Non-Huawei Cloud Terminals Cannot Obtain IP Addresses from an S7700 (DHCP Server)

Publication Date:  2015-06-09 Views:  504 Downloads:  0
Issue Description
The customer has about 200 non-Huawei cloud terminals and replaces the original non-Huawei DHCP server with Huawei S7706. The cloud terminals cannot obtain IP addresses, but desktop PCs can obtain IP addresses.
Handling Process
1. Configure a PC as a DHCP server. Cloud terminals can obtain IP addresses through DHCP. Configure the S7700 as a DHCP server, only the desktop PCs can obtain IP addresses.

The test result indicates that the devices are incompatible.

2. Capture the packets exchanged during IP address obtaining when the S7700 functions as a DHCP server.

The following figure shows the captured packets.

Figure 4-1 Packets captured during IP address obtaining



OFFER packets; however, the S7700 did not receive the packets returned by cloud terminals.

3. Capture DHCP packets when the original DHCP server is used and when the S7700 is used as the DHCP server.

The following figure shows the DHCP OFFER packet returned by the non-Huawei DHCP server.

Figure 4-2 DHCP OFFER packet returned by the non-Huawei DHCP server



The following figure shows DHCP OFFER packet returned by the S7700.

Figure 4-3 DHCP OFFER packet returned by the S7700



Compare the Option fields in two DHCP OFFER packets. The DHCP OFFER packet returned by the non-Huawei DHCP server has more options than the DHCP OFFER packet returned by the S7700. These missing options are not configured on the S7700, and the Option 55 field in the DHCP DISCOVER packet does not request these options.

4. Confirm with the customer whether these options are required. If cloud terminals need to check these options, add these options on the S7700.

Add options as follows:

dns-list 61.128.114.133 
nbns-list 61.128.114.133 
domain-name dd 
option 7 ip-address 192.168.1.1   
force insert option 7 15 44  //The option 55 field in the DHCP DISCOVER packet sent by clients does not request options 7, 15, and 44. Run the command to insert these options forcibly.

5. After the configuration is modified, cloud terminals still fail to obtain IP addresses from the S7700. Continue to compare the two DHCP OFFER packets.
The following figure shows the DHCP OFFER packet returned by the non-Huawei DHCP server.

Figure 4-4 DHCP OFFER packet returned by the non-Huawei DHCP server



The following figure shows DHCP OFFER packet returned by the S7700.

Figure 4-5 DHCP OFFER packet returned by the S7700



The UDP checksum in the DHCP packets sent by S7700 is 0000. According to RFC, UDP checksum with the value being 0000 does not need to be checked. There is a possibility that the cloud terminals still checked the UDP checksum with the value being 0000, causing an error in packet checking. As a result, the cloud terminals do not process the packets sent from the S7700.
Root Cause
According to RFC, clients do not need to check the UDP checksum in the DHCP packets sent by the S7700; however, the cloud terminals still checked the UDP checksum, causing an error.
Solution
It is recommended that the customer coordinate Huawei and the cloud terminal vendor to fix the problem.
Suggestions
Capture and compare packets to find out the differences.

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