In-kernel NAT [ipfw] dropping large UDP return packets

[Andrey V. Elsukov]

On 13.06.2018 23:04, Jeff Kletsky wrote:
>> The kernel version of libalias uses m_megapullup() function to make
>> single contiguous buffer. m_megapullup() uses m_get2() function to
>> allocate mbuf of appropriate size. If size of packet greater than 4k it
>> will fail. So, if you use MTU greater than 4k or if after fragments
>> reassembly you get a packet with length greater than 4k, ipfw_nat()
>> function will drop this packet.
>>
> Thanks!!
> 
> Mystery solved...
> 
> /usr/src/sys/netinet/libalias/alias.c
> 
> #ifdef _KERNEL
> /*
>  * m_megapullup() - this function is a big hack.
>  * Thankfully, it's only used in ng_nat and ipfw+nat.
> 
> suggests that the "old school" approach of natd might resolve this. I'll
> give it a try when I'm close enough to the box to resolve it when I make
> a configuration error.

I didn't look at the rest of libalias, but you, probably, can improve
this hack to use 9k or 16k mbufs. You can replace m_get2() call in
m_megapullup() with the following code:

if (len <= MJUMPAGESIZE)
	mcl = m_get2(len, M_NOWAIT, MT_DATA, M_PKTHDR);
else if (len <= MJUM9BYTES)
	mcl = m_getjcl(M_NOWAIT, MT_DATA, M_PKTHDR, MJUM9BYTES);
else if (len <= MJUM16BYTES)
	mcl = m_getjcl(M_NOWAIT, MT_DATA, M_PKTHDR, MJUM16BYTES);
else
	goto bad;

-- 
WBR, Andrey V. Elsukov

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