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.
Related
Can I write a Windows Task or some kind of configuration script that will choose between two Local Area Connections according to the website I am visiting?
It may not be the best answer to my problem, but if it is possible, at least I know it will work.
The issue in full:
My main ISP currently has an issue routing me to my own websites (all hosted on the same server). It also has a 'sticky IP address' (note, not static) - it will only change your IP address once a fortnight, and they can't (won't) even force a change. Their second line support are working on the issue, but so far, no good, and I cannot access my own websites via their internet connection.
So, currently, I am switching from my main network to my mobile 3G network (tethered) any time I want to work on or view my own websites.
I would like to write a script that will make Windows automatically choose my mobile network for FTP, email and browsing my own websites, but use my main ISP for all other online activity.
Haven't a clue where to start - any help appreciated! Thanks,
Sarah
Well, that was simple, no scripting required. If you're using IPv4 (you can check here https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&ved=0CDoQFjAB&url=http%3A%2F%2Fwww.amiusingipv6.com%2F&ei=Bq4_UpakM4vv0gXboIDwCQ&usg=AFQjCNHiUnyKvVcUe8Z966YwoycLI28urw&bvm=bv.52434380,d.d2k)
Turn Windows features on or off.
Turn on RIP Listener.
Disable your internet connection that cannot connect to the website you are trying to access, and 'tracert' to the website: make a note of the IP address of the website, and the first IP address in the hop list ([FIRST IP]).
Type in 'route print' and make a note of the Interface number of your secondary internet connection.
Type in 'route add -p [WEBSITE IP ADDRESS] mask 255.255.0.0 [FIRST IP] IF [INTERFACE NUMBER] metric 1'
Reboot.
All traffic to the website IP address (be it mail, FTP, whatever) will go through the secondary connection, all other traffic goes through the main connection.
IPv6 instructions here:
http://windows.microsoft.com/en-gb/windows7/configuring-multiple-gateways-on-a-network
My setup is just an iPhone connected via USB as secondary connection and a Home Hub connected via ethernet as primary.
Hope this is useful to someone else - but of course, no one should ever think of using this to get around IP blocks on message boards...
Sarah
As the question states, I have two ethernet devices I need to use. A wifi hotspot for general traffic as well as a LAN connection for local traffic (192.168.1.*). Right now the wifi receives all traffic and disregards the existence of my local LAN devices. I am trying to communicate with this device using python's urllib2 and basic http fetches. The program works partially when I turn one or the other off (turning wifi off makes the LAN code work, and turning LAN off makes wifi/general traffic code work). I believe this is more of an operating system issue than a programming question, but I might be mistaken. I have been messing with the Ethernet setting in system preferences, but nothing has been working so far.
Thanks for the help.
Depending on your needs and degrees of freedom, you can:
1) Easiest: If you can control one or both network ranges, you can put wifi and LAN on different subnets. For example, 192.168.2.* could be LAN traffic, and 192.168.1.* could be wifi. If only the WiFi side has a gateway, then all traffic except 2.* traffic should route through WiFi, and all 1.* traffic goes to the LAN. No change to your computer.
2) Medium: if you don't control the networks, you can define routing rules for the two ports. This lets you say certain IP addresses should be reached thru wifi, vs others thru LAN port. An example in Linux, which I think should work on OS X too: http://linux-ip.net/html/routing-tables.html The trickiest thing is to make sure you won't get in the way when you're in someone else's network. You can do this by creating narrow routing rules, or turning them off when you don't need them. It sounds like you're doing this from a python program, so maybe the program could turn this on and off at start and finish.
3) Slightly more exotic: I wasn't totally sure from your question, but if you're trying to do load balancing (not separate networks), you could create rules to bond together two network ports and spread traffic across them. This isn't something I've done, but real network engineers know how to set this stuff up.
I could expand on one of these if you clarify what you're trying to do and what degrees of freedom you have.
For somebody don't want to do the study you can simply repeat my steps:
Open system preferences
click on Network
(ensure the precedence, first thunderbolt then wifi) click on thunderbolt
Configure IPv4 choose manually
delete router
done!
I have exactly the same user case. But I read this without any network knowledge.
I achieved this by simply following #Nils' 1) instructions. I only understand theses instructions after reading this link
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.
Is there a Mac/Unix commands that lets you see the local network in terms of machines/IP addresses? If there's something on the Mac that is gui-based that would be great too.
ping the broadcast address (the broadcast address is printed as part of the output to ifconfig en0)
The hosts answering are on your local network. (You may also try arp -a but that only keeps track of recently contacted hosts so you may want to run it after the broadcast.)
There is a program called Bonjour Browser that will list well known services that have registered on your local network. I believe that most Macs have one or more registered protocols by default.
The only way to reliably do this is to scan the network using ping sweeps and similar techniques looking for open ports etc on various addresses. You can do that with nmap which is available for OS X. See http://www.netadmintools.com/art406.html for an example.
EDIT: Just to clarify, as diciu pointed out, you can usually ping the broadcast address and/or use your arp cache as well. This will probably work for most home networks where directed broadcast is allowed.
If not, then you would need to run a ping sweep with a tool like nmap to individually check each address for an available host. Many network discovery/scanning tools can check for more than just ping, looking for listening ports, SNMP, etc. as well.
in the days of tiger (10.4) every mac broadcast a 'presence' service on bonjour, which made finding macintoshes on the network a snap. alas, no more...
You can use netdiscover on Mac OS, which is based on ARP packets. It will send ARP requests and scan the response.
For example, run netdiscover command sudo netdiscover -i en0 -r 10.106.0.0/16 in my local network would bring the following result:
Try IP Scanner 2.5 for OS X. http://10base-t.com/ Looking for others, but that's the only real one I've found for OS X.
You can use Nmap but that seems to be a bit much for your stated goals.
OS X ships with netstat, or open Up /Applications/Utilities/Network Utility.app, perhaps this will work for you?
EDIT: oops. netstat doesn't do what I thought.
angry IP scanner for a no mus - no fuss IP scanner with basic port scanning.
NMap and Zenmap for the big power scans.
I actually use both together, as nmap can get easily sidetracked by certain reverse proxy boxes.
alt text http://img440.imageshack.us/img440/6950/problemyd1.png
(The curly lines with dots represent a network route.)
Having a process called "foo.exe", is there a way to forward everything it communicates over TCP/IP to a forwarding proxy located elsewhere? This forwarding should not reflect to other processes.
Another question: if there are multiple network adapters, is it possible to force a process to use one specific adapter.
Since in this example the targethost.com is known, I could just edit "system32\drivers\etc\hosts" to map targethost.com's IP to localhost, where on port 8765 would be the first forwarder waiting for an incoming connection and pass everything forward to proxy.foo.com. I was wondering if there's a more elegant way of doing this.
This is not for malware, I'm doing some network testing with my complex home network. Thank you for warning us.
Some free software for this would be perfect, alternatively a code idea (native or .net). Thank you very much.
It's not too hard if you make your own computer a firewall, then your app connects to a port on your own computer, and that port is forwarded to both the original destination and logged or forwarded on to your spying computer.
Alternatively you can make your other computer the firwall and have it log/forward the info.
Finally you could use a sniffer.
SocksCap will probably do the job (if you're OK with establishing a SOCKS proxy at proxy.foo.com).
You could hook into the TCP stack, for example, by using the Windows Filtering Platform or its predecessors, or you could substitute the network libraries/calls of that particular process.