches with LDWin, a portable and free Windows program published on github:
http://www.sysadmit.com/2016/11/windows-como-saber-la-ip-del-switch-al-que-estoy-conectado.html
LDWin supports the following methods of link discovery: CDP (Cisco Discovery Protocol) and LLDP (Link Layer Discovery Protocol).
You can obtain the model, management IP, VLAN identifier, Port identifier, firmware version, etc.
I often run into problems where I can't get something to connect to something else. I usually forget to check something obvious. Can you help with:
A tip/technique for diagnosing a connection issue
The name of a tool or application that can help (and the situation in which it's useful)
I know the question is a little non-specific, but hopefully the answers can form a useful starting point for anybody who's stuck trying to get computers/programs talking to each other.
Please can you give one answer per answer so the best ones can be voted up.
Simple checks to run when debugging network problems:
Has each machine got an IP address, Go to command prompt and run ipconfig. Key things to check here are the interfaces and ensuring the appropriate ones have IP addresses.
Check both machines IP addresses are in the same range and subnet if you are running it on an internal or Virtual network.
Try pinging each machine from the other to see if they can communicate with each other. Note that some firewalls will block ping requests.
If Pinging fails then check to see if firewalls are active. If the communication is within a 'safe' internal network then try disabling the firewalls and re-pinging.
If the connections are over a wireless network then check signal strength.
If pinging fails and you are connecting through several networks then try running a tracert to see at which will may show you where on the network the connection is failing.
If you are able to ping but not connect then check firewall settings and network connection settings. Windows 2000+ has the capability of setting port an ip access on a connection properties.
Try drawing a network diagram of the connections to help in visualising the problem.
If you are connecting through routers, firewalls and loadbalancers then check that all devices are not tied to any specific ip addresses and that the IP address redirection (if in place) is correct. Also check any NAT logs to see if connections are being received and properly re-directed.
Wireshark
Latest versions of ProcMon
netstat
Wireshark www.wireshark.org
Wireshark is a network protocol analyzer for Unix and Windows.
Features:
Deep inspection of hundreds of protocols, with more being added all the time
Live capture and offline analysis
Standard three-pane packet browser
Multi-platform: Runs on Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, and many others
Captured network data can be browsed via a GUI, or via the TTY-mode TShark utility
The most powerful display filters in the industry
Rich VoIP analysis
Read/write many different capture file formats: tcpdump (libpcap), Catapult DCT2000, Cisco Secure IDS iplog, Microsoft Network Monitor, Network General Sniffer® (compressed and uncompressed), Sniffer® Pro, and NetXray®, Network Instruments Observer, Novell LANalyzer, RADCOM WAN/LAN Analyzer, Shomiti/Finisar Surveyor, Tektronix K12xx, Visual Networks Visual UpTime, WildPackets EtherPeek/TokenPeek/AiroPeek, and many others
Capture files compressed with gzip can be decompressed on the fly
Live data can be read from Ethernet, IEEE 802.11, PPP/HDLC, ATM, Bluetooth, USB, Token Ring, Frame Relay, FDDI, and others (depending on your platfrom)
Decryption support for many protocols, including IPsec, ISAKMP, Kerberos, SNMPv3, SSL/TLS, WEP, and WPA/WPA2
Coloring rules can be applied to the packet list for quick, intuitive analysis
Output can be exported to XML, PostScript®, CSV, or plain text.
work the OSI model from the bottom up
Physical (Do you have a network adapter/connection)
Link layer (arp, ethernet port blocked by network team (I've seen this where locked down environments see two MAC addresses coming from one workstation port and shut down the port)
Network layer (ipconfig, tracert, ping,)
Do you have a network address (DHCP, fixed)
Are you on a proper subnet/have routing between subnets
Is something in the middle blocking you
firewalls, routing tables
When in doubt, check to see if the windows firewall is messing with your communications. 8 times out of 10, it's at fault.
Using tracert is a good start to see how far along the chain you are getting.
For virtual machines it's usally a good idea to make sure you have the loopback adapter set correctly in the Host os.
Most frequently used tool is the ping. It can be used both to test your connection and the availability of a target
Second tool is the tracert if you want to see where the packets get lost.
For more advanced debugging I use the following tools: nmap, wireshark, etc.
Windows has a netstat utility which is pretty similar to the Unix netstat and can do a number of different things that might help you solve network issues.
Random example:
netstat -r displays routing information
netstat /? for usage information
Since you said you're using 2 virtual machines I would hazard a guess that both machines are setup in a NAT configuration (rather than a unique network device) -- In the NAT configuration, neither machine would (typically) be able to ping the other.
If you're familiar with the command line, you can try the "netstat" command.
You can also try "arp -a" to list all the IP/MAC addresses known to your PC.
The "tracert [ip address]" command will show you how many gateways/routers your packets jump through on their way to their destination. (This is probably not helpful if both machines are on the same network, though.)
And don't forget to check your Windows firewall settings.
Otherwise, if you want to get down and dirty, you can try the packet sniffer known as Wireshark: http://www.wireshark.org/ (aka. Ethereal)
Pull the network cable out
If you can get some communications to a device (eg a ping), but can't get your program to talk to a service on the computer. Then, try pulling the network cable out and see if the ping stops. This will verify you're communicating with the computer you really think you are.
On windows i user PortQueryUI : http://www.microsoft.com/en-us/download/details.aspx?id=24009
DNS activity: Portable DNS Cache and Firewall;
General network activity: Wireshark, Network Monitor;
Windows utilities: ping, netstat, nslookup.
You need to be use the process of elimination, for example if you can ping the ip address but not the hostname then there's DNS issues. If you can ping the system but not connect to a share etc.
DNS out of sync
If you're using a virtual machine and you perform a roll-back on it, then it could become out of sync with the DNS (Domain name server). Try to remove and re-add the machine to the domain, or if you've got access to the DNS machine, then get it to flush its cache.