mount.nfs4 clientaddr= option inconsistent behaviour

mount.nfs4 clientaddr= option inconsistent behaviour

[Gabriel Barazer]

Hi,

There is an inconsistent behaviour of mount.nfs4 regarding how to 
detect/handle the client IP address.
The "clientaddr" option is allowed for mount.nfs4, but this address is 
not used for the client IP address, and if there is any DNS failure or 
misconfiguration, mount.nfs4 will fail.

The error is in get_my_ipv4addr() at nfs4mount.c:170 (nfs-utils-1.1.0). 
This function fills *ip_addr with the client address, detected from the 
resolved machine name (gethostname() which is tried to be resolved in 
fill_ipv4_sockaddr). This cannot work if the machine name is not (yet) 
properly resolvable in DNS at mount time (e.g. we are nfs4-mounting the 
root filesystem very early). Using the clientaddr option provided before 
trying to resolve the gethostname() hostname, would be IMHO a more 
consistent behaviour.

This can cause much trouble too with multihomed clients. The right way 
would be to detect the client IP from the server IP (or resolved IP) 
(and deal with clients with multiple addresses per subnet, by taking the 
lowest address only)

Patch for this is easy (the first part, not the ip detection code), and 
can be provided if necessary.

Gabriel

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Re: [NFS] mount.nfs4 clientaddr= option inconsistent behaviour

[Chuck Lever]

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Hi Gabriel-

Gabriel Barazer wrote:
> There is an inconsistent behaviour of mount.nfs4 regarding how to 
> detect/handle the client IP address.
> The "clientaddr" option is allowed for mount.nfs4, but this address is 
> not used for the client IP address, and if there is any DNS failure or 
> misconfiguration, mount.nfs4 will fail.
> 
> The error is in get_my_ipv4addr() at nfs4mount.c:170 (nfs-utils-1.1.0). 
> This function fills *ip_addr with the client address, detected from the 
> resolved machine name (gethostname() which is tried to be resolved in 
> fill_ipv4_sockaddr). This cannot work if the machine name is not (yet) 
> properly resolvable in DNS at mount time (e.g. we are nfs4-mounting the 
> root filesystem very early). Using the clientaddr option provided before 
> trying to resolve the gethostname() hostname, would be IMHO a more 
> consistent behaviour.

I may be mistaken, but I don't think that the Linux NFSv4 implementation 
is made for mounting the root file system.  You may be wandering into an 
area that has had no testing, and in fact, the implementation may not be 
designed to do this at all.

> This can cause much trouble too with multihomed clients. The right way 
> would be to detect the client IP from the server IP (or resolved IP) 
> (and deal with clients with multiple addresses per subnet, by taking the 
> lowest address only)

Yes, this is good analysis, I think.  Neil Brown added some logic to 
mount recently that determines the client's IP address by opening a 
socket to the server, and capturing the local endpoint's address.  This 
takes into account which network interface is used on the client to 
contact the server.  I'm not sure whether this logic is used for 
determining the clientaddr address yet.

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