I was unable to get my E5810A to update using ftp.keysight.com or kaz911's FTP server. Instead, I managed to get my unit updated using a "local" FTP server on a Windows 10 laptop using ftpdmin.
Procedure:
1) Connect the E5810A to a router/switch with DHCP capability or use a crossover cable to connect it to a PC with an Ethernet port.
1.1) The E5810A's IPv4 address is shown on the front panel LCD. Default for my unit was 169.254.58.10 when a DHCP server was absent.
1.2) If connecting directly, configure your PC with an IPv4 address in the same network as the E5810A (e.g., 169.254.58.11).
2) Connect to the E5810A using a web browser (using its IPv4 address as the URL) and reset the unit back to factory defaults.
3) Download the attached e5810.zip file and extract it to your C drive. E.g., C:/e5810.
3.1) Open Windows command prompt (Winkey+R and type cmd). Navigate to the e5810 directory: cd C:/ cd e5810
3.2) Disable all networking cards except for the one needed to connect to the E8510A. !!! This is to guarantee the FTP server will use the correct IPv4 address and NIC !!!
3.3) Run ftpdmin.exe inside the opened command prompt window.
3.4) TEMPORARILY disable Windows firewall. !!! THIS MUST BE DONE TO ALLOW ACCESS TO THE FTP SERVER !!!
At this point the FTP server should be accessible from the E5810A or another PC on the network. NOTE: Only the contents of the folder containing ftpdmin.exe are accessible over FTP.
4) Inside a browser, go to 169.254.58.10/html/update_firmware.html and replace "ftp.agilent.com" with the local FTP server's IPv4 address (if you don't know, its shown in the command prompt window where ftpdmin.exe was ran from).
4.1) Click "Update Firmware"and watch the CMD window containing the FTP server: It should show the E5810A connecting and copying the FW.
4.2) After the E5810A reboots, connect to it using a browser and check the FW version is now A.01.10 and re-configure your E5810A's settings.
4.3) Close the CMD window and re-enable Windows Firewall and delete the e5810 folder.
NOTES:
The simple FTP server was download from:
https://github.com/ngrcld/FtpdminModThe E8510A A.01.10 FW was obtained from xdevs.com:
https://xdevs.com/guide/e5810a/